Alpha Centauri

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Alpha Centauri A^[1]/B^[2]

The position of Alpha Centauri A and Alpha Centauri B
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Centaurus
Alpha Centauri A
Right ascension	14h 39m 36.4951s
Declination	–60° 50′ 02.308″
Apparent magnitude (V)	−0.01
Alpha Centauri B
Right ascension	14h 39m 35.0803s
Declination	–60° 50′ 13.761″
Apparent magnitude (V)	+1.33
Characteristics
Spectral type	G2 V[3][4]
U−B color index	+0.23
B−V color index	+0.69
Characteristics
Spectral type	K1 V[3][4]
U−B color index	+0.63
B−V color index	+0.90
Astrometry
Radial velocity (Rv)	−21.6 km/s
Proper motion (μ)	RA: −3678.19 mas/yr Dec.: 481.84 mas/yr
Parallax (π)	747.1 ± 1.2[5] mas
Distance	4.366 ± 0.007 ly (1.339 ± 0.002 pc)
Absolute magnitude (MV)	4.38 / 5.71
Details
Alpha Centauri A
Mass	1.100[6] M☉
Radius	1.227[6] R☉
Luminosity	1.519[6] L☉
Surface gravity (log g)	4.30[7] cgs
Temperature	5790[6] K
Metallicity	151%[6] Sun
Rotation	~22.5 ± 5.9 days[8]
Age	6 ± 1 Gyr
Alpha Centauri B
Mass	0.907[6] M☉
Radius	0.865[6] R☉
Luminosity	0.500[6] L☉
Surface gravity (log g)	4.37[7] cgs
Temperature	5260[6] K
Metallicity	160%[6] Sun
Rotation	47 days
Orbit[9]
Companion	Alpha Centauri AB
Period (P)	79.91 ± 0.011 yr
Semi-major axis (a)	17.57 ± 0.022"
Eccentricity (e)	0.5179 ± 0.00076
Inclination (i)	79.205 ± 0.041°
Longitude of the node (Ω)	204.85 ± 0.084°
Periastron epoch (T)	1875.66 ± 0.012
Argument of periastron (ω) (secondary)	231.65 ± 0.076°
Other designations
Rigil Kentaurus, Rigil Kent, Toliman, Bungula, FK5 538, CP(D)−60°5483, GC 19728, CCDM J14396-6050 α Cen A α1 Centauri, GJ 559 A, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683 α Cen B α2 Centauri, GJ 559 B, HR 5460, HD 128621, LHS 51, HIP 71681 α Cen C (＝ Proxima Cen) LHS 49, HIP 70890
Database references
SIMBAD	data
Exoplanet Archive	data
ARICNS	data
Extrasolar Planets Encyclopaedia	data

Location of Alpha Centauri in Centaurus (right-click on starmap to enlarge)

Play media

From Earth to Alpha Centauri.

Alpha Centauri (α Centauri, α Cen; also known as Rigil Kent /ˈraɪdʒəl ˈkɛnt/—see Names) or Toliman is the brightest star in the southern constellation of Centaurus and the third brightest star in the night sky.^[10][11] The Alpha Centauri system is located 1.34 parsecs or 4.37 light years from the Sun, making it the closest star system to our Solar System.^[5] Although it appears to the unaided eye as a single object, Alpha Centauri is actually a binary star system (designated Alpha Centauri AB or α Cen AB) whose combined visual magnitude of −0.27 makes it the third brightest star (other than the Sun) seen from Earth after the −1.46 magnitude Sirius and the −0.72 magnitude Canopus.

Its component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of the Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 44.5% of its luminosity. During the pair's 79.91-year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun.

A third star, known as Proxima Centauri, Proxima, or Alpha Centauri C (α Cen C), is probably gravitationally associated with Alpha Centauri AB. Proxima is at the slightly smaller distance of 1.29 parsecs or 4.24 light years from the Sun, making it the closest star to the Sun even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 astronomical units (AU); equivalent to 400 times the size of Neptune's orbit.

The system may also contain at least one planet, the Earth-sized Alpha Centauri Bb, which, if confirmed, will be the closest known exoplanet to Earth. The planet has a mass at least 13% more than Earth's^[12] and orbits Alpha Centauri B with a period of 3.236 days.^[13] Orbiting at a distance of 6 million kilometers from the star,^[12] 4% of the distance of the Earth to the Sun and a tenth of the distance between Mercury and the Sun, the planet has an estimated surface temperature of 1500 K (roughly 1200 °C), too hot to be habitable.^[14] On June 10, 2013, scientists reported that the earlier claims of an Earth-like exoplanet orbiting Alpha Centauri B may not be supported.^[15][16]

Nature and components

Mobile notation diagram of the system

"Alpha Centauri" is the name given to what appears as a single star to the naked eye and the brightest star in the southern constellation of Centaurus. At −0.27v visual magnitude,^[17] it is fainter only than Sirius and Canopus. The next brightest star in the night sky is Arcturus. Actually a multiple star system, its two main stars are Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B), usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri (or Proxima or α Cen C)—has a distance much greater than the observed separation between stars A and B and is probably gravitationally associated with the AB system. As viewed from Earth, it is located at an angular separation of 2.2° from the two main stars. If it were bright enough to be seen without a telescope, Proxima Centauri would appear to the naked eye as a star separate from α Cen AB. Alpha Centauri AB and Proxima Centauri form a visual double star. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be found.^[18] Together all three components make a triple star system, referred to by double-star observers as the triple star (or multiple star), α Cen AB-C.

Artist’s impression of the planet around Alpha Centauri B

View of Alpha Centauri from the Digitized Sky Survey 2

Component sizes and colors. Shows the relative sizes and colors of stars in the Alpha Centauri system and compares them with those of the Sun.

Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous than the Sun. It is a solar-like main-sequence star with a similar yellowish color,^[19] whose stellar classification is spectral type G2 V.^[20] From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger.^[6] The projected rotational velocity ( v·sin i ) of this star is 2.7 ± 0.7 km·s⁻¹, resulting in an estimated rotational period of 22 days,^[8] which gives it a slightly faster rotational period than the Sun's 25 days. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at −0.01 magnitude,^[20] being fractionally fainter than Arcturus at −0.04v magnitude.

Alpha Centauri B is the companion star, or secondary, of the binary system, and is slightly smaller and less luminous than the Sun. It is a main-sequence star of spectral type K1 V,^[4][20] making it more an orange color than the primary star.^[19] Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius.^[6] The projected rotational velocity ( v·sin i ) is 1.1 ± 0.8 km·s⁻¹, resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.)^[21] Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare.^[22] Alpha Centauri B at 1.33v magnitude would be twenty-first in brightness if it could be seen independently of Alpha Centauri A.

Alpha Centauri C, also known as Proxima Centauri, is of spectral class M5 Ve^[20] or M5 VIe, suggesting this is either a small main-sequence star (Type V) or subdwarf (VI) with emission lines. Its B−V color index is +1.90 and its mass is about 0.123 M_☉,^[23] or 129 Jupiter masses.^[24]

Together, the bright visible components of the binary star system are called Alpha Centauri AB (α Cen AB). This "AB" designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) in any multiple star system.^[25] "AB-C" refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Some older references use the confusing and now discontinued designation of A×B. Since the distance between the Sun and Alpha Centauri AB does not differ significantly from either star, gravitationally this binary system is considered as if it were one object.^[26]

Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology), are all consistent with the α Cen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billion years (Gyr).^[27] Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters for α Cen A and/or B have yielded age estimates of 4.85 ± 0.5 Gyr,^[28] 5.0 ± 0.5 Gyr,^[29] 5.2–7.1 Gyr,^[30] 6.4 Gyr,^[31] and 6.52 ± 0.3 Gyr.^[32] Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4–6.5 Gyr, while gyrochronology yields 5.0 ± 0.3 Gyr.^[27]

Observation

The two Alpha Centauri AB binary stars are too close together to be resolved by the naked eye, because the angular separation varies between 2 and 22 arcsec,^[33] but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in) telescopes.^[34]

In the southern hemisphere, Alpha Centauri forms the outer star of The Pointers or The Southern Pointers,^[34] so called because the line through Beta Centauri (Hadar/Agena),^[35] some 4.5° west,^[34] points directly to the constellation Crux — the Southern Cross.^[34] The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.^[36]

South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.^[37] Both stars, including Crux, are too far south to be visible for mid-latitude northern observers. Below about 29° N latitude to the equator (roughly Hermosillo, Chihuahua in Mexico, Galveston, Texas, Ocala, Florida and Lanzarote, the Canary Islands of Spain) during the northern summer, Alpha Centauri lies close to the southern horizon.^[35] The star culminates each year at midnight on 24 April or 9 p.m. on 8 June.^[35][38]

As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB.^[39] This is about four times the angular diameter of the Full Moon, and almost exactly half the distance between Alpha Centauri AB and Beta Centauri. Proxima usually appears as a deep-red star of 13.1v visual magnitude in a poorly populated star field, requiring moderately sized telescopes to see. Listed as V645 Cen in the General Catalogue of Variable Stars (G.C.V.S.) Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly to about 11.0v or 11.09V magnitude.^[20] Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.^[40]

Observational history

English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work Tractatus de Globis, along with Canopus and Achernar, noting "Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third [Alpha Centauri] is in the right foote of the Centaure."^[41]

The binary nature of Alpha Centauri AB was first recognized in December 1689 by astronomer and Jesuit priest Jean Richaud. The finding was made incidentally while observing a passing comet from his station in Puducherry. Alpha Centauri was only the second binary star system to be discovered, preceded only by Alpha Crucis.^[42] By 1752, French astronomer Abbé Nicolas Louis de Lacaille made astrometric positional measurements using a meridian circle while John Herschel, in 1834, made the first micrometrical observations.^[43] Since the early 20th century, measures have been made with photographic plates.^[44]

By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system.^[45] All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris.^[46] Others, like the Belgian astronomer D. Pourbaix (2002), have regularly refined the precision of any new published orbital elements.^[47]

Alpha Centauri A and B resolved over the limb of Saturn, as seen by Cassini–Huygens

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Taken with Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30 seconds.

Alpha Centauri is the closest star system to the Solar System. It lies about 4.37 light-years in distance, or about 41.5 trillion kilometres, 25.8 trillion miles or 277,600 AU. Scottish astronomer Thomas Henderson made the original discovery from many exacting observations of the trigonometric parallaxes of the AB system between April 1832 and May 1833. He withheld the results because he suspected they were too large to be true, but eventually published in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838.^[48] For this reason, Alpha Centauri is considered as the second star to have its distance measured because it was not formally recognized first.^[48] Alpha Centauri is inside the G-cloud, and the nearest known system to it is Luhman 16 at 3.6 light years.^[49]

Scottish astronomer Robert Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a dedicated proper motion survey. This showed the large proper motion and parallax of the star was similar in both size and direction to those of Alpha Centauri AB, suggesting immediately it was part of the system and slightly closer to us than Alpha Centauri AB. Lying 4.24 light-years away, Proxima Centauri is the nearest star to the Sun. All current derived distances for the three stars are from the parallaxes obtained from the Hipparcos star catalog (HIP).^{[50][51][52][53]}

Distance

Alpha Centauri distance estimates

Source	Parallax, mas	Distance, pc	Distance, ly	Ref.
Henderson (1839)	1160 ± 110	0.86+0.09 −0.07	2.81+0.29 −0.24	[54]
Woolley et al. (1970)	743 ± 7	1.346 ± 0.013	4.39 ± 0.04	[55]
Gliese & Jahreiß (1991)	749.0 ± 4.7	1.335 ± 0.008	4.355 ± 0.027	[56]
van Altena et al. (1995)	749.9 ± 5.4	1.334 ± 0.01	4.349+0.032 −0.031	[57]
Perryman et al. (1997) (Hipparcos)	742.12 ± 1.40	1.3475 ± 0.0025	4.395 ± 0.008	[58][59]
Perryman et al. (1997) (Tycho)	(absents)			[60][61]
Söderhjelm (1999)	747.1 ± 1.2	1.3385+0.0022 −0.0021	4.366 ± 0.007	[5]
van Leeuwen (2007) (A)	754.81 ± 4.11	1.325 ± 0.007	4.321+0.024 −0.023	[62]
van Leeuwen (2007) (B)	796.92 ± 25.90	1.25 ± 0.04	4.09+0.14 −0.13	[63]
RECONS TOP100 (2012)	747.23 ± 1.17[nb 1]	1.3383 ± 0.0021	4.365 ± 0.007	[64]

Non-trigonometric distance estimates are marked in italic. The best estimate is marked in bold.

Binary system

Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time ^[9] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. Since the orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days.

With the orbital period of 79.91 years,^[47] the A and B components of this binary star can approach each other to 11.2 astronomical units, equivalent to 1.67 billion km or about the mean distance between the Sun and Saturn, or may recede as far as 35.6 AU (5.3 billion km—approximately the distance from the Sun to Pluto).^[47][65] This is a consequence of the binary's moderate orbital eccentricity e = 0.5179.^[47] From the orbital elements, the total mass of both stars is about 2.0 M_☉^[66]—or twice that of the Sun.^[65] The average individual stellar masses are 1.09 M_☉ and 0.90 M_☉, respectively,^[67] though slightly higher masses have been quoted in recent years, such as 1.14 M_☉ and 0.92 M_☉,^[20] or totalling 2.06 M_☉. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.^[20][44] Stellar evolution theory implies both stars are slightly older than the Sun^[6] at 5 to 6 billion years, as derived by both mass and their spectral characteristics.^[39][68]

Viewed from Earth, the apparent orbit of this binary star means that the separation and position angle (P.A.) are in continuous change throughout the projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsec through the P.A. of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011.^[47] Next closest approach will be in February 2016, at 4.0 arcsec through 300°.^[47][69] Observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.^[70] Widest separation occurred during February 1976 and the next will be in January 2056.^[47]

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbital separation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the two stars is rapidly decreasing, at least until 2019.^[47]

Companion: Proxima Centauri

The much fainter red dwarf star named Proxima Centauri, or simply Proxima, is about 15,000 AU away from Alpha Centauri AB.^[25][39][44] This is equivalent to 0.24 light years or 2.2 trillion kilometres—about 5% the distance between Alpha Centauri AB and the Sun. Proxima is likely gravitationally bound to Alpha Centauri AB, orbiting it with a period between 100,000 and 500,000 years.^[39] However, it is also possible that Proxima is not gravitationally bound and thus moving along a hyperbolic trajectory^[71] with respect to Alpha Centauri AB.^[25] The main evidence for a bound orbit is that Proxima's association with Alpha Centauri AB is unlikely to be accidental, since they share approximately the same motion through space.^[39] Theoretically, Proxima could leave the system after several million years.^[72] It is not yet certain whether Proxima and Alpha are truly gravitationally bound.^[73]
Proxima is an M5.5 V spectral class red dwarf with an absolute magnitude of +15.53, which is only a small fraction of the Sun's luminosity. By mass, Proxima is calculated as 0.123 ± 0.06 M_☉ (rounded to 0.12 M_☉) or about one-eighth that of the Sun.^[74]

High-proper-motion star

Stars closest to the Sun, including Alpha Centauri (25 April 2014).^[75]

All components of Alpha Centauri display significant proper motions against the background sky, similar to the first magnitude stars Sirius and Arcturus. Over the centuries, this causes the apparent stellar positions to slowly change. Such motions define the high-proper-motion stars.^[76] These stellar motions were unknown to ancient astronomers. Most assumed that all stars were immortal and permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle.^[77]

Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.^[78] For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in 1800 years,^[79] as did the brightest star, Sirius, in Canis Major (α CMa).^[80] Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest^[81] whose original data included portions from an earlier catalog by Hipparchos during the 1st century BCE.^[82][83][84] Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th century.^[70]

Scottish-born observer Thomas James Henderson in the 1830s at the Royal Observatory at the Cape of Good Hope discovered the true distance to Alpha Centauri.^[85][54] He soon realised this system displayed an unusually high proper motion,^[86] and therefore its observed true velocity through space should be much larger.^[87][70] In this case, the apparent stellar motion was found using Abbé Nicolas Louis de Lacaille's astrometric observations of 1751–1752,^[88] by the observed differences between the two measured positions in different epochs. Using the Hipparcos Star Catalogue (HIP) data, the mean individual proper motions are −3678 mas/yr or −3.678 arcsec per year in right ascension and +481.84 mas/yr or 0.48184 arcsec per year in declination.^[89][90] As proper motions are cumulative, the motion of Alpha Centauri is about 6.1 arcmin each century, and 61.3 arcmin or 1.02° each millennium. These motions are about one-fifth and twice, respectively, the diameter of the full moon.^[72] Using spectroscopy the mean radial velocity has been determined to be 25.1 ± 0.3 km/s towards the Solar System.^[91][92]

As the stars of Alpha Centauri approach us, the measured proper motion and trigonometric parallax slowly increase.^{[39][72][72][89]} Changes are also observed in the size of the semi-major axis of the orbital ellipse, increasing by 0.03 arcsec per century.^[25] This change slightly shortens the observed orbital period of Alpha Centauri AB by some 0.006 years per century. This small effect is gradually decreasing until the star system is at its closest to us, and is then reversed as the distance increases again.^[25] Consequently, the observed position angles of the stars are subject to changes in the orbital elements over time, as first determined by W. H. van den Bos in 1926.^[93][94][95] Some slight differences of about 0.5% in the measured proper motions are caused by Alpha Centauri AB's orbital motion.^[89]

Apparent motion of Alpha Centauri relative to Beta Centauri

Based on these observed proper motions and radial velocities, Alpha Centauri will continue to gradually brighten, passing just north of the Southern Cross or Crux, before moving northwest and up towards the celestial equator and away from the galactic plane. By about 29,700 AD, in the present-day constellation of Hydra, Alpha Centauri will be 1.00 pc or 3.26 ly away.^[72] Then it will reach the stationary radial velocity (RVel) of 0.0 km/s and the maximum apparent magnitude of −0.86V (which is comparable to present-day magnitude of Canopus). However, even during the time of this nearest approach, the apparent magnitude of Alpha Centauri will still not surpass that of Sirius (which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen from Earth for the next 210,000 years).^[96]

The Alpha Centauri system will then begin to move away from the Solar System, showing a positive radial velocity.^[72] Due to visual perspective, about 100,000 years from now, these stars will reach a final vanishing point and slowly disappear among the countless stars of the Milky Way. Here this once bright yellow star will fall below naked-eye visibility somewhere in the faint present day southern constellation of Telescopium (this unusual location results from the fact that Alpha Centauri's orbit around the galactic centre is highly tilted with respect to the plane of the Milky Way galaxy).^[72]

Apparent movement

In about 4000 years, the proper motion of Alpha Centauri will mean that from the point of view of Earth it will appear close enough to Beta Centauri to form an optical double star. Beta Centauri is in reality far more distant than Alpha Centauri.

Planets

Until the 1990s, technologies did not exist that could detect planets outside the Solar System.^[97] However, some exoplanets may be orbiting the Alpha Centauri system.

The Alpha Centauri B planetary system^[98]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	1.13 ± 0.09 M⊕	0.04	3.2357 ± 0.0008	—	—	—

Alpha Centauri Bb

On 16 October 2012, researchers, mainly from the Observatory of Geneva and from the Centre for Astrophysics of the University of Porto, announced that an Earth-mass planet had been detected in orbit around Alpha Centauri B using the radial velocity technique.^[99][100] Over three years of observations had been needed for the difficult analysis.^[12] The planet has a minimum mass of 1.13 times Earth's mass.^[13] It is not in the habitable zone, orbiting very close to the host star at just 0.04 AU and completing one orbit every 3.236 days.^[13] Its surface temperature is estimated to be 1200 °C (about 1500 K),^[101][102] far too hot for liquid water and also above the melting temperatures of many silicate magmas. For comparison, the surface temperature of Venus, the hottest planet in the Solar System, is 462 °C (735 K).

Possibility of additional planets

The discovery of planets orbiting other star systems, including similar binary systems (Gamma Cephei), raises the possibility that additional planets may exist in the Alpha Centauri system. Such planets could orbit Alpha Centauri A or Alpha Centauri B individually, or be on large orbits around the binary Alpha Centauri AB. Since both the principal stars are fairly similar to the Sun (for example, in age and metallicity), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have
used various radial velocity or star transit methods in their searches around these two bright stars.^[103] All the observational studies have so far failed to find any evidence for brown dwarfs or gas giant planets.^[103][104]

In 2009, computer simulations (then unaware of the close-in planet Bb) showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Certain special assumptions, such as considering that Alpha Centauri A and B may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster) would permit an accretion-friendly environment farther from the star.^[105] Bodies around A would be able to orbit at slightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around A or B make the likelihood of terrestrial planets greater than otherwise.^[97] Theoretical studies on the detectability via radial velocity analysis have shown that a dedicated campaign of high-cadence observations with a 1–m class telescope can reliably detect a hypothetical planet of 1.8 Earth masses in the habitable zone of B within three years.^[106]

Radial velocity measurements of Alpha Centauri B with HARPS spectrograph ruled out planets of more than 4 Earth masses to the distance of the habitable zone of the star (orbital period P = 200 days).^[13]

Alpha Centauri is envisioned as the first target for unmanned interstellar exploration. Crossing the huge distance between the Sun and Alpha Centauri using current spacecraft technologies would take several millennia, though the possibility of solar sail or nuclear pulse propulsion technology could cut this down to a matter of decades.^[107]

Theoretical planets

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B,^{[106][108][109]} but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult.^[105][110] Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.^{[97][111][112][113]}

Some astronomers speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry or lack significant atmospheres. In the Solar System both Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System. Here the comets provided the inner planets with their own source of water and various other ices^[114] but Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming enriching the area round Alpha Centauri A and B with volatile materials.^[115] This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or conversely, Alpha Centauri A for Alpha Centauri B), or if the stars B and A themselves were able to successfully perturb comets into each other's inner system as Jupiter and Saturn presumably have done in the Solar System. Because icy bodies probably also reside in Oort clouds of other planetary systems, when they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars many of these icy bodies then travel starwards.^[72] There is no direct evidence yet of the existence of such an Oort cloud around Alpha Centauri AB, and theoretically this may have been totally destroyed during the system's formation.^[72]

To be in the star's habitable zone, any suspected Earth-like planet around Alpha Centauri A would have to be placed about 1.25 AU away – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone would lie closer at about 0.7 AU (100 million km), approximately the distance that Venus is from the Sun.^[114][116]

With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among the listed "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). Detecting planets as small as three Earth-masses or smaller within two astronomical units of a "Tier 1" target would have been possible with this new instrument.^[117] The SIM mission, however, was cancelled due to financial issues in 2010.^[118]

View from this system

Looking toward the Sun from Alpha Centauri in Celestia

Looking toward the sky around Orion from Alpha Centauri with Sirius near Betelgeuse and the Sun between Perseus and Cassiopeia generated by Celestia

Viewed from near the Alpha Centauri system, the sky would appear very much as it does for earthbound observers, except that Centaurus would be missing its brightest star. The Sun would be a yellow +0.5 visual magnitude star in eastern Cassiopeia at the antipodal point of Alpha Centauri's current RA and Dec. at 02^h 39^m 35^s +60° 50′ (2000). This place is close to the 3.4 magnitude star ε Cassiopeiae. An interstellar or alien observer would find the \/\/ of Cassiopeia had become a /\/\/ shape ^[119] nearly in front of the Heart Nebula in Cassiopeia. Sirius lies less than a degree from Betelgeuse in the otherwise unmodified Orion and is with −1.2 a little fainter than from Earth but still the brightest star in the Alpha Centauri sky. Procyon is also displaced into the middle of Gemini, outshining Pollux, while both Vega and Altair are shifted northwestward relative to Deneb (which barely moves, due to its great distance)- giving the Summer Triangle a more equilateral appearance.

From Proxima itself, Alpha Centauri AB would appear like two close bright stars with the combined magnitude of −6.8. Depending on the binary's orbital position, the bright stars would appear noticeably divisible to the naked eye, or occasionally, but briefly, as single unresolved star. Based on the calculated absolute magnitudes, the visual magnitudes of Alpha Centauri A and B would be −6.5 and −5.2, respectively.^[120]

View from a hypothetical planet

Artist's rendition of the view from a hypothetical airless planet orbiting Alpha Centauri A

An observer on a hypothetical planet orbiting around either Alpha Centauri A or Alpha Centauri B would see the other star of the binary system as an intensely bright object in the night sky, showing a small but discernible disk.

For example, some theoretical Earth-like planet orbiting about 1.25 AU from Alpha Centauri A (so that the star appears roughly as bright as the Sun viewed from the Earth) would see Alpha Centauri B orbit the entire sky once roughly every one year and three months (or 1.3(4) a), the planet's own orbital period. Added to this would be the changing apparent position of Alpha Centauri B during its long eighty-year elliptical orbit with respect to Alpha Centauri A (comparable in speed to Uranus here). Depending on the position on its orbit, Alpha Centauri B would vary in apparent magnitude between −18.2 (dimmest) and −21.0 (brightest). These visual magnitudes are much dimmer than the observed −26.7 magnitude for the Sun as viewed from the Earth. The difference of 5.7 to 8.6 magnitudes means Alpha Centauri B would appear, on a linear scale, 2500 to 190 times dimmer than Alpha Centauri A (or the Sun viewed from the Earth), but also 190 to 2500 times brighter than the −12.5 magnitude full Moon as seen from the Earth.

Also, if another similar Earth-like planet orbited at 0.71 AU from Alpha Centauri B (so that in turn Alpha Centauri B appeared as bright as the Sun seen from the Earth), this hypothetical planet would receive slightly more light from the more luminous Alpha Centauri A, which would shine 4.7 to 7.3 magnitudes dimmer than Alpha Centauri B (or the Sun seen from the Earth), ranging in apparent magnitude between −19.4 (dimmest) and −22.1 (brightest). Thus Alpha Centauri A would appear between 830 and 70 times dimmer than the Sun but some 580 to 6900 times brighter than the full Moon. During such planet's orbital period of 0.6(3) a, an observer on the planet would see this intensely bright companion star circle the sky just as we see with the Solar System's planets. Furthermore, Alpha Centauri A sidereal period of approximately eighty years means that this star would move through the local ecliptic as slowly as Uranus with its eighty-four year period, but as the orbit of Alpha Centauri A is more elliptical, its apparent magnitude will be far more variable. Although intensely bright to the eye, the overall illumination would not significantly affect climate nor influence normal plant photosynthesis.^[114]

An observer on the hypothetical planet would notice a change in orientation to VLBI reference points commensurate with the binary orbit periodicity plus or minus any local effects such as precession or nutation.

Assuming this hypothetical planet had a low orbital inclination with respect to the mutual orbit of Alpha Centauri A and B, then the secondary star would start beside the primary at 'stellar' conjunction. Half the period later, at 'stellar' opposition, both stars would be opposite each other in the sky. Then, for about half the planetary year the appearance of the night sky would be a darker blue – similar to the sky during totality at any total solar eclipse. Humans could easily walk around and clearly see the surrounding terrain, and reading a book would be quite possible without any artificial light.^[114] After another half period in the stellar orbit, the stars would complete their orbital cycle and return to the next stellar conjunction, and the familiar Earth-like day and night cycle would return.

Names

The colloquial name of Alpha Centauri is Rigel Kent or Rigil Kent,^[121] short for Rigil/Rigel Kentaurus,^{[122][nb 2]} the romanization of the Arabic name رجل القنطورس Rijl Qanṭūris,^[121] from the phrase Rijl al-Qanṭūris "the foot of the Centaur".^[123] This is sometimes further abbreviated to Rigel, though that is ambiguous with Beta Orionis, which is also called Rigel. Although the short form Rigel Kent is common in English, the stars are most often referred to by their Bayer designation Alpha Centauri.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future. .

A medieval name is Toliman, whose etymology may be Arabic الظلمان al-Ẓulmān "the ostriches".^[121] During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula,^[124] possibly coined from "β" and the Latin ungula ("hoof").^[121] Together, Alpha and Beta Centauri form the "Southern Pointers" or "The Pointers", as they point towards the Southern Cross, the asterism of the constellation of Crux.^[34]

In Chinese, 南門 Nán Mén, meaning Southern Gate, refers to an asterism consisting of α Centauri and ε Centauri. Consequently, α Centauri itself is known as 南門二 Nán Mén Èr, the Second Star of the Southern Gate.^[125]

To the Australian aboriginal Boorong people^[who?] of northwestern Victoria, Alpha and Beta Centauri are Bermbermgle,^[126] two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula).^[127] The form in Wotjobaluk is Bram-bram-bult.^[126]

Use in modern fiction

Alpha Centauri's relative proximity makes it in some ways the logical choice as "first port of call". Speculative fiction about interstellar travel often predicts eventual human exploration, and even the discovery and colonization of planetary systems. These themes are common to many works of science fiction and video games.

Flexible solar cell woven into fabric

2 September 2014Jon Cartwright

Original link:  http://www.rsc.org/chemistryworld/2014/09/flexible-solar-cell-fabric

The solar cell textile can withstand being bent more than 200 times © Wiley-VCH

Wearable electronics are quickly becoming the fashion. And there could soon be a way to power those electronics indefinitely, now that scientists in China have developed a solar cell ‘textile’ that could be woven into clothes. The textile retains a power-generation efficiency close to 1% even after been bent more than 200 times, and can be illuminated from both sides.

Scientists have been looking into flexible solar cells for decades, partly for coating irregularly shaped objects but also for integrating into wearable fabrics. One popular line of investigation has been dye-sensitized solar cells, in which a pigment absorbs sunlight to generate electrons and their positive counterparts, holes, before passing on those charges to inexpensive semiconductors. These solar cells are cheap and flexible, but the liquid nature of their pigments means that they must be well sealed.

Bend a dye-sensitized solar cell more than a few times and the seals are likely to break, destroying its light-harvesting properties.

That is why Huisheng Peng at Fudan University in Shanghai and colleagues have been exploring another option: polymer solar cells. Although their maximum efficiencies fall below 10% – about half that of crystalline silicon, the most prevalent solar cell – polymer solar cells are lightweight, flexible and easy to manufacture. Peng and colleagues’ solar cell textile consists of microscopic interwoven metal wires coated with an active polymer (to absorb the sunlight), titanium dioxide nanotubes (to conduct the electrons) and another active polymer (to conduct the holes). The researches coated each side of the textile with transparent, conductive sheets of carbon nanotubes,
which complete the circuit.

Because of the textile’s symmetry, the cell can be illuminated on either side. In tests it exhibited a maximum efficiency of 1.08%, which varied by less than 0.03% after 200 cycles of bending. However, the textile is currently only about the size of a fingernail. ‘The main difficulties encountered are how to scale up the solar-cell textile while maintaining high energy-conversion efficiencies,’ says Peng.

Materials scientist Anyuan Cao, who was not involved with the work, believes the results are interesting, particularly the use of carbon nanotube sheets to allow illumination from both sides. But he warns that wearable solar cells are still some way off. ‘Current textiles demonstrated in laboratories are too small and have low energy conversion efficiencies,’ says Cao, who is based at Peking University in China. ‘The materials involved and the fabrication processes are still expensive. Practical use not only requires that the textiles should withstand simple bending, but also that they should sustain much more complex deformations such as folding and twisting, even under dynamic conditions.’

Peng says he and his colleagues are now working to increase the energy-conversion efficiency of their solar cell textile.

References

Z Zhang et al, Angew. Chem., Int. Ed., 2014, DOI: 10.1002/anie.201407688

Small Is Big: How Bacteria Will Make Our World Cleaner and Healthier

By Tim De Chant on Tue, 17 Dec 2013

Original link:  http://www.pbs.org/wgbh/nova/next/nature/microbial-world/

 

During her training as a soil scientist, Janet Jansson never imagined that she’d end up poring over gene sequences from a person’s intestines. The affable native of New Mexico had gone through the usual motions on the way to her first professorship—a Master’s in soil microbiology, a Ph.D. in microbial ecology, and a postdoc in biochemistry to round things out. For 20 years, she used every known trick to coax soil microbes into divulging their secrets. When those didn’t work, she developed her own, including an early and widely-cited method for purifying DNA from bacteria living in soil. It was gratifying work, if not the most high profile.

That would soon change, though. In 2000, Jansson, then a professor at Södertörn University in Sweden, was teaching a class with Charlotta Edlund, a colleague who studied microbes from a medical perspective. The two shared a mutual passion for microscopic life, but their research approaches couldn’t have been more different. Unlike bacteria which thrive in the human body, soil microbes are incredibly difficult to cultivate in the lab—you can’t just inoculate a Petri dish and return in the morning. So Jansson and other soil microbiologists had created an entirely new suite of tools.

Enterococcus faecalis, a common bacterium in the human microbiome

As Jansson and Edlund talked shop over the course of the semester, Jansson recalls her colleague wondering if the techniques used to study soil microbes might be useful in a project on antibiotic resistance that relied on fecal samples. “That was before the human microbiome was cool,” Jansson says. “That’s how it started. And then I got very interested in it.”

Like many ideas whose time is right, Jansson wasn’t alone in her foray into the human microbiome. “There were several soil microbiologists that started to do the same thing as me. Just independently, without knowing,” she recalls. “At that time, methods-wise, technique-wise, the environmental field was farther ahead than the clinical field, whereas in the past, it has been the reverse.”

That reversal would end up changing the way we understand the microbial world. Rather than assuming bacteria and other microorganisms lead lives that occasionally intersect with the macroscopic world, we would come to learn that microbes exert their influence in various and surprising ways. But as we discover more about the remarkable diversity in the microbial world, we’re learning that we may be able to use them as allies in everything from advanced medical treatments to farming and environmental remediation.

A Slick Solution

Jansson’s lab had already expanded well beyond soil microbes by the time an explosion rocked the Deepwater Horizon oil rig in April 2010. Her research group, now at the Lawrence Berkeley National Laboratory in Berkeley, California, was investigating how microbial enzymes might help suck more oil out of a well. To do that, they had been developing complex methods to extract DNA from oil.
“We had already worked out all of those methods at the time the spill happened,” Jansson says.

The Gulf of Mexico is littered with natural oil seeps, which some species of bacteria thrive on. In a way, the spill was a seep of massive proportions, and it had the potential to drastically alter the Gulf’s microbial communities, benefitting some types while harming others. Jansson and one of her collaborators, Terry Hazen, also a microbiologist at the Berkeley Lab, asked BP, which was already funding Jansson’s work on oil and microbes, if they would be willing to back a study of Gulf bacteria populations. They agreed, sending microscopes, freezers, grad students, and postdocs out on boats to survey the waters.

Firefighting ships attempt to put out the blaze on the Deepwater Horizon.

Jansson’s team was in charge of the DNA extraction part, which would help identify which types of bacteria were thriving and where. Comparing water samples taken both in and out of the plume of oil, Jansson and her colleagues noticed distinct differences. Of the 951 taxa that were present in the plume, 16 of those were booming compared with levels in normal sea water. Nearly all of the prospering taxa could either degrade oil or kick their reproduction into high gear when oil is present in cold water.

Their study confirmed what many suspected—that certain members of the Gulf’s natural bacterial communities would chomp away at the oil over time, eventually bringing water quality back up to more normal levels. But more intriguingly, the methods Jansson and her team developed could also be used for environmental monitoring. They showed it is possible to use specific bacterial taxa, or even individual genes, as bioindicators. Instead of using complicated chemical tests to ascertain water quality, scientists in the field could draw a sample of water and run it through a cheap sequencer that would look for particular genetic markers that correlate with oil concentration.

Dolphins swim next a plume of crude released in the Deepwater Horizon oil spill.

Beyond environmental monitoring, Jansson and her group’s work could lead to new ways to clean up oil spills. If we could harness the Gulf’s oil-loving bacteria, they could be used to mop up a spill more quickly. “If you did have an oil spill in the future, you could maybe try to enrich similar microorganisms that have those properties,” Jansson speculates. By building up beneficial populations in and around a spill, the damaging oil could be dissipated more quickly. All by using bacteria that occur naturally in the Gulf.

Living Sensors

Jansson and her colleagues’ work in the Gulf of Mexico is relatively cutting edge, which is to say that it’s not quite ready for widespread use. Jansson’s research is like the early stages of R&D in a long, multi-year development process of a new product. Someday, we may be able to clean up a spill without nasty chemicals or dig up a scoop of soil and run it through a sequencer that can sense subtle but significant changes in microbial communities. But we’re not there yet.

“That should be possible,” says van der Meer, a professor of microbiology at University of Lausanne in Switzerland. “But then you’re talking more about very complicated systems,” he says. “People have tried to develop chips to get very rapid ID of which functional capacity a bacterial community has, but even that requires quite a bit of interpretation before you can get a reasonable signal. We’re speaking about weeks of analysis.”

To bridge that gap, van der Meer has been developing test kits that, rather than sense changes in microbial communities, have the necessary bacteria locked inside. The kits rely on the microbes instead of the more traditional chemicals to test for various contaminants, including arsenic, oil, and
other pollutants, for less cost.

 

The assays cost less than one cent to produce.

One of van der Meer’s bioreporters is a nonpathogenic form of E. coli that has a gene inserted into an appropriate part of its genome that, when in the presence of arsenic, produces a protein that glows green. Green fluorescent protein, or GFP, is a standard technique that’s used in labs across the world to see if a gene is active or not. Van der Meer’s innovation was to take that process out of the lab and into the field.

To get them to the field, van der Meer freeze-dries the cells into a powder. When he needs to use the test, he reconstitutes them by adding a few drops of water. (Other labs transport bacterial spores, which require a few hours to “wake up.”) Once awake, he exposes the bacteria to the water or soil he’s looking to test. If a contaminant like arsenic is present, then the cells will glow green.

Van der Meer’s bioreporters aren’t in mass production currently, but its not hard to imagine that being far off. Once the bacteria have been sensitized to a particular chemical or contaminant, the assays cost less than one cent to produce. Packaging, shipping, and marketing will add to the cost, but with such a low starting point, bioreporters could be far more affordable and thus available to poorer parts of the world. After all, why shouldn’t they know what’s in their drinking water, too?

Agricultural Judo

While van der Meer has been using bacteria to sense what might be wrong with our environment, other scientists have been using strains to encourage what’s right with it. Soil microbiologists—including Jansson—have been studying the intimate relationship between bacteria and plants. The development of genetic surveys has clarified that picture, and over the years, that’s led plant physiologists and soil microbiologists to speculate on various ways to use microbes to boost crop yields.

After decades of research on fertilizers, pesticides, and plant genetics, agricultural science is increasingly focusing its attention on microbes. “You have kind of those three general areas of how we improve crops: Crop genetics, chemicals, and bacteria,” Kloepper says. “We’re in the mature state now where we’re looking at those not as competition, but as a blend of what we offer to growers.”

Soil microbes play an intimate role in plant growth. Boosting beneficial bacteria can raise crop yields.

There are a few different ways we can use microbes in agriculture. Seeds can be coated with a polymer containing beneficial microorganisms, giving the new plant a head start. Microbes can also be mixed in with fertilizer or stirred into irrigation water. Once they form their association with the plant, beneficial bacteria can increase root and shoot growth, fix nitrogen for the plants, and wick up nutrients in the soil that would be otherwise out of reach or unobtainable.

Another approach is called “predictive agriculture.” “This is a relatively very new area,” Jansson says. While still years off, researchers today are using a variety of tools to study which microbes thrive when crop yields are high. Once those populations are characterized, Jansson says, then scientists can ask, “Is there some way we can manipulate the environment to enhance their survival?”

Beneficial bacteria can be applied in a number of ways, including in irrigation water.

While the current tenor of the field has been more early-phase than market-ready, that may change soon. “The benefits of the microbes are being investigated like they’ve never been investigated before,” Kloepper says. “I think we’re on the cusp of making some real breakthroughs.”

If we’ve known about beneficial soil bacteria for years, why has it taken so long for the field to thrive? One reason is certainly our ability to inexpensively sequence DNA and RNA from soil, making it easier than ever to survey microbial communities. Beyond the genetic revolutions, there also may be a more esoteric reason. Kloepper suspects our interest in plant-microbe interactions has been fueled, in part, by our growing familiarity with the human microbiome. “Our food understanding kind of mirrors our understanding in health,” he says. As scientists learn more and the public becomes more accustomed to the idea of microbial helpers, Kloepper says, “why wouldn’t we use it in agriculture?”

The Gut-Heart Connection

Our understanding of microbial worlds has grown quickly under the tutelage of soil scientists and plant microbiologists, but it has positively exploded when medicine has entered the fray. By collaborating with microbiologists like Jansson, doctors and medical researchers have begun to map out and leverage the ecosystem within our bodies.

Unlike Jansson, Stan Hazen stumbled into his study of the human microbiome. Hazen, a cardiovascular specialist and researcher at the Cleveland Clinic, was analyzing archived blood samples of thousands of patients, some of whom suffered a heart attack or stroke or had died, trying to see if any patterns stood out. One that did was the blood level of a compound known as TMAO. The compound changes the way cholesterol is metabolized, causing it to build up in people’s arteries. Hazen and his team had a smoking gun, but they initially didn’t know what was pulling the trigger—what was producing all this TMAO in people with cardiovascular problems.

“We essentially reverse engineered where it came from and discovered that it was a by-product of gut flora metabolism,” Hazen says. More than that, he and his team learned that TMAO levels were higher among red meat eaters, meaning that it wasn’t red meat, per se, that causes heart disease, but its interaction with the bacteria in our gut. “This metabolite alters, literally, how cholesterol is sensed by cells in the artery wall.”

 

“That’s a really a big and fundamental shift in how we think about treating diseases.”

To Hazen, the revelation that microbes in our guts could be causing atherosclerosis changed the way he thought about the human body. Instead of focusing drug development on processes that are entirely human, maybe it should look at the microbiome, too, he thought. “Can we drug the microbiome?” Hazen wonders. He thinks so and is currently testing a drug that will block the enzyme in bacteria that produces TMAO, halting the chain reaction before it can even begin.

“That’s a really a big and fundamental shift in how we think about treating diseases,” Hazen says. “If we give a drug to a bacteria, it’s usually an antibiotic to kill it. Instead of killing it, we’re talking about making an inhibitor of just a specific enzyme.” He’s optimistic the approach will yield benefits outside of atherosclerosis, including other ailments like diabetes and obesity. “Any place where bacteria are thought to be playing a role,” he says.

Drugging the microbiome may not be the only way medicine can use the microbiome to its advantage. Cambridge, Massachusetts-based startup Seres Health is developing a microbe-containing pill to combat infections of Clostridium difficile, a bacterium that causes diarrhea, colon inflammation, and, in some cases, death. The current cutting-edge procedure for treating C. difficile is a fecal transplant, where bacteria from a healthy person’s gut are isolated, cultured, and injected into the patient’s colon. Seres is hoping a simple pill can replace that process.

Probiotic pills have been on the market for years, but the way they were developed and how they are marketed put Seres in a different class. Many existing probiotic pills are sold as supplements, so their claims aren’t verified by the FDA. Seres, though, will be seeking FDA approval for their pill, meaning it will be classified as a drug rather than a supplement.

Clostridium difficile is a bacterial infection that can be fought using transplanted gut bacteria from a healthy donor.

Seres and other companies hoping to produce scientifically-tested, FDA-regulated probiotic pills have a difficult road ahead of them. The human microbiome is dauntingly complex, and we have only begun to understand how many different species it contains and what role they play. Injecting a new player into the mix could disrupt the ecosystem in unexpected ways.

Still, as genetic sequencing techniques improve, our understanding of the human microbiome is likely to improve. Rather than relying on chemical formulas, we may be able to take drugs containing a specific suite of bacteria to subtly alter our microbial ecosystem to treat a number of different diseases, from acute intestinal infections like C. difficile to chronic ailments like clogged arteries.

Assembling the Puzzle

Despite the breakneck pace of discovery, we’re still very much in the early days of learning what we can do with the microbiome and what it can do for us. There’s a long road ahead, and the speed with which we travel down it is dependent on new investigative techniques like the sort that Jansson’s lab are developing. She calls her method an “omics” approach, meaning she’s drawing on genomics, metabolomics, proteomics, and so on, all techniques that rely on huge amounts of data to distill insights into genetics, metabolism, proteins, and more.

Developing lab protocols to purify the necessary materials is a challenge, but even more constraining are computing resources. “It’s hard to even weed through the data,” Jansson says. “You need supercomputing facilities and really massive statistical analyses to be able to go through these data sets.”

Our primitive understanding of the microbiome’s diversity is also holding us back. Where we once thought there were perhaps a few thousand species, now “reasonable estimates are more on the order or millions or hundreds of millions,” according to Jonathan Eisen, an evolutionary biologist at the University of California, Davis.

Jansson is hopeful that by mapping that diversity, we can gain a better understanding of the role bacteria play in the environment and in our bodies—and how we can use them. “A lot of bioinformatics tools are being developed,” she says. “We’re pretty close to being able to deal with massive amounts of these signature gene sequences.”

Sequencing techniques and supercomputing promise to deepen our understanding of microbial communities, but at its heart, the field is dependent on something more personal and less quantifiable—scientific collaboration. After all, without partnerships like Jansson and her colleague Charlotta Edlund’s, we may still be in the dark about the diversity and significance of the microbiome. But with them, we’ve changed the way we treat diseases, clean up the environment, and grow food. Who knows what comes next?

Photo credits: USDA, USGS, NOAA, and CDC

Oort cloud

From Wikipedia, the free encyclopedia

 

An artist's rendering of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.

The Oort cloud (/ˈɔrt/ or /ˈʊərt/;^[1]) or Öpik–Oort cloud,^[2] named after Dutch astronomer Jan Oort, is a spherical cloud of predominantly icy planetesimals believed to surround the Sun at up to 50,000 AU.^[3] This places the cloud a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.^[4]

The Oort cloud is thought to comprise two regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.

Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution.^[3] Although no confirmed direct observations of the Oort cloud are made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well.^[5] The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.^[3] Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.^[3][5]

Hypothesis

In 1932, the Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.^[6] The idea was independently revived by Oort as a means to resolve a paradox.^[7] Over the course of the Solar System's existence the orbits of comets are unstable and eventually dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for almost all of its existence.^[7][8][9]

There are two main classes of comet, short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. All long-period comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky.^[9] Oort noted that there was a peak in numbers of long-period comets with aphelia (their farthest distance from the Sun) of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.^[9] Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.^[9]

Structure and composition

The presumed distance of the Oort cloud compared to the rest of the Solar System

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)^[9] to as far as 50,000 AU (0.79 ly)^[3] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).^[9] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.^[3] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.^[10] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;^[10][11][12] it is seen as a possible source of new comets to resupply the tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.^[13]

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi),^[3] and billions with absolute magnitudes^[14] brighter than 11 (corresponding to approximately 20-kilometre (12 mi) diameter), with neighboring objects tens of millions of kilometres apart.^[5][15] Its total mass is not known, but, assuming that Halley's Comet is a suitable prototype for comets within the outer Oort cloud, roughly the combined mass is 3×10²⁵ kilograms (6.6×10²⁵ pounds), or five times the Earth.^[3][16] Earlier it was thought to be more massive (up to 380 Earth masses),^[17] but improved knowledge of the size distribution of long-period comets led to lower estimates. The mass of the inner Oort Cloud has not been characterized.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.^[18] However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also contain rocky objects.^[19] Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,^[20] a conclusion also supported by studies of granular size in Oort-cloud comets^[21] and by the recent impact study of Jupiter-family comet Tempel 1.^[22]

Origin

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago.^[3] The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.^[3][23] Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart, and it is suggested that many—possibly the majority—of Oort cloud objects were not formed in close proximity to the Sun.^[24] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.^[3]
Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects.
According to the models, about half of the objects scattered travel outward towards the Oort cloud, while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.^[25] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.^[26]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.^[27] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.^[3]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.^[3] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.^[28]

In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster". Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars".^[29]

Comets

Comet Hale–Bopp, an archetypal Oort-cloud comet

Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun. Long-period comets, such as comet Hale–Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are thought to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.^[9] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.^[30] These centaurs are then sent farther inward to become the short-period comets.^[31]

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, it is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.^[8] This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.^[5]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.^[32] Dynamical studies of Oort Cloud comets have shown that their occurrence in the outer-planet region is several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994.^[33]

Tidal effects

Most of the comets seen close to the Sun seem to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon's tidal force bends and deforms the Earth's oceans, causing the tides to rise and fall, the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun. At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.^[34] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.^[9]
Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia.^[35] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.^[36] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.^[37]

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars^[3] or giant molecular clouds.^[33] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.^[38] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.^[38][39]

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt.^[40][41] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.^[42] Thus, the Nemesis hypothesis is no longer needed.^[42]

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort Cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause is a Jupiter-mass object in a distant orbit.^[43] This hypothetical gas giant planet has been nicknamed Tyche. Matese and Whitmire believed that the WISE mission, an all-sky survey using parallax measurements in order to clarify local star distances, was capable of either proving or disproving the Tyche hypothesis.^[42] In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.^[44]

Modified Newtonian dynamics within the Oort cloud

Modified Newtonian dynamics (MOND)^[45][46] suggests that at their distances from the Sun, the objects comprising the Oort cloud should experience accelerations of the order of 10⁻¹⁰ m/s², and thus should be within the realms at which deviations from Newtonian predictions come into effect.
According to this hypothesis, which was proposed to account for the discrepancies in the galaxy rotation curve, which are more commonly attributed to dark matter, acceleration ceases to be linearly proportional to force at very low accelerations.^[45] If correct, this would have significant implications regarding the formation and structure of the Oort cloud. However, the majority of cosmologists do not consider MOND a valid hypothesis.^[47]

Future exploration

Space probes have yet to reach the area of the Oort cloud. One proposal is to use a craft powered by a solar sail that would take around 30 years to reach its destination.^[48]