Four exoplanets orbiting counterclockwise with their host star (HR 8799).
Note that this is not a video of real-time observation, but one
created using 7-10 still images over a decade, and using a computer to
interpolate movement.
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such.
The first confirmation of the detection occurred in 1992. A different
planet, initially detected in 1988, was confirmed in 2003. As of 1
September 2023, there are 5,506 confirmed exoplanets in 4,065 planetary systems, with 878 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and also much more about exoplanets, including composition, environmental conditions and potential for life.
There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy
have found the most, but these methods suffer from a clear
observational bias favoring the detection of planets near the star;
thus, 85% of the exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way,
it can be hypothesized that there are 11 billion potentially habitable
Earth-sized planets in the Milky Way, rising to 40 billion if planets
orbiting the numerous red dwarfs are included.
The least massive exoplanet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium), it is too massive to be a planet and might be a brown dwarf instead. Known orbital times for exoplanets vary from less than an hour
(for those closest to their star) to thousands of years. Some
exoplanets are so far away from the star that it is difficult to tell
whether they are gravitationally bound to it.
Almost all of the planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist. The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone (or sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants, often counted as sub-brown dwarfs. The rogue planets in the Milky Way possibly number in the billions or more.
Definition
IAU
The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets.
The IAU Working Group on Extrasolar Planets issued a position statement
containing a working definition of "planet" in 2001 and which was
modified in 2003. An exoplanet was defined by the following criteria:
Objects with true masses
below the limiting mass for thermonuclear fusion of deuterium
(currently calculated to be 13 Jupiter masses for objects of solar metallicity)
that orbit stars or stellar remnants are "planets" (no matter how they
formed). The minimum mass/size required for an extrasolar object to be
considered a planet should be the same as that used in the Solar System.
Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
Free-floating objects in young star clusters with masses below the
limiting mass for thermonuclear fusion of deuterium are not "planets",
but are "sub-brown dwarfs" (or whatever name is most appropriate).
This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an exoplanet is now as follows:
Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter
masses for objects of solar metallicity) that orbit stars, brown dwarfs
or stellar remnants and that have a mass ratio with the central object
below the L4/L5 instability (M/Mcentral < 2/(25+√621)) are "planets" (no matter how they formed).
The minimum mass/size required for an extrasolar object to be
considered a planet should be the same as that used in our Solar System.
The IAU noted that this definition could be expected to evolve as knowledge improves.
Alternatives
The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of their formation. It is widely thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed.
Brown dwarfs form like stars from the direct gravitational collapse of
clouds of gas and this formation mechanism also produces objects that
are below the 13MJup limit and can be as low as 1MJup.
Objects in this mass range that orbit their stars with wide separations
of hundreds or thousands of AU and have large star/object mass ratios
likely formed as brown dwarfs; their atmospheres would likely have a
composition more similar to their host star than accretion-formed
planets, which would contain increased abundances of heavier elements.
Most directly imaged planets as of April 2014 are massive and have wide
orbits so probably represent the low-mass end of a brown dwarf
formation.
One study suggests that objects above 10MJup formed through gravitational instability and should not be thought of as planets.
Also, the 13-Jupiter-mass cutoff does not have a precise physical
significance. Deuterium fusion can occur in some objects with a mass
below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. As of 2011, the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13MJup in the observed mass spectrum reinforces the choice to forget this mass limit".
As of 2016, this limit was increased to 60 Jupiter masses based on a study of mass–density relationships.
The Exoplanet Data Explorer
includes objects up to 24 Jupiter masses with the advisory: "The 13
Jupiter-mass distinction by the IAU Working Group is physically
unmotivated for planets with rocky cores, and observationally
problematic due to the sin i ambiguity."
The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.
Another criterion for separating planets and brown dwarfs, rather than
deuterium fusion, formation process or location, is whether the core pressure is dominated by Coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.
The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union
(IAU). For exoplanets orbiting a single star, the IAU designation is
formed by taking the designated or proper name of its parent star, and
adding a lower case letter.
Letters are given in order of each planet's discovery around the parent
star, so that the first planet discovered in a system is designated "b"
(the parent star is considered "a") and later planets are given
subsequent letters. If several planets in the same system are discovered
at the same time, the closest one to the star gets the next letter,
followed by the other planets in order of orbital size. A provisional
IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
History of detection
For
centuries scientists, philosophers, and science fiction writers
suspected that extrasolar planets existed, but there was no way of
knowing whether they were real in fact, how common they were, or how
similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star,
but it is now thought that such a spectrum could be caused by the
residue of a nearby exoplanet that had been pulverized into dust by the
gravity of the star, the resulting dust then falling onto the star.
The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992 from the Arecibo Observatory, with the discovery of several terrestrial-mass planets orbiting the pulsarPSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing,
found evidence of planets in a distant galaxy, stating, "Some of these
exoplanets are as (relatively) small as the moon, while others are as
massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly
bound to stars, so they're actually wandering through space or loosely
orbiting between stars. We can estimate that the number of planets in
this [faraway] galaxy is more than a trillion."
On 21st March 2022, the 5000th exoplanet beyond the Solar System was confirmed.
This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.
— Giordano Bruno (1584)
In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that fixed stars are similar to the Sun and are likewise accompanied by planets.
In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia.
Making a comparison to the Sun's planets, he wrote "And if the fixed
stars are the centres of similar systems, they will all be constructed
according to a similar design and subject to the dominion of One."
In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve
wrote that there is no compelling reason that planets could not be much
closer to their parent star than is the case in the Solar System, and
proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.
Discredited claims
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star70 Ophiuchi. In 1855, William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all early reports of detection as erroneous.
In 1991, Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.
2MASS J044144 is a brown dwarf with a companion about 5–10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.
As of 1 September 2023, a total of 5,506 confirmed exoplanets are
listed in the Extrasolar Planets Encyclopaedia, including a few that
were confirmations of controversial claims from the late 1980s.
The first published discovery to receive subsequent confirmation was
made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H.
Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.
Although they were cautious about claiming a planetary detection, their
radial-velocity observations suggested that a planet orbits the star Gamma Cephei.
Partly because the observations were at the very limits of instrumental
capabilities at the time, astronomers remained skeptical for several
years about this and other similar observations. It was thought some of
the apparent planets might instead have been brown dwarfs,
objects intermediate in mass between planets and stars. In 1990,
additional observations were published that supported the existence of
the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
Coronagraphic image of AB Pictoris
showing a companion (bottom left), which is either a brown dwarf or a
massive planet. The data were obtained on 16 March 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.
On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsarPSR 1257+12.
This discovery was confirmed, and is generally considered to be the
first definitive detection of exoplanets. Follow-up observations
solidified these results, and confirmation of a third planet in 1994
revived the topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants
that somehow survived the supernova and then decayed into their current
orbits. As pulsars are aggressive stars, it was considered unlikely at
the time that a planet may be able to be formed in their orbit.
In the early 1990s, a group of astronomers led by Donald Backer, who were studying what they thought was a binary pulsar (PSR B1620−26 b), determined that a third object was needed to explain the observed Doppler shifts. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf
had been measured, giving an estimate of the mass of the third object
that was too small for it to be a star. The conclusion that the third
object was a planet was announced by Stephen Thorsett and his collaborators in 1993.
On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star51 Pegasi.This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational
influence on the motion of their host stars. More extrasolar planets
were later detected by observing the variation in a star's apparent
luminosity as an orbiting planet transited in front of it.
Initially, the most known exoplanets were massive planets that
orbited very close to their parent stars. Astronomers were surprised by
these "hot Jupiters", because theories of planetary formation
had indicated that giant planets should only form at large distances
from stars. But eventually more planets of other sorts were found, and
it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system.
On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".
Before these results, most confirmed planets were gas giants comparable
in size to Jupiter or larger because they were more easily detected,
but the Kepler planets are mostly between the size of Neptune and the size of Earth.
On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.
On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.
This exoplanet, Wolf 503b, is twice the size of Earth and was
discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b
completes one orbit in as few as six days because it is very close to
the star. Wolf 503b is the only exoplanet that large that can be found
near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.
Under the Fulton gap studies, this opens up a new field for
astronomers, who are still studying whether planets found in the Fulton
gap are gaseous or rocky.
In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.
Candidate discoveries
As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.
Planets are extremely faint compared to their parent stars. For
example, a Sun-like star is about a billion times brighter than the
reflected light from any exoplanet orbiting it. It is difficult to
detect such a faint light source, and furthermore, the parent star
causes a glare that tends to wash it out. It is necessary to block the
light from the parent star to reduce the glare while leaving the light
from the planet detectable; doing so is a major technical challenge
which requires extreme optothermal stability. All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent stars.
Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO
will image dozens of gas giants, but the vast majority of known
extrasolar planets have only been detected through indirect methods.
When the star is behind a planet, its brightness will seem to dimIf a planet crosses (or transits)
in front of its parent star's disk, then the observed brightness of the
star drops by a small amount. The amount by which the star dims depends
on its size and on the size of the planet, among other factors. Because
the transit method requires that the planet's orbit intersect a
line-of-sight between the host star and Earth, the probability that an
exoplanet in a randomly oriented orbit will be observed to transit the
star is somewhat small. The Kepler telescope used this method.
As a planet orbits a star, the star also moves in its own small
orbit around the system's center of mass. Variations in the star's
radial velocity—that is, the speed with which it moves towards or away
from Earth—can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less.
When multiple planets are present, each one slightly perturbs
the others' orbits. Small variations in the times of transit for one
planet can thus indicate the presence of another planet, which itself
may or may not transit. For example, variations in the transits of the
planet Kepler-19b suggest the existence of a second planet in the system, the non-transiting Kepler-19c.
When a planet orbits multiple stars or if the planet has moons,
its transit time can significantly vary per transit. Although no new
planets or moons have been discovered with this method, it is used to
successfully confirm many transiting circumbinary planets.
Microlensing occurs when the gravitational field of a star acts
like a lens, magnifying the light of a distant background star. Planets
orbiting the lensing star can cause detectable anomalies in
magnification as it varies over time. Unlike most other methods which
have a detection bias towards planets with small (or for resolved
imaging, large) orbits, the microlensing method is most sensitive to
detecting planets around 1–10 AU away from Sun-like stars.
Astrometry consists of precisely measuring a star's position in
the sky and observing the changes in that position over time. The motion
of a star due to the gravitational influence of a planet may be
observable. Because the motion is so small, however, this method has not
yet been very productive. It has produced only a few disputed
detections, though it has been successfully used to investigate the
properties of planets found in other ways.
A pulsar (the small, ultradense remnant of a star that has exploded as a supernova)
emits radio waves extremely regularly as it rotates. If planets orbit
the pulsar, they will cause slight anomalies in the timing of its
observed radio pulses. The first confirmed discovery of an extrasolar planet
was made using this method. But as of 2011, it has not been very
productive; five planets have been detected in this way, around three
different pulsars.
Like pulsars, there are some other types of stars which exhibit
periodic activity. Deviations from periodicity can sometimes be caused
by a planet orbiting it. As of 2013, a few planets have been discovered
with this method.
When a planet orbits very close to a star, it catches a
considerable amount of starlight. As the planet orbits the star, the
amount of light changes due to planets having phases from Earth's
viewpoint or planets glowing more from one side than the other due to
temperature differences.
Relativistic beaming measures the observed flux from the star
due to its motion. The brightness of the star changes as the planet
moves closer or further away from its host star.
Massive planets close to their host stars can slightly deform
the shape of the star. This causes the brightness of the star to
slightly deviate depending on how it is rotated relative to Earth.
With the polarimetry method, a polarized light reflected off the
planet is separated from unpolarized light emitted from the star. No
new planets have been discovered with this method, although a few
already discovered planets have been detected with this method.
Disks of space dust surround many stars, thought to originate
from collisions among asteroids and comets. The dust can be detected
because it absorbs starlight and re-emits it as infrared
radiation. Features on the disks may suggest the presence of planets,
though this is not considered a definitive detection method.
Planets may form within a few to tens (or more) of millions of years of their star forming.The planets of the Solar System
can only be observed in their current state, but observations of
different planetary systems of varying ages allows us to observe planets
at different stages of evolution. Available observations range from
young proto-planetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in a gaseous protoplanetary disk, they accrete hydrogen/helium envelopes.
These envelopes cool and contract over time and, depending on the mass
of the planet, some or all of the hydrogen/helium is eventually lost to
space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b
which has only about twice the mass of Earth but is almost the size of
Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite
young at a few hundred million years old.
Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.
Some planets orbit one member of a binary star system, and several circumbinary planets have been discovered which orbit both members of a binary star. A few planets in triple star systems are known and one in the quadruple system Kepler-64.
This color–color diagram compares the colors of planets in the Solar System to exoplanet HD 189733b. The exoplanet's deep blue color is produced by silicate droplets, which scatter blue light in its atmosphere.
In 2013, the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color, and Kappa Andromedae b, which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance.
The apparent brightness (apparent magnitude)
of a planet depends on how far away the observer is, how reflective the
planet is (albedo), and how much light the planet receives from its
star, which depends on how far the planet is from the star and how
bright the star is. So, a planet with a low albedo that is close to its
star can appear brighter than a planet with a high albedo that is far
from the star.
The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter
that reflects less than 1% of the light from its star, making it less
reflective than coal or black acrylic paint. Hot Jupiters are expected
to be quite dark due to sodium and potassium in their atmospheres, but
it is not known why TrES-2b is so dark—it could be due to an unknown
chemical compound.
For gas giants,
geometric albedo generally decreases with increasing metallicity or
atmospheric temperature unless there are clouds to modify this effect.
Increased cloud-column depth increases the albedo at optical
wavelengths, but decreases it at some infrared wavelengths. Optical
albedo increases with age, because older planets have higher
cloud-column depths. Optical albedo decreases with increasing mass,
because higher-mass giant planets have higher surface gravities, which
produces lower cloud-column depths. Also, elliptical orbits can cause
major fluctuations in atmospheric composition, which can have a
significant effect.
There is more thermal emission than reflection at some
near-infrared wavelengths for massive and/or young gas giants. So,
although optical brightness is fully phase-dependent, this is not always the case in the near infrared.
Temperatures of gas giants reduce over time and with distance
from their stars. Lowering the temperature increases optical albedo even
without clouds. At a sufficiently low temperature, water clouds form,
which further increase optical albedo. At even lower temperatures,
ammonia clouds form, resulting in the highest albedos at most optical
and near-infrared wavelengths.
Magnetic field
In 2014, a magnetic field around HD 209458 b
was inferred from the way hydrogen was evaporating from the planet. It
is the first (indirect) detection of a magnetic field on an exoplanet.
The magnetic field is estimated to be about one-tenth as strong as
Jupiter's.
The magnetic fields of exoplanets may be detectable by their auroralradio emissions with sensitive enough radio telescopes such as LOFAR.
The radio emissions could enable determination of the rotation rate of
the interior of an exoplanet, and may yield a more accurate way to
measure exoplanet rotation than by examining the motion of clouds.
Earth's magnetic field
results from its flowing liquid metallic core, but on massive
super-Earths with high pressure, different compounds may form which do
not match those created under terrestrial conditions. Compounds may form
with greater viscosities and high melting temperatures, which could
prevent the interiors from separating into different layers and so
result in undifferentiated coreless mantles. Forms of magnesium oxide
such as MgSi3O12 could be a liquid metal at the
pressures and temperatures found in super-Earths and could generate a
magnetic field in the mantles of super-Earths.
Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up (Joule heating)
causing it to expand. The more magnetically active a star is, the
greater the stellar wind and the larger the electric current leading to
more heating and expansion of the planet. This theory matches the
observation that stellar activity is correlated with inflated planetary
radii.
In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic hydrogen form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn
and related exoplanets, since such planets are thought to contain a lot
of liquid metallic hydrogen, which may be responsible for their
observed powerful magnetic fields.
Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733
system. The failure to detect "star-planet interactions" in the
well-studied HD 189733 system calls other related claims of the effect
into question.
In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.
Plate tectonics
In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.
If super-Earths have more than 80 times as much water as Earth, then they become ocean planets
with all land completely submerged. However, if there is less water
than this limit, then the deep water cycle will move enough water
between the oceans and mantle to allow continents to exist.
Volcanism
Large surface temperature variations on 55 Cancri e
have been attributed to possible volcanic activity releasing large
clouds of dust which blanket the planet and block thermal emissions.
Rings
The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.
The brightness of optical images of Fomalhaut b
could be due to starlight reflecting off a circumplanetary ring system
with a radius between 20 and 40 times that of Jupiter's radius, about
the size of the orbits of the Galilean moons.
The rings of the Solar System's gas giants are aligned with their
planet's equator. However, for exoplanets that orbit close to their
star, tidal forces from the star would lead to the outermost rings of a
planet being aligned with the planet's orbital plane around the star. A
planet's innermost rings would still be aligned with the planet's
equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.
In December 2013 a candidate exomoon of a rogue planet was announced. On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.
Clear versus cloudy atmospheres on two exoplanets.
Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.
Sunset studies on Titan by Cassini help understand exoplanet atmospheres (artist's concept).
As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed,
resulting in detection of molecular spectral features; observation of
day–night temperature gradients; and constraints on vertical atmospheric
structure. Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.
In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.
The technology used to determine this may be useful in studying the
atmospheres of distant worlds, including those of exoplanets.
Comet-like tails
KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.
The dust could be ash erupting from volcanos and escaping due to the
small planet's low surface-gravity, or it could be from metals that are
vaporized by the high temperatures of being so close to the star with
the metal vapor then condensing into dust.
In June 2015, scientists reported that the atmosphere of GJ 436 b
was evaporating, resulting in a giant cloud around the planet and, due
to radiation from the host star, a long trailing tail 14 million km
(9 million mi) long.
Insolation pattern
Tidally locked planets in a 1:1 spin-orbit resonance
would have their star always shining directly overhead on one spot,
which would be hot with the opposite hemisphere receiving no light and
being freezing cold. Such a planet could resemble an eyeball, with the
hotspot being the pupil. Planets with an eccentric orbit
could be locked in other resonances. 3:2 and 5:2 resonances would
result in a double-eyeball pattern with hotspots in both eastern and
western hemispheres. Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.
Surface
Surface composition
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy.
Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and
near-infrared may identify magma oceans or high-temperature lavas,
hydrated silicate surfaces and water ice, giving an unambiguous method
to distinguish between rocky and gaseous exoplanets.
Surface temperature
Artist's illustration of temperature inversion in exoplanet's atmosphere.
Measuring the intensity of the light it receives from its parent star
can estimate the temperature of an exoplanet. For example, the planet OGLE-2005-BLG-390Lb
is estimated to have a surface temperature of roughly −220 °C (50 K).
However, such estimates may be substantially in error because they
depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect
may introduce unknown complications. A few planets have had their
temperature measured by observing the variation in infrared radiation as
the planet moves around in its orbit and is eclipsed by its parent
star. For example, the planet HD 189733b has been estimated to have an average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.
As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System. At cosmic distances, life
can only be detected if it is developed at a planetary scale and
strongly modified the planetary environment, in such a way that the
modifications cannot be explained by classical physico-chemical
processes (out of equilibrium processes). For example, molecular oxygen (O 2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means. Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.
The habitable zone around a star is the region where the temperature
is just right to allow liquid water to exist on the surface of a planet;
that is, not too close to the star for the water to evaporate and not
too far away from the star for the water to freeze. The heat produced by
stars varies depending on the size and age of the star, so that the
habitable zone can be at different distances for different stars. Also,
the atmospheric conditions on the planet influence the planet's ability
to retain heat so that the location of the habitable zone is also
specific to each type of planet: desert planets
(also known as dry planets), with very little water, will have less
water vapor in the atmosphere than Earth and so have a reduced
greenhouse effect, meaning that a desert planet could maintain oases of
water closer to its star than Earth is to the Sun. The lack of water
also means there is less ice to reflect heat into space, so the outer
edge of desert-planet habitable zones is further out. Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance.
Planets with larger mass have wider habitable zones because gravity
reduces the water cloud column depth which reduces the greenhouse effect
of water vapor, thus moving the inner edge of the habitable zone closer
to the star.
Planetary rotation rate is one of the major factors determining the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that reflect
more and so can be habitable much closer to their star. Earth with its
current atmosphere would be habitable in Venus's orbit, if it had
Venus's slow rotation. If Venus lost its water ocean due to a runaway greenhouse effect,
it is likely to have had a higher rotation rate in the past.
Alternatively, Venus never had an ocean because water vapor was lost to
space during its formation and could have had its slow rotation throughout its history.
Tidally locked planets (a.k.a. "eyeball" planets)
can be habitable closer to their star than previously thought due to
the effect of clouds: at high stellar flux, strong convection produces
thick water clouds near the substellar point that greatly increase the
planetary albedo and reduce surface temperatures.
Planets in the habitable zones of stars with low metallicity
are more habitable for complex life on land than high metallicity stars
because the stellar spectrum of high metallicity stars is less likely
to cause the formation of ozone thus enabling more ultraviolet rays to
reach the planet's surface.
Habitable zones have usually been defined in terms of surface
temperature, however over half of Earth's biomass is from subsurface
microbes,
and the temperature increases with depth, so the subsurface can be
conducive for microbial life when the surface is frozen and if this is
considered, the habitable zone extends much further from the star, even rogue planets could have liquid water at sufficient depths underground. In an earlier era of the universe the temperature of the cosmic microwave background
would have allowed any rocky planets that existed to have liquid water
on their surface regardless of their distance from a star. Jupiter-like planets might not be habitable, but they could have habitable moons.
The outer edge of the habitable zone is where planets are completely
frozen, but planets well inside the habitable zone can periodically
become frozen. If orbital fluctuations or other causes produce cooling,
then this creates more ice, but ice reflects sunlight causing even more
cooling, creating a feedback loop until the planet is completely or
nearly completely frozen. When the surface is frozen, this stops carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which thaws the planet again. Planets with a large axial tilt
are less likely to enter snowball states and can retain liquid water
further from their star. Large fluctuations of axial tilt can have even
more of a warming effect than a fixed large tilt.Paradoxically, planets orbiting cooler stars, such as red dwarfs, are
less likely to enter snowball states because the infrared radiation
emitted by cooler stars is mostly at wavelengths that are absorbed by
ice which heats it up.
Tidal heating
If a planet has an eccentric orbit, then tidal heating
can provide another source of energy besides stellar radiation. This
means that eccentric planets in the radiative habitable zone can be too
hot for liquid water. Tides also circularize
orbits over time, so there could be planets in the habitable zone with
circular orbits that have no water because they used to have eccentric
orbits.
Eccentric planets further out than the habitable zone would still have
frozen surfaces, but the tidal heating could create a subsurface ocean
similar to Europa's. In some planetary systems, such as in the Upsilon Andromedae
system, the eccentricity of orbits is maintained or even periodically
varied by perturbations from other planets in the system. Tidal heating
can cause outgassing from the mantle, contributing to the formation and
replenishment of an atmosphere.
A review in 2015 identified exoplanets Kepler-62f, Kepler-186f and Kepler-442b as the best candidates for being potentially habitable. These are at a distance of 1200, 490 and 1,120 light-years
away, respectively. Of these, Kepler-186f is in similar size to Earth
with its 1.2-Earth-radius measure, and it is located towards the outer
edge of the habitable zone around its red dwarf star.
When looking at the nearest terrestrial exoplanet candidates, Proxima Centauri b is about 4.2 light-years away. Its equilibrium temperature is estimated to be −39 °C (234 K).
In November 2013, it was estimated that 22±8% of Sun-like stars in the Milky Way galaxy may have an Earth-sized planet in the habitable zone. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarfs are included.
Kepler-186f, a 1.2-Earth-radius planet in the habitable zone of a red dwarf, was reported in April 2014.
Proxima Centauri b, a planet in the habitable zone of Proxima Centauri, the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
In February 2013, researchers speculated that up to 6% of small red
dwarfs may have Earth-size planets. This suggests that the closest one
to the Solar System could be 13 light-years away. The estimated distance
increases to 21 light-years when a 95% confidence interval is used. In March 2013, a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.
Exoplanets are often members of planetary systems of multiple planets
around a star. The planets interact with each other gravitationally and
sometimes form resonant systems where the orbital periods of the
planets are in integer ratios. The Kepler-223 system contains four planets in an 8:6:4:3 orbital resonance.
Some hot Jupiters orbit their stars in the opposite direction to their stars' rotation. One proposed explanation is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.
Search projects
CoRoT Mission to look for exoplanets using the transit method.
Kepler – Mission to look for large numbers of exoplanets using the transit method.
TESS
– To search for new exoplanets; rotating so by the end of its two-year
mission it will have observed stars from all over the sky. It is
expected to find at least 3,000 new exoplanets.
ESPRESSO – A rocky planet-finding, and stable spectroscopic observing, spectrograph mounted on ESO's 4 by 8.2m VLT telescope, sited on the levelled summit of Cerro Paranal in the Atacama Desert of northern Chile.
ANDES
– The ArmazoNes High Dispersion Echelle Spectrograph, a planet finding
and planet characterisation spectrograph, is expected to be fitted onto ESO's ELT 39.3m telescope.
ANDES was formally known as HIRES, which itself was created after a
merger of the consortia behind the earlier CODEX (optical
high-resolution) and SIMPLE (near-infrared high-resolution) spectrograph
concepts.
