A Medley of Potpourri

Thursday, September 17, 2020

Ephemeris

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ephemeris

In astronomy and celestial navigation, an ephemeris (plural: ephemerides) gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly velocity) over time. The etymology is from Latin ephemeris, meaning 'diary' and from Greek ἐφημερίς (ephemeris), meaning 'diary, journal'. Historically, positions were given as printed tables of values, given at regular intervals of date and time. The calculation of these tables was one of the first applications of mechanical computers. Modern ephemerides are often computed electronically, from mathematical models of the motion of astronomical objects and the Earth. However, printed ephemerides are still produced, as they are useful when computational devices are not available.

The astronomical position calculated from an ephemeris is given in the spherical polar coordinate system of right ascension and declination. Some of the astronomical phenomena of interest to astronomers are eclipses, apparent retrograde motion/planetary stations, planetary ingresses, sidereal time, positions for the mean and true nodes of the moon, the phases of the Moon, and the positions of minor celestial bodies such as Chiron.

Ephemerides are used in celestial navigation and astronomy. They are also used by some astrologers.

History

A Latin translation of al-Khwārizmī's zīj, page from Corpus Christi College MS 283

Alfonsine tables

Page from Almanach Perpetuum

1st millennium BC – Ephemerides in Babylonian astronomy.
2nd century AD – the Almagest and the Handy Tables of Ptolemy
8th century AD – the zīj of Ibrāhīm al-Fazārī
9th century AD – the zīj of Muḥammad ibn Mūsā al-Khwārizmī
12th century AD – the Tables of Toledo – based largely on Arabic zīj sources of Islamic astronomy – were edited by Gerard of Cremona to form the standard European ephemeris until the Alfonsine Tables.
13th century AD – the Zīj-i Īlkhānī (Ilkhanic Tables) were compiled at the Maragheh observatory in Persia.
13th century AD – the Alfonsine Tables were compiled in Spain to correct anomalies in the Tables of Toledo, remaining the standard European ephemeris until the Prutenic Tables almost 300 years later.
13th century AD - the Dresden Codex, an extant Mayan ephemeris
1408 – Chinese ephemeris table (copy in Pepysian Library, Cambridge, UK (refer book '1434'); Chinese tables believed known to Regiomontanus).
1474 – Regiomontanus publishes his day-to-day Ephemerides in Nürnberg, Germany.
1496 – the Almanach Perpetuum of Abraão ben Samuel Zacuto (one of the first books published with a movable type and printing press in Portugal)
1504 – While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicted a lunar eclipse for the natives, using the ephemeris of the German astronomer Regiomontanus.
1531 – Work of Johannes Stöffler is published posthumously at Tübingen, extending the ephemeris of Regiomontanus through 1551.
1551 – the Prutenic Tables of Erasmus Reinhold were published, based on Copernicus's theories.
1554 – Johannes Stadius published Ephemerides novae et auctae, the first major ephemeris computed according to Copernicus' heliocentric model, using parameters derived from the Prutenic Tables. Although the Copernican model provided an elegant solution to the problem of computing apparent planetary positions (it avoided the need for the equant and better explained the apparent retrograde motion of planets), it still relied on the use of epicycles, leading to some inaccuracies – for example, periodic errors in the position of Mercury of up to ten degrees. One of the users of Stadius's tables is Tycho Brahe.
1627 – the Rudolphine Tables of Johannes Kepler based on elliptical planetary motion became the new standard.
1679 – La Connaissance des Temps ou calendrier et éphémérides du lever & coucher du Soleil, de la Lune & des autres planètes, first published yearly by Jean Picard and still extant.
1975 – Owen Gingerich, using modern planetary theory and digital computers, calculates the actual positions of the planets in the 16th Century and graphs the errors in the planetary positions predicted by the ephemerides of Stöffler, Stadius and others. According to Gingerich, the error patterns "are as distinctive as fingerprints and reflect the characteristics of the underlying tables. That is, the error patterns for Stöffler are different from those of Stadius, but the error patterns of Stadius closely resemble those of Maestlin, Magini, Origanus, and others who followed the Copernican parameters."

Modern ephemeris

For scientific uses, a modern planetary ephemeris comprises software that generates positions of planets and often of their satellites, asteroids, or comets, at virtually any time desired by the user.

Typically, such ephemerides cover several centuries, past and future; the future ones can be covered because the field of celestial mechanics has developed several accurate theories. Nevertheless, there are secular phenomena which cannot adequately be considered by ephemerides. The greatest uncertainties in the positions of planets are caused by the perturbations of numerous asteroids, most of whose masses and orbits are poorly known, rendering their effect uncertain. Reflecting the continuing influx of new data and observations, NASA's Jet Propulsion Laboratory (JPL) has revised its published ephemerides nearly every year for the past 20 years.

Solar System ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars, and galaxies.

Scientific ephemerides for sky observers mostly contain the positions of celestial bodies in right ascension and declination, because these coordinates are the most frequently used on star maps and telescopes. The equinox of the coordinate system must be given. It is, in nearly all cases, either the actual equinox (the equinox valid for that moment, often referred to as "of date" or "current"), or that of one of the "standard" equinoxes, typically J2000.0, B1950.0, or J1900. Star maps almost always use one of the standard equinoxes.

Scientific ephemerides often contain further useful data about the moon, planet, asteroid, or comet beyond the pure coordinates in the sky, such as elongation to the Sun, brightness, distance, velocity, apparent diameter in the sky, phase angle, times of rise, transit, and set, etc. Ephemerides of the planet Saturn also sometimes contain the apparent inclination of its ring.

Celestial navigation serves as a backup to the Global Positioning System. Software is widely available to assist with this form of navigation; some of this software has a self-contained ephemeris. When software is used that does not contain an ephemeris, or if no software is used, position data for celestial objects may be obtained from the modern Nautical Almanac or Air Almanac.

An ephemeris is usually only correct for a particular location on the Earth. In many cases, the differences are too small to matter. However, for nearby asteroids or the Moon, they can be quite important.

Other modern ephemerides recently created are the EPM (Ephemerides of Planets and the Moon), from the Russian Institute for Applied Astronomy of the Russian Academy of Sciences, and the INPOP (Intégrateur numérique planétaire de l'Observatoire de Paris) by the French IMCCE.

Astrometry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Astrometry

Illustration of the use of interferometry in the optical wavelength range to determine precise positions of stars. Courtesy NASA/JPL-Caltech

Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the Milky Way.

History

Concept art for the TAU spacecraft, a 1980s era study which would have used an interstellar precursor probe to expand the baseline for calculating stellar parallax in support of Astrometry

The history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis and Aristillus to discover Earth's precession. In doing so, he also developed the brightness scale still in use today. Hipparchus compiled a catalogue with at least 850 stars and their positions. Hipparchus's successor, Ptolemy, included a catalogue of 1,022 stars in his work the Almagest, giving their location, coordinates, and brightness.

In the 10th century, Abd al-Rahman al-Sufi carried out observations on the stars and described their positions, magnitudes and star color; furthermore, he provided drawings for each constellation, which are depicted in his Book of Fixed Stars. Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a large astrolabe with a diameter of nearly 1.4 metres. His observations on eclipses were still used centuries later in Simon Newcomb's investigations on the motion of the Moon, while his other observations of the motions of the planets Jupiter and Saturn inspired Laplace's Obliquity of the Ecliptic and Inequalities of Jupiter and Saturn. In the 15th century, the Timurid astronomer Ulugh Beg compiled the Zij-i-Sultani, in which he catalogued 1,019 stars. Like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Beg's catalogue is estimated to have been precise to within approximately 20 minutes of arc.

In the 16th century, Tycho Brahe used improved instruments, including large mural instruments, to measure star positions more accurately than previously, with a precision of 15–35 arcsec. Taqi al-Din measured the right ascension of the stars at the Constantinople Observatory of Taqi ad-Din using the "observational clock" he invented. When telescopes became commonplace, setting circles sped measurements

James Bradley first tried to measure stellar parallaxes in 1729. The stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earth's axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel, the father of modern astrometry. He made the first measurement of stellar parallax: 0.3 arcsec for the binary star 61 Cygni.

Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the filar micrometer. Astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond. This technology made astrometry less expensive, opening the field to an amateur audience.

In 1989, the European Space Agency's Hipparcos satellite took astrometry into orbit, where it could be less affected by mechanical forces of the Earth and optical distortions from its atmosphere. Operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with an unprecedented degree of accuracy. A new "Tycho catalog" drew together a database of 1,058,332 to within 20-30 mas (milliarcseconds). Additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission.

Today, the catalogue most often used is USNO-B1.0, an all-sky catalogue that tracks proper motions, positions, magnitudes and other characteristics for over one billion stellar objects. During the past 50 years, 7,435 Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec.

Applications

Diagram showing how a smaller object (such as an extrasolar planet) orbiting a larger object (such as a star) could produce changes in position and velocity of the latter as they orbit their common center of mass (red cross).

Motion of barycenter of solar system relative to the Sun.

Apart from the fundamental function of providing astronomers with a reference frame to report their observations in, astrometry is also fundamental for fields like celestial mechanics, stellar dynamics and galactic astronomy. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions. It is instrumental for keeping time, in that UTC is essentially the atomic time synchronized to Earth's rotation by means of exact astronomical observations. Astrometry is an important step in the cosmic distance ladder because it establishes parallax distance estimates for stars in the Milky Way.

Astrometry has also been used to support claims of extrasolar planet detection by measuring the displacement the proposed planets cause in their parent star's apparent position on the sky, due to their mutual orbit around the center of mass of the system. Astrometry is more accurate in space missions that are not affected by the distorting effects of the Earth's atmosphere. NASA's planned Space Interferometry Mission (SIM PlanetQuest) (now cancelled) was to utilize astrometric techniques to detect terrestrial planets orbiting 200 or so of the nearest solar-type stars. The European Space Agency's Gaia Mission, launched in 2013, applies astrometric techniques in its stellar census. In addition to the detection of exoplanets, it can also be used to determine their mass.

Astrometric measurements are used by astrophysicists to constrain certain models in celestial mechanics. By measuring the velocities of pulsars, it is possible to put a limit on the asymmetry of supernova explosions. Also, astrometric results are used to determine the distribution of dark matter in the galaxy.

Astronomers use astrometric techniques for the tracking of near-Earth objects. Astrometry is responsible for the detection of many record-breaking Solar System objects. To find such objects astrometrically, astronomers use telescopes to survey the sky and large-area cameras to take pictures at various determined intervals. By studying these images, they can detect Solar System objects by their movements relative to the background stars, which remain fixed. Once a movement per unit time is observed, astronomers compensate for the parallax caused by Earth's motion during this time and the heliocentric distance to this object is calculated. Using this distance and other photographs, more information about the object, including its orbital elements, can be obtained.

50000 Quaoar and 90377 Sedna are two Solar System objects discovered in this way by Michael E. Brown and others at Caltech using the Palomar Observatory's Samuel Oschin telescope of 48 inches (1.2 m) and the Palomar-Quest large-area CCD camera. The ability of astronomers to track the positions and movements of such celestial bodies is crucial to the understanding of the Solar System and its interrelated past, present, and future with others in the Universe.

Statistics

A fundamental aspect of astrometry is error correction. Various factors introduce errors into the measurement of stellar positions, including atmospheric conditions, imperfections in the instruments and errors by the observer or the measuring instruments. Many of these errors can be reduced by various techniques, such as through instrument improvements and compensations to the data. The results are then analyzed using statistical methods to compute data estimates and error ranges.

Computer programs

In fiction

In Star Trek: Voyager, the Astrometrics lab is the set for various scenes.
In 2004 TV series Battlestar Galactica an Astrometrics lab is stated in dialogue multiple times.

Colonization of the asteroids

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Colonization_of_the_asteroids

Asteroids located in the asteroid belt have been suggested as a possible site of human colonization. Some of the driving forces behind this effort to colonize asteroids include the survival of humanity, as well as economic incentives associated with asteroid mining. The process of colonizing asteroids does have many obstacles that must be overcome for human habitation, including transportation distance, lack of gravity, temperature, radiation, and psychological issues.

Driving forces

Survival of humanity

One of the primary arguments for colonizing asteroids is to ensure the long-term survival of the human species. In the event of an existential threat on Earth, such as nuclear holocaust and the subsequent nuclear winter, or supervolcano eruption, a colony on an asteroid would allow the human species to continue on. Michael Griffin, the NASA administrator in 2006, states the importance of pursuing space colonization as follows:

“... the goal isn't just scientific exploration ... it's also about extending the range of human habitat out from Earth into the solar system as we go forward in time ... In the long run a single-planet species will not survive ... If we humans want to survive for hundreds of thousands or millions of years, we must ultimately populate other planets.”

Economics

Another argument for colonization is the potential economic gain from asteroid mining. Asteroids contain a significant amount of valuable materials, including rare minerals, precious metals, and ice which can be mined and transported back to Earth to be sold. 16 Psyche is one such asteroid worth approximately $10 quintillion in metallic iron and nickel. NASA estimates there to be between 1.1 and 1.9 million asteroids within the asteroid belt larger than 1 kilometer in diameter and millions of smaller asteroids. Approximately 8% of those asteroids are similar in composition to 16 Psyche. One company, Planetary Resources, is already aiming to develop technologies with the goal of using them to mine asteroids. Planetary Resources estimates some 30-meter long asteroids to contain as much as $25 to $50 billion worth of platinum.

Transportation

Challenges

The main challenge of transportation to the asteroid belt is the distance from Earth, 204.43 million miles. Scientists currently face a similar challenge in their mission of sending humans to Mars, which is 35.8 million miles from Earth. The trip to Mars took 253 days, based on the Mars rover mission. Additionally, Russia, China, and the European Space Agency ran an experiment, called MARS-500, between 2007 and 2011 to gauge the physical and psychological limitations of manned space flight. The experiment concluded that 18 months of solitude was the limit for a manned space mission. With current technology the journey to the asteroid belt would be greater than 18 months, possibly indicating that a manned mission is beyond our current technological capabilities.

Landing

Asteroids are not large enough to produce significant gravity, making it difficult to land a spacecraft. Humans have yet to land a spacecraft on an asteroid in the asteroid belt, but they have temporarily landed on the asteroid 162173 Ryugu, a near-Earth object of the Apollo group. This was part of the Hayabusa2 mission that was conducted by the Japanese Space Agency. The landing was made possible by using four solar ionic thrusters and four reaction wheels for propulsion. This technology allowed for the orientation control and orbit control of the spacecraft that guided it to land on Ryugu. These technologies may be applied to complete a successful similar landing in the asteroid belt.

Challenges for human habitation

Gravity

Lack of gravity has many adverse effects on human biology. Transitioning gravity fields has the potential to impact spatial orientation, coordination, balance, locomotion, and induce motion sickness. Asteroids, without artificial gravity, have relatively no gravity in comparison to earth. Without gravity working on the human body, bones lose minerals, and bone density decreases by 1% monthly. In comparison, the rate of bone loss for the elderly is between 1-1.5% yearly. The excretion of calcium from bones in space also places those in low gravity at a higher risk of kidney stones. Additionally, a lack of gravity causes fluids in the body to shift towards the head, possibly causing pressure in the head and vision problems.

Overall physical fitness tends to decrease as well, and proper nutrition becomes much more important. Without gravity, muscles are engaged less and overall movement is easier. Without intentional training, muscle mass, cardiovascular conditioning and endurance will decrease.

Artificial gravity

Artificial gravity offers a solution to the adverse effects of zero gravity on the human body. One proposition to implement artificial gravity on asteroids, investigated in a study conducted by researchers at the University of Vienna, involves hollowing out and rotating a celestial body. Colonists would then live within the asteroid, and the centrifugal force would simulate Earth’s gravity. The researchers found that while it may be unclear as to whether asteroids would be strong enough maintain the necessary spin rate, they could not rule out such a project if the dimensions and composition of the asteroid were within acceptable levels.

Currently, there are no practical large-scale applications of artificial gravity for spaceflight or colonization efforts due to issues with size and cost. However, a variety of research labs and organizations have performed a number of tests utilizing human centrifuges to study the effects of prolonged sustained or intermittent artificial gravity on the body in an attempt to determine feasibility for future missions such as long-term spaceflight and space colonization. A research team at the University of Colorado Boulder found that they were able to make all participants in their study feel comfortable at approximately 17 revolutions per minute in a human centrifuge, without the motion sickness that tends to plague most trials of small-scale applications of artificial gravity. This offers an alternative method which may be more feasible considering the significantly reduced cost in comparison to larger structures.

Temperature

Most asteroids are located in the asteroid belt, between Mars and Jupiter. This is a cold region, with temperatures ranging from -73 degrees celsius to -103 degrees. Human life will require a consistent energy source for warmth.

Radiation

In space, cosmic rays and solar flares create a lethal radiation environment. Cosmic radiation has the potential to increase risk of heart disease, cancer, central nervous system disorder, and acute radiation syndrome. On Earth, we are protected by a magnetic field and our atmosphere, but asteroids lack this defense.

One possibility for defense against this radiation is living inside of an asteroid. It is estimated that humans would be sufficiently protected from radiation by burrowing 100 meters deep inside of an asteroid. However, the composition of asteroids creates an issue for this solution. Many asteroids are loosely organized rubble piles with very little structural integrity.

Psychology

Space Travel has a huge impact on human psychology, including changes to brain structure, neural interconnectivity, and behavior.

Cosmic radiation has the ability to impact the brain, and has been studied extensively on rats and mice. These studies show the animals suffer from decreases in spatial memory, neural interconnectivity, and memory. Additionally, the animals had an increase in anxiety and fear.

The isolation of space and difficulty sleeping in the environment also contribute to psychological impacts. The difficulty of speaking with those on earth can contribute to loneliness, anxiety, and depression. A study was used to simulate the psychological impacts of extended space travel. Six healthy males with similar educational backgrounds to astronauts lived inside an enclosed module for 520 days. The members of the survey reported symptoms of moderate depression, abnormal sleep cycles, insomnia, and physical exhaustion.

In addition, NASA reports that missions on the global scale have ended or been halted due to mental issues. Some of these issues include shared mental delusions, depression, and becoming distressed from failed experiments.

However, in many astronauts, space travel can actually have a positive mental impact. Many astronauts report an increase of appreciation for the planet, purpose, and spirituality. This mainly results from the view of Earth from space.

Wednesday, September 16, 2020

Steradian

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Steradian

Steradian
A graphical representation of 1 steradian.The sphere has radius r, and in this case the area A of the highlighted surface patch is r². The solid angle Ω equals [A/r²] sr which is 1 sr in this example. The entire sphere has a solid angle of 4 $π$ sr.
General information
Unit system	SI derived unit
Unit of	Solid angle
Symbol	sr
Conversions
1 sr in ...	... is equal to ...
SI base units	1 m²/m²

The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radians, projected onto a circle, gives a length on the circumference, a solid angle in steradians, projected onto a sphere, gives an area on the surface. The name is derived from the Greek στερεός stereos 'solid' + radian.

The steradian, like the radian, is a dimensionless unit, the quotient of the area subtended and the square of its distance from the center. Both the numerator and denominator of this ratio have dimension length squared (i.e. L²/L² = 1, dimensionless). It is useful, however, to distinguish between dimensionless quantities of a different nature, so the symbol "sr" is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W⋅sr⁻¹). The steradian was formerly an SI supplementary unit, but this category was abolished in 1995 and the steradian is now considered an SI derived unit.

Definition

A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface. For a general sphere of radius r, any portion of its surface with area A = r² subtends one steradian at its center.

The solid angle is related to the area it cuts out of a sphere:

\Omega ={\frac {A}{r^{2}}}\ \mathrm {sr} \,={\frac {2\pi h}{r}}\ \mathrm {sr}

where

A

is the surface area of the spherical cap,

2\pi rh

r

is the radius of the sphere, and

sr is the unit, steradian.

Because the surface area A of a sphere is 4 $π$ r², the definition implies that a sphere subtends 4 $π$ steradians (≈ 12.56637 sr) at its center. By the same argument, the maximum solid angle that can be subtended at any point is 4 $π$ sr.

Other properties

Section of cone (1) and spherical cap (2) that subtend a solid angle of one steradian inside a sphere

If A = r², it corresponds to the area of a spherical cap (A = 2 $π$ rh) (where h stands for the "height" of the cap) and the relationship h/r = 1/2 $π$ holds. Therefore, in this case, one steradian corresponds to the plane (i.e. radian) angle of the cross-section of a simple cone subtending the plane angle 2θ, with θ given by:

{\displaystyle {\begin{aligned}\theta &=\arccos \left({\frac {r-h}{r}}\right)\\&=\arccos \left(1-{\frac {h}{r}}\right)\\&=\arccos \left(1-{\frac {1}{2\pi }}\right)\approx 0.572\,{\text{ rad,}}{\text{ or }}32.77^{\circ }.\end{aligned}}}

This angle corresponds to the plane aperture angle of 2θ ≈ 1.144 rad or 65.54°.

A steradian is also equal to the spherical area of a polygon having an angle excess of 1 radian, to 1/4 $π$ of a complete sphere, or to (180°/ $π$ )²
≈ 3282.80635 square degrees.

The solid angle of a cone whose cross-section subtends the angle 2θ is:

\Omega =2\pi \left(1-\cos \theta \right)\,\mathrm {sr}

SI multiples

Millisteradians (msr) and microsteradians (μsr) are occasionally used to describe light and particle beams. Other multiples are rarely used.

Protoplanetary disk

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Protoplanetary_disk

Atacama Large Millimeter Array image of HL Tauri.

A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.

In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported.

Formation

Fraction of stars that show some evidence of having a protoplanetary disk as a function of stellar age (in millions of years). The samples are nearby young clusters and associations. Figure taken from review of Mamajek (2009).

Protostars form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the rotation to increase as the nebula radius decreases. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk. This occurs because centripetal acceleration from the orbital motion resists the gravitational pull of the star only in the radial direction, but the cloud remains free to collapse in the vertical direction. The outcome is the formation of a thin disc supported by gas pressure in the vertical direction. The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible.

It is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years, before the disk disappears, perhaps being blown away by the young star's stellar wind, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet discovered is 25 million years old.

Protoplanetary disk. Simulated spiral arm vs observational data.

Protoplanetary disks around T Tauri stars differ from the disks surrounding the primary components of close binary systems with respect to their size and temperature. Protoplanetary disks have radii up to 1000 AU, and only their innermost parts reach temperatures above 1000 K. They are very often accompanied by jets.

Protoplanetary disks have been observed around several young stars in our galaxy. Observations by the Hubble Space Telescope have shown proplyds and planetary disks to be forming within the Orion Nebula.

Protoplanetary disks are thought to be thin structures, with a typical vertical height much smaller than the radius, and a typical mass much smaller than the central young star.

The mass of a typical proto-planetary disk is dominated by its gas, however, the presence of dust grains has a major role in its evolution. Dust grains shield the mid-plane of the disk from energetic radiation from outer space that creates a dead zone in which the magnetorotational instability (MRI) no longer operates.

It is believed that these disks consist of a turbulent envelope of plasma, also called the active zone, that encases an extensive region of quiescent gas called the dead zone. The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.

Supernova remnant ejecta producing planet-forming material.

Planetary system

Protoplanetary disk surrounding the young star Elias 2-27, located some 450 light years away.

The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals. This process competes against the stellar wind, which drives the gas out of the system, and gravity (accretion) and internal stresses (viscosity), which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both terrestrial and giant planets.

Some of the moons of Jupiter, Saturn, and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks. The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the planets are arranged in an ecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System likely contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see. The Earth's moon likely formed after a Mars-sized protoplanet obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System.

Debris disks

Artist's impression of the water snowline around the star V883 Orionis.

Gas-poor disks of circumstellar dust have been found around many nearby stars—most of which have ages in the range of ~10 million years (e.g. Beta Pictoris, 51 Ophiuchi) to billions of years (e.g. Tau Ceti). These systems are usually referred to as "debris disks". Given the older ages of these stars, and the short lifetimes of micrometer-sized dust grains around stars due to Poynting Robertson drag, collisions, and radiation pressure (typically hundreds to thousands of years), it is thought that this dust is from the collisions of planetesimals (e.g. asteroids, comets). Hence the debris disks around these examples (e.g. Vega, Alphecca, Fomalhaut, etc.) are probably not truly "protoplanetary", but represent a later stage of disk evolution where extrasolar analogs of the asteroid belt and Kuiper belt are home to dust-generating collisions between planetesimals.

Relation to abiogenesis

Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets.

Gallery

20 protoplanetary discs captured by the High Angular Resolution Project (DSHARP).
A shadow is created by the protoplanetary disc surrounding the star HBC 672 within the nebula.
Protoplanetary disc AS 209 nestled in the young Ophiuchus star-forming region.
Protoplanetary disk HH 212.
By observing dusty protoplanetary discs, scientists investigate the first steps of planet formation.
Concentric rings around young star HD 141569A, located some 370 light-years away.
Debris disks detected in HST images of young stars, HD 141943 and HD 191089 - images at top; geometry at bottom.
Protoplanetary disk HH-30 in Taurus - disk emits the reddish stellar jet.
Artist's impression of a protoplanetary disk.
A proplyd in the Orion Nebula.
Play media

Video shows the evolution of the disc around a young star like HL Tauri (artist concept).