Nebular hypothesis

From Bharatpedia, an open encyclopedia

The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting the Sun which clumped up together to form the planets. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular theory is the solar nebular disk model (SNDM) or solar nebular model.[1] It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular theory are echoed in modern theories of planetary formation, but most elements have been superseded.

According to the nebular theory, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk (proplyd) around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10–100 million years.[2]

The protoplanetary disk is an accretion disk that feeds the central star.[3] Initially very hot, the disk later cools in what is known as the T Tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains eventually may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes approximately 100 million to a billion years.[2]

The formation of giant planets is a more complicated process. It is thought to occur beyond the frost line, where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk.[4] The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses (M🜨) it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants such as Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.[2]

History[edit | edit source]

Main article: History of Solar System formation and evolution hypotheses

There is evidence that Emanuel Swedenborg first proposed parts of the nebular theory in 1734.[5][6] Immanuel Kant, familiar with Swedenborg's work, developed the theory further in 1755, publishing his own Universal Natural History and Theory of the Heavens, wherein he argued that gaseous clouds (nebulae) slowly rotate, gradually collapse and flatten due to gravity, eventually forming stars and planets.[1]

Pierre-Simon Laplace independently developed and proposed a similar model in 1796[1] in his Exposition du systeme du monde. He envisioned that the Sun originally had an extended hot atmosphere throughout the volume of the Solar System. His theory featured a contracting and cooling protosolar cloud—the protosolar nebula. As this cooled and contracted, it flattened and spun more rapidly, throwing off (or shedding) a series of gaseous rings of material; and according to him, the planets condensed from this material. His model was similar to Kant's, except more detailed and on a smaller scale.[1] While the Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties. The main problem involved angular momentum distribution between the Sun and planets. The planets have 99% of the angular momentum, and this fact could not be explained by the nebular model.[1] As a result, astronomers largely abandoned this theory of planet formation at the beginning of the 20th century.

According to some, a major critique came during the 19th century from James Clerk Maxwell (1831–1879), who in some sources is claimed to have maintained that different rotation between the inner and outer parts of a ring could not allow condensation of material.[7] However, both the critique and the attribution to Maxwell have been deemed to be incorrect upon further investigation, with the original error being made by George Gamow in some popular publications and propagated continually ever since.[8] Astronomer Sir David Brewster also rejected Laplace, writing in 1876 that "those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process". He argued that under such view, "the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere".[9] Brewster claimed that Sir Isaac Newton's religious beliefs had previously considered nebular ideas as tending to atheism, and quoted him as saying that "the growth of new systems out of old ones, without the mediation of a Divine power, seemed to him apparently absurd".[10]

The perceived deficiencies of the Laplacian model stimulated scientists to find a replacement for it. During the 20th century many theories addressed the issue, including the planetesimal theory of Thomas Chamberlin and Forest Moulton (1901), the tidal model of James Jeans (1917), the accretion model of Otto Schmidt (1944), the protoplanet theory of William McCrea (1960) and finally the capture theory of Michael Woolfson.[1] In 1978 Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the modern Laplacian theory.[1] None of these attempts proved completely successful, and many of the proposed theories were descriptive.

The birth of the modern widely accepted theory of planetary formation—the solar nebular disk model (SNDM)—can be traced to the Soviet astronomer Victor Safronov.[11] His 1969 book Evolution of the protoplanetary cloud and formation of the Earth and the planets,[12] which was translated to English in 1972, had a long-lasting effect on the way scientists think about the formation of the planets.[13] In this book almost all major problems of the planetary formation process were formulated and some of them solved. Safronov's ideas were further developed in the works of George Wetherill, who discovered runaway accretion.[1] While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe; as of Template:Extrasolar planet counts astronomers have discovered Template:Extrasolar planet counts extrasolar planets in our galaxy.[14]

Solar nebular model: achievements and problems[edit | edit source]

Achievements[edit | edit source]

Dusty disks surrounding nearby young stars in greater detail.[15]

The star formation process naturally results in the appearance of accretion disks around young stellar objects.[16] At the age of about 1 million years, 100% of stars may have such disks.[17] This conclusion is supported by the discovery of the gaseous and dusty disks around protostars and T Tauri stars as well as by theoretical considerations.[18] Observations of these disks show that the dust grains inside them grow in size on short (thousand-year) time scales, producing 1 centimeter sized particles.[19]

The accretion process, by which 1 km planetesimals grow into 1,000 km sized bodies, is well understood now.[20] This process develops inside any disk where the number density of planetesimals is sufficiently high, and proceeds in a runaway manner. Growth later slows and continues as oligarchic accretion. The end result is formation of planetary embryos of varying sizes, which depend on the distance from the star.[20] Various simulations have demonstrated that the merger of embryos in the inner part of the protoplanetary disk leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.[21]

Current issues[edit | edit source]

The physics of accretion disks encounters some problems.[22] The most important one is how the material, which is accreted by the protostar, loses its angular momentum. One possible explanation suggested by Hannes Alfvén was that angular momentum was shed by the solar wind during its T Tauri star phase. The momentum is transported to the outer parts of the disk by viscous stresses.[23] Viscosity is generated by macroscopic turbulence, but the precise mechanism that produces this turbulence is not well understood. Another possible process for shedding angular momentum is magnetic braking, where the spin of the star is transferred into the surrounding disk via that star's magnetic field.[24] The main processes responsible for the disappearance of the gas in disks are viscous diffusion and photo-evaporation.[25][26]

Multiple star system AS 205.[27]

The formation of planetesimals is the biggest unsolved problem in the nebular disk model. How 1 cm sized particles coalesce into 1 km planetesimals is a mystery. This mechanism appears to be the key to the question as to why some stars have planets, while others have nothing around them, not even dust belts.[28]

The formation timescale of giant planets is also an important problem. Old theories were unable to explain how their cores could form fast enough to accumulate significant amounts of gas from the quickly disappearing protoplanetary disk.[20][29] The mean lifetime of the disks, which is less than ten million (107) years, appeared to be shorter than the time necessary for the core formation.[17] Much progress has been done to solve this problem and current models of giant planet formation are now capable of forming Jupiter (or more massive planets) in about 4 million years or less, well within the average lifetime of gaseous disks.[30][31][32]

Another potential problem of giant planet formation is their orbital migration. Some calculations show that interaction with the disk can cause rapid inward migration, which, if not stopped, results in the planet reaching the "central regions still as a sub-Jovian object."[33] More recent calculations indicate that disk evolution during migration can mitigate this problem.[34]

Formation of planets[edit | edit source]

Rocky planets[edit | edit source]

According to the solar nebular disk model, rocky planets form in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to prevent condensation of water ice and other substances into grains.[35] This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals.[lower-alpha 1][35] Such conditions are thought to exist in the inner 3–4 AU part of the disk of a Sun-like star.[2]

After small planetesimals—about 1 km in diameter—have formed by one way or another, runaway accretion begins.[20] It is called runaway because the mass growth rate is proportional to R4~M4/3, where R and M are the radius and mass of the growing body, respectively.[36] The specific (divided by mass) growth accelerates as the mass increases. This leads to the preferential growth of larger bodies at the expense of smaller ones.[20] The runaway accretion lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter.[20] Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals.[20][36] In addition, the influence of larger bodies stops further growth of smaller bodies.[20]

The next stage is called oligarchic accretion.[20] It is characterized by the dominance of several hundred of the largest bodies—oligarchs, which continue to slowly accrete planetesimals.[20] No body other than the oligarchs can grow.[36] At this stage the rate of accretion is proportional to R2, which is derived from the geometrical cross-section of an oligarch.[36] The specific accretion rate is proportional to M−1/3; and it declines with the mass of the body. This allows smaller oligarchs to catch up to larger ones. The oligarchs are kept at the distance of about 10·Hr (Hr=a(1-e)(M/3Ms)1/3 is the Hill radius, where a is the semimajor axis, e is the orbital eccentricity, and Ms is the mass of the central star) from each other by the influence of the remaining planetesimals.[20] Their orbital eccentricities and inclinations remain small. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them.[20] Sometimes nearby oligarchs merge. The final mass of an oligarch depends on the distance from the star and surface density of planetesimals and is called the isolation mass.[36] For the rocky planets it is up to 0.1 M🜨, or one Mars mass.[2] The final result of the oligarchic stage is the formation of about 100 Moon- to Mars-sized planetary embryos uniformly spaced at about 10·Hr.[21] They are thought to reside inside gaps in the disk and to be separated by rings of remaining planetesimals. This stage is thought to last a few hundred thousand years.[2][20]

The last stage of rocky planet formation is the merger stage.[2] It begins when only a small number of planetesimals remains and embryos become massive enough to perturb each other, which causes their orbits to become chaotic.[21] During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for 10 to 100 million years, is the formation of a limited number of Earth-sized bodies. Simulations show that the number of surviving planets is on average from 2 to 5.[2][21][37][38] In the Solar System they may be represented by Earth and Venus.[21] Formation of both planets required merging of approximately 10–20 embryos, while an equal number of them were thrown out of the Solar System.[37] Some of the embryos, which originated in the asteroid belt, are thought to have brought water to Earth.[35] Mars and Mercury may be regarded as remaining embryos that survived that rivalry.[37] Rocky planets which have managed to coalesce settle eventually into more or less stable orbits, explaining why planetary systems are generally packed to the limit; or, in other words, why they always appear to be at the brink of instability.[21]

Giant planets[edit | edit source]

The dust disk around Fomalhaut—the brightest star in Piscis Austrinus constellation. Asymmetry of the disk may be caused by a giant planet (or planets) orbiting the star.

The formation of giant planets is an outstanding problem in the planetary sciences.[22] In the framework of the solar nebular model two theories for their formation exist. The first one is the disk instability model, where giant planets form in the massive protoplanetary disks as a result of its gravitational fragmentation (see above).[39] The second possibility is the core accretion model, which is also known as the nucleated instability model.[22][34] The latter scenario is thought to be the most promising one, because it can explain the formation of the giant planets in relatively low-mass disks (less than Template:Solar mass).[34] In this model giant planet formation is divided into two stages: a) accretion of a core of approximately 10 M🜨 and b) accretion of gas from the protoplanetary disk.[2][22][40] Either method may also lead to the creation of brown dwarfs.[31][41] Searches as of 2011 have found that core accretion is likely the dominant formation mechanism.[41]

Giant planet core formation is thought to proceed roughly along the lines of the terrestrial planet formation.[20] It starts with planetesimals that undergo runaway growth, followed by the slower oligarchic stage.[36] Hypotheses do not predict a merger stage, due to the low probability of collisions between planetary embryos in the outer part of planetary systems.[36] An additional difference is the composition of the planetesimals, which in the case of giant planets form beyond the so-called frost line and consist mainly of ice—the ice to rock ratio is about 4 to 1.[29] This enhances the mass of planetesimals fourfold. However, the minimum mass nebula capable of terrestrial planet formation can only form 1–2 M🜨 cores at the distance of Jupiter (5 AU) within 10 million years.[36] The latter number represents the average lifetime of gaseous disks around Sun-like stars.[17] The proposed solutions include enhanced mass of the disk—a tenfold increase would suffice;[36] protoplanet migration, which allows the embryo to accrete more planetesimals;[29] and finally accretion enhancement due to gas drag in the gaseous envelopes of the embryos.[29][32][42] Some combination of the above-mentioned ideas may explain the formation of the cores of gas giant planets such as Jupiter and perhaps even Saturn.[22] The formation of planets like Uranus and Neptune is more problematic, since no theory has been capable of providing for the in situ formation of their cores at the distance of 20–30 AU from the central star.[2] One hypothesis is that they initially accreted in the Jupiter-Saturn region, then were scattered and migrated to their present location.[43] Another possible solution is the growth of the cores of the giant planets via pebble accretion. In pebble accretion objects between a cm and a meter in diameter falling toward a massive body are slowed enough by gas drag for them to spiral toward it and be accreted. Growth via pebble accretion may be as much as 1000 times faster than by the accretion of planetesimals.[44]

Once the cores are of sufficient mass (5–10 M🜨), they begin to gather gas from the surrounding disk.[2] Initially it is a slow process, increasing the core masses up to 30 M🜨 in a few million years.[29][42] After that, the accretion rates increase dramatically and the remaining 90% of the mass is accumulated in approximately 10,000 years.[42] The accretion of gas stops when the supply from the disk is exhausted.[40] This happens gradually, due to the formation of a density gap in the protoplanetary disk and to disk dispersal.[34][45] In this model ice giants—Uranus and Neptune—are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post-runaway-gas-accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion.[45] Migration is caused by the interaction of the planet sitting in the gap with the remaining disk. It stops when the protoplanetary disk disappears or when the end of the disk is attained. The latter case corresponds to the so-called hot Jupiters, which are likely to have stopped their migration when they reached the inner hole in the protoplanetary disk.[45]

During the accretion of gas via streams, a giant planet can be surrounded by a circumplanetary disk. This circumplanetary disk also carries solids and can form satellites. The Galilean moons are thought to have formed in such a circumplanetary disk.[40]

In this artist's conception, a planet spins through a clearing (gap) in a nearby star's dusty, planet-forming disc.

Giant planets can significantly influence terrestrial planet formation. The presence of giants tends to increase eccentricities and inclinations (see Kozai mechanism) of planetesimals and embryos in the terrestrial planet region (inside 4 AU in the Solar System).[37][38] If giant planets form too early, they can slow or prevent inner planet accretion. If they form near the end of the oligarchic stage, as is thought to have happened in the Solar System, they will influence the merges of planetary embryos, making them more violent.[37] As a result, the number of terrestrial planets will decrease and they will be more massive.[46] In addition, the size of the system will shrink, because terrestrial planets will form closer to the central star. The influence of giant planets in the Solar System, particularly that of Jupiter, is thought to have been limited because they are relatively remote from the terrestrial planets.[46]

The region of a planetary system adjacent to the giant planets will be influenced in a different way.[38] In such a region, eccentricities of embryos may become so large that the embryos pass close to a giant planet, which may cause them to be ejected from the system.[lower-alpha 2][37][38] If all embryos are removed, then no planets will form in this region.[38] An additional consequence is that a huge number of small planetesimals will remain, because giant planets are incapable of clearing them all out without the help of embryos. The total mass of remaining planetesimals will be small, because cumulative action of the embryos before their ejection and giant planets is still strong enough to remove 99% of the small bodies.[37] Such a region will eventually evolve into an asteroid belt, which is a full analog of the asteroid belt in the Solar System, located from 2 to 4 AU from the Sun.[37][38]

Exoplanets[edit | edit source]

Thousands of exoplanets have been identified in the last twenty years, with, at the very least, billions more, within our observable universe, yet to be discovered.[47] The orbits of many of these planets and systems of planets differ significantly from the planets in the Solar System. The exoplanets discovered include hot-Jupiters, warm-Jupiters, super-Earths, and systems of tightly packed inner planets.

The hot-Jupiters and warm-Jupiters are thought to have migrated to their current orbits during or following their formation. A number of possible mechanisms for this migration have been proposed. Type I or Type II migration could smoothly decrease the semimajor axis of the planet's orbit resulting in a warm- or hot-Jupiter.

Meaning of accretion[edit | edit source]

Use of the term "accretion disk" for the protoplanetary disk leads to confusion over the planetary accretion process. The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge.[48] In an accretion disk, there is a net flux of mass from larger radii toward smaller radii.[23]

However, that meaning should not be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high-energy collisions between sizable planetesimals.[20]

In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word.[49] The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted into the giant planet's regular moons.[50]

Notes[edit | edit source]

  1. ↑ The planetesimals near the outer edge of the terrestrial planet region—2.5 to 4 AU from the Sun—may accumulate some amount of ice. However the rocks will still dominate, like in the outer main belt in the Solar System.[35]
  2. ↑ As a variant they may collide with the central star or a giant planet.

Reference[edit | edit source]

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Woolfson, M.M. (1993). "Solar System – its origin and evolution". Q. J. R. Astron. Soc. 34: 1–20. Bibcode:1993QJRAS..34....1W. For details of Kant's position, see Stephen Palmquist, "Kant's Cosmogony Re-Evaluated", Studies in History and Philosophy of Science 18:3 (September 1987), pp.255–269.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Cite error: Invalid <ref> tag; no text was provided for refs named Montmerle2006
  3. ↑ Andrews, Robin George (10 August 2022). "Astronomers May Have Found the Galaxy's Youngest Planet - The Webb telescope soon will help measure the world, which may offer insights into how our own formed". The New York Times. Retrieved 11 August 2022.
  4. ↑ D'Angelo, G.; Bodenheimer, P. (2013). "Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal. 778 (1): 77 (29 pp.). arXiv:1310.2211. Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77. S2CID 118522228.
  5. ↑ Swedenborg, Emanuel (1734). (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works). Vol. I.
  6. ↑ Baker, Gregory L. "Emanuel Swenborg – an 18th century cosomologist". The Physics Teacher. October 1983, pp. 441–446.
  7. ↑ George H. A. Cole (2013). Planetary Science: The Science of Planets around Stars, Second Edition, Michael M. Woolfson, p. 190
  8. ↑ Petzold, Charles (February 2005). "Maxwell, Molecules, and Evolution". Retrieved 2023-01-03.
  9. ↑ Brester, David (1876), "More Worlds Than One: The Creed of the Philosopher and the Hope of the Christian", Chatto and Windus, Piccadilly, p. 153
  10. ↑ As quoted by David Brewster, "More worlds than one : the creed of the philosopher and the hope of the Christian", Fixed stars and binary systems. p. 233
  11. ↑ Henbest, Nigel (1991). "Birth of the planets: The Earth and its fellow planets may be survivors from a time when planets ricocheted around the Sun like ball bearings on a pinball table". New Scientist. Retrieved 2008-04-18.
  12. ↑ Safronov, Viktor Sergeevich (1972). Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. Israel Program for Scientific Translations. ISBN 978-0-7065-1225-0.
  13. ↑ Wetherill, George W. (1989). "Leonard Medal Citation for Victor Sergeevich Safronov". Meteoritics. 24 (4): 347. Bibcode:1989Metic..24..347W. doi:10.1111/j.1945-5100.1989.tb00700.x.
  14. ↑ Schneider, Jean (10 September 2011). "Interactive Extra-solar Planets Catalog". Extrasolar Planets Encyclopaedia. Retrieved 2011-09-10.
  15. ↑ "SPHERE Reveals Fascinating Zoo of Discs Around Young Stars". www.eso.org. Retrieved 11 April 2018.
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Andre1994
  17. ↑ 17.0 17.1 17.2 Cite error: Invalid <ref> tag; no text was provided for refs named Haisch2001
  18. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Padgett1999
  19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Kessler-Silacci2006
  20. ↑ 20.00 20.01 20.02 20.03 20.04 20.05 20.06 20.07 20.08 20.09 20.10 20.11 20.12 20.13 20.14 Kokubo, Eiichiro; Ida, Shigeru (2002). "Formation of protoplanet systems and diversity of planetary systems". The Astrophysical Journal. 581 (1): 666–680. Bibcode:2002ApJ...581..666K. doi:10.1086/344105.
  21. ↑ 21.0 21.1 21.2 21.3 21.4 21.5 Raymond, Sean N.; Quinn, Thomas; Lunine, Jonathan I. (2006). "High-resolution simulations of the final assembly of earth-like planets 1: terrestrial accretion and dynamics". Icarus. 183 (2): 265–282. arXiv:astro-ph/0510284. Bibcode:2006Icar..183..265R. doi:10.1016/j.icarus.2006.03.011. S2CID 119069411.
  22. ↑ 22.0 22.1 22.2 22.3 22.4 Cite error: Invalid <ref> tag; no text was provided for refs named Wurchterl2004
  23. ↑ 23.0 23.1 Lynden-Bell, D.; Pringle, J. E. (1974). "The evolution of viscous discs and the origin of the nebular variables". Monthly Notices of the Royal Astronomical Society. 168 (3): 603–637. Bibcode:1974MNRAS.168..603L. doi:10.1093/mnras/168.3.603.
  24. ↑ Devitt, Terry (January 31, 2001). "What Puts The Brakes On Madly Spinning Stars?". University of Wisconsin-Madison. Retrieved 2013-04-09.
  25. ↑ Dullemond, C.; Hollenbach, D.; Kamp, I.; D'Alessio, P. (2007). "Models of the Structure and Evolution of Protoplanetary Disks". In Reipurth, B.; Jewitt, D.; Keil, K. (eds.). Protostars and Planets V. Tucson, AZ: University of Arizona Press. pp. 555–572. arXiv:astro-ph/0602619. Bibcode:2007prpl.conf..555D. ISBN 978-0816526543.
  26. ↑ Clarke, C. (2011). "The Dispersal of Disks around Young Stars". In Garcia, P. (ed.). Physical Processes in Circumstellar Disks around Young Stars. Chicago, IL: University of Chicago Press. pp. 355–418. ISBN 9780226282282.
  27. ↑ "Worlds with many suns". www.eso.org. Retrieved 11 February 2019.
  28. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Youdin2002
  29. ↑ 29.0 29.1 29.2 29.3 29.4 Inaba, S.; Wetherill, G.W.; Ikoma, M. (2003). "Formation of gas giant planets: core accretion models with fragmentation and planetary envelope" (PDF). Icarus. 166 (1): 46–62. Bibcode:2003Icar..166...46I. doi:10.1016/j.icarus.2003.08.001. Archived from the original (PDF) on 2006-09-12.
  30. ↑ Lissauer, J. J.; Hubickyj, O.; D'Angelo, G.; Bodenheimer, P. (2009). "Models of Jupiter's growth incorporating thermal and hydrodynamic constraints". Icarus. 199 (2): 338–350. arXiv:0810.5186. Bibcode:2009Icar..199..338L. doi:10.1016/j.icarus.2008.10.004. S2CID 18964068.
  31. ↑ 31.0 31.1 Bodenheimer, P.; D'Angelo, G.; Lissauer, J. J.; Fortney, J. J.; et al. (2013). "Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion". The Astrophysical Journal. 770 (2): 120 (13 pp.). arXiv:1305.0980. Bibcode:2013ApJ...770..120B. doi:10.1088/0004-637X/770/2/120. S2CID 118553341.
  32. ↑ 32.0 32.1 D'Angelo, G.; Weidenschilling, S. J.; Lissauer, J. J.; Bodenheimer, P. (2014). "Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope". Icarus. 241: 298–312. arXiv:1405.7305. Bibcode:2014Icar..241..298D. doi:10.1016/j.icarus.2014.06.029. S2CID 118572605.
  33. ↑ Papaloizou 2007 page 10
  34. ↑ 34.0 34.1 34.2 34.3 D'Angelo, G.; Durisen, R. H.; Lissauer, J. J. (2011). "Giant Planet Formation". In S. Seager. (ed.). Exoplanets. University of Arizona Press, Tucson, AZ. pp. 319–346. arXiv:1006.5486. Bibcode:2010exop.book..319D.
  35. ↑ 35.0 35.1 35.2 35.3 Raymond, Sean N.; Quinn, Thomas; Lunine, Jonathan I. (2007). "High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability". Astrobiology. 7 (1): 66–84. arXiv:astro-ph/0510285. Bibcode:2007AsBio...7...66R. doi:10.1089/ast.2006.06-0126. PMID 17407404. S2CID 10257401.
  36. ↑ 36.0 36.1 36.2 36.3 36.4 36.5 36.6 36.7 36.8 Thommes, E.W.; Duncan, M.J.; Levison, H.F. (2003). "Oligarchic growth of giant planets". Icarus. 161 (2): 431–455. arXiv:astro-ph/0303269. Bibcode:2003Icar..161..431T. doi:10.1016/S0019-1035(02)00043-X. S2CID 16522991.
  37. ↑ 37.0 37.1 37.2 37.3 37.4 37.5 37.6 37.7 Bottke, William F.; Durda, Daniel D.; Nesvorny, David; et al. (2005). "Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion" (PDF). Icarus. 179 (1): 63–94. Bibcode:2005Icar..179...63B. doi:10.1016/j.icarus.2005.05.017.
  38. ↑ 38.0 38.1 38.2 38.3 38.4 38.5 Petit, Jean-Marc; Morbidelli, Alessandro (2001). "The Primordial Excitation and Clearing of the Asteroid Belt" (PDF). Icarus. 153 (2): 338–347. Bibcode:2001Icar..153..338P. doi:10.1006/icar.2001.6702. Archived from the original (PDF) on 2007-02-21. Retrieved 2008-03-18.
  39. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Boss2003
  40. ↑ 40.0 40.1 40.2 D'Angelo, G.; Lissauer, J. J. (2018). "Formation of Giant Planets". In Deeg H., Belmonte J. (ed.). Handbook of Exoplanets. Springer International Publishing AG, part of Springer Nature. pp. 2319–2343. arXiv:1806.05649. Bibcode:2018haex.bookE.140D. doi:10.1007/978-3-319-55333-7_140. ISBN 978-3-319-55332-0. S2CID 116913980.
  41. ↑ 41.0 41.1 Janson, M.; Bonavita, M.; Klahr, H.; Lafreniere, D.; et al. (2011). "High-contrast Imaging Search for Planets and Brown Dwarfs around the Most Massive Stars in the Solar Neighborhood". Astrophys. J. 736 (89): 89. arXiv:1105.2577. Bibcode:2011ApJ...736...89J. doi:10.1088/0004-637x/736/2/89. S2CID 119217803.
  42. ↑ 42.0 42.1 42.2 Fortier, A.; Benvenuto, A.G. (2007). "Oligarchic planetesimal accretion and giant planet formation". Astron. Astrophys. 473 (1): 311–322. arXiv:0709.1454. Bibcode:2007A&A...473..311F. doi:10.1051/0004-6361:20066729. S2CID 14812137.
  43. ↑ Thommes, Edward W.; Duncan, Martin J.; Levison, Harold F. (1999). "The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System" (PDF). Nature. 402 (6762): 635–638. Bibcode:1999Natur.402..635T. doi:10.1038/45185. PMID 10604469. S2CID 4368864.
  44. ↑ Lambrechts, M.; Johansen, A. (August 2012). "Rapid growth of gas-giant cores by pebble accretion". Astronomy & Astrophysics. 544: A32. arXiv:1205.3030. Bibcode:2012A&A...544A..32L. doi:10.1051/0004-6361/201219127. S2CID 53961588.
  45. ↑ 45.0 45.1 45.2 Papaloizou, J. C. B.; Nelson, R. P.; Kley, W.; et al. (2007). "Disk-Planet Interactions During Planet Formation". In Bo Reipurth; David Jewitt; Klaus Keil (eds.). Protostars and Planets V. Arizona Press. p. 655. arXiv:astro-ph/0603196. Bibcode:2007prpl.conf..655P.
  46. ↑ 46.0 46.1 Levison, Harold F.; Agnor, Craig (2003). "The role of giant planets in terrestrial planet formation" (PDF). The Astronomical Journal. 125 (5): 2692–2713. Bibcode:2003AJ....125.2692L. doi:10.1086/374625. S2CID 41888579.
  47. ↑ "Are We Alone? Discovery of Billions of Earth-Like Planets May Hold the Answer". SciTechDaily. July 8, 2020.
  48. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Yorke1999
  49. ↑ D'Angelo, G.; Podolak, M. (2015). "Capture and Evolution of Planetesimals in Circumjovian Disks". The Astrophysical Journal. 806 (1): 29pp. arXiv:1504.04364. Bibcode:2015ApJ...806..203D. doi:10.1088/0004-637X/806/2/203. S2CID 119216797.
  50. ↑ Canup, Robin M.; Ward, William R. (2002). "Formation of the Galilean Satellites: Conditions of Accretion" (PDF). The Astronomical Journal. 124 (6): 3404–3423. Bibcode:2002AJ....124.3404C. doi:10.1086/344684. S2CID 47631608.
Retrieved from "https://en.bharatpedia.org/w/index.php?title=Nebular_hypothesis&oldid=467277"