Giant Planet Formation by Gravitational Instability

“As a result, hybrid exoplanet population synthesis models (e.g., Boss 1998; Nayakshin 2010), combining both core accretion (e.g., Mizuno 1980; Pollack et al. 1996) and disk instability (e.g., Cameron 1978; Boss 1997), may be needed to explain the plethora of exoplanetary system discoveries.”

“For jovian planets, the two competing, most commonly used theories are known as the ‘cold’ and ‘hot start’ scenarios, corresponding to the formation and evolution via core accretion and gravitational instability, respectively (Pollack et al. 1996; Boss 1997; Burrows et al. 1997; Marley et al. 2007).”

“…with disk gravitational fragmentation is the possibility for the formation of massive gas giant planet and brown dwarf companions to solar-type stars (Boss 1997, 2011; Stamatellos and Whitworth 2009; Boley et al. 2010; Nayakshin 2010; Kratter and Murray-Clay 2011; Rogers and Wadsley 2012).”

“The timescale for gas dissipation in the inner disk will determine which formation mechanisms for giant planets, i.e., gravitational instability (Boss 1997; Mayer et al. 2002) or core accretion (Bodenheimer & Lin 2002) are plausible.”

“This gravitational instability (GI, e.g., Boss 1997) scenario is a possible channel for planet formation around A or B stars, but direct observational evidence is scarce.”

“…which dust coagulates into larger and larger bodies which become the cores of planets (e.g. Pollack et al. 1996); and gravitational instability (GI), in which the self-gravity of a circumstellar disk triggers the fragmentation and collapse of gas into a gas giant planet directly (e.g. Boss 1997).”

“As with primordial dust, the presence of primordial gas would also, therefore, be unexpected in main-sequence stars of this age, particularly if planet formation occurs much more rapidly, as suggested by revived gravitational instability theories (Pollack et al. 1996; Boss 1997; Mayer et al. 2002).”

“…rapid accretion and other models such as the formation of Neptune from a small number of relatively large protoplanets (e.g., Brunini & Fernández 1999) or by gravitational instability in the solar nebula (Cameron 1978; Cameron, DeCampli, & Bodenheimer 1982; Boss 1997) also deserve investigation.”

“…formation mechanism of disk fragmentation (also referred to as “gravitational instability”), for instance, can form substellar companions with Jupiter-like and greater masses at wide orbital separations (Adams & Benz 1992; Boss 1997; Durisen et al. 2007; Kratter et al. 2010; Zhu et al. 2012).”

“Alternative mechanisms for the formation of wide-orbit planets include formation by gravitational collapse 50 of a portion of the protoplanetary disk, in situ accretion within a very massive, distant ring of planetesimals 51 , and scattering from the giant-planet region followed by orbital…”

“…very early ages ( ≲ 1 Myr) at up to ≳ 100 au from the central star, as in the case of HL Tau (ALMA Partnership et al. 2015), presents significant challenges for the two models of planet formation which are most favored, core accretion and gravitational instability (Pollack et al. 1996; Boss 1997).”

“…models generally predict that planets form either by core accretion ( J. B. Pollack et al. 1996 ) or by direct gravitational collapse ( A. P. Boss 1997 ) , and then cool over time until they reach thermal equilibrium with the radiation from the host star and with heat generated by…”

“…are more common with an occurrence rate about 5 − 15% (Johnson et al. 2010; Wittenmyer et al. 2020; Fulton et al. 2021; Sabotta et al. 2021), which is predicted by both the core accretion (Pollack et al. 1996; Liu & Ji 2020) and the gravitational instability (Boss 1997) planet formation theory.”

“Most planets are understood to form via core accretion in a primordial gaseous disk (Pollack et al. 1996), while disk fragmentation via gravitational instability has been proposed as an alternative mechanism for some planets (e.g., Boss 1997).”

“Whilst they may form in situ through disc fragmentation (Boss 1997; Mayer et al. 2002), they may have formed closer in to their host stars and subsequently moved via dynamical processes, either with other planets in the system (Davies et al. 2013), or through the direct (Laughlin & Adams 1998;…”

“Furthermore, if gas giants formed from gravitational instability of gas fragments in the protosolar disk, which could be an alternative path for giant planet formation (e.g., Boss 1997; Vorobyov & Elbakyan 2018), gas giants’ initial eccentricities are not well determined.”

“The Jeans mass is the mass of a sphere of uniform density gas with a radius equal to the Jeans length, where the Jeans length is the critical wavelength for self-gravitational collapse of an isothermal gas (e.g., Boss 1997b, 2005).”

“While the circumstances surrounding the birth of each planet may be unique – from fragmentation via gravitational instability (e.g. Kuiper 1951; Boss 1997; Inutsuka et al. 2010; Nayakshin 2010) to streaming instability of large dust reservoirs (e.g. Youdin&Goodman 2005;Youdin& Johansen 2007;…”

“Grains with initial radii larger than the considered ones are not considered, because they are unrealistically large for initially formed protoplanets via disk instability (Boss 1997).”

“To explain the formation of gas giants, the following two models have been mainly proposed: the core accretion model (e.g., Hayashi et al. 1985; Pollack et al. 1996) and the gravitational instability model (e.g., Boss 1997).”

“These clumps, in turn, can contract to form gravitationally bound sub condensations [6, 7].”

“E P] 1 8 A indicate the likely building mechanism, i.e., core accretion (Pollack et al. 1996) vs. gravitational instability (Boss 1997).”

“Gravitational fragmentation of a massive protoplanetary disk can also result in a metal enriched atmosphere (Boss 1997; Boley et al. 2011).”

“…Bodenheimer & Pollack 1986; Pollack et al. 1996; Kokubo & Ida 2002; Thommes et al. 2003; Chambers 2006a; Fortier et al. 2009; Ormel & Kobayashi 2012; Chambers 2013) and the disk instability model (e.g., Cameron 1978; Boss 1997; Mayer et al. 2002; Rafikov 2005; Durisen et al. 2007; Zhu et al. 2012).”

“They showed that microscopic grains grow and sediment to the centre of such gas clumps within a few thousand years (cf. also Boss 1997).”

“There are two major planet formation models: the core accretion model (Pollack et al. 1996), more likely to form planets in the inner disk, and the disk instability model (Boss 1997), which is in better agreement with the conditions in the extended disk.”

“…by Santos et al. (2017), Schlaufman (2018) and Narang et al. (2018) who found a mass upper limit for giant planets around 4 – 10 M J , and interpreted this boundary as the threshold where the dominant formation mechanism transitions from core accretion to gravitational instability (Boss 1997).”

“The morphology and location of AB Aur b in the disk is consistent with that expected for a planet being formed by rapid gravitational collapse (see also Boss 1997; Speedie et al. 2024).”

“The two browndwarfs(BDs)PZTelBand η TelBareatratherlargerseparations (tens to hundreds au) and may belong to a separate group with a different formation mechanism (most likely disk instability 46,47 ).”

“…for two different formation mechanisms where lower-mass giants may form via core accretion (Pollack et al. 1996) and higher mass giants may form via disk instability (Boss 1997), as formation via disk instability is not as metallicity dependent as core-accretion (Boss 2002; Cai et al. 2006).”

“The metallicity-planet correlation has posed interesting challenges to the current planet formation scenarios, since it appears to favour the core accretion model (Alibert, Mordasini & Benz 2004) over the planetary formation as a result of disk instabilities (Boss 1997).”

“A planet would have a similar composition to its parent star only if the gravitational instability model (Boss, 1997) were the dominant method of planetary formation in the system.”

“On the other hand, disk instability [8, 4, 2] can form dense clumps via gravitational instabilities (GIs) in times comparable to the dynamic time of the disk (.”

“A useful selection of articles might include Heemskerk et al. (1992), Woodwardet al. (1994), Tomleyet al. (1994), Boss (1997), and Nelsonet al. (1998).”

“Gravita-tional instability in the disk is one alternative forma-† NASA FINESST Fellow tion pathway that consists of massive clump formation at > 100 au (Boss 1997; Mayer et al. 2002), followed by inward migration and potential disruption (Kratter & Lodato 2016; Nayakshin 2017).”

“While core accretion remains the favored formation mechanism for gas giants, alternative processes such as gravitational instability ( A. P. Boss 1997, 2006 ) may be responsible for the formation of some GEMS planets and should not be excluded.”

“…and consistently finds that the secondary bodies have masses of order M ∼ 10 M Jup and form at large radial distances from the star (e.g., Adams & Benz 1992; Boss 1997; Mayor et al. 2004; Kratter et al. 2010; Stamatellos & Whitworth 2008; Inutsuka et al. 2010; Zhu et al. 2012; and many others).”

“Alternatively, if the brown dwarf formed after the stellar components via disk fragmentation (e.g., Cameron 1978; Boss 1997; Kratter et al. 2010), then dynamical scattering with the inner binary could also account for the wide orbit of Hii 1348B (e.g., Heggie 1975; Reipurth & Clarke 2001; Veras et…”

“When cooling is sufficiently rapid, fragmentation can occur (Boss 1997; Gammie 2001; Mayer et al. 2002).”

“The most prominent ones are either fragmentation in the proto-stellar cloud (e.g., Bate et al. 1995; Kroupa 1995; Lomax et al. 2015; Moe et al. 2019) and gravitational instability in the proto-stellar or circumstellar disk (e.g., Boss 1997; Kratter & Lodato 2016).”

“Considering the ‘classical’ planet formation schemes: core accretion (Ida & Lin 2004) and disk instability (Boss 1997), we cannot conclude that the characteristics of the planets around giant stars favor either of the two schemes.”

“An alternative model for planetary formation is the disk instability mechanism which is based on the formation and survival of self-gravitating clumps of gas and dust in a marginally gravitationally unstable protoplanetary disk ( Boss 1997; reviewed by Durisen et al. 2007).”

“In the alternative “top-down” model of disk instability, disks that are sufficiently massive and cool are expected to fragment into low-mass self-gravitating objects ( A. P. Boss 1997 ) .”

“An alternative route via gravitational instability has been proposed ( e.g., A. P. Boss 1997, 2006; A. P. Boss & S. Kanodia 2023 ) but this mechanism typically requires a very massive, cool disk early in the protostellar phase, conditions that are currently not empirically well constrained for…”

“Rapid planet formation at wide orbits has been the domain of gravitational instabilities (GI) and disk fragmentation, whereby a massive enough disk can be unstable to self-gravity of the gas into spiral structures (Shu et al., 1987) or bound fragments (Boss, 1997).”

“Gas giant planets form through runaway gas accretion onto a rocky core ( H. Mizuno et al. 1978; J. B. Pollack et al. 1996 ) or via gravitational instability of the protoplanetary disk ( A. P. Boss 1997 ) .”

“As an alternative mechanism, direct in-situ formation via disk fragmentation due to gravitational instability (GI) has been proposed [5] although planet-planet/star scattering may be another possibility [6].”

“RU Lup is also undergoing sudden increases in brightness (i.e., Giovannelli et al. 1991) which points to outbursting FU Orionis system category that can be related to gravitational instability (GI; Boss 1997; Gam-mie 2001).”

“There are two widely discussed models for the formation of massive planets: Core Accretion (CA) (Pollack et al. 1996) and Gravitational Instability (GI) (Boss 1997).”

“…mass (of order 1 M J ) collapse to form puffed-up protoplanets that may be massive enough to clear a gap and avoid rapid inward (type I) migration down to their own tidal disruption separations (e.g. Boss 1997, 1998; Zhu et al. 2012; Schleicher and Dreizler 2014; Lichtenberg and Schleicher 2015).”

“On the other hand, the alternative model of giant planet formation, the disk gravitational instability model [17], can form giant planets efficiently beyond 50 au.”

“Simulations show that such disk instabilities lead to spiral arm formation, which can fragment directly to form large protoplanets ( Boss 1997).”

“In contrast, giant planets formed via gravitational instability should be more common at larger radii and around less-massive stars, as instabilities are more likely in the cold outer disk regions (e.g., Boss 1997).”

“We consider the formation mechanism in more detail in Section 2.3, but the scenario is generally similar to the formation of a Jovian planet by gravitational instability (Boss 1997).”

“They may have formed via another mechanism such as gravitational instability (GI) within the protoplanetary disk (Cameron 1978; Boss 1997) or within collapsing clouds, i.e. like binary systems.”

“…et al. 2010; Nayakshin 2010a), planet formation begins with gravitational instability (GI; e.g., Kuiper 1951; Cameron, Decampli & Bodenheimer 1982; Boss 1997) of a massive young protoplanetary disc when the latter hatches ∼ 1 Jupiter mass gas fragments at distances of tens to hundreds of AU…”

“It also marks the distance where gas giants like Jupiter and Saturn form, by accreting hydrogen from the proto-planetary nebula onto ice-giant cores, but other formation mechanisms for these planets are also possible (Boss 1997).”

“The idea that planets form through gravitational fragmentation of protostellar discs goes back at least to Cameron [10] and has been the subject of ongoing study since that time [11].”

“Using equation (2), one gets a sedimentation time τsed ≈ R vd ∼ 6 × 103 yr ( 1 gcm−3 ρsp ) ( 1 cm Rsp ) , (66) in rough agreement with an estimate of Boss (1997).”

“(24) Here we have assumed particle motion to take place in the radial-azimuthal plane and zero gas velocity, ux = uy = 0, in the friction terms.”

“In principle, planets can form purely from gravity, via direct gravitational instability in the initial circumstellar disc (Boss 1997).”

“Although there is some debate about exactly how planets form, whether by core accretion (e.g., Pollack et al. 1996; Chambers 2004) or disk instability (e.g., Boss 1997, 2000, 2001), it is generally accepted that planets form out of disks of circumstellar material accreting onto the star.”

“Do they form via the bottom-up core accretion process ( J. B. Pollack et al. 1996; M. Lambrechts & A. Johansen 2012 ) or top-down gravitational collapse ( A. P. Boss 1997; K. Kratter & G. Lodato 2016 ) ?”

“…of a collapsar accretion disk, the outskirts of which can become gravitationally unstable and form neutron star fragments ( A. L. Piro & E. Pfahl 2007; B. D. Metzger et al. 2024 ) , akin to the gravitational-instability channel of giant planet formation in protoplanetary disks ( A. P. Boss 1997 ) .”

“In contrast, gravitational instability, which depends primarily on disk mass and cooling rates rather than solid content, could still operate efficiently in such a metal-poor environment (A. P. Boss 1997, 2006).”

“The disk-instability scenario ( e.g., A. P. Boss 1997 ) proposes that parts of the protoplanetary disk can collapse under gravity, forming massive gaseous protoplanets within a few hundred years.”

“(e.g., Cameron 1978; Boss 1997; Durisen et al. 2007; Boley & Durisen 2010; Paardekooper 2012; Xu & Kunz 2021).”

“It is possible that some giant planets, especially those approaching the 13 M J threshold that conventionally demarcates planets from brown dwarfs, instead formed via disk instability (Boss 1997).”

“…into four main categories: the currently more generally-favored model of core accretion (Pollack et al. 1996; Hubickyj et al. 2005), the theory of disk instability (Boss 1997; Boss et al. 2002b), the photoevaporation model (Guillot and Hueso 2006), and the CO snowline model (Ali-Dib et al. 2014).”