Giant Planet Formation by Gravitational Instability

“Numerical experiments with fluid disks often assume a fixed temperature profile or a fixed entropy profile (Boss 1997, 1998; Nelson et al. 1998).”

“The only other theory that is occasionally discussed is gravitational instability, in which the gaseous disk collapses directly into giant planets (Boss 1997).”

“Alternatively, the “disk instability” scenario can potentially form giant protoplanets via the fragmentation of a scaled-up MMSN disk on timescales as short as ∼1000 yr (Cameron 1978; Boss 1997; Mayer er al.”

“In the disc instability scenario, gravitational instabilities in the protoplanetary disc excite dense spiral arms which fragment directly into gas giant planets (Boss 1997).”

“Giant planets or low-mass brown dwarf companions could be formed relatively rapidly from a fragmentation process in a gravitationally unstable disk (e.g., Boss 1997, 1998; Rice et al. 2003b).”

“Revived two decades ago (Boss 1997), this mode of formation has been extensively studied afterward (e.g., Durisen et al. 2007; Durisen 2011), and the prevailing 18 D’Angelo and Lissauer consensus is now that it probably favors the formation of massive objects, brown dwarfs or low-mass stellar…”

“The first two modes are relevant to disk evolution, while the possibility of fragmentation is of great interest as a mechanism for forming massive planets or substellar objects (Boley 2009; Boss 1997; Stamatellos, Hubber & Whitworth 2007).”

“Gravitational instability in the disk was also proposed as a viable scenario to form planets, especially in the outer disk (Boss 1997, 1998; Durisen 2007).”

“It may not arise naturally from the gravitational instability scenario (Boss 1997).”

“The second relies on gravitational instability in a gaseous protostellar disk (Kuiper 1951 ; Cameron 1978 ; Boss 1997), which produces a subcondensation with composition close to that of the central star with little, if any, solid core (Stevenson 1982 ; see also Boss 1998).”

“In both the core accretion (Pollack et al. 1996) and disk instability (Boss 1997) models, the overall efficiency of the giant planet formation process scales with the amount of mass available in the disk.”

“E P] 1 8 Ju l 2 instability (Boss 1997) is the main working mechanism for the formation of giant planets.”

“In contrast, Boss (1997) argued that the direct collapse of clumps from gravitational instabilities can create massive gas giant planets (see also Durisen et al. 2007 and references therein).”

“Such cores are a “smoking gun” of the core-accretion bottom-up formation scenario for giant planets (Pollack et al. 1996), but are generally not expected in the gravitational instability scenario (Boss 1997).”

“An alternative scenario involves direct fragmentation of the disc (Boss 1997), which is probably only possible in the outermost regions of the disc (Boley 2009).”

“An alternative mechanism has been proposed and investigated, where gas giant planets form rapidly through a gravitational instability of the gaseous portion of the disk (Boss, 1997, 2001).”

“Such a two-step process is suggested because nebular temperatures are estimated to be too high for the inferred disk areal mass densities to allow direct gravitational instability by the Toomre condition (Boss 1997, 2001), akin to the Jeans criterion for star formation.”

“…a totally different theory of protoplanetary formation relies on gravitational instability in a massive gaseous protostellar disk (Kuiper 1951; Cameron 1978; Boss 1997), which produces a subcondensation with a composition close to that of the central star and having little, if any, solid core.”

“In light of the new constraints on disk densities presented here and in Paper I, it is natural to compare them with the two basic recipes for giant planet formation; disk instability (e.g., Boss 1997) and core accretion (e.g., Pollack et al. 1996).”

“…be sufficient to explain the core masses of Jupiter and Saturn, removing one of the primary objections to the GGPP mechanism Considering the present(Boss 1997b). uncertainties in giant planet core masses, it is unclear if the core masses of the giant planets truly are similar to each other or to…”

“An alternate model proposes an even faster process: a gravitational instability in the nebula triggers a collapse that forms the giant planets in about 0.1 Myr (Boss 1997, 2000).”

“…mass and eccentricity constraints from direct imaging surveys to show that planetary-mass gas giants likely form via core accretion (Pollack et al. 1996), while more massive brown dwarfs and other substellar companions likely form via gravitational instability in protoplanetary disks (Boss 1997).”

“Numerical calculations on gravitationally unstable disks by Adams and Benz (1992) and Boss (1997) suggest that if a disk can be produced with appropriate conditions, fragmentation into objects of ∼ 10 MJ rather than ∼ 1 MJ islikely to occur.”

“A sufficiently cold gas disk is gravitationally unstable and can collapse directly into giant gaseous planets (Boss 1997, Mayer et al. 2002).19 Jupiter and Saturn could have formed in this way, but only if the surface density was greater than the MMSN value by more than a factor of 10 (Guillot &…”

“For example, the models presented here assume “locally isothermal” thermodynamics, an assumption that errs strongly in favor of clumping and merits considerable further scrutiny (Boss 1997; Pickett et al. 1998, 2000a; Nelson, Benz, & Ruzmaikina 2000).”

“The two main models of giant planet formation are core accretion (Pollack et al. 1996) and gravitational instability (Boss 1997, 2001).”

“Boss (1997, 1998) has reopened the possibility that Jupiter and Saturn actually formed very quickly by a gravitational instability in the gaseous disk.”

“These ages provide time-scales with which to constrain the physical processes driving, for example, disc dissipation and planet formation (e.g. core accretion versus gravitational collapse; Pollack 1984; Boss 1997).”

“The enhanced disk mass may save conventional giant planet core accretion models that otherwise suffer from formation times in excess of the disk-clearing timescale (Boss 1997).”

“In the present study, we have chosen QðrÞ 1 in the radial region 0:8 r=Req 1, in order to mimic the Q(r) distribution in Boss (1997, 1998).”

“Given these difficulties, it is natural to search for other formation mechanisms, and GI (Boss 1997) stands out as a promising alternative.”

“We first assume an edge-on, co-planar geometry with i = 90◦, and in situ formation of the planets directly from the disk gas, e.g. Boss (1997).”

“, the collapse of a region of the disk under self-gravity [Boss, 1997], there is no requirement for a core, although a core could still form at a later stage [Helled et al.”

“Both core accretion plus gas capture (e.g., Pollack et al. 1996) and direct formation by disk instability (Cameron 1978; Boss 1997) can form gas giant planets in principle.”

“Alternatively, planets could form by disk instability (Boss 1997, and later papers), where the protoplanetary disk is massive and gravitationally unstable, forming multiple self-gravitating clumps of gas that coalesce into bound, planet-mass objects.”

“In situ formation via gravitational instability (Boss 1997) is another possibility, but it would require a rather massive (&0.2-0.35 M⋆) disk.”

“In particular, the large spread in eccentricities poses a problem for planet formation, which is believed to occur in discs around young stars (see e.g. Pollack et al. 1996; Boss 1997).”

“This work does not consider planet formation via gravitational instability (Boss 1997) or tidal downsizing (Nayakshin 2010a,b, 2011).”

“However, and as discussed in Santos et al. (2004b), current data still cannot exclude that giant planets may also be formed by the disk instability model (Boss 1997, 2002; Mayer et al. 2002), in particular planets orbiting low metallicity stars.”

“…sizes for conditions in the outer solar system range from ∼< 1 m up to several hundred km depending on details of both microscopic and macroscopic physics (e.g., Goldreich & Ward 1973; Tremaine 1990; Cuzzi et al. 1993a, 1993b; Bailey 1994; Weidenschilling 1997; Boss 1997; Wurm & Blum 1998).”

“Two mechanisms have been proposed to explain giant planet formation: core accretion and disc instability (e.g. Pollack et al. 1996; Boss 1997; Mayer et al. 2002).”

“…core accretion model (Safronov 1969; Wetherill 1980; Bodenheimer & Pollack 1986) and the gravitational instability model (Kuiper 1951; Cameron 1978; Boss 1997), although it is not clear that the latter should produce core-less planets given accretion of solids plus subsequent gravitational…”

“2007) or, in the case of proto-planetary discs, low-mass companions (Stamatellos et al. 2007) or even giant planets (Boss 1997, E-mail: peter.cossins@astro.le.ac.uk 1998).”

“The rapid formation of gas giant planets by gravitational instabilities in a protoplanetary disk (Kuiper 1951; Cameron 1978; Boss 1997) is an appealing alternative to the conventional scenario of accretion of gas onto preexisting large rocky cores formed by accumulation of planetesimals (Wetherill…”

““Hot-start” models (e.g., Burrows et al. 1997; Saumon & Marley 2008) slowly radiate their initial gravitational potential energy over time and therefore best represent formation via disk instability (e.g., Boss 1997; Mayer 2002).”

“Secondly, giant planets can be formed either by gas accretion onto a solid core (Pollack et al. 1996) or by direct gravitational fragmentation of the disk (Boss 1997).”

“Our study of metallicity distribution of BDs host stars seems to support the disc-instability mechanism (Boss 1997, 2002, 2006) for those BDs formed in the planetary discs, while it is inconsistent with the prediction of core-accretion formation mechanism (Pollack et al. 1996; Ida & Lin 2004;…”

“However, early calculations (Vorobyov & Basu 2005, 2006) used a polytropic equation of state which can artificially enhance fragmentation (Boss 1997; Pickett et al. 2000; Gammie 2001).”

“…atmospheric metallicities of gas giants formed through disk instability, the proposed alternative mechanism of giant planet formation (Cameron 1978; Boss 1997), but recent simulations by Helled & Bodenheimer (2010) suggest that the metallicity of the HR 8799 planets should be similar to HR 8799…”

“…(e.g., Pollack et al. 1996; Alibert et al. 2005; Lambrechts & Johansen 2012, 2014), gravitational instabilities in protoplanetary disks (e.g., Boss 1997; Durisen et al. 2007; Vorobyov 2013), fragmentation of collapsing molecular cloud cores (e.g., Boss 2001; Bate et al. 2002; Bate 2009) and…”

“The primary mechanisms posited for transport are gravitational instability (Cameron 1978; Boss 1997) and the magnetorotational instability (MRI; Velikhov 1959; Chandrasekhar 1960; Balbus & Hawley 1991).”

“This may cause the truncation of circumnuclear disks (Goodman 2003) or the formation of planets (e.g., Boss 1997 and references therein).”

“A completely different approach assumes planet formation by disk instability (e.g., Boss 1997a).”

“An alternative planet formation model that has been hypothesized is that of gravitational instability (Zel’Dovich, 1970; Boss, 1997).”

“…that Jupiter-sized planets can not be formed close to their parent stars, at a distance of ∼ 0.05 AU, but instead must be formed behind the ’ice line’ of the protoplanetary disk, at a distance of ∼ 5 AU (e.g., Bodenheimer & Pollack 1986; but see, however, Boss 1997 for a heretical approach).”

“Another in-situ formation scenario is based on the gravitational instability (e.g., Cameron 1978; Boss 1997).”

“…most formation models of planets or low-mass brown dwarf companions – core accretion (e.g. Pollack et al. 1996), gravitational istability (e.g. Boss 1997) and fragmentation of a pre-stellar core (e.g. Padoan & Nordlund 2002; Bate et al. 2003) – cannot easily explain the existence of such…”

“Both the core-accretion and gravitational collapse models predict that giant planets are likely to form at large orbital distances, beyond the snow line, which itself moves inward in time (Pollack et al. 1996; Boss 1997; Bodenheimer et al. 2000; Sasselov & Lecar 2000; Mayer et al. 2002).”

“In the disk instability picture (e.g. Boss 1997; Mayer et al. 2002), gravitational instabilities of a sufficiently massive disk causes it to fragment into clumps out of which planets form.”

“Contrary to the locally adiabatic model of Boss (1997), which produced clumps, the present locally adiabatic models do not form clumps, only spiral arms.”

“Observational estimates are essential for constraining theories such as the core accretion (Pollack et al. 1996) and gravitational instability (Boss 1997) models.”

“The alternative mechanism for gas giant planet formation is a disk instability, where a gravitationally unstable region in a protoplanetary disk forms self-gravitating clumps of gas and dust, within which the dust grains coagulate and sediment to form a central core (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.”

“The competing model, gravitational instability in a massive circumstellar disk (e.g., Boss 1997, 2000; Mayer et al. 2002) does not predict such a relationship.”

“In this paper we examine the possibility of forming gas giant planets around M dwarfs by the competing mechanism of disk instability (Cameron 1978; Boss 1997, 1998, 2003, 2004, 2005; Mayer et al. 2002, 2004).”

“…mass, dust size distribution, and gas content all have profound impacts on the two leading theories of giant planet formation: the gravitational instability model (Boss 1997, Mayer et al. 2002), and the core accretion model (Mizuno 1980, Pollack 1996, Laughlin et al. 2005, Alibert et al. 2005).”

“The possibility that gas giant planets form from fragmentation of a gaseous disk was suggested by Cameron (1978) and advocated by Boss (e.g., Boss 1997).”

“GI theories investigate the fragmentation of the protoplanetary disk into bound clumps (Cameron 1978; Boss 1997).”

“While typical gas-rich disks around young stars may extend to 100 AU, or more in a few extreme cases, the outer reaches are too tenuous to form planetary cores within the lifetime of the disk, unless the disk is massive enough to become gravitationally unstable (Boss 1997; Marois et al. 2008).”

“Theory predicts that planets form in circumstellar disks over a period of 106 to 108 years (Pollack et al., 1996; Boss, 1997).”

“Most disks, however, are well below the Toomre criterion for global gravitational instability (Q < 1, Toomre 1964; Boss 1997), with a typical Mdisk/M∗ of only ∼ 5±3%.”

“Disk instability (Boss 1997) is then the remaining candidate mechanism for forming wide gas giant planets (Dodson-Robinson et al. 2009; Boley 2009).”

“Those parameters are important for improving theoretical models of the composition and origin of extrasolar planets (e.g., Saumon et al. 1996 ; Guillot et al. 1997 ; Ward 1997 ; Boss 1997, 1998 ; Trilling et al. 1998 ; Sandquist et al. 1998 ; Marley et al. 1999 ; Seager, Whitney, & Sasselov 1999).”

“Such as planet formation (Lissauer 1993; Boss 1997), grain growth and settling (Dullemond & Dominik 2004b, 2005), photo-evaporation (e.g. Alexander et al. 2006; Gorti et al. 2009), viscous accretion (Hartmann et al. 1998), the magneto-rotational instability (Chiang & Murray-Clay 2007), and…”

“This scenario is compatible with the gravitational instability mechanism: a gravitationally unstable region in a protoplanetary disk forms self-gravitating clumps of gas and dust within which the dust grains coagulate and sediment to form a central core (Boss 1997).”

“Formation scenarios of Jovian planets fall into two broad categories: a) as a direct condensation from the proto-solar nebula (Boss 1997), and b) in a two phase process where a rocky core is formed, and a gaseous envelope is subsequently accreted to form the planet’s atmosphere (e.g. Bodenheimer &…”

“Disk instability models (e.g. Boss 1997), on the other hand, do not predict, in general, the dependence of the planet formation on metallicity (Boss 2002) and they also don’t seem to depend strongly on stellar mass, at least in the case of M dwarfs (Boss 2006).”