Giant Planet Formation by Gravitational Instability

“…systems.6 Two main mechanisms for the formation of wide-orbit giant companions within protoplanetary disks have been suggested and explored: 1) top-down formation by gravitational disk instability (GI: e.g., Boss 1997), and 2) bottom-up formation by core accretion (CA: e.g., Pollack et al. 1996).”

“In contrast, the primary formation channel of more massive, distant bodies continues to be somewhat uncertain, since direct gravitational collapse (Boss 1997) remains a distinct possibility at large stello-centric radii, where the natal gaseous nebulae would have been comparatively colder.”

“…planetary migration, and more – have been identified (e.g., Johansen et al. 2007; Birnstiel et al. 2010; Turner & Sano 2008; Pollack et al. 1996; Boss 1997; Tanaka et al. 2002) but there is no consensus as to their relative importance as numerical models struggle to match the increasingly…”

“It is hypothesized that this planet must have formed via core accretion (Pollack 1984; Pollack et al. 1996), rather than through gravitational instability (Boss 1997).”

“Indeed, several recent results have shown that giant planets may form in less than 1 Myr via either the bottom-up, core-accretion scenario (Rice & Armitage 2003; Alibert, Mordasini & Benz, 2004; Hubickyj et al. 2005) or the top-down, fragmentation scenario (Boss 1997; Mayer et al. , 2002).”

“The disk instability scenario [64, 65, 66, 317, 316] invokes a localized region of the disk that becomes gravitationally unstable.”

“Later studies were radiative three-dimensional calculations that included simplified ideal magnetic fields calculations (e.g. Boss, 1997, 1999), or solved the complete MHD equations (e.g. Boss, 2002, 2005, 2007, 2009; Commerçon et al., 2010; Tomida et al., 2010, 2013; Bate et al., 2014).”

“However, an alternative scenario for the formation of giant planets is the disk instability model (Boss, 1997, 2001), in which the giant planets form from the direct contraction of a gas clump resulting from local gravitational instability in the disk.”

“The gravitational disc instability model for planet formation (e.g., Boss 1997) was thought to form planets only at distances greater than 50-100 AU (e.g., Rafikov 2005) due to inefficient cooling at closer distances (Gammie 2001; Mayer et al. 2004; Rafikov 2005; Meru & Bate 2011).”

“The disk instability mechanism for giant planet formation 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).”

“This roughly corresponds to the mass limit for having gravitational instabilities in a disk around a 1 M⊙ star such as HD 107146 (e.g. Boss 1997; Rice et al. 2005).”

“Unlike with core accretion theory, a metal-dominated composition is inconsistent with the gravitational instability mechanism for giant planet formation (e.g., Boss 1997, 2000, 2003).”

“In the disk instability picture (e.g., Cameron 1978; Boss 1997; Mayer et al. 2002), gravitational instability in a sufficiently massive disk fragments it into the clumps out of which planets may form.”

“For example, temperature determines whether the gas is susceptible to gravitational instabilities (see, e.g., a review by Kratter & Lodato 2016), a potential mechanism to form giant planets, stellar companions and accretion bursts (e.g., Boss 1997; Boley 2009; Vorobyov 2009; Tobin et al. 2016a).”

“…of giant planets is that the Jupiters (MP ≤ 4MJ) are formed via core accretion whereas super-Jupiters (MP > 4MJ) are formed via the disk instability or the gravitational instability (e.g., Boss 1997; Rafikov 2005; Stamatellos & Whitworth 2008; Boley 2009; Cai et al. 2010; Boss 2010, 2011).”

“On the other hand, giant planets around intermediate-mass stars might be efficiently formed by the disk instability mechanism (Boss 1997).”

“Alternatively, distant bound planets could have formed in situ by gravitational instability (Boss 1997).”

“As noted by Sato et al. (2005), such a big core points toward formation of the planet in the core accretion scenario (e.g., Mizuno 1980; Bodenheimer & Pollack 1986), rather than in the disk instability scenario (e.g., Boss 1997).”

“Such clumps can contract to form giant gaseous protoplanets (Boss 1997; Boss 1998b).”

“Gravitational instabilities in disks could potentially form planets on significantly shorter timescales than core accretion (e.g. Boss 1997; Durisen et al. 2007; Boss 2011), and could therefore explain how these features are seen at such early times.”

“Therefore, it seems unlikely that these companions formed from the protoplanetary disk material via either of the popular gi nt planet formation mechanisms of core accretion (Pollack et al. 1996) or disk fragmentation (Kuiper 1951; Cameron 1978; Boss 1997; Durisen et al. 2007).”

“…AU; e.g., Lafrenière et al. 2008, 2011; Biller et al. 2011; Ireland et al. 2011) may already pose a challenge to the core accretion model, and it has been suggested that the gravitational instability model (e.g., Boss 1997) may better explain their formation (e.g., Dodson-Robinson et al. 2009).”

“Fragmentation has been proposed as an alternative way to form the most massive gas giant planets by Mayer et al. (2002); Boss (1997, 2004, 2006).”

“Core accretion (e.g., Pollack et al. 1996; Hubickyj et al. 2005) and direct formation by disk instability (e.g., Cameron 1978; Boss 1997, 1998) are both viable modes of gas giant planet formation and both may be required to explain the diversity of observed planetary systems.”

“, 2005), in the gravitational instability model (Cameron, 1978; Boss, 1997) the planet’s enrichment with solids is so far considered as an independent part of the actual formation mechanism.”

“In protoplanetary discs where the self-gravity of the gas is dynamically important, direct gravitational collapse of locally Jeans-unstable over-densities within the disc (Boss, 1997Boss, 1998Durisen et al., 2007) would also produce giant planets very rapidly, on the local dynamical timescale.”

“Thus, we consider our simulations a reasonable approximation of terrestrial accretion even if giant planets form more quickly than embryos (e.g., Boss 1997).”

“The main competition to the standard model comes from “rapid collapse” scenarios, which posit that disks that are cool and massive enough can form gas giants directly through gravitational instabilities (Boss 1997).”

“While in situ formation of super-Jovian planets may be possible via gravitational instability (Boss 1997; Kratter & Lodato 2016), this could not lead to the formation of Neptune-or super Earth-mass planets.”

“It is also possible that they form quickly through gravitational instability (Boss 1997) in the young, compact circumstellar disk.”

“, [53,261], GI/TD model of Nayakshin [61] can also explain the formation of low-mass planets.”

“Giant planets are thought to form either via core-accretion (Pollack et al. 1996) or gravitational instability scenario (Boss 1997).”

“4.2: it constrains the physics of gas accretion and potentially even the mode of formation, namely core accretion (Perri and Cameron 1974; Mizuno 1980; Bodenheimer and Pollack 1986) or gravitational instability (Toomre 1981; Boss 1997; Durisen et al. 2007).”

“The competing mechanism model for planet formation (Boss 1997, disk instability model) should not be very sensitive to the metallicity of the host star, so this has also been suggested as evidence in favor the core-accretion scenario (e.g., see Ida & Lin 2004; Mordasini et al. 2009).”

“The determination of the critical value of β is important in models of giant planet formation that rely on disc instability (e.g. Boss 1997; Boley et al. 2010).”

“…stage to make meaningful statements on the relative roles of the two proposed mechanisms for gas giant planet formation, i.e. core accretion (e.g. Lissauer 1993; Pollack et al. 1996; Alibert, Mordasini & Benz 2004) and disc instability (e.g. Boss 1997, 2001; Mayer et al. 2002; Rice et al. 2003).”

“In particular, positive nonrotational velocities are severely damped for numerical stability in Boss (1997, 1998).”

“Dynamical time scales at these radii are prohibitive for core formation and the formation of planets, although planet formation via gravitational instability is a possibility (Boss 1997).”

“E P] 1 regarding the formation of giant planets: disk gravitational instability (Boss, 1997) and core accretion (Pollack et al., 1996).”

“The second scenario [8, 52] assumes that giant planets are formed by gravitational instabilities in the massive proto-Solar Nebula [7], leading to the rapid formation of density enhancements—or clumps—with solar chemical compositions and masses probably larger than the present ones of the giant planets.”

“The alternative for the core accretion model is the gravitational instability scenario, in which a massive disk becomes gravitational unstable and fragments into giant planets (Boss 1997; Mayer et al. 2004).”

“One possibility is that the AS 209 disk was gravitationally unstable in the past and the planet formed via gravitational instability (GI; Boss 1997).”

“…orbital distances from the star, disk fragmentation may form bound objects such as giant planets, brown dwarfs, or stellar mass companions (e.g., Boss 1997; Rice et al. 2005; Rafikov 2005; Stamatellos & Whitworth 2009; Kratter et al. 2010; Meru & Bate 2011; Machida et al. 2011b; Vorobyov & Basu…”

“This single-star-environment tropism does also affect the alternative planet-forming scenario by gravitational instabilities (Boss, 1997), which has witnessed a renewed interest after the discovery of jovian exoplanets at large radial distances from their star (Boss, 2011).”

“There are two main pathways to form substellar companions around young stars (Lissauer 1987; Boss 1997; Boley 2009).”

“The gravitational instability scenario (hereafter GI; e.g., Boss 1997; Forgan & Rice 2013) has been proposed as an alternative to solve that issue.”

“The two widely accepted planet formation theories, core accretion (Pollack et al. 1996) and gravitational instability (Boss 1997) both predict that circumplanetary disks form around giant planets (e.g. Quillen & Trilling 1998; Canup & Ward 2002; Ward & Canup 2010).”

“A competing theory involves collapse of a region of the disk under self-gravity, e.g. (Boss 1997), but may have difficulty explaining significant enrichment of refractory elements (Hubbard et al. 2002; Guillot 2005).”

“One currently favored explanation is that core-accretion and other processes related to gravitational disk instabilities are responsible for building each group respectively (Pollack et al. 1996; Ida and Lin 2004; Boss 1997; Durisen et al. 2007).”

“Since this is uncomfortably close to a typical disk life time, it was considered as one of the weakest points of core accretion scenario (e.g. Boss 1997).”

“Observations are needed to determine the importance of effects such as gravitational instability (Boss 1997), streaming instability (Johansen et al. 2007), dust growth (Birnstiel et al. 2010), core accretion (Pollack et al. 1996), planetary migration (Tanaka et al. 2002), and more.”

“Dodson-Robinson et al. (2009) claim that 35 AU is a rough limit beyond which massive gas giant planets must form by an alternate mechanism, such as gravitational instability in the disc (e.g. Cameron 1978; Boss 1997); Boley (2009) claims that this limit is approximately 100 AU.”

“A completely different formation mechanism for gas giant, based on direct gravitational collapse of young protoplaneraty discs, has also been proposed (Boss 1997).”

“Another mechanism for giant planet formation is disk fragmentation as a consequence of the gravitational instability (Kuiper 1951; Cameron 1978; Boss 1997, see also the recent review by Durisen et al. 2007 and Stamatellos & Whitworth Electronic address: dnero@physics.utoledo.edu 2009 for recent…”

“Regarding the third point, the mass accumulation at the edge of a dead zone, or even at the edge of a gap opened by a planet, may lead to planet formation via gravitational instability, which requires a dense, cold region (e.g. Boss 1997; Mayer et al. 2002).”

“This would be an issue giant planets rapidly formed via a mechanism (Boss, 1997) but not if formed around a slowly accreting Mizuno, 1980).”

“Boss (1997) has argued that planet formation could occur gravitationally in some circumstances in timescales of about 103 yr, whereas the accretion onto a rocky core process (Pollack et al. 1996) would require 106È107 yr ; i.e., we cannot come to a conclusion on the viability of planet formation…”

“Gas- and ice-giant planets may also form by gravitational instability (Boss 1997, 2000; Pickett et al. 2003; Durisen et al. 2007).”

“Current giant-planet formation models assume that they are created at distances beyond the stability point of ice (e.g. Lissauer 1987; Boss 1997), which implies conditions not suitable to surface habitability.”

“Knowledge about the core masses of planets is highly desired because it is thought to help to distinguish between the possible planet formation scenarios of core accretion (see e.g. Pollack et al. 1996) and gravitational instability (see e.g. Boss 1997).”

“The two leading scenarios for planet formation, core accretion and direct gravitational collapse, should lead to signif- icantly different initial orbital outcomes for young planets (e.g., Lissauer 1993; Boss 1997; Papaloizou & Terquem 2006).”

“To explain the ice giants Uranus and Neptune, Boss (2003) has advocated a model in which these planets are the remnants of ∼2 MJ gas giants ablated by a sustained ionizing flux of photons from nearby OB type stars.”

“Present address: Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA gions of the solar nebula, before migrating in (Boss 1997, 2002; Boley et al. 2010).”

“Fragmentation through gravitational instability (GI) is a model which forms stellar companions quickly and typically only works in the cool outer reaches of the circumstellar disk (Kuiper, 1951; Boss, 1997).”

“If, however, inner disks dissipate rapidly on the order of 1 Myr, the disk instability mechanism proposed by Boss (1997) may be required to explain these subAU gas giants.”

“…et al. 2004), where gas from the protoplanetary disk is accreted around a previously formed rocky/icy core, and the disk instability model (e.g. Boss 1997; Mayer et al. 2002), where a planet is formed ⋆ The data presented herein are based on observations collected at the La Silla Parana…”

“Core accretion plus gas capture (e.g., Pollack et al. 1996) and direct formation by disk instability (e.g., Cameron 1978; Boss 1997, 1998) are both viable formation models for gas giant planets (see D’Angelo et al. 2010 for a review).”

“Moreover, spiral shocks have been found in simulations of protosolar discs, and have been proposed as a means of converting the gaseous protostellar disc into orbiting planetesimals (Boss 1997).”

“Renewed interest in disc instability as a planet-forming mechanism is largely due to the advancement of Boss (1997) and to recent discoveries of extrasolar planets c© 0000 RAS ar X iv :1 20 9.”

“Boss (1997) argued that giant planets may form this way.”

“Furthermore, both core accretion (Pollack et al. 1996) and gravitational insta- MNRAS 000, 1–18 (2020) bility (Boss 1997) population synthesis models have shown that at separations beyond 50 au, where JWST is most sensitive, planetarymass companions are increasingly uncommon (Forgan&Rice 2013;…”

“Indeed, massive disks can become gravitationally unstable in their outer parts, which can trigger giant planet formation through gravitational collapse (Boss, 1997).”

“…the disk instability mechanism, where gas-phase gravitational instabilities (GI’s) produce spiral waves that fragment into protoplanetary clumps (Boss 1997, 1998, 2000, 2001, 2002, 2003, 2004, 2005, 2007, 2009; Pickett et al. 1998, 2000a,b, 2003; Nelson et al. 1998, 2000; Nelson 2006; Gammie…”

“Since the timescale for the core accretion model (Pollack et al. 1996) is longer than formation due to disk instability (Boss 1997), a detection of a young planet around a T Tauri star should also provide clues to the formation mechanism itself.”

“The main scenarios currently considered for planet formation are the core accretion model (e.g. Pollack et al. 1996) and disc instability (e.g. Boss 1997).”

“However, the birthplace of planets are expected to be inside several 10AU both in the core-accretion model (e.g., Pollack et al. 1996) and in the disk instability scenario (e.g., Boss 1997).”

“…& Nordlund 2004; Hennebelle E-mail: dstamatellos@uclan.ac.uk & Chabrier 2008, 2009; Hopkins 2013), (ii) by fragmentation of protostellar discs (Boss 1997; Stamatellos, Hubber & Whitworth 2007a; Attwood et al. 2009; Boley 2009; Stamatellos & Whitworth 2009a), which may not even be…”

“The alternative formation mechanism, disk instability (Boss, 1997), seems to require a disk mass of at least 0.04 M⊙ to operate (Boss, 2010).”

“It is also possible that formation at moderate periods is not the only mode of planet formation, with at least some wide-orbit planets forming in situ by disk instability during the earliest stages of disk evolution (Boss 1997; Boley 2009).”

“There is an ongoing debate about whether GI makes very massive disks fragment (Boss 1997, 2003; Cai et al. 2006, 2009), or simply favor accretion and mass transport (Hartmann et al. 2006).”

“The core-accretion theory (Pollack et al. 1996) is currently prefered to the gravitational instability theory (Boss 1997) as the origin mechanism of most planets detected by velocimetry because the latter mechanism is not expected to produce giant planets at distances <∼ 10 au (Rafikov 2005).”

“At radii greater than approximately 50AU, planet formation through direct fragmentation of these spiral overdensities into bound objects is possible (Boss 1997, 1998; Clarke 2009; Rafikov 2009; Cossins et al. 2010).”

“1 and 2).”

“The alternative so-called gravitational instability scenario for planet formation (Boss 1997) has been shown to be excluded, both on theoretical (Rafikov 2005, 2007) and numerical (Boley et al. 2007, see Dullemond et al. 2009 for a recent review) grounds, at least for planets within about 50 to 100…”

“This, in turn, can be used to test predictions of planetary formation and evolution theories (Pollack et al. 1996, Boss 1997).”

“The knowledge of the deep composition of Solar System Giant Planets is essential to constrain their formation models (Pollack et al. 1996; Boss 1997; Owen et al. 1999; Gautier & Hersant 2005).”

“…that metallicity variation makes very little difference to the occurrence of fragmentation (Boss 2002). c© 2016 The Authors In the GI scenario, gas giant planets and brown dwarfs form by direct gravitational collapse in the gaseous protostellar disc (Kuiper 1951; Cameron 1978; Boss 1997, 1998).”

“In the GI, contrastingly, planets form in gravitationally unstable disks as stars form in gravitationally unstable molecular clouds (e.g., Boss 1997; Gammie 2001; Rafikov 2005; Cai et al. 2006; Kratter et al. 2010; Meru & Bate 2010).”