Giant Planet Formation by Gravitational Instability

“Furthermore, it has been shown that the alternative scenario, giant planet formation via gravitational instability in the protoplanetary disk (Boss 1997; Mayer et al. 2002; Boss 2004) does not carry a metallicity dependency that can explain the observed relation.”

“…at separations of < 100 au are thought to be the consequence of either core accretion (Lissauer & Stevenson 2007) or gravitational instabilities (Boss 1997, 2011) acting at the Class II stage (i.e., T Tauri stage), planets at larger separations are believed to be products of disk fragmentation…”

“Gravitational fragmentation of protostellar disks is an alternative formation scenario (Kuiper 1951; Cameron 1978; Boss 1997; Boley 2009).”

“In the frame of the gravitational instability model, gas giant protoplanets form rapidly through a gravitational instability of the gaseous portion of the disk and then more slowly contract to planetary densities (Boss 1997, 2005).”

“…HD 47186b [Bouchy et al. 2008] and HD 40307bcd [Mayor et al. 2008]), a situation similar to that of our solar system and consistent with the “best of both worlds” scenario (Boss 1997), where rocky planets form by collisional accumulation inside the orbits of gas giants formed by disk instability.”

“In particular, the fragmentation of accretion discs has been invoked to explain both the formation of gas giant planets (e.g. Boss 1997; Mayer et al. 2002), and also to explain the formation of young, massive stars observed close to super-massive black holes at the centres of galaxies (e.g.,…”

“…explained by fragmentation that forms gas clump in the gas disc, following by gravitational instability of the fragment within the protosolar disc (Boss, 1997; Durisen et al., 2007; Vorobyov and Elbakyan, 2018), instead of the conventional core-accretion (e.g. Pollack et al., 1996; Lissauer et…”

“In GI, the outer regions of gravitationally unstable protoplanetary discs may fragment into Jupiter mass and larger clumps that were previously believed to share the composition of their parent discs (e.g., Boss 1997).”

“On the other hand, if the cooling is overly efficient the disc fragments into dense clumps that may be the precursors of gaseous giant planets or stars (Cameron 1978; Boss 1997).”

“Although timescales favour the dispersal of protoplanetary disks from the inside-out (Hayashi et al. 1985; Strom et al. 1989), some disk dispersal processes such as gravitational instability may be more efficient in the outer, colder parts of the disk (Boss 1997; Rice et al. 2006).”

“…also lead to disk fragmentation, which has been invoked to explain the formation of stellar/sub-stellar companions or giant planets at large radii (Boss 1997; Kratter & Matzner 2006; Stamatellos & Whitworth 2009; Helled et al. 2014) Studying these non-linear phenomena requires direct numerical…”

“Fragmentation of a disk is a possible mechanism for planet formation and may be aided by binary perturbations (Boss 1997, 2006).”

“If relatively close, i.e., at separations 100 au, PMCs may form through the classical mechanisms of core accretion (Lissauer & Stevenson 2007) or Class II gravitational instabilities (Boss 1997, 2011).”

“If this planet formed through gravitational instability (Boss 1997), this heavy elemental concentration would require either an enormous loss of its original gaseous envelope or an extensive acquisition of heavy elemental material through the post-formation accretion of solid planetesimals and…”

“Two mechanisms have been proposed to form giant, large-separation exoplanets: the core-accretion model,(28) and the gravitational instability model.(29) These two models predict different thermodynamic conditions in the newly formed exoplanets; spectra covering the full wavelength range from 0.”

“An alternative model for giant planet formation is disk instability (Boss 1997; Mayer et al. 2002).”

“The alternative model of gravitational instability can create a protoplanet very rapidly, on dynamical (orbital) timescales (Boss 1997; Rice, Lodato & Armitage 2005), provided that the disc-cooling time is similarly short (Gammie 2001; Rafikov 2005).”

“T130, 1 (2008) [3]Boss, A.”

“to maintain themselves by self-gravity (Boss, 1997).”

“…of different planet formation mechanisms such as core accretion, dynamical scattering, disk instability, and cloud fragmentation, all of which act over different orbital distance regimes (e.g., Boss 1997; Chabrier et al. 2014; Helled et al. 2014; Helling et al. 2014; Kratter & Lodato 2016).”

“The cloud is however much less massive, e.g. its mass is around the opacity fragmentation limit of ∼10MJ (Rees 1976; Nayakshin 2010b; Forgan & Rice 2011), and is born inside a gas disc orbiting the parent star (e.g. Boss 1997).”

“…associated with star formation, such as the collapse of protostellar cores (e.g. Goodwin et al. 2004), the fragmentation of protostellar discs (e.g. Boss 1997), and on the largest scale the gravo-turbulent collapse of a molecular cloud to form stars (e.g. Bate et al. 2003; Bate & Bonnell 2005).”

“This lends supports to the core-accretion model (e.g. Mizuno 1980; Pollack et al. 1996) as the main planet formation mechanism (e.g. Ida & Lin 2004b; Benz et al. 2006), in opposition to the alternative disk-instability model (Boss 1997, 2002; Mayer et al. 2002).”

“Perhaps the companion to DI Tau A formed rapidly through gravitational instabilities in a massive circumstellar disk (see, e.g., Boss 1997).”

“There are two ways of forming planetary mass objects, a companion can form through gravitational instability (GI) in disks (Boss 1997; Lodato & Rice 2004; Durisen et al. 2007; Kratter et al. 2010).”

“…et al. 1996) leading to terrestrial planet formation, while in the outer disk, core accretion, fragmentation, and instabilities are the-orized to form giant gaseous protoplanets (Kuiper 1951; Cameron 1978; Boss 1997; Bate et al. 2002, 2003; Johnson & Gammie 2003; Rafikov 2005; Cai et al. 2006).”

“However, it is still unclear if giant planets around low-mass stars form in this way, or if a competing theory, which is the disk instability theory (Cameron 1978; Boss 1997), offer a better explanation.”

“This class of close-orbiting giant planets must be corroborated by theories of planet formation, which originally predicted giant planets to form and accrete material further out beyond their host star’s “ice line” (e.g., Boss 1997; Rubie et al. 2015; Morbidelli et al. 2015).”

“In the corresponding top-down scenario, planetary cores can be created by gravitational instabilities leading to the collapse of dense regions in the protoplanetary disk (Boss 1997, 2011; Rafikov 2005; Durisen et al. 2007; Kratter et al. 2010; Kratter & Lodato 2016).”

“It has been hypothesized that such self-gravitating over-densities could be precursors to giant planets (Boss 1997, 1998).”

“Gravitational instability is said to occur when the protoplanetary disk rapidly cools and collapses into planetary-mass fragments (Boss, 1997).”

“Discovery of substellar companions at hundreds of AU from their host stars in direct imaging surveys has posed challenges to classical formation mechanisms like core accretion (Pollack et al. 1996) and disk instability (Boss 1997).”

“Another avenue for giant planet formation is the gravitational instability model (e.g., Boss 1997) wherein instabilities in the circumstellar disk can lead to fragmentation and eventual collapse.”

“Fragmentation of disks due to fast cooling may be an important planet formation mechanism under some conditions (e.g., Boss 1997, 2000, 2001; Mayer et al. 2002, 204; Stamatellos & Whitworth 2009; Boley 2009; Clarke 2009; Rafikov 2009).”

“The rapid dispersion timescales of disks around massive stars from high-energy radiation may cause disk lifetimes to be shorter than giant planets formation timescales (Boss 1997).”

“Alternatively, the disk instability scenario (e.g. Boss 1997) suggests that a disk can fragment into a dense core that can contract to form giant gaseous protoplanets in a few hundred years.”

“There are two major scenarios of gas giant planet formation, core-accretion (CA; Pollack et al. 1996) and gravitational instability (GI; Boss 1997,Mayer et al. 2002).”

“Note that dust particles do not necessarily reach this size and barriers such as fragmentation, drift, bouncing etc. tend to limit their growth, unless these are overcome by means of other mechanisms, e.g. streaming (Youdin, & Goodman 2005) or gravitational (Boss 1997) instability.”

“The importance of gravitational instabilities (GI) in the evolution of protoplanetary disks (PPDs) and in planet formation remains hotly debated (Cameron 1978; Boss 1997; Durisen et al. 2007; Boley & Durisen 2010; Paardekooper 2012).”

“…it is a slow process in outer regions, forming planetary cores with masses > 10M⊕ at timescales longer than the lifetime of the disk (106 to 107 years), whereas the timescale for generating dense gaseous clumps by gravitational instability is significantly shorter at around 1000 years (Boss, 1997).”

“For example, massive planets are also proposed to be able to form by disk collapse (Boss 1997) or by protostellar cloud fragmentation followed inward migration by disk capture (Font-Ribera et al. 2009).”

“Both major contending theories of planet formation, core accretion and gravitational instability, require collection of solid material into a dense mid-plane layer within circumstellar disks (e.g., Pollack et al. 1996; Boss 1997; Schräpler & Henning 2004, and references therein).”

“…disks (e.g., Ida & Lin 2004; Alibert et al. 2005), forming mostly planetary companions, and gravitational fragmentation in molecular clouds or protoplanetary disks (e.g., Boss 1997; Bate 2000; Rice et al. 2003; Stamatellos & Whitworth 2009), forming mostly stellar or brown dwarf-mass companions.”

“…(Kornet, Stepinski & Różyczka 2001), the appearance of vortices (Barge & Sommeria 1995; Bracco et al. 1998; Klahr & Bodenheimer 2003) and clumps (Boss 1997, 2001; Mayer et al. 2004a,b; Rice et al. 2004; Durisen et al. 2005), and also the formation of pressureenhanced regions due to the…”

“There is, moreover, the possibility that planets may form in the disk by gravitational collapse, as some recent simulations suggest (Boss 1997, 1998; Mayer et al. 2002), though this notion is not entirely free of controversy (Tohline & Hachisu 1990; Pickett et al. 2000b).”

“…( M BD < 42.5 M Jup ) and low-mass-ratio ( q < 0.01 ) brown-dwarf companions may form through gravitational instability within circumstellar disks ( Boss 1997 ) , while high-mass ( M BD > 42.5 M Jup ) and high-mass-ratio ( M BD / M * > 0.01 ) brown-dwarf companions instead form more like stars,…”

“An alternative to the accretion theories of formation is gravitational instability (GI; Boss 1997).”

“However, it is still unclear whether brown dwarfs found at close-in separations to stars form like stellar binaries or by disk gravitational instabilities (Boss 1997).”

“Its mass and semi-major axis are compatible with predictions from disk gravitational instabilities (Boss 1997; Forgan & Rice 2013) and from collapse and A177, page 8 of 13 A.-L. Maire et al.: VLT/SPHERE astrometric confirmation and orbital analysis of the brown dwarf companion HR 2562 B Fig.”

“Stellar mass companions are believed to form early on through the cooling and contraction of a fragment of the primary’s protostellar cloud, or slightly later from a fragment of its protoplanetary disk (Kuiper 1951; Cameron 1978; Boss 1997; Boley 2009; Vorobyov 2013).”

“The original approach assumed that giant planets started cooling from an arbitrary high entropy state (Stevenson 1982; Burrows et al. 1997), with such “hot start” models now typically associated with the outcome of fragmentation in the protoplanetary disc (Boss 1997).”

“Since the core accretion model (Pollack et al. 1996) predicts longer formation timescales than the disk instability model (Boss 1997) then a detection of a young planet would provide information about the formation mechanism itself.”

“Whether the planets were formed in situ through gravitationl instability(Boss 1997), or formed through outward migration or scattering, is still not clear.”

“Gravitational instability of a massive disk has been invoked as an alternative, rapid formation mechanism for Jovian-type planets (Boss 1997), particularly at very large (>50 AU) orbital radii.”

“Gravitational instability models for gas giant planet formation, which rely on collective processes in a massive disc acting on global scales, suggest that these planets can form on much smaller time-scales, as low as a few thousand years (Boss 1997, 2008).”

“…addressing such problems as reconciling the evolutionary cooling models (hot/warm/cold start; Spiegel & Burrows 2012) with the relevant physical processes (e.g., core accretion and disk instability; Pollack et al. 1996; Boss 1997) are essential to improving our understanding of planet formation.”

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

“Arcs may reveal that pebbles are concentrated by vortices, while spirals or other non-axisymmetric structure may hint at the presence of gravitationally unstable discs (Toomre 1964; Boss 1997; Kratter & Lodato 2016) or massive companions (Dong et al. 2015; Zhu et al. 2015; Bae & Zhu 2018).”

“In addition to the core accretion paradigm, some planets could be produced through gravitational instability in sufficiently massive disks (see, e.g., Boss 1997; Wagner et al. 2019).”

“%) is quite large and challenging to explain in a disk gravitational instability scenario (Boss 1997) without additional mechanisms (non-in situ formation with migration, mass accretion after formation).”

“It will then break up into discrete mass concentrations which can continue to compact under their own self gravity, leading to a large number of giant planets (Boss 1997).”

“The two oldest formation theories are gravitational instability (Kuiper 1951; Cameron 1978; Boss 1997) (see Durisen et al. 2007, Helled et al. 2014, Kratter & Lodato 2016 and Rice 2016 for reviews), and core accretion (Safronov & Zvjagina 1969; Goldreich & Ward 1973; Perri & Cameron 1974; Harris…”

“Their atmospheric abundances provide a test of theories such as core accretion (Pollack et al. 1996) and disk instabilities (Boss 1997).”

“The competing gravitational instability model (e.g., Kuiper 1951; Cameron 1978; Boss 1997, 1998; Durisen et al. 2007) posits circumstellar disks which are massive enough to fragment as a result of their own gravity and form Jupiter mass planets on a much more rapid timescale than is typical in…”

“For the latter scenario two models have been proposed, the core-accretion scenario (Safronov & Zvjagina 1969; Goldreich & Ward 1973; Pollack et al. 1996) and the disc instability scenario (Cameron 1978; Boss 1997).”

“…atmosphere on a rocky core, e.g. Wetherill 1980; Bodenheimer & Pollack 1986; Lissauer 1993), which will be considered in forthcoming studies. that they form through local gravitational instabilities in the outer parts of a protoplanetary disk (Kuiper 1951; Boss 1997, 1998a,b; Helled et al. 2008).”

“Based on the older variants of disc instability models (e.g., Boss 1997) that do not include planet migration and tidal stripping of the fragments, it is tempting to use the presence of a core in a gas giant as prima facie evidence of formation by core accretion plus gas capture, as core accretion…”

“Short dissipation timescales (∼1000 yr) favor the formation of planets via gravitational instability (e.g., Boss 1997) while longer dissipation timescales allow for core accretion, which takes a few Myr (e.g., Lissauer & Stevenson 2007).”

“Nelson (2000) showed that the formation of giant Send offprint requests to: M. Montalto planets is unlikely in equal mass binaries with semi-major axis ∼ 50 AU, both by means of the disk instability (Boss 1997) and the core accretion mechanisms (Pollack et al. 1996).”

“Gas giant planet formation by disk instability is an alternate mechanism which leads to rapid planet formation (Boss 1997, 1998).”

“…gravitational instability, whereby instabilities in massive circumstellar disks allow for the material to directly collapse into giant planet cores on short timescales (∼103 – 104 years after the disk becomes unstable Boss 1997), could be a viable alternative formation mechanism for TOI-4201 b.”

“These disk conditions, in turn, are expected to influence where, when, and how planets form and migrate, as well as their composition (e.g., Stevenson & Lunine 1988; Boss 1997; Kokubo & Ida 2002).”

“In models of disk instability, companions form rapidly through local gravitational collapse in a protoplanetary disk (Boss 1997, 2006; Dodson-Robinson et al. 2009; Vorobyov 2013).”

“Alternative planet formation theories do not require a core at all (Kuiper 1951; Cameron 1978; Boss 1997, 1998, 2000).”

“This mechanism requires a very massive disk that can cool effectively on timescales of a few local orbital periods (Boss 1997; Mayer et al. 2002; Rafikov 2005; Durisen et al. 2007).”

“Such a clump can contract to form giant gaseous protoplanet (Boss 1997, 1998).”

“Numerical hydrodynamics simulations have established that disks which are evolved under the condition of local isothermality will fragment into small dense clumps due to gravitational instabilities when the Toomre stability parameter Q is sufficiently low.”

“…the mostly convective interiors of giant planets occur after their formation by core accretion ( Pollack et al. 1996; D ’ Angelo et al. 2003; Li et al. 2021 ) or disk instability ( Boss 1997 ) , and are regulated by the radiative properties of the thin layers of atmosphere on the planets ’ surface.”

“…formation theory, a hot Jupiter can be formed if a gravitational instability creates bound clumps from the fragments of the proto-planetary disk (Boss 1997; Durisen et al. 2007), or if the protoplanetary core accretes large quantities of gas from the proto-planetary disk (Perri & Cameron 1974;…”

“As κ And b is at a separation where forming massive planets by core accretion is exceptionally difficult, it likely formed through other means, such as gravitational instability.(40)”

“…scenarios, Core Accretion (CA; Pollack et al. 1996; Ida & Lin 2004a; Mordasini et al. 2012) and Gravitational Instability (GI; Kuiper 1951; Boss 1997; Kratter & Lodato 2016), vie to explain the formation of gas giant planets (for a review of advantages and disadvantages of the theories…”

“…from dust particles accumulating into pebbles, planetesimals, and then solid cores (Lissauer & Stewart 1993; Pollack et al. 1996; Morbidelli et al. 2012), some massive giant planets and brown dwarfs may form by top-down collapse in fragmenting protostellar disks (Kratter & Lodato 2016; Boss 1997).”

“A second way to form gas giants is by gravitational fragmentation of protostellar discs (Kuiper 1951; Cameron 1978; Boss 1997).”