Giant Planet Formation by Gravitational Instability

“In the TD model (Kuiper 1951; Cameron 1978; Boss 1997, 2007; Meru and Bate 2010; Nayakshin et al. 2011; Nayakshin 2013, 2014), the planet formation takes place inside massive gaseous clumps of mass ~10 MJ (Jupiter mass) that are formed due to gravitational instabilities in the outer cold disk.”

“Disk GI has long been suggested to be an alternative theory for giant planet formation (Boss 1997).”

“In TD, Gravitational Instability (GI) of cold massive protoplanetary disc hatches gas fragments at distances of tens to hundreds of AU from the host star (Kuiper 1951; Boss 1997; Rice et al. 2005; Helled et al. 2014).”

“According to the disk instability model (Boss, 1997; Mayer, 2002), planets form due to gravitational collapse in the accretion disk, similar to the formation of stars.”

“Furthermore, some of the fragments could give birth to Earth-mass or more massive cores due to grain sedimentation (e.g., Boss 1997; Helled & Schubert 2008; Nayakshin 2011).”

“Gravitational disc Instability (GI) theory for giant planet formation (e.g., Kuiper 1951; Cameron et al. 1982; Boss 1997, 1998) posits that gravitational instability of the disc leads to formation of self-gravitating gas fragments that later contract into present day planets.”

“According to Boss (1997), several gaseous protoplanets can rapidly form by disk instability in a marginally gravitationally unstable protoplanetary disk.”

“Their formation mechanisms remain under debate as they are not easily compatible with either in-situ core accretion or gravitational instability (Pollack et al. 1996; Boss 1997; Chabrier 2003; Kratter & Lodato 2016).”

“Among existing theories, gravitational instability is proposed as a preferred method for the formation of giant planets beyond the snow line (Boss 1997; Mayer et al. 2002).”

“Planets this small are unlikely to form via disk instability (Boss 1997; DodsonRobinson et al. 2009; Boley 2009), so the most likely pathway to a planet like HIP 41378 f is via core accretion.”

“The formation of gas-giant planets is thought to occur either through the gradual bottom-up growth of planetary embryos, followed by rapid gas accretion (core accretion), or through the direct formation of gas giants, following gravitational instability (Mizuno 1980; Boss 1997; Armitage 2010).”

“The second is gravitational instability (GI) (Kuiper 1951; Cameron 1978; Boss 1997, 1998), where entire regions of protoplanetary discs collapse under their own gravity (fragmentation) to form bound clumps consisting mainly of gas (fragments).”

“To date, there are two main planet formation theories: the core accretion mechanism (Safronov 1969; Mizuno 1980; Bodenheimer & Pollack 1986) and the gravitational instability model (Boss 1997).”

“Two representative formation scenarios for Jupiter have been proposed: core accretion (Perri & Cameron 1974; Mizuno 1980; Pollack et al. 1996) and disk instability (Kuiper 1951; Boss 1997; Mayer et al. 2002).”

“…coagulate growing into planets, which can later accrete gas (e.g. Mizuno 1980; Lissauer 1993; Pollack et al. 1996) and (2) gravitational instability where the protoplanetary disc fragments in gaseous clumps, which condense into planets (e.g. Adams, Ruden & Shu 1989; Boss 1997; Durisen et al. 2007).”

“E P] 3 0 M ar 2 01 7 ter 2016), mostly motivated by the idea of forming gas giants via GI (Boss 1997, 1998; Durisen et al. 2007; Zhu et al. 2012).”

“…core accretion models, the disk instability model, once in vague, has been reformulated with fragmentation from massive protoplanetary disks and has been advanced through the investigations of many authors (e.g., Boss, 1997; Mayer et al., 2002, 2004; Boley et al., 2010; Cha and Nayakshin, 2011).”

“…Pollack et al. 1996),was also discussed in models for HR 8799 bcde by Close (2010), who argued that the inner planet was most likely formed by core accretion, while for the outer planets the gravitational instability of the disk (Cameron 1978; Boss 1997) is the more probable formation scenario.”

“Such a planet formation scenario may exhibit a positive planet–metallicity correlation for giant planets, in contrast to the usual route to forming giant planets via radiative cooling and contraction of the gas fragments (e.g. Boss 1997).”

“Currently, there are two well-established theories for giant planet formation: the core-accretion scenario (Pollack et al. 1996) and disk gravitational instability (Boss 1997).”

“However, it may be rele- vant to planet formation theories requiring a self-gravitating disc (e.g., Boss 1997; Nayakshin 2010, 2013).”

“Formation via gravitational instability (Boss 1997), which is believed to form very massive giant planets and low-mass stars efficiently, may be able to form objects as low-mass as 2 M J (e.g., Matsuo et al. 2007).”

“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).”

“…formation scenario known as “gravitational instability,” wherein the gaseous component of the protoplanetary disk becomes unstable to gravitational collapse and subsequently fragments into the observed gas giant planets (Boss 1997; see also Kratter & Lodato 2016 for a more recent review).”

“Finally, it is worth noting that the outer planets in our Solar System, and also superjovian planets large enough to burn deuterium, could form via the direct collapse of a portion of the protoplanetary disk, rather than core accretion [144].”

“Since the dawn of observational exoplanet detection in recent decades the ever-growing number and variety of planetary species gives rise to renewed theoretical efforts for understanding the formation of these bodies (e.g., Boss 1997; Bodenheimer et al. 2000).”

“If the disc is self-gravitating, its dynamical evolution can rapidly form giant planets and brown dwarfs bydisc fragmentation (Kuiper 1951; Boss 1997; Stamatellos et al. 2007).”

“We have found that this effect is likely to be relevant even when the disk mass is lower than the qD > 0.1−0.2 value at which significant GIs are generally thought to occur (see, e.g. Shu et al. 1990; Boss 1997, LR96).”

“…survey of 150 stars detected 13 substellar companions with masses between 1 and 75 MJup and semimajor axes between 5 and 300 au, finding that both core accretion (CA; Mizuno 1980) and disk instability (DI; Boss 1997) appeared necessary to explain the detections for the FGK stars in their sample.”

“In the gravitational instability model, planetary formation begins with the protosolar nebula (PSN), breaking up due to its own self-gravity into giant protoplanets of solar composition, which then accrete planetesimals from the PSN (Cameron 1978; Boss 1997; Helled et al. 2006).”

“In the gravitational instability (GI) model of planet formation (Boss 1997), unstable regions of the disc may directly collapse to rapidly form giant gaseous protoplanets and brown dwarfs.”

“In sufficiently massive disks shocks may trigger the gravitational instability (GI), which is a possible theory to form giant planets (Boss 1997).”

“…such as the different mechanisms of planetary formation (e.g., core accretion, Pollack et al. 1996; gravitational insta- bility, Cameron 1978, Boss 1997), the initial conditions of the environments where planets are formed (Benz et al. 2014; Mordasini 2018), and the dynamical interactions…”

“…(Hall et al. 2019) quasi-steady state in which the instability acts to transport angular momentum (Paczynski 1978; Laughlin & Bodenheimer 1994; Lodato & Rice 2004), or it can become sufficiently unstable that it fragments to form bound objects, potentially of planetary mass (Boss 1997, 1998).”

“As the decline of accretion disk frequency appears to be gradual, i.e., not all stars lose their gas disk at the same age, this implies that gaseous planets may form on even shorter timescales of .1 Myr, in agreement with disk fragmentation theories (e.g., Boss 1997).”

“For example, the strength of MRI turbulence directly affect planetesimal dynamics in protoplanetary disks (Yang et al. 2012; Gressel et al. 2012); while GI can potentially form giant planets directly through disk fragmentation (Boss 1997, 1998; Gammie 2001; Vorobyov 2013; Helled et al. 2013).”

“Note that giant planets can be formed much faster within the context of gravitational instability theory, but it requires too high disk masses (> 0.1M ) and an efficient cooling mechanism to operate (Boss 1997).”

“Finally, MGU disks are required for the disk instability mechanism of giant planet formation to operate (e.g., Boss 1997).”

“…planet formation theories propose drastically different time scales for the formation of planets, with core-accretion (Goldreich & Ward 1973; Mizuno 1980) requiring up to 8 Myr (Mordasini et al. 2007) and gravitational disk instability (Kuiper 1951) requiring less than 0.1 Myr (Boss 1997).”

“Another possibility is that this planet formed by direct collapse of the gaseous material in the disk (Cameron 1978; Boss 1997).”

“It is the detection of these planets in formation that gives us key insights into the primary formation mechanisms that are at work in the earliest phases of system evolution, be it through core accretion (Pollack et al. 1996) or gravitational instability (Boss 1997).”

“The two leading theories of giant planet formation are core accretion (Mizuno 1980; Bodenheimer & Pollack 1986; Pollack et al. 1996; Ikoma et al. 2000) and gravitational instability (Boss 1997; Boss & Kanodia 2023).”

“The alternate mode of formation is the gravitational instability ( GI ) scenario ( A. P. Boss 1997, 2006 ) , which occurs in the protostellar phase of massive disks where the disk fragments into gravitationally bound clumps.”

“As a corollary, an alternative route to formation would be direct gravitational collapse within an unstable disk (Boss 1997; Durisen et al. 2007).”

“Besides core accretion, direct collapse in gravitation-ally unstable disks may also form giant planets at early stages (e.g., Boss 1997; Mercer & Stamatellos 2020; Longarini et al. 2023; Boss & Kanodia 2023), when those class 0/I prostellar disks own higher disk mass than their later stage…”

“These objects are more likely to form like stars in a top-down fashion through the so-called “gravitational instability”(GI) mechanism (Boss, 1997).”

“Meanwhile, more massive objects (&10-20 MJup) are likely formed predominantly via gravitational disk instability (e.g., Boss 1997; Forgan et al. 2018).”

“The first is a top-down approach where the circumstellar disc fragments due to gravitational instabilities (GI), and the fragments collapse into protoplanetary cores (Boss 1997).”

“These observations are the outcome of planet formation which occurs in 4 protoplanetary disks (Pollack et al. 1996; Boss 1997; Matsuo et al. 2007).”

“The ubiquity of such objects represents a challenge to our planetary-system formation theories (Boss 1997; Nagasawa et al. 2008; Coleman & Nelson 2014).”

“Boss (1997, 2002) proposed this mechanism to explain the formation of giant planets before the dissipation of gas in the disc.”

“…clouds in disk galaxies (e.g., Goldreich & Lynden-Bell 1965a; Elmegreen 1987; Kim et al. 2003), gas giant planets in protoplanetary disks (e.g., Boss 1997; Durisen et al. 2007), and bound clumps resulting from fragmentation of shocked layers in star-forming regions (e.g., Elmegreen & Lada…”

“…the currently favored core accretion model, the disk instability model, for explaining the formation of gaseous giant planets suggests that these planets are formed as a result of gravitational fragmentation in a massive protoplanetary disk surrounding a young star (Boss, 1997; Mayer et al., 2004).”

“…(e.g. Ida & Lin 2004, Alibert et al. 2005), which is thought to be the main mechanism of giant planet formation (e.g. Fischer & Valenti 2005, and references therein), and disk instability model (e.g. Boss 1997), which may dominate the formation processes of brown dwarf or substellar companions.”

“They discovered excess emission at∼ 65 au, which they identified as a candidate protoplanet in the earliest stagesof formation, a possible example of protoplanetary disc fragmentation forming a bound object (Boss 1997).”

“The second is gravitational instability in a gaseous disk, where a Jovian planet forms directly by gravitational collapse of a gas clump in a disk (Boss 1997, 2007).”

“Gonzalez (1997) suggested that the stars have become polluted during planet formation, though we find at least equally attractive the idea that having lots of refractory material around makes it easier for planets to form, even Jupiters and Saturns, since they have rocky cores (Boss 1997).”

“Therefore, it is possible for these planets to form through core-accretion in disks that are just massive enough but are not gravitationally unstable and prone to instabilities (Boss 1997, 2006).”

“On the other hand, gravitational instability theory of planet formation (Boss 1997) consists of a rapid formation mechanism that involves the break-up of a massive protostellar disc into clumps under their self-gravity.”

“An alternative could be top-down formation of the giant planet by fragmentation of gravi-tationally unstable disk (Boss 1997; Mayer et al. 2002).”

“…brown dwarfs would be based on formation process, as planets are expected to form bottom up from core accretion, while brown dwarf companions form from gravitational collapse, however this is challenging to distinguish for individual objects (Pollack et al. 1996; Boss 1997; Alibert et al. 2005).”

“Gas-giant planets are thought to have formed from either core accretion, in which runaway gas accretion takes place onto the massive,∼10M⊕, core (Perri & Cameron 1974; Bodenheimer & Pollack 1986), or from a direct collapse from the gas disk (Mizuno 1980; Boss 1997; Armitage 2010).”

“There is, however, an alternative for gas giant planet formation, the top-down mechanism of gas disk gravitational instability (Boss 1997), where a massive protoplanetary disk forms spiral arms, which grow and interact, resulting in the formation of self-gravitating clumps of disk gas and dust that…”

“• Are there multiple formation channels for giant exoplanets? The two most common models for the giant planet formation(7) are the bottom-up core accretion or nucleated instability model [103], and the top-down gravitational instability model [15].”

“On the other hand, the core accretion mechanism (Mordasini et al. 2009; Tanigawa & Tanaka 2016) and that of gravitational instability (Boss 1997, 2001) are also able to produce companions of BD masses in protoplanetary disks.”

“There are two main theories as to how giant planets form in protoplanetary discs, the core accretion and gravitational instability theories (Pollack et al. 1996; Boss 1997; Helled et al. 2014), both of which contain their own problems that can be difficult to solve.”

“Alternatively, S-type planets, giant or terrestrial, in close eclipsing binaries could build up through other formation scenarios, such as gravitational instabilities (Boss 1997) or in-situ core-accretion (Boss 2006; Mayer et al. 2007; Duchêne 2010).”

“…the system saturates in a gravitoturbulent state that can transport significant angular momentum (Gammie 2001; Rice et al. 2014), while more efficient cooling causes the system to fragment into dense clumps that may serve as the pre-cursors of gas giant planets (Cameron 1978; Boss 1997).”

“Contemporary formation models typically fall into the broad categories of core accretion (Mizuno et al. 1980) or disk instability models (Boss 1997), neither of which has yet emerged as a dominate driver.”

“The origin of those planets is still being debated, as prevalent planet formation theories (see Pollack et al. 1996; Boss 1997) do not allow for in situ formation of such objects.”

“Since then many theories with regard to the planetary formation both inside and outside the solar system have been advanced through the works of many authors (e.g., McCrea and Williams, 1965; Cameron, 1978; Boss 1997; Boley et al., 2010; Cha and Nayakshin, 2011).”

“Both core accretion (Pollack et al. 1996) and disk instability (Boss 1997) theories advocated in the past can, in principle, form gas giant planets.”

“It is believed that extrasolar planets, detected in orbit around an increasing number of nearby stars may have formed either by core accretion or by disk instability (e.g. Boss, 1997).”

“It has been quite some time since the idea was first suggested (Kuiper 1951; Cameron 1978) and revived by Boss (1997, 1998a) that the dense clumps in a disk fragmented by GIs may become self-gravitating precursors to gas giant planets.”

“But in these disks, gravitational instability may also lead to the rapid formation of gas giant (Boss 1997).”

“Theory suggests a lower mass limit, the Jeans Mass, where the thermal pressure and other dynamical effects overpower gravitational attraction and a clump of gas dissipates before it can collapse (Low & Lynden-Bell 1976; Boss 1997).”

“Another hypothesis includes the gravitational instability mechanism, in which the solar nebula fragments through its own self-gravity into globs of gas and dust which then shrink and collapse to form giant planets (Boss 1997).”

“This is an open access a collapse and contract to form gas giant protoplanets [6, 7].”

“These stars play a critical role in understanding planet formation and evolution as the two competing planet formation paradigms, core accretion (Pollack et al. 1996) and gravitational instability (Boss 1997), predict very different planet frequencies for the smallest stars.”