THE FORMATION MECHANISM OF GAS GIANTS ON WIDE ORBITS

By S. Dodson-Robinson, D. Veras, Eric B. Ford, C. Texas, U. Florida, Nasa Exoplanet Science InstituteCaltech. 2009

166 citations recorded by the source corpus.

The recent discoveries of massive planets on ultra-wide orbits of HR 8799 and Fomalhaut present a new challenge for planet formation theorists. Our goal is to figure out which of three giant planet formation mechanisms—core accretion (with or without migration), scattering from the inner disk, or gravitational instability—could be responsible for Fomalhaut b, HR 8799 b, c and d, and similar planets discovered in the future. This paper presents the results of numerical experiments comparing the long-period planet formation efficiency of each possible mechanism in model A star, G star, and M star disks. First, a simple core accretion simulation shows that planet cores forming beyond 35 AU cannot reach critical mass, even under the most favorable conditions one can construct. Second, a set of N-body simulations demonstrates that planet–planet scattering does not create stable, wide-orbit systems such as HR 8799. Finally, a linear stability analysis verifies previous work showing that global spiral instabilities naturally arise in high-mass disks. We conclude that massive gas giants on stable orbits with semimajor axes a ≳ 35 AU form by gravitational instability in the disk. We recommend that observers examine the planet detection rate as a function of stellar age, controlling for the planets’ dimming with time. Any age trend would indicate that planets on wide orbits are transient relics of scattering from the inner disk. If planet detection rate is found to be independent of stellar age, it would confirm our prediction that gravitational instability is the dominant mode of producing detectable planets on wide orbits. We also predict that the occurrence ratio of long-period to short-period gas giants should be highest for M dwarfs due to the inefficiency of core accretion and the expected small fragment mass (∼10 MJup) in their disks.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/9a886b583d283be1dda8ac100859f63c328fd15b

Citations in the corpus

• A COMBINED SUBARU/VLT/MMT 1–5 μm STUDY OF PLANETS ORBITING HR 8799: IMPLICATIONS FOR ATMOSPHERIC PROPERTIES, MASSES, AND FORMATION (2011, unknown): “HR 8799’s planets are often described as confounding either planet formation model (e.g. Marois et al. 2011) or being clear examples of disk instability-formed planets, as claimed by Dodson-Robinson et al. (2009).”
• Rapid growth of gas-giant cores by pebble accretion (2012, unknown): “The grey dotted curves correspond to classical planetesi-mal accretion (PA), where the faster growth corresponds to 2D accretion of planetesimal fragments (Rafikov 2004) and the slower to 3D accretion of planetesimals (e.g. Dodson-Robinson et al. 2009).”
• TRANSITIONAL DISKS AS SIGNPOSTS OF YOUNG, MULTIPLANET SYSTEMS (2011, unknown): “Notably, Crida et al. (2006) found that even in strongly accreting disks with kinematic viscosity of order 1015cm2 s−1 at 1 AU, corresponding to an accretion rate of Ṁ∗ ≈ 5 × 10 −7M⊙ yr −1 (Dodson-Robinson et al. 2009a), a Jupiter-mass planet (1MJ) can open a gap in only 1000 orbits.”
• THE RUNTS OF THE LITTER: WHY PLANETS FORMED THROUGH GRAVITATIONAL INSTABILITY CAN ONLY BE FAILED BINARY STARS (2009, unknown): “Dodson-Robinson et al. (2009) have investigated the possibility of forming the HR 8799 system via scattering in the absence of dynamically important gas, but find that putting three massive planets into such closely spaced yet wide orbits is unlikely.”
• PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS (2014, unknown): “…collapse of a massive protoplanetary disk has consumed much of the discussion about planet formation for the past decade, especially after the discovery of the HR 8799 planets (e.g., Boss 2007; Durisen et al. 2007; Boley 2009; Dodson-Robinson et al. 2009; Nero & Bjorkman 2009; Kratter et al. 2010).”
• CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b (2011, unknown): “…focused on the orbital dynamics and formation of the system; however, the conclusions about the planetary masses and long term stability of the system from these works are mixed (Fabrycky & Murray-Clay 2010; Reidemeister et al. 2009; Goździewski & Migaszewski 2009; Dodson-Robinson et al. 2009).”
• The great escape: how exoplanets and smaller bodies desert dying stars (2011, unknown): “Regardless, such planets are very unlikely to have formed in these environments; neither core accretion nor gravitational instability formation models can fully form planets beyond ∼100 au (Dodson-Robinson et al. 2009).”
• SPECTRAL AND PHOTOMETRIC DIAGNOSTICS OF GIANT PLANET FORMATION SCENARIOS (2011, unknown): “For instance, the planets found in the HR8799 system (Marois et al. 2008, 2010) appear to be inconsistent with classical “cold-start” models (Dodson-Robinson et al. 2009; Bowler et al. 2010; Janson et al. 2010; Currie et al. 2011; Madhusudhan et al. 2011).”
• NEAR-INFRARED SPECTROSCOPY OF THE EXTRASOLAR PLANET HR 8799 b (2010, unknown): “The agreement with the “hot-start” models may suggest that the HR 8799 planets were formed through disk instability, as has been suggested by Dodson-Robinson et al. (2009) based on an analysis of plausible theoretical formation mechanisms.”
• EXOPLANET ALBEDO SPECTRA AND COLORS AS A FUNCTION OF PLANET PHASE, SEPARATION, AND METALLICITY (2010, unknown): “The existence of giant planets at separations larger than ∼ 10 AU is difficult to account for in standard core accretion models (Pollack et al. 1996; Ida & Lin 2005; Dodson-Robinson et al. 2009).”
• DISCOVERY OF A PROBABLE 4–5 JUPITER-MASS EXOPLANET TO HD 95086 BY DIRECT IMAGING (2013, unknown): “At 56 AU, the fragmentation of the protostellar disk of HD95086 (1.6 M⊙) can occur (Dodson-Robinson et al. 2009) but such a low mass planet might not be formed through direct collapse.”
• RESOLVED IMAGES OF LARGE CAVITIES IN PROTOPLANETARY TRANSITION DISKS (2011, unknown): “Moreover, this disk instability mechanism seems to naturally favor formation at wide separations from the primary (Boss 2006; Boley 2009; Rafikov 2009; Clarke 2009; Dodson-Robinson et al. 2009).”
• THE GEMINI/NICI PLANET-FINDING CAMPAIGN: THE FREQUENCY OF PLANETS AROUND YOUNG MOVING GROUP STARS (2013, unknown): “Such widely separated companions pose a challenge for the accepted model of core-accretion formation, which likely formed the closer-in (<10 AU) population of planets detected to date via radial velocity studies (e.g., Mordasini et al. 2009; Janson et al. 2012; Dodson-Robinson et al. 2009).”
• Separating gas-giant and ice-giant planets by halting pebble accretion (2014, unknown): “This result follows from the assumption that the planetesimal velocity dispersion is equal to the Hill speed (Dodson-Robinson et al. 2009; Dodson-Robinson & Bodenheimer 2010), vH = ΩrH, and gravitational focusing occurs from a radius (rc/rH)1/2rH, which is smaller than the planetesimal scale height…”
• DIRECT IMAGING OF A COLD JOVIAN EXOPLANET IN ORBIT AROUND THE SUN-LIKE STAR GJ 504 (2013, unknown): “A recent study (Dodson-Robinson et al. 2009) suggested that even a locally stable (but globally unstable) disk with an order of magnitude higher mass than the minimum mass solar nebula (Hayashi 1981) around a G-type star like GJ 504 was unable, in simulations, to form giant planets beyond 30 AU…”
• Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, unknown): “In situ formation is hard to explain in the context of planetesimaldriven core accretion theory, because planetesimal accretion rates are too slow in wide orbits (Dodson-Robinson et al. 2009).”
• Possible planet-forming regions on submillimetre images (2011, unknown): “Dodson-Robinson et al. (2009) investigated three possible mechanisms – core accretion (with or without migration), scattering from the inner disc or gravitational instability – which might result in massive planets at wide ( 35 au) orbits discovered by high-contrast direct imaging observations of…”