FORMATION AND STRUCTURE OF LOW-DENSITY EXO-NEPTUNES

By Leslie A. Rogers, P. Bodenheimer, Jack J. Lissauer, Sara Seager. 2011

179 citations recorded by the source corpus.

Kepler has found hundreds of Neptune-size (2–6 R⊕) planet candidates within 0.5 AU of their stars. The nature of the vast majority of these planets is not known because their masses have not been measured. Using theoretical models of planet formation, evolution, and structure, we explore the range of minimum plausible masses for low-density exo-Neptunes. We focus on highly irradiated planets with Teq ⩾ 500 K. We consider two separate formation pathways for low-mass planets with voluminous atmospheres of light gases: core-nucleated accretion and outgassing of hydrogen from dissociated ices. We show that Neptune-size planets at Teq = 500 K with masses as small as a few times that of Earth can plausibly be formed by core-nucleated accretion coupled with subsequent inward migration. We also derive a limiting low-density mass–radius relation for rocky planets with outgassed hydrogen envelopes but no surface water. Rocky planets with outgassed hydrogen envelopes typically have computed radii well below 3 R⊕. For both planets with H/He envelopes from core-nucleated accretion and planets with outgassed hydrogen envelopes, we employ planet interior models to map the range of planet mass–envelope mass–equilibrium temperature parameter space that is consistent with Neptune-size planet radii. Atmospheric mass loss mediates which corners of this parameter space are populated by actual planets and ultimately governs the minimum plausible mass at a specified transit radius. We find that Kepler's 2–6 R⊕ planet candidates at Teq = 500–1000 K could potentially have masses ≲ 4 M⊕. Although our quantitative results depend on several assumptions, our qualitative finding that warm Neptune-size planets can have masses substantially smaller than those given by interpolating the masses and radii of planets within our Solar System is robust.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/986368e2b6f83767d93ddf5f8e8004a35a2bcc85

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Citations in the corpus

• IN SITU FORMATION AND DYNAMICAL EVOLUTION OF HOT JUPITER SYSTEMS (2015, methodology): “…discussed in the literature, and has been used to calculate the formation phases of both, solar system planets (e.g. Pollack et al. 1996; Hubickyj et al. 2005; Helled et al. 2014), as well as extrasolar planets 3 (Bodenheimer et al. 2003; Dodson-Robinson and Bodenheimer 2009; Rogers et al. 2011).”
• Characterization of exoplanets from their formation - I. Models of combined planet formation and evolution (2012, background): “An exact lower boundary in mass for the application of the model is difficult to specify, because we find (in agreement with Rogers et al. 2011) that planets of just a few Earth masses can accrete non-negligible amounts of H2/He during the presence of the nebula, provided that the grain opacity in…”
• SUPER-EARTH ATMOSPHERES: SELF-CONSISTENT GAS ACCRETION AND RETENTION (2015, methodology): “It is noteworthy to mention Rogers et al. (2011), who also coupled self-consistently core-nucleated accretion with the subsequent post-dispersal evolution, including mass loss.”
• HUBBLE SPACE TELESCOPE NEAR-IR TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH HD 97658B (2014, background): “…based on mod- 2 els of planet formation and atmospheric mass loss for close-in planets, this still leaves a wide range of plausible models (e.g., Rogers & Seager 2010b; Rogers et al. 2011; Nettelmann et al. 2011; Heng & Kopparla 2012; Lopez et al. 2012; Lopez & Fortney 2013; Fortney et al. 2013).”
• PROBABILISTIC MASS–RADIUS RELATIONSHIP FOR SUB-NEPTUNE-SIZED PLANETS (2015, background): “Furthermore, the mere presence of otherwise layered exoplanets produces a range of radii at a given mass due only to differences in the layers’ compositions (e.g. Seager et al. 2007; Fortney et al. 2007; Rogers et al. 2011).”
• ZODIACAL EXOPLANETS IN TIME (ZEIT). III. A SHORT-PERIOD PLANET ORBITING A PRE-MAIN-SEQUENCE STAR IN THE UPPER SCORPIUS OB ASSOCIATION (2016, background): “Mann et al. (2016) suggested these large radii could be due to the initial heat of formation as well as inflation and escape of the atmosphere under the influence of the young, active host star (e.g., Rogers et al. 2011; Ehrenreich et al. 2015).”
• THE MASS OF Kepler-93b AND THE COMPOSITION OF TERRESTRIAL PLANETS (2014, background): “…predict that planets intermediate in size between Earth and Neptune could be gaseous “mini-Neptunes,” water worlds, or rocky “Super-Earths” (Kuchner 2003; Léger et al. 2004; Valencia et al. 2006; Seager et al. 2007; Fortney et al. 2007; Rogers et al. 2011; Lopez et al. 2012; Zeng & Sasselov 2013).”
• DENSITY AND ECCENTRICITY OF KEPLER PLANETS (2012, background): “(8), also see Rogers et al. 2011).”
• MAKE SUPER-EARTHS, NOT JUPITERS: ACCRETING NEBULAR GAS ONTO SOLID CORES AT 0.1 AU AND BEYOND (2014, background): “…range from ∼0.1–100 M⊕ over ∼0.05–100 AU.6 In timedependent models, Lacc is a prescribed function of time that enables one to follow the envelope’s quasi-static contraction and mass gain from the nebula (Pollack et al. 1996; Bodenheimer et al. 2000; Movshovitz et al. 2010; Rogers et al. 2011).”
• HOW ROCKY ARE THEY? THE COMPOSITION DISTRIBUTION OF KEPLER’S SUB-NEPTUNE PLANET CANDIDATES WITHIN 0.15 AU (2014, background): “Studies of such low-mass planets include investigations of “ocean worlds” (Léger et al. 2004), low-density irradiated exoNeptunes (Rogers et al. 2011), and scaling relations between mass and radius for sub-Jovian planets of varying compositions, including iron, silicates, water ice, carbon…”
• Characterization of exoplanets from their formation - II. The planetary mass-radius relationship (2012, methodology): “We have compared our results with Rogers et al. (2011), and found good agreement (Sect.”
• UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “Are these Neptune-like planets that form beyond the snow-line and contain large amounts of volatile ices (Rogers et al. 2011), or are these scaled up terrestrial worlds with H/He envelopes that formed close to their current orbits (Hansen & Murray 2013; Chiang & Laughlin 2013)?”
• DISSIPATIVE DIVERGENCE OF RESONANT ORBITS (2012, background): “The number of well-characterized systems within theKepler sample remains limited and estimation of planetary masses from radii alone is generally risky (Stevenson 1982; Rogers et al. 2011).”
• Formation of planetary systems by pebble accretion and migration (2018, background): “The envelope mass fraction that could be expected from outgassing alone is in the range of at most a few percent (Rogers et al. 2011).”
• An asteroseismic view of the radius valley: stripped cores, not born rocky (2017, background): “Indeed, if sub-Neptune planets formed beyond the snowline, they would have large amounts of water and volatile ices (Rogers et al. 2011), which may completely eliminate the presence of any radius gap (Lopez & Fortney 2013).”
• CHARACTERIZING THE COOL KOIs. IV. KEPLER-32 AS A PROTOTYPE FOR THE FORMATION OF COMPACT PLANETARY SYSTEMS THROUGHOUT THE GALAXY (2012, background): “For a planet this small with such a high equilibrium temperature the atmospheric mass fraction would have to be very small, ∼ 10−5 (Rogers et al. 2011).”
• THE FLAT TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ1214b FROM WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE (2011, background): “A high bulk volatile content would point to GJ1214b forming beyond the snow line and migrating inward, although any such statements about GJ1214b’s past are subject to large uncertainties in the atmospheric mass loss history (see Rogers et al. 2011).”
• A REVISED ESTIMATE OF THE OCCURRENCE RATE OF TERRESTRIAL PLANETS IN THE HABITABLE ZONES AROUND KEPLER M-DWARFS (2013, background): “Planets with radius > 1.4 R⊕ are thought to have either homogeneous composition of water ice, silicate or iron or some differential composition of these compounds (Seager et al. 2007; Rogers et al. 2011; Lopez et al. 2012).”