Formation and Structure of Low-Density Exo-Neptunes

Abstract

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 T eq ≥ 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 T eq = 500 K with masses as small as a few times that of Earth can plausibly be formed 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 T eq = 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.

An independent planet search in theKeplerdataset

“…However, the above solution exhibits strong self-consistency between different determinations of some of the variables: the stellar mass from the spectroscopy agrees with the stellar mass from the dynamical modeling, and the semi-major axes from the light curve fitting + stellar model agree with the semi-major axes from dynamical model. From a theoretical stand point, Rogers et al (2011) attempted to put limits on the masses of similarly sized Kepler candidates and found that even low-mass low-density planets were possible in the general framework of core-nucleated accretion using plausible disk configurations. They found that a planet with a radius of 6 R ⊕ like Kepler-87 c and an equilibrium temperature of 500 K would have a mass of 6.4 M ⊕ if ≈20% of its mass were made of a gaseous envelope (assuming an ice-rock interior, and H/He in protosolar proportions).…”

Characterization of the planetary system Kepler-101 with HARPS-N

“…Detailed modeling of the internal structure of the Earth-sized planet Kepler-101c is not possible because of the weak constraint on its mass of M p < 3.8 M ⊕ (<8.7 M ⊕ ) at 1σ (2σ). We are only able to exclude a composition of pure iron with a probability of 68.3%, according to the models for solid planets by Seager et al (2007) and Zeng & Sasselov (2013), and any H/He envelope from the planetary radius constraint (Rogers et al 2011).…”

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…This is particularly true for gas giant planet formation, with the final mass of the giant planet closely linked to the gas dispersal time (e.g., Lissauer et al 2009, Movshovitz et al 2010, Rogers et al 2011.…”