FORMATION AND STRUCTURE OF LOW-DENSITY EXO-NEPTUNES

Journal Article · Thu Sep 01 04:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/738/1/59· OSTI ID:21582943

Rogers, Leslie A; Seager, Sara ^[1]; Bodenheimer, Peter ^[2]; Lissauer, Jack J ^[3]

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1. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
2. UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
3. Space Science and Astrobiology Division, MS 245-3, NASA-Ames Research Center, Moffett Field, CA 94035 (United States)

Kepler has found hundreds of Neptune-size (2-6 R{sub +}) 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{sub 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{sub eq} = 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{sub +}. 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{sub +} planet candidates at T{sub eq} = 500-1000 K could potentially have masses {approx}< 4 M{sub +}. 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.

• OSTI ID: 21582943
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 738; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ELEMENTS
EVOLUTION
HYDROGEN
MASS
NONMETALS
PLANETS