Formation and Structure of Low-Density Exo-Neptunes
Kepler has found hundreds of Neptune-size (2-6 R [subscript ⊕]) 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 explor...
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| Format: | Article |
| Language: | en_US |
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IOP Publishing
2012
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| Online Access: | http://hdl.handle.net/1721.1/74167 https://orcid.org/0000-0003-0638-3455 https://orcid.org/0000-0002-6892-6948 |
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| author | Bodenheimer, Peter Lissauer, Jack J. Seager, Sara Rogers, Leslie Anne |
| author2 | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences |
| author_facet | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Bodenheimer, Peter Lissauer, Jack J. Seager, Sara Rogers, Leslie Anne |
| author_sort | Bodenheimer, Peter |
| collection | MIT |
| description | Kepler has found hundreds of Neptune-size (2-6 R [subscript ⊕]) 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 [subscript 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 [subscript 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 [subscript ⊕]. 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 [subscript ⊕] planet candidates at T [subscript eq] = 500-1000 K could potentially have masses lsim 4 M [subscript ⊕]. 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. |
| first_indexed | 2024-09-23T15:40:36Z |
| format | Article |
| id | mit-1721.1/74167 |
| institution | Massachusetts Institute of Technology |
| language | en_US |
| last_indexed | 2024-09-23T15:40:36Z |
| publishDate | 2012 |
| publisher | IOP Publishing |
| record_format | dspace |
| spelling | mit-1721.1/741672022-09-29T15:25:40Z Formation and Structure of Low-Density Exo-Neptunes Bodenheimer, Peter Lissauer, Jack J. Seager, Sara Rogers, Leslie Anne Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology. Department of Physics Seager, Sara Rogers, Leslie Anne Kepler has found hundreds of Neptune-size (2-6 R [subscript ⊕]) 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 [subscript 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 [subscript 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 [subscript ⊕]. 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 [subscript ⊕] planet candidates at T [subscript eq] = 500-1000 K could potentially have masses lsim 4 M [subscript ⊕]. 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. 2012-10-22T13:06:06Z 2012-10-22T13:06:06Z 2011-08 2011-04 Article http://purl.org/eprint/type/JournalArticle 0004-637X 1538-4357 http://hdl.handle.net/1721.1/74167 Rogers, Leslie A. et al. “Formation and Structure of Low-Density Exo-Neptunes.” The Astrophysical Journal 738.1 (2011): 59. © 2011 IOP Publishing https://orcid.org/0000-0003-0638-3455 https://orcid.org/0000-0002-6892-6948 en_US http://dx.doi.org/10.1088/0004-637x/738/1/59 Astrophysical Journal Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. application/pdf IOP Publishing IOP |
| spellingShingle | Bodenheimer, Peter Lissauer, Jack J. Seager, Sara Rogers, Leslie Anne Formation and Structure of Low-Density Exo-Neptunes |
| title | Formation and Structure of Low-Density Exo-Neptunes |
| title_full | Formation and Structure of Low-Density Exo-Neptunes |
| title_fullStr | Formation and Structure of Low-Density Exo-Neptunes |
| title_full_unstemmed | Formation and Structure of Low-Density Exo-Neptunes |
| title_short | Formation and Structure of Low-Density Exo-Neptunes |
| title_sort | formation and structure of low density exo neptunes |
| url | http://hdl.handle.net/1721.1/74167 https://orcid.org/0000-0003-0638-3455 https://orcid.org/0000-0002-6892-6948 |
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