Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition

Lopez, Eric; Fortney, Jonathan J.

doi:10.1088/0004-637x/792/1/1

Cited by 713 publications

(983 citation statements)

References 68 publications

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“…For H/He dominated atmospheric envelopes, R R p p D is roughly equivalent to the relative mass fraction of envelope (Lopez & Fortney 2014). In general, we find that the planets with lower relative envelope fractions are more highly irradiated than the planets with large relative envelope fractions.…”

Section: Discussionmentioning

confidence: 70%

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The Kepler-454 System: A Small, Not-Rocky Inner Planet, a Jovian World, and a Distant Companion

Gettel

Charbonneau

Dressing

et al. 2016

ApJ

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Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 day orbit. From spectroscopy, we estimate the stellar temperature to be 5687±50 K, its metallicity to be [m/H] = 0.32±0.08, and the projected rotational velocity to be v sin i<2.4 km s −1 . We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be M 1.028 0.03 0.04 -+  , the radius to be 1.066±0.012 R e , and the age to be 5.25 1.39Gyr. We estimate the radius of the 10.6 day planet as 2.37±0.13 R ⊕ . Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 observations made with the HIRES spectrograph at the Keck Observatory, we measure the mass of this planet to be 6.8±1.4 M ⊕ . We also detect two additional nontransiting companions, a planet with a minimum mass of 4.46±0.12 M J in a nearly circular 524 day orbit and a massive companion with a period >10 years and mass >12.1 M J . The 12 exoplanets with radii <2.7 R ⊕ and precise mass measurements appear to fall into two populations, with those <1.6 R ⊕ following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76±0.73 g cm −3 , Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.

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Section: Discussionmentioning

confidence: 70%

“…iron covered by a low density envelope. Employing the models of Lopez & Fortney (2014) and assuming a system age of 5 Gyr, the observed mass and radius of Kepler-454b could be explained if the planet is shrouded by a solar metallicity H/He and a total mass equal to roughly 1% of the total planetary mass.…”

Section: Discussionmentioning

confidence: 99%

The Kepler-454 System: A Small, Not-Rocky Inner Planet, a Jovian World, and a Distant Companion

Gettel

Charbonneau

Dressing

et al. 2016

ApJ

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“…During the formation phase, the luminosity resulting from the accretion of planetesimals is anyway usually much bigger than the core's cooling. But it leads to an underestimation of the luminosity of low-mass, core-dominated planets with masses 30M ⊕ during the evolutionary phase at constant mass (Lopez & Fortney 2014). We therefore restrict this work mostly to giant planets where the luminosity is strongly dominated by the cooling of the H/He envelope also during evolution.…”

Section: Thermodynamics Of the Accretion Processmentioning

confidence: 99%

Characterization of exoplanets from their formation

Mordasini

Marleau

Mollière

2017

A&A

116

142

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Context. This paper continues a series in which we predict the main observable characteristics of exoplanets based on their formation. In Paper I we described our global planet formation and evolution model that is based on the core accretion paradigm. In Paper II we studied the planetary mass-radius relationship with population syntheses. Aims. In this paper we present an extensive study of the statistics of planetary luminosities during both formation and evolution. Our results can be compared with individual directly imaged extrasolar (proto)planets and with statistical results from surveys. Methods. We calculated three populations of synthetic planets assuming different efficiencies of the accretional heating by gas and planetesimals during formation. We describe the temporal evolution of the planetary mass-luminosity relation. We investigate the relative importance of the shock and internal luminosity during formation, and predict a statistical version of the post-formation mass vs. entropy "tuning fork" diagram. Because the calculations now include deuterium burning we also update the planetary mass-radius relationship in time.Results. We find significant overlap between the high post-formation luminosities of planets forming with hot and cold gas accretion because of the core-mass effect. Variations in the individual formation histories of planets can still lead to a factor 5 to 20 spread in the post-formation luminosity at a given mass. However, if the gas accretional heating and planetesimal accretion rate during the runaway phase is unknown, the post-formation luminosity may exhibit a spread of as much as 2-3 orders of magnitude at a fixed mass. As a key result we predict a flat log-luminosity distribution for giant planets, and a steep increase towards lower luminosities due to the higher occurrence rate of low-mass (M 10-40 M ⊕ ) planets. Future surveys may detect this upturn. Conclusions. Our results indicate that during formation an estimation of the planetary mass may be possible for cold gas accretion if the planetary gas accretion rate can be estimated. If it is unknown whether the planet still accretes gas, the spread in total luminosity (internal+accretional) at a given mass may be as large as two orders of magnitude, therefore inhibiting the mass estimation. Due to the core-mass effect even planets which underwent cold accretion can have large post-formation entropies and luminosities, such that alternative formation scenarios such as gravitational instabilities do not need to be invoked. Once the number of self-luminous exoplanets with known ages and luminosities increases, the resulting luminosity distributions may be compared with our predictions.

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“…From the resulting probability distribution, we constrain CMF>0.96 and M core > 10.8 Å M at 99.7% confidence (3σ). One potential limitation of our method is that the Lopez & Fortney (2014) models assume that the planet incident flux is constant. However, the luminosity of K2-66 has increased by a factor of ∼2 since evolving off of the main sequence, and therefore the planet incident flux was twice as low for most of its lifetime.…”

Section: K2-66mentioning

confidence: 99%

“…The gaseous envelopes of planets at extreme temperatures are subjected to photoevaporation by the incident radiation from their host stars (e.g., Owen & Wu 2013;Lopez & Fortney 2014). Probing planets at extreme temperatures is crucial to understanding these sculpting effects and the formation histories of planets close to their host stars.…”

Section: Introductionmentioning

confidence: 99%

K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

Sinukoff

Howard

Petigura

et al. 2017

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We report precise mass and density measurements of two extremely hot sub-Neptune-size planets from the K2 mission using radial velocities, K2 photometry, and adaptive optics imaging. K2-66 harbors a close-in subNeptune-sized ( -+ 2.49 0.24 0.34 Å R ) planet (K2-66b) with a mass of  21.3 3.6 Å M . Because the star is evolving up the subgiant branch, K2-66b receives a high level of irradiation, roughly twice the main-sequence value. K2-66b may reside within the so-called "photoevaporation desert," a domain of planet size and incident flux that is almost completely devoid of planets. Its mass and radius imply that K2-66b has, at most, a meager envelope fraction (<5%) and perhaps no envelope at all, making it one of the largest planets without a significant envelope. K2-106 hosts an ultra-short-period planet (P=13.7 hr) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius ( -+ 1.82 0.14 0.20 Å R ) and mass (  9.0 1.6 Å M ) are consistent with a rocky composition, as are all other small ultra-short-period planets with well-measured masses. K2-106 also hosts a larger, longer-period planet (R p = -+ 2.77 0.23 0.37 Å R , P=13.3 days) with a mass less than 24.4 Å M at 99.7% confidence. K2-66b and K2-106b probe planetary physics in extreme radiation environments. Their high densities reflect the challenge of retaining a substantial gas envelope in such extreme environments.

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Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition

Cited by 713 publications

References 68 publications

The Kepler-454 System: A Small, Not-Rocky Inner Planet, a Jovian World, and a Distant Companion

The Kepler-454 System: A Small, Not-Rocky Inner Planet, a Jovian World, and a Distant Companion

Characterization of exoplanets from their formation

K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

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