Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres

By E. Adams, Sara Seager, L. Elkins‐Tanton. 2007

186 citations recorded by the source corpus.

The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters—which are difficult or impossible to constrain from measurements alone—produce planets with the same average densities but widely varying bulk compositions. We find that adding a gas envelope equivalent to 0.1%-10% of the mass of a solid planet causes the radius to increase 5%-60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet), or alternatively a large rocky iron core and some H and/or He. For example, a wide variety of compositions can explain the observed radius of GJ 436b, although all models require some H/He. We conclude that the identification of water worlds based on the mass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/31889a05d3385322e7a2480d1e8ad483d18c20ca

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. Jon Fortney
2. N. Madhusudhan
3. J. Owen
4. D. Charbonneau
5. Sara Seager
6. C. Mordasini
7. A. Showman
8. H. Knutson

In Instrumentation

1. Jon Fortney
2. J. Owen
3. D. Charbonneau
4. N. Madhusudhan
5. Timothy Morton
6. H. Knutson
7. A. Showman
8. C. Mordasini

Citations in the corpus

• PLANET OCCURRENCE WITHIN 0.25 AU OF SOLAR-TYPE STARS FROM KEPLER (2011, unknown): “Adding gas to any of the models increases Rp and decreases ρ (Adams et al. 2008).”
• Composition and fate of short-period super-Earths: The case of CoRoT-7b (2009, unknown): “While the combination of mass (M) and radius (R) measurements alone does not yield a unique solution for the composition of a planet (Valencia et al., 2007b; Adams et al., 2008), the short period of CoRoT-7b and consequently the strong irradiation on the planet, may help constrain its composition.”
• THE OPTICAL AND NEAR-INFRARED TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ 1214b: FURTHER EVIDENCE FOR A METAL-RICH ATMOSPHERE (2011, background): “This is because knowledge of only the mass and radius of planets in this regime is not sufficient to determine their bulk composition due to degeneracies that exist in theoretical models (Adams et al. 2008; Rogers & Seager 2010a).”
• The Evaporation Valley in the Kepler Planets (2017, background): “However, due to degeneracies present in the mass-radius plane at low-masses (e.g. Adams et al. 2008; Rogers & Seager 2010), we cannot infer the compositions just based on the current measured mass and radius for the majority of the observed planets.”
• A FRAMEWORK FOR QUANTIFYING THE DEGENERACIES OF EXOPLANET INTERIOR COMPOSITIONS (2009, methodology): “(16) Similar fiducial choices of T0 have been made in other studies that used Equation (4) to describe the gas layer temperature profiles of low-mass exoplanets (Adams et al. 2008; Miller-Ricci et al. 2009).”
• THE ATMOSPHERIC SIGNATURES OF SUPER-EARTHS: HOW TO DISTINGUISH BETWEEN HYDROGEN-RICH AND HYDROGEN-POOR ATMOSPHERES (2008, background): “Mass-radius measurements may provide the first constraints on atmospheric mass for super-Earths, however there are large degeneracies in the models that will be difficult to overcome (Adams et al. 2008).”
• KEPLER PLANETS: A TALE OF EVAPORATION (2013, background): “Variations in the planetary structure Low-mass planets could contain volatile-rich atmospheres, they could also contain a significant amount of iron and/or ice in their cores, (Adams et al. 2008; Rogers & Seager 2010).”
• Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation (2017, background): “Such low-mass planets can have a large radius even with a tenuous envelope (Adams et al. 2008; Rogers et al. 2011; Mordasini et al. 2012b), and can be easily evaporated to bare cores at close-in orbits (Lopez & Fortney 2013; Jin et al. 2014).”
• DENSITIES AND ECCENTRICITIES OF 139 KEPLER PLANETS FROM TRANSIT TIME VARIATIONS (2013, background): “Unfortunately, the water content is difficult to deduce, since the density is very sensitive to the presence of hydrogen; e.g., a 5M⊕ rocky planet with a hydrogen atmosphere that is only ∼ 1% of its mass would have the same density as a pure water planet (Adams et al. 2008; Wu & Lithwick 2013).”
• MULTIWAVELENGTH CONSTRAINTS ON THE DAY–NIGHT CIRCULATION PATTERNS OF HD 189733b (2008, background): “…on the atmospheric dynamics of these planets will be applicable to smaller, core-dominated planets such as HD 149026b and GJ 436b (Butler et al. 2004; Sato et al. 2005; Fortney et al. 2006a; Gillon et al. 2007; Deming et al. 2007; Torres et al. 2007; Adams et al. 2008; Winn et al. 2008).”
• DENSITY AND ECCENTRICITY OF KEPLER PLANETS (2012, background): “Although density measurements of individual planets cannot break this inherent degeneracy (see, e.g., Adams et al. 2008), we attempt to break it with our statistical sample.”
• PLANETARY POPULATION SYNTHESIS COUPLED WITH ATMOSPHERIC ESCAPE: A STATISTICAL VIEW OF EVAPORATION (2014, background): “The large differences between the radii of the cores and the total planetary radii show that the planetary envelope with only 0.1% of the planetary mass produces a large increase in the planetary radius, which is a well-known effect (e.g., Adams et al. 2008).”
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (2017, background): “In addition, a few-percent-by-mass secondary atmosphere can be outgassed during planet formation and evolution (Adams et al. 2008).”
• A PLATEAU IN THE PLANET POPULATION BELOW TWICE THE SIZE OF EARTH (2013, background): “For example, Adams et al. (2008) found that adding a H/He gas envelope equivalent to 0.2–20% of the mass of a solid 5 ME planet increases the radius 8–110% above the gas-free value.”