UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION

By E. Lopez, Jon Fortney. 2013

412 citations recorded by the source corpus.

Transiting planet surveys like Kepler have provided a wealth of information on the distribution of planetary radii, particularly for the new populations of super-Earth- and sub-Neptune-sized planets. In order to aid in the physical interpretation of these radii, we compute model radii for low-mass rocky planets with hydrogen–helium envelopes. We provide model radii for planets 1–20 M⊕, with envelope fractions 0.01%–20%, levels of irradiation 0.1–1000 times Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide simple analytic fits that summarize how radius depends on each of these parameters. Most importantly, we show that at fixed H/He envelope fraction, radii show little dependence on mass for planets with more than ∼1% of their mass in their envelope. Consequently, planetary radius is to a first order a proxy for planetary composition, i.e., H/He envelope fraction, for Neptune- and sub-Neptune-sized planets. We recast the observed mass–radius relationship as a mass–composition relationship and discuss it in light of traditional core accretion theory. We discuss the transition from rocky super-Earths to sub-Neptune planets with large volatile envelopes. We suggest ∼1.75 R⊕ as a physically motivated dividing line between these two populations of planets. Finally, we discuss these results in light of the observed radius occurrence distribution found by Kepler.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1cc0188fd565193c01ceaf77f30c36187bfa1cfe

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In Astronomy and Astrophysics

1. J. Owen
2. Jon Fortney
3. Timothy Morton
4. N. Madhusudhan
5. D. Charbonneau
6. C. Mordasini
7. J. Johnson
8. J. Winn

In Instrumentation

1. J. Owen
2. Timothy Morton
3. D. Charbonneau
4. J. Johnson
5. Jon Fortney
6. J. Winn
7. Andrew W. Howard
8. G. Marcy

Citations in the corpus

• AN ANCIENT EXTRASOLAR SYSTEM WITH FIVE SUB-EARTH-SIZE PLANETS (2015, background): “Although photometry alone does not yield the masses of the planets, planetary thermal evolution models (Lopez & Fortney 2014) predict that the composition of planets with radii less than 0.8R⊕ are highly likely rocky.”
• PROBABILISTIC FORECASTING OF THE MASSES AND RADII OF OTHER WORLDS (2016, background): “For example, one might reasonably expect that the Neptune-like planets follow a different MR relation from the terrestrial planets, since the voluminous gaseous envelope of the former dominates their radius (Lopez & Fortney 2014).”
• The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B (2017, methodology): “mmer et al. 2013; Owen and Wu 2013; Koskinen et al. 2014; Lammer et al. 2014; 1 kevin.j.zahnle@NASA.gov 2 dcatling@u.washington.edu arXiv:1702.03386v2 [astro-ph.EP] 13 Jul 2017 2 Zahnle & Catling Lopez and Fortney 2014; Tian 2015; Erkaev et al. 2015, 2016; Owen and Alvarez 2016, the list is not ”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, background): “The heating due to radioactive decay in the planet’s core is included in the planet’s luminosity budget while the effect of the thermal cooling of the core is currently neglected (Lopez & Fortney 2014).”
• Dearth of short-period Neptunian exoplanets - a desert in period-mass and period-radius planes (2016, background): “…processes and crossed their death line were doomed because they lost a major part of their masses, either due to the stellar insolation (e.g., Lopez & Fortney 2014; Tian 2015) or because of Roche-lobe overflow (Kurokawa & Nakamoto (2014); Article number, page 6 of 8 Matsakos & Konigl (2016),…”
• THE MASS–RADIUS RELATION FOR 65 EXOPLANETS SMALLER THAN 4 EARTH RADII (2013, background): “Perhaps the mass diversity at a given radius results from different core masses in planets with large gaseous envelopes (as argued in Lopez & Fortney 2013); the size of the planet is determined by the fraction of gas, but the mass is determined by the size of the rocky core.”
• HOW ROCKY ARE THEY? THE COMPOSITION DISTRIBUTION OF KEPLER’S SUB-NEPTUNE PLANET CANDIDATES WITHIN 0.15 AU (2014, methodology): “First, a substantial fraction of these planets have radii that are just too large to be explained by rock/ice/water combinations (e.g, Lopez & Fortney 2014; Rogers 2014).”
• 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): “For them, the most distinct feature is that their location in the mass-density plot reveals their envelope mass fraction (Lopez & Fortney 2014).”
• Multiple Disk Gaps and Rings Generated by a Single Super-Earth (2017, background): “Their compositions, as inferred from their bulk densities, span a wide range; some are consistent with pure rock while others have 1–10% of their mass in voluminous H/He envelopes (e.g., Lopez & Fortney 2014; Rogers 2015; Lee & Chiang 2016).”
• THE RADIUS DISTRIBUTION OF PLANETS AROUND COOL STARS (2013, background): “Understanding the shape of the planet radius distribution in detail inaccessible to histogram binning is especially important since it is known that the physical properties of planets likely change dramatically between ∼1 and ∼2 R⊕ (Weiss & Marcy 2014; Marcy et al. 2014; Lopez & Fortney 2013).”
• Kepler Planet Masses and Eccentricities from TTV Analysis (2016, methodology): “Here we convert observed masses and radii to envelope masses by interpolating the tables of Lopez & Fortney (2014).”
• BREEDING SUPER-EARTHS AND BIRTHING SUPER-PUFFS IN TRANSITIONAL DISKS (2015, background): “50 days, radii ∼4–10R⊕, masses ∼2–6M⊕, and inferred GCRs & 20% (e.g., Masuda 2014; Lopez & Fortney 2014).”
• THE OCCURRENCE OF POTENTIALLY HABITABLE PLANETS ORBITING M DWARFS ESTIMATED FROM THE FULL KEPLER DATASET AND AN EMPIRICAL MEASUREMENT OF THE DETECTION SENSITIVITY (2015, result): “This result agrees with previous fits to measured exoplanet masses and radii by Weiss & Marcy (2014) and simulations by Lopez & Fortney (2014).”