Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 116, Issue 20, Pages 9723-9728Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1812905116
Keywords
exoplanets; bimodal distribution; ices; water worlds; planet formation
Categories
Funding
- Simons Foundation [Simons Collaboration on the Origins of Life] [337090]
- Harvard Faculty of Arts and Sciences Dean's Competitive Fund for Promising Scholarship
- Sandia Z Fundamental Science Program by the Department of Energy National Nuclear Security Administration [DE-NA0001804, DE-NA0002937]
- Harvard University
- US Department of Energy's National Nuclear Security Administration [DE-NA-0003525]
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The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets ( likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R-circle plus) still remains. They may either have a rocky core enveloped in a H-2-He gaseous envelope ( gas dwarfs) or contain a significant amount of multicomponent, H2O-dominated ices/fluids ( water worlds). Planets in the mass range of 10-15 M-circle plus, if half-ice and half-rock by mass, have radii of 2.5 R-circle plus, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R. range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are water worlds.
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