Occurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars
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Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board April 16, 2014 (received for review January 24, 2014)

Significance
Among the nearly 4,000 planets known around other stars, the most common are 1–4× the size of Earth. A quarter of Sun-like stars have such planets orbiting within half an Earth’s orbital distance of them, and more surely orbit farther out. Measurements of density show that the smallest planets are mostly rocky while the bigger ones have rocky cores fluffed out with hydrogen and helium gas, and likely water, befitting the term ‘‘mini-Neptunes.’’ The division between these two regimes is near 1.5 R⊕. Considering exoplanet hospitality, 11% of Sun-like stars have a planet of 1–2× the size of Earth that receives between 1.0–4.0× the incident stellar light that our Earth enjoys. However, we remain ignorant of the origins of, and existence of, exobiology, leaving the location of the habitable zone uncertain.
Abstract
Small planets, 1–4× the size of Earth, are extremely common around Sun-like stars, and surprisingly so, as they are missing in our solar system. Recent detections have yielded enough information about this class of exoplanets to begin characterizing their occurrence rates, orbits, masses, densities, and internal structures. The Kepler mission finds the smallest planets to be most common, as 26% of Sun-like stars have small, 1–2 R⊕ planets with orbital periods under 100 d, and 11% have 1–2 R⊕ planets that receive 1–4× the incident stellar flux that warms our Earth. These Earth-size planets are sprinkled uniformly with orbital distance (logarithmically) out to 0.4 the Earth–Sun distance, and probably beyond. Mass measurements for 33 transiting planets of 1–4 R⊕ show that the smallest of them, R < 1.5 R⊕, have the density expected for rocky planets. Their densities increase with increasing radius, likely caused by gravitational compression. Including solar system planets yields a relation:
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: gmarcy{at}berkeley.edu.
Author contributions: G.W.M., L.M.W., E.A.P., H.I., A.W.H., and L.A.B. contributed to this work by acquiring data at telescopes, analyzing those data, and providing final data products or graphs of their results; and G.W.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. A.S.B. is a guest editor invited by the Editorial Board.
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