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Physical Sciences

Occurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars

Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard, and Lars A. Buchhave
  1. aDepartment of Astronomy, University of California, Berkeley, CA 94720;
  2. bInstitute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822; and
  3. cHarvard-Smithsonian Center for Astrophysics, Harvard University, Cambridge, MA 02138

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PNAS September 2, 2014 111 (35) 12655-12660; first published May 27, 2014; https://doi.org/10.1073/pnas.1304197111
Geoffrey W. Marcy
aDepartment of Astronomy, University of California, Berkeley, CA 94720;
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  • For correspondence: gmarcy@berkeley.edu
Lauren M. Weiss
aDepartment of Astronomy, University of California, Berkeley, CA 94720;
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Erik A. Petigura
aDepartment of Astronomy, University of California, Berkeley, CA 94720;
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Howard Isaacson
aDepartment of Astronomy, University of California, Berkeley, CA 94720;
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Andrew W. Howard
bInstitute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822; and
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Lars A. Buchhave
cHarvard-Smithsonian Center for Astrophysics, Harvard University, Cambridge, MA 02138
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  1. Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board April 16, 2014 (received for review January 24, 2014)

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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: ρ=2.32+3.19R/R⊕ [g cm−3]. Larger planets, in the radius range 1.5–4.0 R⊕, have densities that decline with increasing radius, revealing increasing amounts of low-density material (H and He or ices) in an envelope surrounding a rocky core, befitting the appellation ‘‘mini-Neptunes.’’ The gas giant planets occur preferentially around stars that are rich in heavy elements, while rocky planets occur around stars having a range of heavy element abundances. Defining habitable zones remains difficult, without benefit of either detections of life elsewhere or an understanding of life’s biochemical origins.

  • extrasolar planets
  • astrobiology
  • SETI

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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Occurrence and core-envelope structure of planets
Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard, Lars A. Buchhave
Proceedings of the National Academy of Sciences Sep 2014, 111 (35) 12655-12660; DOI: 10.1073/pnas.1304197111

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Occurrence and core-envelope structure of planets
Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard, Lars A. Buchhave
Proceedings of the National Academy of Sciences Sep 2014, 111 (35) 12655-12660; DOI: 10.1073/pnas.1304197111
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  • Physical Sciences
  • Earth, Atmospheric, and Planetary Sciences
Proceedings of the National Academy of Sciences: 111 (35)
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  • Article
    • Abstract
    • Occurrence Rates of 1–4 R⊕ Planets
    • Properties: Masses, Radii, and Densities
    • Structure: Core-Envelope Model of 1–4 R⊕ Planets
    • Interiors, Formation, and Evolution
    • Correlations with Heavy Element Abundance
    • Habitable Zone: Humility and Hubris
    • Acknowledgments
    • Footnotes
    • References
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