A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models

Guðmundur Stefánsson; Suvrath Mahadevan; Yamila Miguel; Paul Robertson; Megan Delamer; Shubham Kanodia; Caleb I Cañas; Joshua N Winn; Joe P Ninan; Ryan C Terrien; Rae Holcomb; Eric B Ford; Brianna Zawadzki; Brendan P Bowler; Chad F Bender; William D Cochran; Scott Diddams; Michael Endl; Connor Fredrick; Samuel Halverson; Fred Hearty; Gary J Hill; Andrea S J Lin; Andrew J Metcalf; Andrew Monson; Lawrence Ramsey; Arpita Roy; Christian Schwab; Jason T Wright; Gregory Zeimann

doi:10.1126/science.abo0233

A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models

Science. 2023 Dec;382(6674):1031-1035. doi: 10.1126/science.abo0233. Epub 2023 Nov 30.

Authors

Guðmundur Stefánsson¹, Suvrath Mahadevan^{2

3

4}, Yamila Miguel^{5

6}, Paul Robertson⁷, Megan Delamer^{2

3}, Shubham Kanodia⁸, Caleb I Cañas⁹, Joshua N Winn¹, Joe P Ninan¹⁰, Ryan C Terrien¹¹, Rae Holcomb⁷, Eric B Ford^{2

3

12

13}, Brianna Zawadzki^{2

3}, Brendan P Bowler¹⁴, Chad F Bender¹⁵, William D Cochran^{16

17}, Scott Diddams^{18

19

20}, Michael Endl^{14

16}, Connor Fredrick^{19

20}, Samuel Halverson²¹, Fred Hearty^{2

3}, Gary J Hill^{14

17}, Andrea S J Lin^{2

3}, Andrew J Metcalf^{19

20

22}, Andrew Monson¹⁵, Lawrence Ramsey^{2

3}, Arpita Roy^{23

24}, Christian Schwab²⁵, Jason T Wright^{2

3

26}, Gregory Zeimann^{17

27}

Affiliations

¹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08540, USA.
² Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA.
³ Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802, USA.
⁴ Institute for Particle Physics and Astrophysics, Eidgenössische Technische Hochschule Zurich, 8092 Zurich, Switzerland.
⁵ Leiden Observatory, Leiden University, 2300 RA Leiden, Netherlands.
⁶ Space Research Organisation of the Netherlands, NL-3584 CA Utrecht, Netherlands.
⁷ Department of Physics and Astronomy, University of California Irvine, Irvine, CA 92697, USA.
⁸ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA.
⁹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
¹⁰ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India.
¹¹ Department of Physics and Astronomy, Carleton College, Northfield, MN 55057, USA.
¹² Center for Astrostatistics, The Pennsylvania State University, University Park, PA 16802, USA.
¹³ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA.
¹⁴ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA.
¹⁵ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA.
¹⁶ Center for Planetary Systems Habitability, The University of Texas at Austin, Austin, TX 78712, USA.
¹⁷ McDonald Observatory, The University of Texas at Austin, Austin, TX 78712, USA.
¹⁸ Electrical, Computer and Energy Engineering, University of Colorado, Boulder, CO 80305, USA.
¹⁹ National Institute of Standards and Technology, Boulder, CO 80305, USA.
²⁰ Department of Physics, University of Colorado, Boulder, CO 80309, USA.
²¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
²² Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, NM 87117, USA.
²³ Space Telescope Science Institute, Baltimore, MD 21218, USA.
²⁴ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA.
²⁵ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia.
²⁶ Penn State Extraterrestrial Intelligence Center, The Pennsylvania State University, University Park, PA 16802, USA.
²⁷ Hobby-Eberly Telescope, University of Texas at Austin, Austin, TX 78712, USA.

PMID: 38033084
DOI: 10.1126/science.abo0233

Abstract

Theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of Neptune. We used radial velocity observations to detect a Neptune-mass exoplanet orbiting LHS 3154, a star that is nine times less massive than the Sun. The exoplanet's orbital period is 3.7 days, and its minimum mass is 13.2 Earth masses. We used simulations to show that the high planet-to-star mass ratio (>3.5 × 10^-4) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. In the core-accretion simulations, we show that close-in Neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars.