An 11 Earth-mass, Long-period Sub-Neptune Orbiting a Sun-like Star

Andrew W. Mayo, Vinesh M. Rajpaul, Lars A. Buchhave, Courtney D. Dressing, Annelies Mortier, Li Zeng, Charles D. Fortenbach, Suzanne Aigrain, Aldo S. Bonomo, Andrew Collier Cameron, David Charbonneau, Adrien Coffinet, Rosario Cosentino, Mario Damasso, Xavier Dumusque, A. F.Martinez Fiorenzano, Raphaëlle D. Haywood, David W. Latham, Mercedes López-Morales, Luca MalavoltaGiusi Micela, Emilio Molinari, Logan Pearce, Francesco Pepe, David Phillips, Giampaolo Piotto, Ennio Poretti, Ken Rice, Alessandro Sozzetti, Stephane Udry

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)


Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with M =0.892-0.035 +0.051 and R = 0.8717-0.0061 +0.0064 R o. Kepler-538b is a 2.215-0.034 +0.040 R sub-Neptune with a period of P = 81.73778 ± 0.00013 days. It is the only known planet in the system. We collected radial velocity (RV) observations with the High Resolution Echelle Spectrometer (HIRES) on Keck I and High Accuracy Radial velocity Planet Searcher in North hemisphere (HARPS-N) on the Telescopio Nazionale Galileo (TNG). We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross-correlation function FWHM observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of K = 1.68-0.38 +0.39 m s-1 and a planet mass of Mp = 10.6-2.4 +2.5 M . Kepler-538b is the smallest planet beyond P = 50 days with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

Original languageEnglish
Article number165
JournalAstronomical Journal
Issue number4
Publication statusPublished - 2019

Bibliographical note

Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved..


  • methods: data analysis
  • planets and satellites: composition
  • planets and satellites: detection
  • planets and satellites: fundamental parameters
  • planets and satellites: gaseous planets
  • techniques: photometric
  • techniques: radial velocities

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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