Vincent Van Eylen, Simon Albrecht, Davide Gandolfi, Fei Dai, Joshua N. Winn, Teriyuki Hirano, Norio Narita, Hans Bruntt, Jorge Prieto-Arranz, Víctor J.S. Béjar, Grzegorz Nowak, Mikkel N. Lund, Enric Palle, Ignasi Ribas, Roberto Sanchis-Ojeda, Liang Yu, Pamela Arriagada, R. Paul Butler, Jeffrey D. Crane, Rasmus HandbergHans Deeg, Jens Jessen-Hansen, John A. Johnson, David Nespral, Leslie Rogers, Tsuguru Ryu, Stephen Shectman, Tushar Shrotriya, Ditte Slumstrup, Yoichi Takeda, Johanna Teske, Ian Thompson, Andrew Vanderburg, Robert Wittenmyer

Research output: Contribution to journalArticlepeer-review

33 Scopus citations


We report on the discovery and characterization of the transiting planet K2-39b (EPIC 206247743b). With an orbital period of 4.6 days, it is the shortest-period planet orbiting a subgiant star known to date. Such planets are rare, with only a handful of known cases. The reason for this is poorly understood but may reflect differences in planet occurrence around the relatively high-mass stars that have been surveyed, or may be the result of tidal destruction of such planets. K2-39 (EPIC206247743) is an evolved star with a spectroscopically derived stellar radius and mass of 3.88 0.42 0.48 R and 1.53+0.12 0.13 M, respectively, and a very close-in transiting planet, with a R = 3.4. Radial velocity (RV) follow-up using the HARPS, FIES, and PFS instruments leads to a planetary mass of 50.3 9.4 9.7 M. In combination with a radius measurement of 8.3 1.1 R, this results in a mean planetary density of 0.50+0.17 0.29 gcm-3. We furthermore discover a long-term RV trend, which may be caused by a longperiod planet or stellar companion. Because K2-39b has a short orbital period, its existence makes it seem unlikely that tidal destruction is wholly responsible for the differences in planet populations around subgiant and mainsequence stars. Future monitoring of the transits of this system may enable the detection of period decay and constrain the tidal dissipation rates of subgiant stars.

Original languageEnglish (US)
Article number143
JournalAstronomical Journal
Issue number5
StatePublished - Nov 2016
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • planetary systems
  • planets and satellites: detection
  • planets and satellites: dynamical evolution and stability
  • stars: fundamental parameters
  • stars: individual (K2-39)


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