KElt-18b: Puffy planet, hot host, probably perturbed

Kim K. McLeod, Joseph E. Rodriguez, Ryan J. Oelkers, Karen A. Collins, Allyson Bieryla, Benjamin J. Fulton, Keivan G. Stassun, B. Scott Gaudi, Kaloyan Penev, Daniel J. Stevens, Knicole D. Colón, Joshua Pepper, Norio Narita, Ryu Tsuguru, Akihiko Fukui, Phillip A. Reed, Bethany Tirrell, Tiffany Visgaitis, John F. Kielkopf, David H. CohenEric L.N. Jensen, Joao Gregorio, Özgür Baştürk, Thomas E. Oberst, Casey Melton, Eliza M.R. Kempton, Andrew Baldrige, Y. Sunny Zhao, Roberto Zambelli, David W. Latham, Gilbert A. Esquerdo, Perry Berlind, Michael L. Calkins, Andrew W. Howard, Howard Isaacson, Lauren M. Weiss, Paul Benni, Thomas G. Beatty, Jason D. Eastman, Matthew T. Penny, Robert J. Siverd, Michael B. Lund, Jonathan Labadie-Bartz, G. Zhou, Ivan A. Curtis, Michael D. Joner, Mark Manner, Howard Relles, Gaetano Scarpetta, Denise C. Stephens, Chris Stockdale, T. G. Tan, D. L. Depoy, Jennifer L. Marshall, Richard W. Pogge, Mark Trueblood, Patricia Trueblood

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


We report the discovery of KELT-18b, a transiting hot Jupiter in a 2.87-day orbit around the bright (V = 10.1), hot, F4V star BD+60 1538 (TYC 3865-1173-1). We present follow-up photometry, spectroscopy, and adaptive optics imaging that allow a detailed characterization of the system. Our preferred model fits yield a host stellar temperature of K and a mass of, situating it as one of only a handful of known transiting planets with hosts that are as hot, massive, and bright. The planet has a mass of, a radius of, and a density of, making it one of the most inflated planets known around a hot star. We argue that KELT-18b's high temperature and low surface gravity, which yield an estimated ∼600 km atmospheric scale height, combined with its hot, bright host, make it an excellent candidate for observations aimed at atmospheric characterization. We also present evidence for a bound stellar companion at a projected separation of ∼1100 au, and speculate that it may have contributed to the strong misalignment we suspect between KELT-18's spin axis and its planet's orbital axis. The inferior conjunction time is 2457542.524998 ± 0.000416 (BJDTDB) and the orbital period is 2.8717510 ± 0.0000029 days. We encourage Rossiter-McLaughlin measurements in the near future to confirm the suspected spin-orbit misalignment of this system.

Original languageEnglish (US)
Article number263
JournalAstronomical Journal
Issue number6
StatePublished - Jun 2017
Externally publishedYes


  • methods: observational
  • planets and satellites: detection
  • planets and satellites: gaseous planets
  • stars: individual (BD+60 1538)
  • techniques: photometric
  • techniques: radial velocities

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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