KELT-21b: A Hot Jupiter Transiting the Rapidly Rotating Metal-poor Late-A Primary of a Likely Hierarchical Triple System

Marshall C. Johnson, Joseph E. Rodriguez, George Zhou, Erica J. Gonzales, Phillip A. Cargile, Justin R. Crepp, Kaloyan Penev, Keivan G. Stassun, B. Scott Gaudi, Knicole D. Colón, Daniel J. Stevens, Klaus G. Strassmeier, Ilya Ilyin, Karen A. Collins, John F. Kielkopf, Thomas E. Oberst, Luke Maritch, Phillip A. Reed, Joao Gregorio, Valerio BozzaSebastiano Calchi Novati, Giuseppe D'Ago, Gaetano Scarpetta, Roberto Zambelli, David W. Latham, Allyson Bieryla, William D. Cochran, Michael Endl, Jamie Tayar, Aldo Serenelli, Victor Silva Aguirre, Seth P. Clarke, Maria Martinez, Michelle Spencer, Jason Trump, Michael D. Joner, Adam G. Bugg, Eric G. Hintz, Denise C. Stephens, Anicia Arredondo, Anissa Benzaid, Sormeh Yazdi, Kim K. McLeod, Eric L.N. Jensen, Daniel A. Hancock, Rebecca L. Sorber, David H. Kasper, Hannah Jang-Condell, Thomas G. Beatty, Thorsten Carroll, Jason Eastman, David James, Rudolf B. Kuhn, Jonathan Labadie-Bartz, Michael B. Lund, Matthias Mallonn, Joshua Pepper, Robert J. Siverd, Xinyu Yao, David H. Cohen, Ivan A. Curtis, D. L. Depoy, Benjamin J. Fulton, Matthew T. Penny, Howard Relles, Christopher Stockdale, Thiam Guan Tan, Steven Villanueva

Research output: Contribution to journalArticlepeer-review

32 Scopus citations


We present the discovery of KELT-21b, a hot Jupiter transiting the V = 10.5 A8V star HD 332124. The planet has an orbital period of P = 3.6127647 ± 0.0000033 days and a radius of 1.586-0.040+0.039 RJ. We set an upper limit on the planetary mass of at confidence. We confirmed the planetary nature of the transiting companion using this mass limit and Doppler tomographic observations to verify that the companion transits HD 332124. These data also demonstrate that the planetary orbit is well-aligned with the stellar spin, with a sky-projected spin-orbit misalignment of λ = -5.6-1.91.7. The star has Teff = 7598-8481 K, Mz.ast; = 1.458-0.028+0.029 M, Rz.ast; = 1.638 ± 0.034 R and v sin I = 146 km s-1, the highest projected rotation velocity of any star known to host a transiting hot Jupiter. The star also appears to be somewhat metal poor and α-enhanced, with [Fe/H] = -405-0.033+0.032 and [α/Fe] = 0.145 ± 0.053; these abundances are unusual, but not extraordinary, for a young star with thin-disk kinematics like KELT-21. High-resolution imaging observations revealed the presence of a pair of stellar companions to KELT-21, located at a separation of 1.″2 and with a combined contrast of ΔKs = 6.39 ± 0.06 with respect to the primary. Although these companions are most likely physically associated with KELT-21, we cannot confirm this with our current data. If associated, the candidate companions KELT-21 B and C would each have masses of ∼0.12 M, a projected mutual separation of ∼20 au, and a projected separation of ∼500 au from KELT-21. KELT-21b may be one of only a handful of known transiting planets in hierarchical triple stellar systems.

Original languageEnglish (US)
Article number100
JournalAstronomical Journal
Issue number2
StatePublished - Feb 2018
Externally publishedYes


  • methods: observational
  • planets and satellites: detection
  • planets and satellites: gaseous planets
  • stars: individual (HD 332124)
  • techniques: photometric
  • techniques: radial velocities

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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