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Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star

Petigura, Erik A. and Schlieder, Joshua E. and Crossfield, Ian J. M. and Howard, Andrew W. and Deck, Katherine M. and Ciardi, David R. and Sinukoff, Evan and Allers, Katelyn N. and Best, William M. J. and Liu, Michael C. and Beichman, Charles A. and Isaacson, Howard and Sinukoff, Brad M. S. and Lépine, Sébastien (2015) Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star. Astrophysical Journal, 811 (2). Art. No. 102. ISSN 0004-637X.

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Discoveries from the prime Kepler mission demonstrated that small planets (<3 R_⊕) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located $65\pm 6$ pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASA's K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, R_P/R_* = 2.60 ± 0.14% and 3.15 ± 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 ± 0.43 R_⊕ and 1.92 ± 0.53 R_⊕, respectively, straddling the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes. The planets have orbital periods of 9.32414 days and 15.50120 days, respectively, and a period ratio P_c/P_b = 1.6624, very near to the 5:3 mean motion resonance, which may be a record of the system's formation history. Transit timing variations due to gravitational interactions between the planets may be detectable using ground-based telescopes. Finally, this system offers a convenient laboratory for studying the bulk composition and atmospheric properties of small planets with low equilibrium temperatures.

Item Type:Article
Related URLs:
URLURL TypeDescription
Petigura, Erik A.0000-0003-0967-2893
Schlieder, Joshua E.0000-0001-5347-7062
Crossfield, Ian J. M.0000-0002-1835-1891
Howard, Andrew W.0000-0001-8638-0320
Ciardi, David R.0000-0002-5741-3047
Sinukoff, Evan0000-0002-5658-0601
Allers, Katelyn N.0000-0003-0580-7244
Best, William M. J.0000-0003-0562-1511
Liu, Michael C.0000-0003-2232-7664
Beichman, Charles A.0000-0002-5627-5471
Isaacson, Howard0000-0002-0531-1073
Lépine, Sébastien0000-0002-2437-2947
Additional Information:© 2015. The American Astronomical Society. Received 2015 July 29; accepted 2015 August 20; published 2015 September 28. We thank Yoram Lithwick, Kimberly M. Aller, and Brendan Bowler for helpful conversations that improved the manuscript. We thank Lauren M. Weiss for conducting HIRES observations. Support for this work was provided by NASA through Hubble Fellowship grant HST-HF2-51365.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract NAS 5-26555. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under contract DE-AC02-05CH11231. This work made use of the SIMBAD database (operated at CDS, Strasbourg, France), NASA's Astrophysics Data System Bibliographic Services, and data products from the 2MASS, the APASS database, the SDSS-III project, and the Digitized Sky Survey. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. Some of the data presented herein were obtained at the W. M. Keck Observatory (which is operated as a scientific partnership among Caltech, UC, and NASA) and at the Infrared Telescope Facility (IRTF, operated by UH under NASA contract NNH14CK55B). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Kepler, K2, IRTF (SPEX), Keck-II (NIRC2), Keck-I (HIRES).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51365.001-A
NASANAS 5-26555
Department of Energy (DOE)DE-AC02-05CH11231
Subject Keywords:planets and satellites: detection; stars: individual (EPIC-206011691) ; techniques: photometric; techniques: spectroscopic
Issue or Number:2
Record Number:CaltechAUTHORS:20151120-080318445
Persistent URL:
Official Citation:Erik A. Petigura et al 2015 ApJ 811 102
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:62270
Deposited By: Ruth Sustaita
Deposited On:20 Nov 2015 19:45
Last Modified:04 Nov 2019 21:24

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