The K2-138 System: A Near-resonant Chain of Five Sub-Neptune Planets Discovered by Citizen Scientists

February 2018 • 2018AJ....155...57C

Authors • Christiansen, Jessie L. • Crossfield, Ian J. M. • Barentsen, Geert • Lintott, Chris J. • Barclay, Thomas • Simmons, Brooke. D. • Petigura, Erik • Schlieder, Joshua E. • Dressing, Courtney D. • Vanderburg, Andrew • Allen, Campbell • McMaster, Adam • Miller, Grant • Veldthuis, Martin • Allen, Sarah • Wolfenbarger, Zach • Cox, Brian • Zemiro, Julia • Howard, Andrew W. • Livingston, John • Sinukoff, Evan • Catron, Timothy • Grey, Andrew • Kusch, Joshua J. E. • Terentev, Ivan • Vales, Martin • Kristiansen, Martti H.

Abstract • K2-138 is a moderately bright (V = 12.2, K = 10.3) main-sequence K star observed in Campaign 12 of the NASA K2 mission. It hosts five small (1.6-3.3 {R}_\oplus) transiting planets in a compact architecture. The periods of the five planets are 2.35, 3.56, 5.40, 8.26, and 12.76 days, forming an unbroken chain of near 3:2 resonances. Although we do not detect the predicted 2-5 minute transit timing variations (TTVs) with the K2 timing precision, they may be observable by higher-cadence observations with, for example, Spitzer or CHEOPS. The planets are amenable to mass measurement by precision radial velocity measurements, and therefore K2-138 could represent a new benchmark system for comparing radial velocity and TTV masses. K2-138 is the first exoplanet discovery by citizen scientists participating in the Exoplanet Explorers project on the Zooniverse platform.

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Jessie Christiansen

Associate Scientist