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Published April 2017 | Published
Journal Article Open

A Low-mass Exoplanet Candidate Detected by K2 Transiting the Praesepe M Dwarf JS 183


We report the discovery of a repeating photometric signal from a low-mass member of the Praesepe open cluster that we interpret as a Neptune-sized transiting planet. The star is JS 183 (HSHJ 163, EPIC 211916756), with T_(eff) = 3325 ± 100 K, M* = 0.44 ± 0.04 M⊙, R* = 0.44 ± 0.03 R⊙, and log g* = 4.82 ± 0.06. The planet has an orbital period of 10.134588 days and a radius of R_P = 0.32 ± 0.02 R_J. Since the star is faint at V = 16.5 and J = 13.3, we are unable to obtain a measured radial velocity orbit, but we can constrain the companion mass to below about 1.7 M J, and thus well below the planetary boundary. JS 183b (since designated as K2-95b) is the second transiting planet found with K2 that resides in a several-hundred-megayear open cluster; both planets orbit mid-M dwarf stars and are approximately Neptune sized. With a well-determined stellar density from the planetary transit, and with an independently known metallicity from its cluster membership, JS 183 provides a particularly valuable test of stellar models at the fully convective boundary. We find that JS 183 is the lowest-density transit host known at the fully convective boundary, and that its very low density is consistent with current models of stars just above the fully convective boundary but in tension with the models just below the fully convective boundary.

Additional Information

© 2017 The American Astronomical Society. Received 2016 July 8; revised 2017 February 2; accepted 2017 February 5; published 2017 March 24. J.P. would like to thank Ian Crossfield for useful discussions. T.D. is supported by an NSF Graduate Research Fellowship under grant DGE1144469. B.J.S.P. would like to thank Balliol College and the Clarendon Fund for their financial support of this work. S.A. and H.P. acknowledge funding from the Leverhulme Trust. S.A. received support from the UK Science and Technology Facilities Council (STFC). K.G.S. acknowledges partial support through NSF PAARE grant AST-1358862. J.L.-B. acknowledges support from the Marie Curie Actions of the European Commission (FP7-COFUND).

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