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Published May 2018 | Published + Accepted Version
Journal Article Open

An Improved Transit Measurement for a 2.4 R⊕ Planet Orbiting A Bright Mid-M Dwarf K2–28


We present a new Spitzer transit observation of K2–28b, a sub-Neptune (Rp = 2.45 ± 0.28 R⊕) orbiting a relatively bright (V_(mag) = 16.06, K_(mag) = 10.75) metal-rich M4 dwarf (EPIC 206318379). This star is one of only seven with masses less than 0.2 M⊙ known to host transiting planets, and the planet appears to be a slightly smaller analogue of GJ 1214b (2.85 ± 0.20 R⊕). Our new Spitzerobservations were taken two years after the original K2 discovery data and have a significantly higher cadence, allowing us to derive improved estimates for this planet's radius, semimajor axis, and orbital period, which greatly reduce the uncertainty in the prediction of near future transit times for the James Webb Space Telescope (JWST) observations. We also evaluate the system's suitability for atmospheric characterization with JWST and find that it is currently the only small (<3 R⊕) and cool (<600 K) planet aside from GJ 1214b with a potentially detectable secondary eclipse. We also note that this system is a favorable target for near-infrared radial velocity instruments on larger telescopes (e.g., the Habitable Planet Finder on the Hobby–Eberly Telescope), making it one of only a handful of small, cool planets accessible with this technique. Finally, we compare our results with the simulated catalog of the Transiting Exoplanet Survey Satellite (TESS) and find K2–28b to be representative of the kind of mid-M systems that should be detectable in the TESS sample.

Additional Information

© 2018 The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2018 January 29; revised 2018 April 6; accepted 2018 April 9; published 2018 May 3. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. H.A.K. and C.D.D. acknowledge support from the Sloan Foundation and from NASA through an award issued by JPL/Caltech. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

Attached Files

Published - Chen_2018_AJ_155_223.pdf

Accepted Version - 1801.10177.pdf


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