Spitzer observations confirm and rescue the habitable-zone super-earth K2-18b for future characterization

Benneke, Bjorn and Werner, Michael and Petigura, Erik and Knutson, Heather and Dressing, Courtney and Crossfield, Ian J. M. and Schlieder, Joshua E. and Livingston, John and Beichman, Charles and Christiansen, Jessie and Krick, Jessica and Gorjian, Varoujan and Howard, Andrew W. and Sinukoff, Evan and Ciardi, David R. and Akeson, Rachel L. (2017) Spitzer observations confirm and rescue the habitable-zone super-earth K2-18b for future characterization. The Astrophysical Journal, 834 (2). ISSN 0004-637X

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The recent detections of two transit events attributed to the super-Earth candidate K2-18b have provided the unprecedented prospect of spectroscopically studying a habitable-zone planet outside the solar system. Orbiting a nearby M2.5 dwarf and receiving virtually the same stellar insolation as Earth, K2-18b would be a prime candidate for the first detailed atmospheric characterization of a habitable-zone exoplanet using the Hubble Space Telescope (HST)and James Webb Space Telescope (JWST). Here, we report the detection of a third transit of K2-18b near the predicted transit time using the Spitzer Space Telescope. The Spitzer detection demonstrates the periodic nature of the two transit events discovered by K2, confirming that K2-18 is indeed orbited by a super-Earth in a 33 day orbit, ruling out the alternative scenario of two similarly sized, long-period planets transiting only once within the 75 day Kepler Space Telescope (K2) observation. We also find, however, that the transit event detected by Spitzer occurred 1.85 hr () before the predicted transit time. Our joint analysis of the Spitzer and K2 photometry reveals that this early occurrence of the transit is not caused by transit timing variations, but the result of an inaccurate ephemeris due to a previously undetected data anomaly in the K2 photometry. We refit the ephemeris and find that K2-18b would have been lost for future atmospheric characterizations with HST and JWST if we had not secured its ephemeris shortly after the discovery. We caution that immediate follow-up observations as presented here will also be critical for confirming and securing future planets discovered by the Transiting Exoplanet Survey Satellite (TESS), in particular if only two transit events are covered by the relatively short 27-day TESS campaigns.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Access to published version in accordance with the copyright policy of the publisher.
Faculty/School / Institute/Centre: No Faculty
Faculty/School / Institute/Centre: No Faculty
Date Deposited: 08 Nov 2017 01:46
Last Modified: 19 Apr 2018 00:39
Uncontrolled Keywords: astrobiology; planets and satellites; detection; planets and satellites; individual (K2-18b); techniques; photometric
Fields of Research (2008): 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020108 Planetary Science (excl. Extraterrestrial Geology)
Fields of Research (2020): 51 PHYSICAL SCIENCES > 5199 Other physical sciences > 519999 Other physical sciences not elsewhere classified
Identification Number or DOI: https://doi.org/10.3847/1538-4357/834/2/187
URI: http://eprints.usq.edu.au/id/eprint/32082

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