Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization

TL;DR

This study confirms the periodic transits of habitable-zone super-Earth K2-18b using Spitzer, refines its orbital parameters, and highlights the importance of follow-up observations for future atmospheric characterization with HST, JWST, and TESS.

Contribution

The paper provides the first confirmation of K2-18b's periodic transits with Spitzer and refines its ephemeris, ensuring its suitability for future atmospheric studies.

Findings

Confirmed K2-18b's 33-day orbit through Spitzer detection

Refined the planet's transit timing, correcting previous ephemeris errors

Demonstrated the necessity of follow-up observations for accurate planet characterization

Abstract

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 HST and 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 and ruling out the alternative scenario of two similarly-sized, long-period planets transiting only once within the 75-day K2 observation. We also find, however, that the transit event detected by Spitzer occurred 1.85 hours (7-sigma) 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 (TTVs), but the result of an inaccurate K2 ephemeris due to a previously undetected data anomaly in the K2 photometry likely caused by a cosmic ray hit. 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 in confirming and securing future planets discovered by TESS, in particular if only two transit events are covered by the relatively short 27-day TESS campaigns.