Abstract
Abstract Observations from the Kepler and K2 missions have provided the astronomical community with unprecedented amounts of data to search for transiting exoplanets and other astrophysical phenomena. Here, we present K2-288, a low-mass binary system (M2.0 ± 1.0; M3.0 ± 1.0) hosting a small (R p = 1.9 R ⊕), temperate (T eq = 226 K) planet observed in K2 Campaign 4. The candidate was first identified by citizen scientists using Exoplanet Explorers hosted on the Zooniverse platform. Follow-up observations and detailed analyses validate the planet and indicate that it likely orbits the secondary star on a 31.39-day period. This orbit places K2-288Bb in or near the habitable zone of its low-mass host star. K2-288Bb resides in a system with a unique architecture, as it orbits at >0.1 au from one component in a moderate separation binary (a proj ∼ 55 au), and further follow-up may provide insight into its formation and evolution. Additionally, its estimated size straddles the observed gap in the planet radius distribution. Planets of this size occur less frequently and may be in a transient phase of radius evolution. K2-288 is the third transiting planet system identified by the Exoplanet Explorers program and its discovery exemplifies the value of citizen science in the era of Kepler, K2, and the Transiting Exoplanet Survey Satellite.
Highlights
With the discovery and validation of over 300 planets spanning the ecliptic as of 2018 October, the retired K2 Mission has continued the exoplanet legacy of Kepler by providing highcadence continuous light curves for tens of thousands of stars for more than a dozen ∼80 day observing campaigns (Howell et al 2014; NASA Exoplanet Archive 2018)
K2-288 (EPIC 210693462, LP 413-32, NLTT 11596, 2MASS J03414639+1816082) was proposed as a target in K2 Campaign 4 (C4) by four teams in K2 GO Cycle 1.23 The target was subsequently observed at 30-minute cadence for 75 days in C4, which ran from 2015 February 7 until 2015 April 23
We model the Spitzer systematics using the pixel-level decorrelation (PLD) method proposed by Deming et al (2015), which uses a linear combination of the normalized pixel light curves to model the systematic noise caused by motion of the point-spread function (PSF) on the detector
Summary
With the discovery and validation of over 300 planets spanning the ecliptic as of 2018 October, the retired K2 Mission has continued the exoplanet legacy of Kepler by providing highcadence continuous light curves for tens of thousands of stars for more than a dozen ∼80 day observing campaigns (Howell et al 2014; NASA Exoplanet Archive 2018). When Exoplanet Explorers was launched, candidate transits were uploaded as soon as planet searches in new K2 campaigns had completed, and citizen scientists were examining these new candidates simultaneously with our team This process began with K2 Campaign 12 and Exoplanet Explorers immediately had success with its first discovery, K2-138, a system hosting five transiting sub-Neptunes in an unbroken chain of near 3:2 resonances (Christiansen et al 2018). Another system simultaneously identified by our team and citizen scientists on Planet Hunters and Exoplanet Explorers is K2233, a young K dwarf hosting three small planets (David et al 2018).
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