Ultra Short Period Planets in K2: SuPerPiG Results for Campaigns 0-5

Abstract

We have analyzed data from Campaigns 0-5 of the K2 mission and report 19 ultra-short-period candidate planets with orbital periods of less than 1 day (nine of which have not been previously reported). Planet candidates range in size from 0.7-16 Earth radii and in orbital period from 4.2 to 23.5 hours. One candidate (EPIC 203533312, Kp=12.5) is among the shortest-period planet candidates discovered to date (P=4.2 hours), and, if confirmed as a planet, must have a density of at least rho=8.9 g/cm^3 in order to not be tidally disrupted. Five candidates have nominal radius values in the sub-Jovian desert (R_P=3-11 R_E and P<=1.5 days) where theoretical models do not favor their long-term stability; the only confirmed planet in this range is in fact thought to be disintegrating (EPIC 201637175). In addition to the planet candidates, we report on four objects which may not be planetary, including one with intermittent transits (EPIC 211152484) and three initially promising candidates that are likely false positives based on characteristics of their light curves and on radial velocity follow-up. A list of 91 suspected eclipsing binaries identified at various stages in our vetting process is also provided. Based on an assessment of our survey's completeness, we estimate an occurrence rate for ultra-short period planets among K2 target stars that is about half that estimated from the Kepler sample, raising questions as to whether K2 systems are intrinsically different from Kepler systems, possibly as a result of their different galactic location.

13 pages, 8 figures, accepted to AJ on 2016 May 20

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• The K2-HERMES Survey. I. Planet Candidate Properties from K2 Campaigns 1-3
• K2-66b and K2-106b: Two extremely hot sub-Neptune-size planets with high densities
• TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes
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• Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars IV: Updated Properties for 86 Cool Dwarfs Observed During Campaigns 1-17
• Disproval of the validated planets K2-78b, K2-82b, and K2-92b
• 37 New Validated Planets in Overlapping K2 Campaigns
• Scaling K2. IV. A Uniform Planet Sample for Campaigns 1-8 and 10-18
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• The resilience of Kepler systems to stellar obliquity
• Scaling K2. III. Comparable Planet Occurrence in the FGK Samples of Campaign 5 and Kepler
• TOI-1075 b: A Dense, Massive, Ultra-Short Period Hot Super-Earth Straddling the Radius Gap
• K2-HERMES II. Planet-candidate properties from K2 Campaigns 1-13
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• Accretion of Planetary Material onto Host Stars
• Search for Exoplanetary Transits in the Galactic Bulge
• Radius and mass distribution of ultra-short period planets
• TFAW survey II: 6 Newly Validated Planets and 13 Planet Candidates from K2