A Cool Earth-sized Planet Candidate Transiting a Tenth Magnitude K-dwarf From K2

TL;DR

This study demonstrates that the high-precision photometry of bright stars by K2 enables detection of single transits from temperate, Earth-sized planets around Sun-like stars. By combining K2 photometry with HARPS RVs, Hipparcos-Gaia astrometry, and imaging for HD 137010, the authors validate a shallow 10-hour transit as a likely transiting planet candidate, HD 137010 b. The inferred radius is approximately Earth's and the orbital period is around one year, placing the planet near the outer edge of the habitable zone with an incident flux near 0.3 I_earth. This work highlights the feasibility and significance of single-transit detections in bright systems and informs follow-up strategies for confirming temperate Earth analogs with future missions.

Abstract

The transit method is currently one of our best means for the detection of potentially habitable "Earth-like" exoplanets. In principle, given sufficiently high photometric precision, cool Earth-sized exoplanets orbiting Sun-like stars could be discovered via single transit detections; however, this has not previously been achieved. In this work, we report a 10-hour long single transit event which occurred on the $V=10.1$ K-dwarf HD 137010 during K2 Campaign 15 in 2017. The transit is comparatively shallow ($225\pm10$ ppm), but is detected at high signal-to-noise thanks to the exceptionally high photometric precision achieved for the target. Our analysis of the K2 photometry, historical and new imaging observations, and archival radial velocities and astrometry strongly indicate that the event was astrophysical, occurred on-target, and can be best explained by a transiting planet candidate, which we designate HD 137010 b. The single observed transit implies a radius of $1.06^{+0.06}_{-0.05}$ $R_\oplus$, and assuming negligible orbital eccentricity we estimate an orbital period of $355^{+200}_{-59}$ days ($a=0.88^{+0.32}_{-0.10}$ AU), properties comparable to Earth. We project an incident flux of $0.29^{+0.11}_{-0.13}$ $I_\oplus$, which would place HD 137010 b near the outer edge of the habitable zone. This is the first planet candidate with Earth-like radius and orbital properties that transits a Sun-like star bright enough for substantial follow-up observations.

Figures (2)

• Figure 1: (Top) Systematics-corrected K2 Campaign 15 light curve of HD 137010 (EPIC 249661074). The individual K2 long-cadence flux measurements are marked by grey points, while the blue curve represents a spline model to detrend the long-term photometric variability. A noticeable drop in flux, consistent with a planetary transit, is visible around BJD $\approx$ 2458055.1. (Middle) K2 light curve with long-term variability removed. A best-fit transit model has been applied to the transit event. (Bottom) Inset focusing on the transit event. The event has a depth of $\sim$225 ppm and $\sim$10 hr duration, and we estimate it is detected at a high signal-to-noise of $\approx$30. The transit event is consistent with an Earth-sized exoplanet with a $\approx$1 yr orbital period.
• Figure 2: Archival imaging of HD 137010 over time. The location of the star at the epoch of K2 observations is marked by a red cross, and the adopted K2 photometric aperture is shown by the red outline. (Far left) POSS-I blue-wavelength photographic plate image from 1953. (Centre left) AAO-SES red-wavelength photographic plate image from 1993. (Centre right) Pan-STARRS z-band image from 2012. (Far right) Summed image from K2 observations. We do not observe any background stars brighter than $\approx$19$^{\text{th}}$ magnitude that fall within the K2 photometric aperture, leading us to conclude that background flux contamination is negligible.