Transit least-squares survey

René Heller,Kai Rodenbeck,Michael Hippke

doi:10.1051/0004-6361/201935600

Abstract

The extended Kepler mission (K2) has revealed more than 500 transiting planets in roughly 500 000 stellar light curves. All of these were found either with the box least-squares algorithm or by visual inspection. Here we use our new transit least-squares (TLS) algorithm to search for additional planets around all K2 stars that are currently known to host at least one planet. We discover and statistically validate 17 new planets with radii ranging from about 0.7 Earth radii (R⊕) to roughly 2.2 R⊕ and a median radius of 1.18 R⊕. EPIC 201497682.03, with a radius of 0.692+0.059−0.048, is the second smallest planet ever discovered with K2. The transit signatures of these 17 planets are typically 200 ppm deep (ranging from 100 ppm to 2000 ppm), and their orbital periods extend from about 0.7 d to 34 d with a median value of about 4 d. Fourteen of these 17 systems only had one known planet before, and they now join the growing number of multi-planet systems. Most stars in our sample have subsolar masses and radii. The small planetary radii in our sample are a direct result of the higher signal detection efficiency that TLS has compared to box-fitting algorithms in the shallow-transit regime. Our findings help in populating the period-radius diagram with small planets. Our discovery rate of about 3.7% within the group of previously known K2 systems suggests that TLS can find over 100 additional Earth-sized planets in the data of the Kepler primary mission.

Highlights

After the continuous monitoring of its prime observing field from 2009 to 2013, the repurposed Kepler telescope (Borucki et al 2010) performed another 19 campaigns of different target fields along the ecliptic, covered for about 75 d each from 2014 until 2018 (Howell et al 2014)
We recently presented the transit least-squares (TLS) algorithm, which is optimized for the detection of shallow periodic transits (Hippke & Heller 2019)
We inspected the K2 aperture masks used by EVEREST and compared them to high-resolution images from Pan-STARRS to evaluate the apparent separation of any possible contaminants

Summary

Introduction

After the continuous monitoring of its prime observing field from 2009 to 2013, the repurposed Kepler telescope (Borucki et al 2010) performed another 19 campaigns of different target fields along the ecliptic, covered for about 75 d each from 2014 until 2018 (Howell et al 2014). Luger et al (2016) constructed EVEREST, an automated K2 photometric extraction pipeline based on a pixel level decorrelation technique (Deming et al 2015) in combination with a Gaussian process optimization (Aigrain et al 2015). These improvements of the K2 stellar photometry calibration allowed the confirmation of 359 exoplanets and 472 more candidates, in addition to the 2331 confirmed planets and 2425 candidates yet to be confirmed from the Kepler primary mission. All K2 planets and candidates were found using either a direct application of the box least-squares (BLS) (Kovács et al 2002) transit search algorithm, for example, for the searches by Adams. The popularity of BLS is well founded in its good detection efficiency of shallow transits (Tingley 2003a,b) and computational speed, for example, compared to algorithms using artificial intelligence (Mislis et al 2016; Pearson et al 2018; Zucker & Giryes 2018; Armstrong et al 2018)

Methods

Results

Discussion

Conclusion