Alleviating the transit timing variation bias in transit surveys

I. RIVERS: Method and detection of a pair of resonant super-Earths around Kepler-1705^★

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Versoix, Switzerland
e-mail: adrien.leleu@unige.ch
² Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
³ Disaitek, 10 rue Achille Antheaume 95190 Fontenay-en-Parisis, France
⁴ School of Physics and Astronomy, Monash University, Victoria 3800, Australia

Received: 4 June 2021
Accepted: 13 October 2021

Abstract

Transit timing variations (TTVs) can provide useful information for systems observed by transit, as they allow us to put constraints on the masses and eccentricities of the observed planets, or even to constrain the existence of non-transiting companions. However, TTVs can also act as a detection bias that can prevent the detection of small planets in transit surveys that would otherwise be detected by standard algorithms such as the Boxed Least Square algorithm if their orbit was not perturbed. This bias is especially present for surveys with a long baseline, such as Kepler, some of the TESS sectors, and the upcoming PLATO mission. Here we introduce a detection method that is robust to large TTVs, and illustrate its use by recovering and confirming a pair of resonant super-Earths with ten-hour TTVs around Kepler-1705 (prev. KOI-4772). The method is based on a neural network trained to recover the tracks of low-signal-to-noise-ratio (S/N) perturbed planets in river diagrams. We recover the transit parameters of these candidates by fitting the light curve. The individual transit S/N of Kepler-1705b and c are about three times lower than all the previously known planets with TTVs of 3 h or more, pushing the boundaries in the recovery of these small, dynamically active planets. Recovering this type of object is essential for obtaining a complete picture of the observed planetary systems, and solving for a bias not often taken into account in statistical studies of exoplanet populations. In addition, TTVs are a means of obtaining mass estimates which can be essential for studying the internal structure of planets discovered by transit surveys. Finally, we show that due to the strong orbital perturbations, it is possible that the spin of the outer resonant planet of Kepler-1705 is trapped in a sub- or super-synchronous spin–orbit resonance. This would have important consequences for the climate of the planet because a non-synchronous spin implies that the flux of the star is spread over the whole planetary surface.