Extrasolar Binary Planets II: Detectability by Transit Observations

TL;DR

This paper explores the potential to detect binary gas-giant planets via transit light curve modulations, based on formation models and simulations, highlighting their possible presence among known exoplanets and detection with current data.

Contribution

It demonstrates that binary planets formed through orbital instability can be detected by analyzing transit light curve variations, providing a new method for identifying such systems.

Findings

Binary planets form with a probability of ~10% in certain systems.

Transit light curve modulations can reveal binary planet presence.

Binary planets may explain some known inflated gas giants or false positives.

Abstract

We discuss the detectability of gravitationally bounded pairs of gas-giant planets (which we call "binary planets") in extrasolar planetary systems that are formed through orbital instability followed by planet-planet dynamical tides during their close encounters, based on the results of N-body simulations by Ochiai, Nagasawa and Ida (Paper I). Paper I showed that the formation probability of a binary is as much as $\sim 10\%$ for three giant planet systems that undergo orbital instability, and after post-capture long-term tidal evolution, the typical binary separation is 3--5 times the sum of physical radii of the planets. The binary planets are stable during main sequence lifetime of solar-type stars, if the stellarcentric semimajor axis of the binary is larger than 0.3 AU. We show that detecting modulations of transit light curves is the most promising observational method to detect binary planets. Since the likely binary separations are comparable to the stellar diameter, the shape of the transit light curve is different from transit to transit, depending on the phase of the binary's orbit. The transit durations and depth for binary planet transits are generally longer and deeper than those for the single planet case. We point out that binary planets could exist among the known inflated gas giant planets or objects classified as false positive detections at orbital radii > 0.3 AU, propose a binary planet explanation for the CoRoT candidate SRc01 E2 1066, and show that binary planets are likely to be present in, and could be detected using Kepler-quality data.