Discovery of TOI-1260d and the characterisation of the multi-planet system

TL;DR

This paper reports the discovery of a third transiting planet in the TOI-1260 system, refining the system's parameters and analyzing the planets' compositions and atmospheric loss histories.

Contribution

It presents the discovery of a third planet in the TOI-1260 system and provides detailed characterization and interior composition modeling of all three planets.

Findings

The third planet has a radius of approximately 3.09 R⊕.

All three planets contain some fraction of gas in their interiors.

The innermost planet likely lost its primordial hydrogen envelope.

Abstract

We report the discovery of a third planet transiting the star TOI-1260, previously known to host two transiting sub-Neptune planets with orbital periods of 3.127 and 7.493 days, respectively. The nature of the third transiting planet with a 16.6-day orbit is supported by ground-based follow-up observations, including time-series photometry, high-angular resolution images, spectroscopy, and archival imagery. Precise photometric monitoring with CHEOPS allows to improve the constraints on the parameters of the system, improving our knowledge on their composition. The improved radii of TOI-1260b, TOI-1260c are $2.36 \pm 0.06 \rm R_{\oplus}$, $2.82 \pm 0.08 \rm R_{\oplus}$, respectively while the newly discovered third planet has a radius of $3.09 \pm 0.09 \rm R_{\oplus}$. The radius uncertainties are in the range of 3\%, allowing a precise interpretation of the interior structure of the three planets. Our planet interior composition model suggests that all three planets in the TOI-1260 system contains some fraction of gas. The innermost planet TOI-1260b has most likely lost all of its primordial hydrogen-dominated envelope. Planets c and d were also likely to have experienced significant loss of atmospheric through escape, but to a lesser extent compared to planet b.