Validation of a Third Earth-sized Planet in the TOI-2267 Binary System

TL;DR

TOI-2267 is a nearby M-dwarf binary hosting two Earth-sized planets near a $3:2$ resonance, with a third candidate (TOI-2267 d) previously unvalidated due to lacking independent observations. The authors combine two ground-based Palomar/WIRC transits in the $J$ band with archival TESS data and use the TRICERATOPS+ framework to statistically validate TOI-2267 d, while attempting to constrain its stellar host. The analysis yields dilution-corrected planet parameters consistent across bands and transit nights, with an FPP of $4.7\times10^{-6}$ and NFPP of 0, but cannot conclusively assign TOI-2267 d to TOI-2267A or TOI-2267B. Depending on the host, TOI-2267 d would be $0.98\pm0.09\,R_{\oplus}$ with $T_{eq}\approx503$ K or $1.77\pm0.43\,R_{\oplus}$ with $T_{eq}\approx422$ K, implying very different atmospheric histories; the system offers a unique testbed for how close binarity influences planet formation and atmospheric mass loss in sub-Earth to Earth-sized planets.

Abstract

We report the validation of a third terrestrial exoplanet in the nearby (22 pc) TOI-2267 system. TOI-2267 is a binary system with stellar components TOI-2267A (M5, 3030 K) and TOI-2267B (M6, 2930 K), with an on-sky separation of 0.$^{\prime\prime}$384 (8 au projected separation). TOI-2267 hosts two Earth-sized planets (TOI-2267 b, $1.00\pm0.11 R_{\oplus}$, and TOI-2267 c, $1.14\pm0.13 R_{\oplus}$, if orbiting the primary star; or $1.22\pm0.29 R_{\oplus}$ and $1.36\pm0.33 R_{\oplus}$, respectively, if orbiting the secondary star) with orbital periods of 2.3 and 3.5 days. This system also contains a third Earth-sized planet candidate with an orbital period of 2.0 days that was previously identified as a likely planet with a low false-positive probability, but could not be firmly validated due to the lack of independent observations beyond TESS data. We combine two new transit observations from the 5.1m Hale Telescope at Palomar Observatory with archival TESS data and high-resolution imaging to statistically validate the planetary nature of TOI-2267 d ($0.98\pm0.09 R_{\oplus}$ if orbiting the primary star, or $1.77\pm0.43 R_{\oplus}$ if orbiting the secondary star) using the updated TRICERATOPS+ pipeline. We attempt to determine the host star for TOI-2267 d using transit shape stellar density analysis, but are unable to conclusively assign a host. Our validation of TOI-2267 d suggests that TOI-2267 is either the first known double transiting M dwarf binary system, or hosts three planets in an extremely compact orbital configuration.

Figures (4)

• Figure 1: Phased-folded and 2-minute binned TESS light curve (left panel) and detrended Palomar/WIRC light curves (middle and right panels) for the two transit observations of TOI-2267 d. Residuals after the best-fit transit light curve model has been subtracted are shown in the lower panels. Unbinned data are shown as grey circles, with 10 minute binned points overplotted as black circles. The best joint-fit transit models are overplotted as blue lines for the TESS data and red lines for the Palomar data, with 100 random draws from the posterior distribution to illustrate the typical model uncertainty. All detrended light curves are available electronically as data behind the figure.
• Figure 2: Empirical stellar density constraints derived from our transit shape analysis (red) compared to the stellar density measurements from the SED fitting of ZP25 (blue), for the primary (top) and secondary star (bottom) fits. Our empirical constraints on the stellar density are not precise enough to conclusively rule out either star as the host of TOI-2267 d.
• Figure 3: Posterior distribution of transit model parameters in our joint transit fit, assuming the primary host star, using both nights of Palomar/WIRC data and the phase-folded TESS photometry for TOI-2267 d.
• Figure 4: Posterior distribution of transit model parameters in our joint transit fit, assuming the secondary host star, using both nights of Palomar/WIRC data and the phase-folded TESS photometry for TOI-2267 d.