LTT 1445 - Nearby triple M dwarf system with a transiting terrestrial planet

Three Red Suns in the Sky: A Transiting, Terrestrial Planet in a Triple M Dwarf System at 6.9 Parsecs
https://arxiv.org/abs/1906.10147

Quote :

We present the discovery from TESS data of LTT 1445Ab. At a distance of 6.9 parsecs, it is the second nearest transiting exoplanet system found to date, and the closest one known for which the primary is an M dwarf. The host stellar system consists of three mid-to-late M dwarfs in a hierarchical configuration, which are blended in one TESS pixel. We use follow-up observations from MEarth and the centroid offset analysis in the TESS data validation report to determine that the planet transits the primary star in the system. The planet has a radius 1.35 R_Earth, an orbital period of 5.35882 days, and an equilibrium temperature of 428 K. With radial velocities from HARPS, we place a three-sigma upper mass limit of 8.4 M_Earth on the candidate. The planet provides one of the best opportunities to date for the spectroscopic study of the atmosphere of a terrestrial world. The presence of stellar companions of similar spectral type may facilitate such ground-based studies by providing a calibration source to remove telluric variations. In addition, we present a detailed characterization of the host stellar system. We use high-resolution spectroscopy and imaging to rule out the presence of any other close stellar or brown dwarf companions. Nineteen years of photometric monitoring of A and BC indicates a moderate amount of variability, in agreement with the observed low-level, short-term variability in the TESS light curve data. We derive a preliminary astrometric orbit for the BC pair that reveals an edge-on and eccentric configuration. The presence of a transiting planet in this system raises the possibility that the entire system is co-planar, which implies that the system may have formed from the early fragmentation of an individual protostellar core.

I've split the post on this system off from the main TESS News and Results thread since this system is rather nearby and has some decent amount of potential for being a high-value system compare to many of the other Tess Objects of Interest.

A second planet candidate seems to have been identified in the TESS data for LTT 1445 A. It has an orbital period of 3.12 days (interior to the confirmed planet), and a transit depth of ~0.001, corresponding to a radius of ~0.9 Earth-radii.

LTT 1445 ExoFOP page.

https://arxiv.org/abs/2107.14737

A Second Planet Transiting LTT~1445A and a Determination of the Masses of Both Worlds

Using TESS data, we present the discovery of a second planet in the LTT 1445 system, with an orbital period of 3.1 days. We combine radial velocity measurements obtained from the five spectrographs ESPRESSO, HARPS, HIRES, MAROON-X, and PFS to establish that the new world also orbits LTT 1445A. We determine the mass and radius of LTT 1445Ab to be 2.87+/-0.25 M_Earth and 1.304^{+0.067}_{-0.060} R_Earth, consistent with an Earth-like composition of 33% iron and 67% magnesium silicate. For the newly discovered LTT 1445Ac, we measure a mass of 1.54^{+0.20}_{-0.19} M_Earth and a minimum radius of 1.15 R_Earth, but we cannot determine the radius directly as the signal-to-noise of our light curve permits both grazing and non-grazing configurations.

Lavie et al. "Planetary system around LTT 1445A unveiled by ESPRESSO: Multiple planets in a triple M-dwarf system"
https://arxiv.org/abs/2210.09713

They also find the transiting 3.12-day planet, plus a third non-transiting planet in a 24.3-day orbit.

Already posted but worth repeating for EPE updates.

Transmission spectrum for LTT 1445 Ab.

Diamond-Lowe et al. "Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab"
https://arxiv.org/abs/2210.11809

The story will be familiar from similar studies of other low-mass planets orbiting M dwarfs:

Quote :

With the data presented in this work we rule out a clear, low mean molecular weight atmosphere at 10 bars of surface pressure (3.4σ) and 1 bar of surface pressure (2.9σ). This result joins previous work on GJ 1132b (Diamond-Lowe et al. 2018; Mugnai et al. 2021; Libby-Roberts et al. 2022), TRAPPIST-1a–f (de Wit et al. 2016, 2018), LHS 3844 (Kreidberg et al. 2019; Diamond-Lowe et al. 2020b), and L 98-59b (Damiano et al. 2022; Zhou et al. 2022) to strongly suggest that highly irradiated terrestrial exoplanets orbiting M dwarfs are not capable of retaining low mean molecular weight atmospheres, if they are able to accrete them in the first place. It is possible, however, that LTT 1445Ab possesses a high mean molecular weight or cloudy/hazy atmosphere, which would fall below our detection limits, or no atmosphere at all.

HST/WFC3 Light Curve Confirms the Closest Exoplanet to Transit an M Dwarf is Terrestrial
https://arxiv.org/abs/2307.02970

Refinement of the radius of LTT 1445 Ac, confirmation that it is indeed terrestrial.