LTT 1445

LTT 1445 is a triple M-dwarf system 22.4 ly distant in the constellation Eridanus. The primary LTT 1445 A hosts two exoplanets—one discovered in 2019 that transits the star every 5.36 days, and another found in 2021 that transits the star every 3.12 days, close to a 12:7 resonance. As of October 2022 it is the second closest transiting exoplanet system discovered, with the closest being HD 219134 bc.

Stellar system
All three stars in the system are M-dwarfs, with masses between 0.16 and 0.26. LTT 1445 A and LTT 1445 BC are separated by about 34 astronomical units and orbit each other with a period of about 250 years. The BC pair orbit each other about every 36 years in an eccentric orbit (e= ~0.5). The alignment of the three stars and the edge-on orbit of the BC pair suggests co-planarity of the system. The existence of transiting planets suggests that the entire system is co-planar, with orbits in one plane.

The TESS light curve showed stellar flares and rotational modulation due to starspots, likely on either the B or C component.

LTT 1445 Ab
LTT 1445 Ab is an exoplanet located approximately 22 light years away from Earth. Astrophysicists of the Harvard Center for Astrophysics discovered it in June 2019 with data from the Transiting Exoplanet Survey Satellite. The team obtained follow-up observations, including HARPS radial velocity measurements to constrain the mass of the planet.

LTT 1445 Ab takes 5 days to orbit its star, which in turn orbits two sibling stars, making a total of three stars in the system.

In July 2021, the mass of the planet was measured as $11.22$ Earth masses, confirming an Earth-like composition.

In 2022, a planetary transmission spectrum showed no evidence for an atmosphere, although an atmosphere with high altitude hazes cannot be ruled out yet. LTT 1445 Ab likely has a rocky composition, and because it orbits close to the M-dwarf, it has an equilibrium temperature of $11.37$ (431 K).

LTT 1445 Ac
A second planet, LTT 1445 Ac, was also found in 2021 on a 3.12 day orbital period, with a mass of $-5.74$ Earth masses. Although it transits the star too, its smaller size made it difficult to detect before the radial velocity measurements, and still makes it difficult to estimate its exact size. The planets orbit near a 12:7 orbital resonance with one another - Ac orbiting 11.988 times for every 7 orbits Ab makes - oscillating one full orbit away from a 'perfect' resonance every 104 years. The planet's existence was independently confirmed in 2022.

In 2023, observations with the Hubble Space Telescope allowed a more precise determination of the planet's size, supporting a rocky composition for both planets. Its equilibrium temperature is $36.2$ (516 K).

LTT 1445 Ad
A third planetary candidate on a 24.3-day orbit, LTT 1445 Ad, was found in 2022. This is a possibly rocky super-Earth orbiting within the habitable zone.