Planetary system LHS 1140 revisited with ESPRESSO and TESS

Abstract

Context. LHS 1140 is an M dwarf known to host two transiting planets at orbital periods of 3.77 and 24.7 days. They were detected with HARPS and Spitzer. The external planet (LHS 1140 b) is a rocky super-Earth that is located in the middle of the habitable zone of this low-mass star. All these properties place this system at the forefront of the habitable exoplanet exploration, and it therefore constitutes a relevant case for further astrobiological studies, including atmospheric observations. Aims. We further characterize this system by improving the physical and orbital properties of the known planets, search for additional planetary-mass components in the system, and explore the possibility of co-orbitals. Methods. We collected 113 new high-precision radial velocity observations with ESPRESSO over a 1.5-yr time span with an average photon-noise precision of 1.07 m s−1. We performed an extensive analysis of the HARPS and ESPRESSO datasets and also analyzed them together with the new TESS photometry. We analyzed the Bayesian evidence of several models with different numbers of planets and orbital configurations. Results. We significantly improve our knowledge of the properties of the known planets LHS 1140 b (Pb ~ 24.7 days) and LHS 1140 c (Pc ~ 3.77 days). We determine new masses with a precision of 6% for LHS 1140 b (6.48 ± 0.46 M⊕) and 9% for LHS 1140 c (mc = 1.78 ± 0.17 M⊕). This reduces the uncertainties relative to previously published values by half. Although both planets have Earth-like bulk compositions, the internal structure analysis suggests that LHS 1140 b might be iron-enriched and LHS 1140 c might be a true Earth twin. In both cases, the water content is compatible to a maximum fraction of 10–12% in mass, which is equivalent to a deep ocean layer of 779 ± 650 km for the habitable-zone planet LHS 1140 b. Our results also provide evidence for a new planet candidate in the system (md = 4.8 ± 1.1M⊕) on a 78.9-day orbital period, which is detected through three independent methods. The analysis also allows us to discard other planets above 0.5 M⊕ for periods shorter than 10 days and above 2 M⊕ for periods up to one year. Finally, our co-orbital analysis discards co-orbital planets in the tadpole and horseshoe configurations of LHS 1140 b down to 1 M⊕ with a 95% confidence level (twice better than with the previous HARPS dataset). Indications for a possible co-orbital signal in LHS 1140 c are detected in both radial velocity (alternatively explained by a high eccentricity) and photometric data (alternatively explained by systematics), however. Conclusions. The new precise measurements of the planet properties of the two transiting planets in LHS 1140 as well as the detection of the planet candidate LHS 1140 d make this system a key target for atmospheric studies of rocky worlds at different stellar irradiations.

Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b

“…The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability (Spinelli et al 2019), making this planet one of the most interesting targets for the search of biosignatures in the future although Galactic cosmic rays could impact this (Herbst et al 2020). Measurements and interior modeling by Lillo-Box et al (2020) suggest that the planet could possess a substantial mass of water. Modeling suggests that, if it were to have a surface ocean, LHS 1140b may be in a Note.…”

“…Finally, a recent study used ESPRESSO and TESS data to provide updated system parameters (Lillo-Box et al 2020). Hence we tried fitting the light curves using their parameters (a/R s = 96.4, i = 89.877°) as well as performing retrievals using their revised mass (M p = 6.38 M ⊕ ).…”

Detectability of biosignatures on LHS 1140 b

“…In this study, we simulate the potential atmosphere of the habitable zone planet LHS 1140 b by using a radius of 1.727±0.032 R ⊕ , a mass of 6.98±0.89 M ⊕ and a surface gravity, g, of 23.7±2.7 ms −2 (Ment et al 2019). We do not expect that our results would change significantly when using the slightly lower planetary mass of 6.48±0.46 M ⊕ suggested by Lillo-Box et al (2020). The planet receives an incident flux of 0.503±0.03 S and orbits its host star in ∼24.7 days.…”

Warm terrestrial planet with half the mass of Venus transiting a nearby star

“…This kernel is independent of the one used for the RV, but it uses the same hyperparameters, except for the amplitude (A FWHM ). This approach, inspired by Suárez Mascareño et al ( 2020) and subsequently Lillo-Box et al (2020), relies on the assumption that the variations in FWHM are solely due to stellar activity and that their periodicity and coherence are the same as the stellar activity component of the RV. Under these assumptions, the joint fit of the RV and FWHM data sets allows constraining the hyperparameters of the quasi-periodic kernel better.…”