LHS 1140 b

LHS 1140 b was detected using both the radial velocity method (which measures the mass of a companion object) and transit photometry (which determines radius). Because of this, LHS 1140 b is one of very few potentially habitable exoplanets with a determined mass and radius, the others all being those around TRAPPIST-1. The planet's radius is well-constrained at 1.43±0.10 R_⊕, equivalent to about 9,110 km. It is one of the smallest radius values for any potentially habitable planet and similar to that of Kepler-62f. Using the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph on the ESO 3.6m telescope at La Silla Observatory in Chile, the discovery team was able to determine the mass of LHS 1140 b, and found that it is around 6.65 M_⊕, with an uncertainty parameter of ±1.82 M_⊕. If it were to have an Earth-like composition, the mass of the planet would instead be closer to 3 M_⊕. The density of LHS 1140 b comes out to be around 12.5 g/cm³, one of the highest ever observed for a rocky planet and over twice the density of Earth. This gives it a surface gravity of 3.25 times that of Earth, even higher than that of Jupiter. To escape the gravity of LHS 1140 b, a craft would have to move at 24 km/s, or nearly 54,000 mph. LHS 1140 b's incredibly high density suggests an iron-rich composition, with an iron-nickel core taking up to 75% of the planet's total mass.[2] For comparison, Earth's core comprises about 32.5% of its mass, less than half that of LHS 1140 b.[4]

The orbit of LHS 1140 b takes 24.737 days to complete, much quicker than Earth's year of 365 days. Its orbital radius is at 0.0875 AU, or 8.75% the distance between Earth and the Sun. While this is quite close, the star LHS 1140 is so dim that the planet only gets 0.41 times the incident flux of Earth at this distance. With an albedo of 0, LHS 1140 b has an equilibrium temperature of 230 K (−43 °C; −46 °F), compared to Earth's at 255 K (−18 °C; −1 °F). If LHS 1140 b has an albedo similar to that of Earth, the equilibrium temperature would be even lower, at 201 K (−72 °C; −98 °F). However, with a greenhouse effect at least as strong as Earth's LHS 1140 b would have a surface temperature greater than 266 K (−7 °C; 19 °F) for an albedo of 0. Due to the high mass of the planet, it likely has a thicker atmosphere with a more powerful greenhouse effect. Like many other potentially habitable red dwarf planets, the orbit of LHS 1140 b is quite circular: the eccentricity is measured to be lower than 0.29 to a 90% confidence. However, unlike many other red dwarf planets, the circularization of the orbit cannot be explained by stellar tides, and thus the circularity of the orbit is likely to be natal.[2]