Kepler-138 Exoplanet System

Twin water worlds and a Mars-mass planet around a red dwarf // Kepler-138, also known as KOI-314, is a real system of four known exoplanets located 218 light-years away in the constellation Lyra. The host star Kepler-138 is a fairly young red dwarf with an estimated age of 2. 21 billion years. It possesses an M1V spectral type with ~0. 5 solar masses and radii, corresponding to a surface temperature of 3841 K (3568 °C) and a luminosity ~6% of the Sun's. Three exoplanets were confirmed around this star in 2014, followed by a fourth in 2022. In order of orbital distance, they are Kepler-138 b, c, d and e. The planets form a curious configuration that resembles a more compact version of the inner Solar System (Mercury, Venus, Earth, Mars), with the smallest, b, being followed by two larger twins, c and d and then likely another small outer planet, e. Examining the system sequentially, the first planet is Kepler-138 b, a Mars analogue and one of the smallest known exoplanets, with just 7% of Earth’s mass and 64% of its radius. It orbits its star every 10. 3 days. Kepler-138 b's true nature remains uncertain. Earlier studies suggested the planet may contain a substantial volatile layer rather than being purely rocky. Still, those conclusions relied on an older mass estimate of 0. 187 Earth masses, which has since been revised downward. If the newer estimate is correct, it is difficult to explain how such a low-mass planet could retain a significant envelope, pointing either to substantial systematic uncertainties or inaccuracies in the earlier mass–radius measurements. The next two bodies, Kepler-138 c and d, are twin super-Earths. They are around 50% larger than our home planet, with 2. 3 and 2. 1 times the mass. The duo were initially considered strikingly different, with c being thought of as rocky and d having high volatile content like a miniature gas giant. A recent study instead suggests that they are both ocean worlds with 2000-kilometre-deep water mantles. Water is be