A Mini-Neptune with Earth-Like Insolation

Observations by K2, an extension of the Kepler mission using the repurposed Kepler space telescope, has led to the detection of a mini-Neptune that receives a similar amount of insolation from its host star as Earth gets from the Sun. The planet is identified as EPIC 201912552.01. Its host star, being a red dwarf star, is smaller and much less luminous than the Sun. EPIC 201912552.01 receives an Earth-like insolation because it orbits its host star much closer than Earth is from the Sun. In fact, the planet has an orbital period of only 33 days. Being so close to its host star, the planet is most likely tidally-locked, with the same hemisphere perpetually facing its host star, resulting in permanent day and night sides.

By measuring the dip in brightness when EPIC 201912552.01 transits in front of its host star, the size of the planet is estimated to be 2.24 ± 0.25 times the diameter of Earth. With such a size, EPIC 201912552.01 is too large to be a rocky planet like Earth. Instead, the planet is most probably a mini-Neptune. The equilibrium temperature of the planet is estimated to be 271 ± 16 K. Both the planet and its host star are located at a distance of approximately 110 light-years. The proximity and relative brightness of the system makes EPIC 201912552.01 a good target for its atmosphere to be characterised by future space-based telescopes such as the James Webb Space Telescope (JWST).

Reference:
Montet et al. (2015), “Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 18 Systems, Including a Planet Receiving Earth-like Insolation”, arXiv:1503.07866 [astro-ph.EP]