K2-415b

Centauri Dreams posts on K2-415b; a transit uncovered from the second Kepler mission. This transit is also found by TESS which catalog is TOI-5557.

It is the closest of Kepler's transits at 22 parsecs (70 ly). TRAPPIST-1 at 12 pc still wears the crown for transits overall; and there's 55 Cancri at 12.587 pc and so on. Of course other planets have been spotted closer still by the radialvelocity. Transits offer (so far) the only constraint on physical size of a star-orbiting small planet.

And K2-415b is small: about the size of Earth.

Sadly the star seems a bit flarey, as so many Ms are, so even with the inclination nailed to 89.32 ±0.41° the constraint on mass is laughable: 3.0 ±2.7 M_⊕. Which the authors point out: just by assuming near-solid iron (like Mercury) the upper bound should be 2.2 Earth masses.

On to lowerbounds, starting with... clouds. The 4.83 ±.43 S_⊕ insolation would strip away the obvious, hydrogen and helium. Moving on to oceans: yeah, no water here, either; and carbon-dioxide is supercritical at the surface and not liquid. These lads get BIG ups for calculating insolation, and thence pegging possible albedi of 0 or 0.3 before getting into temperature. Sudarsky fails on Venus irradiance which is 1.911 S_⊕ but I doubt even acid clouds can survive five times Earth's. Given that the transiting circle is near-enough the rocky basalt(?) crust of the planet, lower bound mass cannot be much less than Earth mass.

As the Chief Dreamer points out, this is not a holiday destination. But per Haruka Hoshino and Eiichiro Kokubo many systems churn out similarly-sized planets at similarly-spaced semimajors (20 Hill radii); so this star may host another Earth-mass orbiting 10-15 days. But no *-415c transits so finding such needs (probably) radial-velocity measurements, which - as noted - the star makes difficult to constrain.

BACKDATE 2/15