Super-Earths and Red-Dwarfs

Super Earth in GJ667 Trinary System
From Physics Today

The European Southern Observatory (ESO) has reported, after studying nearby red-dwarfs, that ~40% (range 28%-95%) have Super-Earth planets in their habitable zones, which is quite remarkable considering how small a red-dwarf’s habitable zone is (from 0.25 AU and smaller.) That means 80% of the stars in our Galaxy – at least our bit of it – have large planets in their habitable zones. By “Habitable Zone” the ESO researchers use the “Recent Venus-Early Mars” limits – in other words from an insolation of ~1.7 – 0.4 times Earth. For really “Earth-like” the Zone is much more restricted, from 1.1 – 0.9 Earth insolation. In that case the numbers crash to <10%, but the sample size is still pretty small, so more observations are needed.

The original ESO release is here: Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way

The research is part of a series of papers publishing findings from the ESO’s HARPS instrument. The series is:

The HARPS search for southern extra-solar planets
In particular…
XXXI. The M-dwarf sample
XXXV.Super-Earths around the M-dwarf neighbors Gl 433 and Gl 667C

So the figures are 9 Super-Earths found around 102 Red-dwarfs via the radial velocity method, 2 of which are in the (broad) habitable zone. Once all the probabilities are worked out, plus the odds of transit, the ~41% of red-dwarfs figure is produced. How Earth-like are “Super-Earths?” is the relevant question. Being more massive, their gravity is higher, but not overly so. A 4.5 Earth mass super-Earth has 2 gee surface gravity, if it has Earth-like internal structure. If it’s an Ocean Planet – thus very Un-Earth-like – then the gravity is even lower, about 1.26 gee. The Broad Habitable Zone is from Venus-like (up until it lost its ocean c. 1 Gya) to Early Mars (when it still had an ocean – still a maybe.) Alternatively a Desert planet can remain stable to 1.77 Earth insolation, while a Hydrogen planet (with water oceans) can be stable to 0.01 or less.

A more meaningful way of understanding “habitable” is “biocompatible” – usually phrased as “can liquid water exist on its surface?” A broad range of environments are compatible with this, but most aren’t Earth-like. Truly Earth-like restricts the range to a much narrower span of orbit, reducing the Earth-like planets to a small fraction of the “Biocompatible Planets”. Instead of ~40%, the Earth-like worlds are more like ~4% or less.

By necessity a red-dwarf habitable zone planet has been significantly braked by tidal forces. If it’s too close and it formed with significant eccentricity, then the planet is probably inhospitable from the energy released by tidal braking. The usual end-state is for one face of the planet to always face its star, and the other to have eternal night. However neither Mercury or Venus ended up so, and exhibit other possible final states after significant tidal evolution. Mercury orbits 3 times for every 2 Solar days (its sidereal day is 2/3 its orbital period), while Venus rotates backwards compared to its orbital motion, which results in a Solar day that is significantly shorter than its sidereal day (116.7 vs 243.1 Earth days.) The actual final spin-state of a red-dwarf planet might be something like one of those, thus precluding the Endless Day/Endless Night dichotomy beloved by SF fans.