Are planets tidally locked to red dwarfs habitable? It's complicated
Astronomers are keenly interested in red dwarfs and the planets that orbit them. Up to 85% of the stars in the Milky Way could be red dwarfs, and 40% of them might host Earth-like exoplanets in their habitable zones, according ...
But there are some problems with their potential habitability. One of those problems is tidal locking.
Red dwarf stars are sometimes called "M-dwarfs," but the terms can get a bit fuzzy.
The confusion between the terms red dwarf and M-dwarf stems from temperature and mass. One definition of a red dwarf is synonymous with an M-dwarf, based on a maximum temperature of 3,900 K and a maximum mass of 0.6 solar masses.
Another definition of a red dwarf includes hotter stars with a maximum temperature of 5,200 K and a maximum mass of 0.8 solar masses. This definition includes all K-type main-sequence stars, which are also called K-dwarfs.
Another red dwarf definition includes only part of the K-dwarf classification, and yet another includes some brown dwarfs.
The Hertzsprung-Russell diagram shows how the definitions overlap.
In general, red dwarfs are the smallest and coolest main-sequence stars. Because they're such low-mass objects, they develop slowly and live a long time. The least massive among them can maintain a constant luminosity for trillions of years, but there are no red dwarfs that old in the Universe yet.
However astronomers define them, there are an awful lot of them. If the estimate of 250 billion stars in the Milky Way is accurate, then 212 billion of them might be red dwarfs. Astronomers think that the red dwarf stellar population hosts a lot of rocky planets in their habitable zones. This is why the astronomy community makes such a pointed effort to study red dwarfs: they're the Milky Way's exoplanet cookie jar.
Artist's impression of a habitable exoplanet orbiting a red dwarf star. The habitability of the planets of red dwarf stars is conjectural. Credit: ESO/M. Kornmesser, public domain
The definition of a red dwarf can be a bit fuzzy. Sometimes the definition included the brightest brown dwarfs, and sometimes it includes all or part of the K-dwarf classification. Credit: Spacepotato – Modified version of Image: HR-diag-no-text.svg, written by Rursus and modified by Bhutajata, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2093830
This is an artist’s illustration of the nearby red dwarf AD Leonis, also known as Gliese 388. Like other red dwarfs, it’s known to flare violently, but flaring wasn’t part of this study. Credit: National Astronomical Observatory of Japan
This figure from the study shows how variables can have competing effects. (a) shows the surface temperature and winds. (b) shows the mid-troposphere height and wind patterns. (c) shows top-of-atmosphere albedo. (d) shows the surface evaporation rates and precipitation reaching the surface in yellow contours. (c) shows that regions with abundant clouds reflect a larger fraction of incoming SW (shortwave flux or stellar radiation), reducing warming in the stellar eye but also increasing the greenhouse effect. Credit: Lobo et al. 2022