Planets Around Red Dwarfs

If you picked a star at random in the Milky Way, it would most likely be a red dwarf — an M-type star smaller, cooler and much dimmer than the Sun. Roughly three-quarters of nearby stars are red dwarfs, and rocky planets around them appear to be common. If life is widespread in the galaxy, a lot of it might live under red skies.

Famous red dwarf planets

• Proxima Centauri b — discovered in 2016, it orbits the nearest star to the Sun, about 4.2 light-years away. It is at least about as massive as Earth and lies in its star's habitable zone, with a year of just over 11 days.
• TRAPPIST-1 — a tiny red dwarf about 40 light-years away with seven Earth-sized planets, announced in 2017. Several of them orbit in or near the habitable zone. All seven would fit inside the orbit of Mercury.

The advantages

• Very long lives. Red dwarfs burn their fuel so slowly that they can shine for hundreds of billions of years or more. Life would have plenty of time.
• Lots of them. More stars means more chances.
• Easier to study. Small stars make it easier to detect small planets and to analyse their atmospheres when they pass in front of the star.

The challenges

Tidal locking

A habitable zone close to the star means strong tidal forces. Many such planets are probably tidally locked, with one side always facing the star — permanent day on one hemisphere, permanent night on the other. A thick atmosphere or deep ocean could carry heat around and keep the night side from freezing solid; climate models suggest this is possible, but it depends on many details.

Flares

Red dwarfs, especially young ones, can be magnetically violent, producing flares far stronger relative to their size than the Sun's. Repeated flares and stellar winds could erode a planet's atmosphere over time.

A rough childhood

Young red dwarfs stay bright for hundreds of millions of years before settling down. Planets that will later be in the habitable zone may spend this early period too hot, risking the loss of their water before conditions improve.

What telescopes are finding

The James Webb Space Telescope has been studying the TRAPPIST-1 planets. Early measurements of the two innermost planets, TRAPPIST-1 b and c, published in 2023, did not find signs of thick carbon dioxide atmospheres. Whether the cooler planets farther out have kept their air is one of the most watched questions in exoplanet science, and observations are ongoing.

Builder's verdict

A red dwarf is a high-risk, high-reward choice. Your planet will have an extremely long-lived star, but it needs to be massive enough, wet enough and well-shielded enough to survive the early flares — and you will probably have to design for one side always facing the sun.