Exoplanets around red dwarfs: Rocky-water worlds hold good potential for life outside Earth

Red dwarfs are cool, dim and common – and no, I’m not talking about the iconic British sci-fi sitcom.

They are the smallest and longest-lived hydrogen burning stars and make up almost three quarters of objects in our Milky Way galaxy. They are also frequently home to small exoplanets. A new study has divided these planets into three categories – rocky, gassy and watery – and surprisingly that all three could be potentially habitable.

Tess looks for exoplanets
TESS is a mission which seeks to identify exoplanets around the brightest dwarf stars. Credit: NASA

Small exoplanets were thought to have a bimodal distribution in their radius – that is come in two main types – rocky with thin hydrogen-helium atmospheres, and gassy with thick atmospheres.

In this study, the researchers used radius and mass measurements from 34 exoplanets detected by TESS – the Transiting Exoplanet Survey Satellite —and found that they fit into three categories (rather than the expected two): rocky, gassy and watery.

Read more: Rocky exoplanet found in habitable zone of star next door

The authors suggest that these planets actually formed as icy planets further out from the star, but migrated inwards over time, which, according to Johanna Teske, from the Carnegie Institute for Science, and an author on the paper, “is important for piecing together the complex puzzle of small planets’ formation and evolution.”

Exoplanet orbit habitable zones
Red dwarfs (lower panel) have habitable zones (green) much closer in to the parent star than hotter, larger stars. Credit: NASA/Kepler Mission/Dana Berry

The final category – ‘watery exoplanets’ – is particularly exciting for the potential for finding life as the density predictions of this group suggest these watery exoplanets are likely composed of rock and water in a 50-50 ratio.

“Although the presence of watery small exoplanets is particularly enticing, all three types of planets around red dwarfs could present potentially habitable conditions for life,” says Johanna Teske, from the Carnegie Institute for Science.

These exoplanets will be good targets for follow-up observations with a technique known as transmission spectroscopy, which hunts for the chemical signatures of life in exoplanetary atmospheres.

But don’t hold your breath while waiting to find life in another solar system, as chemical signatures are not definitive proof of life.

Perhaps we ought to take Red Dwarf’s advice:

We’re getting a signal. It’s probably nothing but I just thought I’d mention it.


Oh God, aliens… Your explanation for anything slightly peculiar is aliens, isn’t it? You lose your keys – it’s aliens. A picture falls off the wall – it’s aliens. That time we used up a whole bog roll in a day, you thought that was aliens as well!

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