The least massive stars are called "ultra-cool dwarf stars". They are about the size of Jupiter. But compared with our Sun, they are more than twice as cold, ten times less massive and a hundred times less luminous. Their lifespan is over a hundred times longer than that of our star. And they’ll be the last stars to shine when the Universe turns cold and dark.
Although they are far more common in the Universe than Sun-like stars, their low luminosity means they are still poorly understood. In particular, very little is known about their planetary population, which represents a significant fraction of the planets in our Milky Way.
Against this backdrop, the SPECULOOS project - led by the University of Liège and involving researchers from Belgium, France, Switzerland, the UK and the USA - has uncovered a new Earth-sized planet orbiting an ultra-cool dwarf star : SPECULOOS-3.
| The SPECULOOS project The SPECULOOS project (Search for Planets EClipsing ULtra-cOOl Stars), initiated and led by astronomer Michael Gillon of the University of Liège, has been specially designed to search for exoplanets around the nearest ultra-cool dwarf stars. These stars are scattered across the sky, so they need to be observed one by one, over a period of weeks, to have a good chance of detecting transiting planets. This requires a dedicated network of professional robotic telescopes. This is the concept behind SPECULOOS, jointly run by the Universities of Liège, Cambridge, Birmingham and Berne, MIT and ETH Zürich. |
This is only the second planetary system discovered around this type of star, after the famous TRAPPIST-1 system in 2017, which boasts seven Earth-sized planets, including several potentially habitable ones.
The SPECULOOS-3b exoplanet lies around 55 light-years from Earth (which is very close on a cosmic scale, with our galaxy stretching over 100,000 light-years). Its size is practically the same as that of our planet.
Singularity
On SPECULOOS-3b, a year - the time it takes for the planet to orbit its star - lasts around 17 hours. And the days and nights may never end... The planet is thought to rotate synchronously with its star, like the Moon in orbit around the Earth. One side, called the "day side", always faces the star, while the "night side" is locked in endless darkness.
The host star, SPECULOOS-3, is more than twice as cold as our Sun, with an average temperature of around 2,600°C. Thanks to its extremely short orbit, the planet receives far more energy per second than the Earth does from the Sun : almost 16 times more. It is literally bombarded by high-energy radiation. In such an environment, the presence of an atmosphere around the planet is highly unlikely.
However, without an atmosphere, SPECULOOS-3b proves to be an excellent target for the James Webb, the giant space telescope launched in 2021.
"With the James Webb Telescope, we could study the mineralogy of the planet’s surface," explains Elsa Ducrot, a former researcher at the University of Liège, currently a postdoc at Observatoire de Paris-PSL in the Laboratoire d’études spatiales et d’instrumentation en astrophysique.
SPECULOOS-3b would be the first exoplanet to reveal its surface composition, making it a prime target for observations by the James Webb Telescope.
Reference :
Gillon, M., Pedersen, P.P., Rackham, B.V. et al. Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3. Nature Astronomy (2024).
▶ https://doi.org/10.1038/s41550-024-02271-2