Paris (AFP)
A disk of gas and dust surrounding a planet outside our solar system: this observation by astronomers, the first unequivocal, will help verify the theories on the formation of planets and moons.
It is the culmination of a quest started in 2018 with the discovery of PDS 70b, a young planet in formation close to the star PDS 70. Its system is very close to ours, "only" 370 light years away, in the constellation Centauri.
The following year, astronomers detected there for the first time a disk of gas and dust encircling a second planet, PDS 70c, discovered with the Very Large Telescope (VLT) of the European Southern Observatory (ESO).
By combining these observations with those they made with the ALMA radio telescope, scientists then assume that the disk of matter allows moons to form around the planet PDS 70c.
Today, new observations made with ALMA "present the clear detection of a disc in which satellites could form", says Myriam Benisty, astronomer at the University of Grenoble and main author of a study on the subject published. Thursday, in The Astrophysical Journal Letter.
Astronomers knew since 2006 that the star PDS 70 was surrounded by a very large disc of matter, but the limits of their instruments just let them suppose the presence of at least one planet between the star and the disc.
- Two exoplanets in formation -
The two planets which were discovered there are of great interest because they belong to a juvenile star system.
Their star, PDS 70, is only about 5.4 million years old.
By comparison, our Sun is a great old man, with more than 4.6 billion years on the clock.
"More than 4,000 exoplanets (planets existing outside our solar system) have been identified to date, but they have all been detected in mature systems," says astronomer Miriam Keppler, researcher at the Institute of Astronomy Max Planck, co-author of the study.
She was the one who discovered PDS 70b in 2018.
With its companion PDS 70c, these two planets "are the only two exoplanets detected to date which are still at the stage of their formation", she explains in the ESO press release.
These gas giants, like Jupiter, have a mass at least equal to the latter.
The disc of matter that surrounds PDS 70c contains enough of it to form the equivalent of three of our Moon.
Jupiter, which is much older, has four moons, and dozens of smaller satellites.
These new observations "are very important to prove the theories on the formation of the planets which one could not verify until now", underlines the astronomer Jaehan Bae, of the Laboratory for the Earth and the Planets of Carnegie, quoted in the ESO press release.
Planets form in the disks of dust and gas that surround young stars.
They can then be themselves surrounded by a disk of dust and gas which feeds their formation.
And it is in this disc that the moons orbiting giant gas planets can be formed.
But it will still take many observations, and more powerful instruments, to understand exactly the process at work.
© 2021 AFP