Paris (AFP)
The Cheops space telescope has unveiled the cosmic ballet of five exoplanets, a phenomenon never before observed with stars whose variety calls into question the theories on their formation, according to a study.
About 200 light years from Earth, these exoplanets - so called because they are outside our solar system - revolve on the same plane around their star, TOI-178, with the regularity of a clock .
They follow a "resonance chain" movement, identified by the astronomer Laplace at the end of the 18th century with three of Jupiter's moons.
When the most distant, Ganymede, goes around the planet, it takes two at Europa, closer, and four at Io, even closer to Jupiter.
The same celestial mechanics are at work with TOI-178.
The first planet in the chain circles the star in about three days, and subsequent ones, each more distant than the previous one, do so in 6, 10, 15 and 20 days, respectively, aligning regularly for some 'between them.
One, in music.
A sixth exoplanet, the closest to the star, orbiting too quickly to resonate with the following ones.
"We only know of five other systems with such a configuration", in which several exoplanets agree in an orbital harmony around their star, remarks to AFP Adrien Leleu, astronomer at the Observatory of the University of Geneva , and lead author of the study published this week in Astronomy and Astrophysics.
- "Frozen" -
Their interest is to be the witness of "a state which one thinks quasi original", as explained the astrophysicist at the University of Bern Yannick Alibert, co-author of the article.
It evokes "systems as frozen since the end of their phase of formation, which lasts millions of years".
Because miraculously, nothing has come to disturb this "rather fragile" balance during the more than two billion years which have ensued until our days.
And this is where things get complicated.
Because normally, the further one moves away from the star, the more the density of the planets should decrease.
However, the density of the third in the chain, in particular, is much higher than that of the second.
"We have a great variation, which is surprising", according to Mr. Leleu.
"On the one hand we have the impression of having something that has not moved for billions of years, the periods of the planets are set to within a few minutes, it is ordered, and on the other hand there are these variations in density ", which challenge commonly accepted theories on the formation of planetary systems.
Observations will continue thanks to Cheops, the telescope of the European Space Agency (ESA), in orbit since the end of 2019, and dedicated to the characterization of exoplanets.
While waiting to use one day the future successor of Hubble, the James Webb space telescope, which will be launched this year.
- "Rosetta stone" -
"The TOI-178 system has the advantage of being bright enough and some of its planets large enough to precisely study their atmospheres from Earth and space," Head of State Prof. Kate Isaak told AFP. scientist from the Cheops mission at ESA, who co-signed the study.
With a better understanding of the formation of planetary systems, and possibly the detection of exoplanets orbiting TOI-178 at a greater distance.
"We want to find out whether there are planets beyond, which would be in the habitable zone, beyond the sixth" exoplanet, "says Prof. Leleu.
And in the meantime, explore this system called by astronomers the "Rosetta Stone", providing as many answers as questions on the formation of the planets.
© 2021 AFP