Caught in the act, a star swallows a planet

They had already seen the premises and consequences of such an event. "What we lacked was to surprise the star at that moment, when you have a planet that suffers such a fate," says Kishalay De, a postdoctoral fellow at the Massachusetts Institute of Technology's Kavli Institute (MIT) and first author of the study published Wednesday in Nature.

By the way, this is what awaits the Earth, but in about 5 billion years, when the Sun, coming to the end of its existence as a yellow dwarf, will swell into a red giant. At best, its size and temperature will turn the blue planet into a big molten rock. At worst, it will disappear body and well.

It all began in May 2020, when Kishalay De observed with a special camera at the Caltech Observatory a star that began to shine a hundred times brighter than usual for about ten days. It is located in the galaxy, some 12,000 light years from Earth.

He then expects, because this is what he is looking for, to observe a binary star system, in which two stars are orbiting each other. The most massive tears the envelope of the lighter, and emits light with each bite.

"It looked like a merger of stars," said the astronomer during a briefing with co-authors of the study, dependent on Harvard-Smithsonian and Caltech, two American research institutes. But the analysis of the light emitted by the star will reveal the presence of clouds of molecules too cold to be the result of a merger of stars.

Above all, the team will determine that the star, "similar to the Sun", released an amount of energy a thousand times lower than expected if it merged with another star. This amount of detected energy corresponds to that of a planet like Jupiter.

A quick end

On a cosmic scale, which can be counted in billions of years, its end was extremely rapid. Especially since it was "very close to the star, it circled it in less than a day," according to Mr. De.

Observations indicate that the planet's envelope was torn apart by the star's gravitational forces for a few months at most, before being absorbed. It was this last phase that produced the luminous glow for about ten days.

"The star ingests an object much cooler than its own surface, by several thousand degrees," Miguel Montargès, an astrophysicist at the LESIA of the Observatoire de Paris-PSL, told AFP: "It's like putting an ice cube in a boiling pan, which is better not to do, because of temperature exchanges."

In this reaction, the star ejected large clouds of gas into interstellar space that then cooled for months turning into dust clouds.

The digestion went smoothly, according to Morgan MacLeod, co-author of the study and astrophysicist at the Harvard-Smithsonian Center for Astrophysics. Scientists saw afterwards "the star slowly contract and return to its previous size," he said.

Astronomers estimate that such an event could occur up to several times a year in our galaxy, which has at least one hundred billion stars and probably at least as many planets.

"It is likely that now that we have observed this event, we will observe many others, and all these events will help us better understand the future of the solar system," Montargès said.

© 2023 AFP