This exoplanet that didn’t want to die

A giant exoplanet orbits a long-exploded star. This is the first time that a planet that has survived this type of event has been observed, but many more could exist.

In a few billion years, our Sun will transform into a red giant, grow bigger and bigger before eventually exploding, leaving behind only a white dwarf. What will remain of the Earth and the other planets in the solar system? A priori, not much, even if the doubt persists.

A study published in Nature October 13, 2021 claims to have found an exoplanet (dubbed MOA2010BLG477Lb) that survived a similar event and still orbits its stripped star. The authors are interested in a signal detected in 2010 and discovered that it was a system containing a white dwarf and a Jupiter-like planet.

« The signal was first spotted using the gravitational microlens technique, explains one of the authors Camilla Danielski, from the Institute of Astrophysics of Andalusia in Granada. But we have added another method based on direct infrared observation. That’s how we were able to find this couple. The astrophysicist is linked to the Institut d’astrophysique de Paris, many of whose members co-signed the study.

“We are sure of ourselves”

Previous observations confirmed that this configuration was theoretically possible, but this is the first time that a planet has been observed with certainty around a white dwarf. Last year, another study had appeared on the same subject, but the authors acknowledged that there was uncertainty, as the planet was 14 times the mass of Jupiter. In other words, it was very possible that it was not a planet, but a small star like a brown dwarf. ” here, assure Camilla Danielski, we know the planet is roughly the mass of Jupiter, so we are sure of ourselves. »

In addition, the authors are convinced, the planet was there before the transformation of the star into a white dwarf, because there is no binary system. Models of the formation of planets would have great difficulty in explaining how a new world can form around a white dwarf.

As far as the star is concerned, additional checks have been made, but in all cases, it is impossible to deal with a star in its main sequence, before its transformation into a red giant. This one is far from being sufficiently brilliant. It is therefore neither a brown dwarf, nor a neutron star, nor a black hole. By elimination, it is indeed a white dwarf!

A hypothetical system, with a star comparable to the Sun and a white dwarf. // Source: Flickr/CC/Kevin Gill

There remains an uncertainty around the distance between the two stars. For comparison, Jupiter and the Sun are 5.6 astronomical units apart (one AU corresponds to the Earth-Sun distance). In this system, observers can only say that the distance is greater than 2.8 AU. Camilla Danielski specifies: “ That doesn’t tell us anything about the distance between the two during the main sequence. The planet may have migrated after the transformation into a white dwarf, but it is also unclear when this occurred. »

How to survive the Apocalypse

So how can a planet survive such events? Hard to say, but some studies suggest that half of white dwarfs orbit planets that have survived their evolution. To put it simply, it depends on the mass of the star, that of the planet and the distance between the two. In 2007, a study suggested that this was possible, as long as the planet was outside the star as it expanded and transformed into a red giant. A step not necessarily easy since, in the case of our Sun, the diameter would be multiplied by approximately 200, which would include Mercury, Venus and probably Earth. Except in the case where the release of its energy would push back the orbits of the planets.

Currently, it is difficult to find out more, as white dwarfs are much smaller and less bright than stars in their main sequence. They are also much more difficult to detect, especially if it is necessary to find stars which orbit around. ” We know that 97% of the stars in our galaxy will turn into a white dwarf, assure Camilla Danielski. But we are currently limited by the means of detection. »

But that could change in the near future. The Vera C. Rubin Observatory will be operational in 2023 if all goes according to plan. Its very sensitive detectors will be able to pick up faint objects and we can expect an increase in the number of discoveries of white dwarfs. The Nancy-Grace-Roman space telescope, whose launch is scheduled for 2025, will work in the infrared and should also bring beautiful stones to the building.

The Vera-C.-Rubin observatory under construction. // Source: Wikimedia / CC / Frossie (cropped photo)

For Camilla Danielski, there are many possible advances, but we have to change the methods: “ Our study was made possible because we crossed two detection techniques. This is something that must become a habit in astronomy to hope to learn more about the Universe. “The researcher hopes, thanks to these advances, to have a better knowledge of all the life of the stars:” We have long been interested in the stars of their main sequence. But technologies will allow us to know the younger stars, the oldest … But also the planets that are around. What will teach us about the evolution of all these stars. »

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