The giant planet around a small star "should not exist"

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University of Bern

Astronomers have discovered a giant planet that, according to current theories, should not exist.

The Jupiter-like world is unusually large compared to its host star, which contradicts a widely held idea about planet formation.

The star, located at 284 billion km, is a red dwarf type M – the most prevalent type in our galaxy.

An international team of astronomers published its findings in the journal Science.

"It's exciting because we have long wondered if giant planets like Jupiter and Saturn can form around so small stars," said Professor Peter Wheatley, of the University of Warwick, in the UK Who did not participate in the last study.

"I think the general impression was that these planets simply did not exist, but we could not be sure because the small stars are very weak, which makes them difficult to study, even if they are much more common than stars like the Sun, "he told BBC News.

The researchers used telescopes in Spain and the United States to track the gravitational accelerations of the star that could be caused by planets orbiting it.

The red dwarf has a larger mass than its orbiting planet – named GJ 3512b. But the size difference is much smaller than between, for example, the Sun and Jupiter.

The distant star has a mass at most 270 times greater than that of the planet. For comparison, the Sun is about 1050 times more massive than Jupiter.

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Calar Alto Observatory

Astronomers use computer simulations to explain their theories about the formation of planets in clouds, or "disks", of gas and dust orbiting young stars. These simulations predict that many small planets should gather around tiny D-type dwarf stars.

"Around such stars, there should be only Earth-sized planets or a little more massive super-lands," said co-author Christoph Mordasini, a professor at the University of Bern, in Swiss.

A concrete example of a planetary system consistent with the theory is that around a star called Trappist-1.

This star, located 369 trillion km (39 light years) from the Sun, hosts a system of seven planets, whose masses are roughly equal to or slightly less than those of the Earth.

"GJ 3512b, however, is a giant planet whose mass is about twice as large as that of Jupiter, and therefore at least an order of magnitude more massive than the planets predicted by theoretical models for such a small star, "said Professor Mordasini.

The discovery calls into question the widespread idea of ​​the formation of the planet called fundamental accretion.

"Generally, we think that the giant planets that start their lives are an icy core, gravitating around a gas disk surrounding the young star, and then developing rapidly by drawing gas on itself," said the Professor Wheatley.

"But the authors argue that the discs surrounding the small stars do not provide enough material for it to work, rather they consider that it is more likely that the planet will have formed suddenly when part of the disc s & rsquo; Is collapsed because of its own gravity. "

The authors of the Science article suggest that such a collapse can occur when the disk of gas and dust has more than one tenth of the mass of the parent star. Under these conditions, the gravitational effect of the star becomes insufficient to maintain the stable disk.

The material of the disc is drawn inward to form a gravitational block that develops over time to become a planet.

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ESO / M Kornmesser / Nick Risinger

Professor Wheatley co-authored a study in 2017 that described a gas giant called NGTS-1b, discovered with British telescopes in Chile. NGTS-1b is also very large compared to the size of its parent star – another red dwarf of type M located 600 light-years away (five quadrillion km).

"The parent star, NGTS-1, is small, but not as small as this new example (GJ 3512.) It is possible that NGTS-1 represents the smallest star that can form nearby planets via the core accumulation, and that smaller stars only form giant planets farther away by the gravitational collapse model advocated by the authors, "said Professor Wheatley.

"This type of prediction is extremely useful for guiding future research and allowing us to test these models."

Indeed, the authors of the study in Science suggest that the GJ 3512b must have migrated a long distance to its current position of less than one astronomical unit (150 million km, 93 million miles).

With its 204-day oval orbit around the star, GJ 3512b spends most of its time closer to its red dwarf parent than that of Mercury to the sun. The eccentric orbit of the gas giant suggests the presence of giant planets farther away that could have distorted its orbit.

Professor Wheatley said the new study would give "a tremendous boost to our understanding of planet formation," adding, "It may be that NGTS-1 represents the smallest star that can form nearby planets through the accumulation of of nuclei, and that the smaller stars form only giant planets farther away according to the gravitational collapse model advocated by the authors.

"This type of prediction is extremely useful for guiding future research and allowing us to test these models."

The co-author Hubert Klahr of the Max Planck Institute for Astronomy in Heidelberg, Germany, said: "Until now, the only planets whose training was compatible with disc instabilities were a handful of young planets hot and very massive, away from their hosts.

"With GJ 3512b, we now have an extraordinary candidate for a planet that could have emerged from the instability of a disk around a star with very little mass." This discovery prompts us to revisit our models. "

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