Astronomers have discovered a giant planet that, they say, should not exist based on existing theories.
The Jupiter-like planet, considered odd in size because it is larger than its star, contradicts theories related to the process of planet formation.
The star, 284 trillion kilometers away, is an M-type red dwarf – a very common type in our galaxy.
An international team of astronomers has reported the findings in the journal Science.
“It’s interesting because we’ve been wondering for a long time whether giant planets like Jupiter and Saturn could form around such a small star,” said Professor Peter Wheatley, from the University of Warwick, UK, who was not involved in the new study.
“I think the general impression is that these planets don’t exist, but we can’t be sure because small stars are so weak they’re difficult to study, even though they’re actually much more common than stars like the Sun.”
The researchers used telescopes in Spain and the US to track the increase in the star’s gravity that might cause the planets to orbit it.
The red dwarf star has a greater mass than the planet that orbits it – named GJ 3512b. But the difference in size is much smaller than in, for example, the Sun and Jupiter.
The distant star has a mass 270 times greater than the planet. While the Sun is 1,050 times denser than Jupiter.
Astronomers use computer simulations to explain their theories about how planets form from the clouds, or disks, of gas and dust that orbit young stars.
This simulation predicts many small planets clustered around M-type dwarf stars.
“Around such a star there should be only Earth-sized planets or denser Super-Earths,” said study co-author Christoph Mordasini, professor at the University of Bern, Switzerland.
One example that actually occurs from a planetary system that fits the theory is one orbiting a star known as Trappist-1.
The star, 367 trillion km (38 light km) from the Sun, has seven planets, all with a mass equal to or slightly less than Earth.
“But GJ 3512b is a giant planet with a mass about half that of Jupiter’s, and thus means, at least, one unit of magnitude larger than the planets predicted by theoretical models for such a small star,” said Professor Mordasini.
This finding calls into question the general idea of planet formation known as core accretion.
“Usually we would think of a giant planet starting out as an ice-core, orbiting away from the gas disk around a young star, and then growing rapidly by pulling gas toward it,” Wheatley said.
“But the authors of the report say the disk around a small star does not provide enough material for this process to occur. They then estimate it is more likely for a planet to suddenly form, when part of the disk collapses under its own gravity.”
The authors of the paper in the journal Science suggest that collapses can occur when a disk of gas and dust has a mass of more than about one-tenth the mass of the host star. Under these circumstances the gravitational influence of the star is not strong enough to stabilize the disk.
Part of the disk moves inward, forming a mass formed by gravity, which then develops into a planet. This thinking suggests that these collapses occurred far from the star, while planets could have formed by accreting nuclei much closer.
Professor Wheatley was co-author of a 2017 study on the gas giant NGTS-1b, which the British telescope discovered in Chile. NGTS-1b is also very large compared to its parent star – another M-type red dwarf 600 light-years away.
“The parent star, NGTS-1, is small in size, but not as small as this new example, GJ 3512. This is likely because NGTS-1 represents the smallest star that can form adjacent planets via core-growth, and smaller stars can only form planets. giants are far from the authors’ supported gravitational collapse model,” Wheatley said.
“Forecasts like this are invaluable in determining future searches, as they allow us to test these various models.”
Researchers writing a report in the journal Science do suggest that GJ 3521b should have moved a great distance, so that it arrived at its current position under 1 astronomical unit or 150 million km.
With an oval orbit of 204 days around the star, GJ 3521b spends most of its time closer to its red dwarf than Mercury is to the Sun.
The report’s co-author Hubert Klahr of the Max Planck Institute for Astronomy in Heidelberg, Germany said: “Until now, planets whose formation corresponded to disk instability were only a small number of very large, hot and young planets, which were far from their parent stars.
“Through GJ 3521b, we now have an extraordinary candidate planet that could arise from the instability of the disk around a very small star. This finding prompts us to re-examine existing models.”
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