Young stars more than twice the mass of the Sun generate extreme radiation that makes it very difficult for planets to fuse dust and gas. And yet a handful of exoplanets more massive than Jupiter and in orbits more distant than that of Pluto have been discovered around these O and B stars. New research published in Royal Astronomical Society Monthly Notices suggests that these planets may have originally formed around nearby smaller stars and then been stolen by more massive ones.
“Essentially, this is a planetary heist,” co-author Emma Daffern-Powell said in a statement. Daffern-Powell is an astronomer at the University of Sheffield in the UK. “We used computer simulations to show that stealing or capturing these [planets] occurs on average once in the first 10 million years of a star forming region’s evolution.
Astronomers theorize that stars like our Sun build planets from a swirling disk of dust and gas. (Many observations support this theory.) Gravitational instabilities in the disk cause small clumps of planetary matter to collapse inward and more matter to accrete. However, extreme stellar radiation such as that produced by O and B stars (at least twice but sometimes more than 20 or 50 times the mass of the Sun) can disrupt accretion through a process called photoevaporation.
“It’s hard to find planets around O and B stars, which are bright stars,” said lead author Richard Parker, an astrophysicist at the University of Sheffield in the UK. “There’s nothing at first sight to stop O [and] B stars form planets… However, intense far-ultraviolet and extreme-ultraviolet radiation is strong enough to evaporate gas from protoplanetary disks, and if there is no gas, then it is impossible to form planets of the mass of Jupiter in the disk.
This does not mean that massive stars do not host exoplanets. A recent survey of a young stellar group, the Scorpius-Centaurus Association (Sco-Cen), which contains dozens of O and B stars, has confirmed at least two exoplanets and a candidate planet all larger than Jupiter around these inhospitable stars.
In trying to explain these planets, Parker and Daffern-Powell explored the likelihood that the planets formed elsewhere and then worked their way into orbits around massive stars. They conducted computer simulations that modeled a cluster of young stars, some of which received simulated exoplanets, and tracked the fate of these exoplanets for 10 million years.
The simulation showed that when a less massive star with an exoplanet got too close to a more massive star, the massive star ripped the planet out of its orbit. Either the more massive star would immediately steal the planet for itself, or the planet would float freely within the group; this planet could later be captured by another massive star. After repeating the simulation several times, the researchers found that, on average, O and B stars steal or capture about one exoplanet every 10 million years from a lower-mass star.
Form planetary systems from the outside
The simulations revealed that captured planets can have orbits ranging in size from 4 times the Earth-Sun distance (astronomical units or AU) to 10,000 AU. Stolen planets were more likely to orbit within 200 AU. Two of the three exoplanets discovered so far in Sco-Cen orbit more than 200 AU from their stars, suggesting they were captured rather than stolen from the stars that grew them. (Sco-Cen also contains a large number of floating or rogue planets, which can eventually be captured.)
“Their scenario seems totally plausible,” commented Sean Raymond, an astronomer at the Laboratoire d’Astrophysique de Bordeaux in France who was not involved in the study. “It’s new and interesting and carefully simulated. I just think it’s really cool.
The researchers acknowledged that due to computational limitations, their simulations did not include binary stars, which are very common in young star clusters. With stronger gravity, two stars are more likely to capture or steal a planet than a star, Parker explained, so it’s possible that more planets around stars O and B will be stolen or captured than these simulations show. suggest. The team plans to include binary stars in future simulations.
“I think the role of star clusters on planetary systems is really undervalued,” Raymond said. “This work shows that instead of looking inside a planetary system, we should sometimes think about what happened on the outside.”
—Kimberly MS Cartier (@AstroKimCartier), personal writer
Quote: Cartier, KMS (2022), Massive stars may commit grand theft planet, Eos, 103, https://doi.org/10.1029/2022EO220462. Posted September 23, 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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