Young stars with a mass greater than twice the Sun generate extreme radiation that makes it very difficult for planets to merge due to dust and gas. Yet a handful of exoplanets more massive than Jupiter and on orbits farther than Pluto have been discovered around these stars O and B. New research published in Royal Astronomical Society Monthly Notices suggests that these planets may have originally formed around smaller nearby stars and were later stolen from more massive ones.
“Essentially, this is a planetary robbery,” 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 prove the theft or capture of these [planets] it occurs on average once in the first 10 million years of the evolution of a star-forming region.
Thieves of the planet
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 lumps of planetary material to collapse inward and accumulate more material. However, extreme stellar radiation such as that produced by stars O and B (at least double but sometimes more than 20 or 50 times the mass of the Sun) can interrupt accretion through a process called photoevaporation.
“It’s hard to find planets around stars O and B, which are bright stars,” said lead author Richard Parker, an astrophysicist at the University of Sheffield in the UK. “There is nothing at first sight to stop O [and] B stars forming planets … However, intense far ultraviolet and extreme ultraviolet radiation is powerful enough to evaporate gas from protoplanetary disks, and if there is no gas, then it is impossible to form planets of Jupiter’s mass in the disk .
This does not mean that massive stars do not host exoplanets. A recent investigation of a young star group, the Scorpius-Centaurus (Sco-Cen) association, which contains dozens of O and B stars, 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 planets formed elsewhere and later found their way into orbits around massive stars. They conducted computer simulations that modeled a cluster of young stars, some of which were given simulated exoplanets, and charted the fates 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 tore the planet out of its orbit. Either the most massive star would immediately steal the planet for itself, or the planet would be allowed to float freely within the group; that planet may later be captured by a different massive star. After repeating the simulation several times, the researchers found that on average stars O and B would steal or capture about one exoplanet every 10 million years from a star of lower mass.
Form planetary systems from the outside
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. The 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 by the stars that raised them. (Sco-Cen also contains a large hoard of floating or rogue planets, which could possibly be captured.)
“Their scenario seems entirely plausible,” commented Sean Raymond, an astronomer from the Laboratoire d’Astrophysique de Bordeaux in France who was not involved in the study. “It’s new, interesting and carefully simulated. I just think it’s really cool.
The researchers recognized that due to computational limitations, their simulations did not include binary stars, which are very common in young star groups. With higher 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 could be stolen or captured than these simulations suggest. The team plans to include binary stars in future simulations.
“I think the role of star clusters on planetary systems is really underestimated,” Raymond said. “This work shows that instead of looking inside a planetary system, we should sometimes think about what happened on the out. “
– Kimberly MS Cartier (@AstroKimCartier), Staff Writer