Two astronomers in the UK have shown that some giant planets orbiting far from their host stars have likely been captured from the planetary systems of other stars. Using computer simulations, Richard Parker and Emma Daffern-Powell at the University of Sheffield showed that giant planets recently discovered by the BEAST mission – and dubbed “BEASTies” – were probably ejected from their original systems shortly after their formation, and were then captured by other stars.
The planetary systems so far discovered by astronomers display a remarkable diversity. In systems like TRAPPIST-1, several small, rocky planets can be tightly packed together in orbits close to their host stars. In contrast, Jupiter-sized planets have been discovered in orbits hundreds of astronomical units (au, the distance from Earth to the Sun) from their hosts – often challenging astronomers’ preconceptions about how planetary systems form.
In 2021, the B-star Exoplanet Abundance Study (BEAST) discovered two Jupiter-sized planets orbiting OB-type stars. These are hot stars with masses at least 2.4 times that of the Sun. Current theories suggest that the intense radiation emitted by OB-type stars should have evaporated the discs of planet-forming material that originally surrounded them – precluding the formation of planets. Adding to the mystery of their existence, one of the BEASTies orbits its host at a distance of 556 au, which is well over 10 times greater than the distance between Pluto and the Sun.
Now, Parker and Daffern-Powell have developed an explanation for the formation of the BEASTies. As suggested in previous studies, it should be possible for planets to be exchanged between planetary systems. This could occur after a planet is somehow ejected from its original host star and captured by another star as it wanders through interstellar space. Another possibility is that a planet is stolen as two stars pass close to each other.
Sparsely populated regions
These scenarios seem highly unlikely at first glance, especially because OB stars tend to exist in more sparsely populated regions of the galaxy. However, some astronomers believe that OB stars may have formed in nurseries with far higher stellar densities. This was followed by a period when the stars moved apart rapidly. In such a scenario, exchanges of planets between stars could have occurred far more readily within these dense regions.
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To explore this idea, the Sheffield duo did computer simulations of stellar nurseries to estimate how readily these planetary heists could occur. Their results showed that on average, a capture occurred once within the first 10 million years of the evolution of a dense star-forming region. The simulations also suggest that the BEASTies were more likely to have been captured as free-floating planets than stolen directly, given the shapes and sizes of their orbits.
This discovery strengthens the idea that planets orbiting at distances greater than 100 au from their host stars are no longer occupying the systems where they originally formed. The duo’s results offer important guidance for future observations of the BEAST mission and help to better explain the immense diversity of planetary systems we observe today.
The research is described in Monthly Notices of the Royal Astronomical Society.
