The range of configurations in which planets can form and survive has expanded a little with the announcement of a system containing two stars and at least three giant planets. The find may be somewhat overshadowed by the recent news of a planet circling Proxima Centauri, but nevertheless represents a significant addition to our catalogue of planets around other stars.
In the Astrophysical Journal (pre-print on arXiv), a team led by Dr Johanna Teske of the Carnegie Institution have announced the discovery of planets around two stars with the smallest separation we have yet seen.
HD 133131A and HD 133131B, however, are still not close. These two stars, which are similar in size and brightness to the Sun, orbit each other at a distance of 360 astronomical units, or AU (1 AU is the distance between the Earth to the Sun). Only a handful of non-cometary objects are known at such distances from the Sun, as worlds orbiting one would receive little heat from the other.
Nevertheless, this is still a much closer relationship than previous stellar pairs that were found to have their own planets. The next closest known example is almost three times further apart. At the opposite extreme, there are planets known to orbit both stars at once where the stars are very close indeed.
Dig out an astronomy book from 20 or 30 years ago and there is a fair chance it will question whether planets could even exist in star systems containing more than one star. The gravitational distortion produced by the additional star was thought likely to disrupt planetary formation, or throw any planet that formed out of its orbit.
In recent years, however, many planets have been found in systems with two, and even four, stars. These have all been widely spaced systems, such that the influence of one star on planets orbiting the other would be small. Astronomers are keen to know how close stars can be without planets getting disrupted.
The orbits of the three planets discovered in the HD 133131B. Teske et al/The Astrophysical Journal
Almost certainly there is a midrange where the distance between stars makes planets impossible, but HD 133131A indicates this zone is narrower than might have been feared.
The discovery has other interesting features. For a start, two planets have been identified around HD 133131A and one around HD 133131B, and none of them are in exceptionally close orbits that may be the result of a disruption in planets that formed further out.
Both stars are more than twice the Sun's age and very metal-poor. Metal-rich stars, formed from the remnants of previous generations of supernovae, are far more likely to host planets, making this pair a puzzle.
Binary stars usually form from the same gas cloud and share near identical composition, but HD 133131A has slightly less heavy elements than its companion. The authors raise the possibility that one or more planets fell into HD 133131B, enriching it with metals.
All three planets are gas giants and therefore unsuited to life. The smallest has a minimum of half Jupiter’s mass; both the others are substantially heavier than Jupiter.