In July, Maciej Konacki, then at the California Institute of Technology in Pasadena and now at the Nicolaus Copernicus Astronomical Center in Torun, Poland, reported finding a planet inside a triple star system called HD 188753, about 150 light years from Earth.
The planet is a "hot Jupiter" - a gas giant that orbits much closer to its parent star than Jupiter does. It circles its sun-like star every 3.35 days. Farther out in the same system is a pair of smaller stars. The alien world defied all explanation because planets like this should not be able to form in a triple star system.
This is because astronomers believe a typical hot Jupiter forms in the cool outer regions of a dusty disc around its parent star and then migrates inwards due to drag from the disc. According to this model, Konacki's hot Jupiter should have formed at least three times the Earth-sun distance from the host star, or 3 astronomical units (AU). But the gravity of the binary pair farther out should have caused the planet-forming disc to shrink so that it was contained within 1.5 AU of the main star, leaving little material to form such a large planet or drag it inwards.
Now Simon Portegies Zwart of the University of Amsterdam in the Netherlands and Steve McMillan of Drexel University in Philadelphia, Pennsylvania, think they have solved the riddle. They point out that like most stars, HD 188753 probably formed inside an "open cluster" of several hundred stars. They argue that it is possible the planet first formed around the host star and then the star drew it into a tight orbit. Subsequently, a close encounter in the cluster hooked them up with the binary system.
To calculate the chances of the main star capturing the binary pair, they modelled stars zooming around within typical open clusters. Their results suggest that, on average, each cluster should contribute five or six systems like HD 188753 to our galaxy. They estimate that among the 10 million single, double and triple star systems that lie within 1600 light years of Earth, there should be 1200 triple stars with a planet - rare, but not impossible to find (Astrophysical Journal Letters, vol 633, p 141).
"Nature is always more crazy than we are; it invents things that we cannot envision at all," says Portegies Zwart. "Only after we see them and think very deeply can we come to the solution. But this is an easy solution - there's no magic involved. I'm sure we will find more of these systems in the next few years."
If many more are discovered, their orbital configurations could act as a "fossil record" of star clusters that wandered the Milky Way billions of years ago, revealing the speeds and the distribution of stars inside.
Author: Hazel Muir.
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