The heftier a star is, the more likely it is to host a Jupiter-like planet, suggest observations of stars more massive than the Sun. The new work bolsters the leading theory of how planets form, but raises new mysteries, since massive stars do not appear to host the 鈥渉ot Jupiters鈥� found in tight orbits around many lower-mass stars.
Most of the 236 known extrasolar planets have been found by searching for the gravitational wobbles that the planets induce in their host stars as they orbit. Searches have focused on Sun-like stars, since stars less massive than the Sun are dimmer and more difficult to observe, and higher-mass stars tend to rotate very quickly 鈥� every 12 hours or so 鈥� making it hard to spot the signature of any planets.
But astronomers have recently begun to observe a class of stars about twice as massive as the Sun that have slowed down with age, rotating slowly enough to reveal the presence of planets. Called subgiant, or 鈥渞etired鈥�, A stars, they are about 2 billion years old 鈥� nearly at the end of their lives 鈥� and rotate just once every 100 days or so.
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Six of these stars had been found to host planets in the past, and now John Johnson of the University of California in Berkeley, US, has found four more using data from the Keck Observatory in Hawaii, and the Lick Observatory in California, both in the US. So far, the statistics suggest that 8 to 9% of these ageing, massive stars host planets 鈥� twice the rate for Sun-like stars.
鈥淭his is a very strong indication that stellar mass has an integral role in planet formation,鈥� says Johnson. 鈥淭here鈥檚 probably a treasure trove of planets around these retired A stars.鈥�
Raw material
The reason may be simple. Planets are thought to form from discs of gas and dust surrounding young stars. Since more massive stars are thought to have more massive discs, there should be more raw material from which to build planets around hefty stars.
But not everything about the new discoveries is so easily explained. None of the planets lie close to their host stars 鈥� all are at least 0.8 astronomical units away (1 AU is the distance between Earth and the Sun). 鈥淭his gap is rather striking around A stars,鈥� says Johnson, explaining that many extrasolar planets around smaller stars are much closer in than that.
These tightly orbiting planets 鈥� most of which are gas giants dubbed 鈥渉ot Jupiters鈥� 鈥� are thought to have migrated there from more distant orbits, although theorists do not understand what makes them stop at a particular distance from their stars.
Mysterious process
鈥淭here may be a different migration scenario around high-mass stars,鈥� says Johnson. 鈥淏ut the exact process that causes this 鈥� it鈥檚 very much a mystery.鈥�
Alan Boss, an extrasolar planet theorist at the Carnegie Institution of Washington in Washington, DC, US, admits that theorists cannot explain the observations. 鈥淚 think this is a field being driven by observers,鈥� he says. 鈥淭he score is something like observers: 230, theorists: 0.鈥�
The new work suggests that planets can form around many different types of star: from low-mass red dwarfs to high-mass A stars to pulsars 鈥� the dense stars left behind after some supernova explosions. Planets have been found around every sort of star we鈥檝e looked at, says Boss. 鈥淎ll of the studies are showing us that the frequency of planetary systems is larger than we would have guessed.鈥�
The research was presented on Monday at a meeting of the American Astronomical Society in Honolulu, Hawaii, US.
