Published online 5 June 2007 | Nature | doi:10.1038/news070604-4

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Two new planets lack heavy foundations

A pair of Jupiter-sized planets has been put together from a surprisingly light mixture.

Can you make a Jupiter without plenty of heavy elements? Seems so.Can you make a Jupiter without plenty of heavy elements? Seems so.NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Astronomers have discovered a star that has managed to build two giant planets despite a dearth of the ingredients often thought to be needed for the job.

The star HD 155358, 140 light years away in the constellation of Hercules, is in some ways similar to the Sun — just a bit hotter and smaller. But one thing is very different: it is very short of the heavy elements — such as oxygen, silicon, carbon, iron and the like — out of which most planets and their cores are made.

HD 155358's comparative lack of elements heavier than primordial hydrogen and helium (which astronomers rather eccentrically lump together as 'metals') means that, according to some theories, planets would have been hard put to form out of the collapsing cloud of gas that gave the star birth.

But William Cochran and his co-workers, working at the McDonald Observatory at the University of Texas, Austin, have discovered two planets in orbit around it1.

Both are fairly big; one is about 90% the mass of Jupiter, the other about half that size. Intriguingly, they are close enough together that they feel each other's gravitational pull quite markedly with one planet's orbit becoming more and more circular while the other becomes elongated until a limit is reached and the roles are reversed. This pattern repeats every 2,500 years or so; Cochran's colleague Michael Endl compares it to a dance.

Getting to the core

Stars this poor in metals that nevertheless harbour planets are extremely rare. Among the more than 100 stars with planets so far identified there is only one other example, a giant star called HD 47536. Such metal-poor stars with planets could provide important clues about how planets form.

There are two leading theories for this. In the first, a dense core of heavy elements condenses out of the disk of gas and dust that surrounds a new star, and this core then gathers up gas from the disk.

The alternative to this 'core accretion' model is a process in which the disk itself breaks up into clumps that gradually collapse into planets. This process can happen very quickly, and it could in principle produce planets with no solid core at all.

Core accretion is generally the favoured theory for the formation of the small, Earth-like planets in our own Solar System, but some researchers think that disk instability might account for our gas-giant neighbours, such as Jupiter.

Cochran and his colleagues say that either of these planet-formation models might work for the planets around HD 155358, despite its scarcity of heavy elements.

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However, they say that this is only true if the disk around the star contained a lot more material than the one from which our Solar System emerged. Looked at in this way, "the HD 155358 system is an indication that disk masses can vary significantly", says Endl.

However, Alan Boss, a champion of disk-instability models who works at the Carnegie Institution of Washington, in DC, thinks disk-instability looks to be a more likely formation mechanism for the HD 155358 planets, as it would work in a metal-poor disk even if the disk were barely any larger than that from which our Solar System formed: "Disk instability does not need a more massive disk to make these guys," he says. "I think these two new planets present a major challenge for the core-accretion model."

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  • References

    1. Cochran W. D., Endl M., Wittenmyer R. A. & Bean J. L. Astrophys. J. (in the press).