Two new planets lack heavy foundations

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.

Star birth

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 it.

Planet's orbit

Both are fairly big; one is about 90 per cent of 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.

Dance like

This pattern repeats every 2,500 years or so; Cochran's colleague Michael Endl compares it to a dance. Stars this poor in metals that nevertheless harbour planets are extremely rare. 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.

Disk masses

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, 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.


Nature News Service

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