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The planet that
shouldn't exist


Posted: August 27, 2009

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A planet ten times the mass of Jupiter found orbiting its star in less than one Earth day could provide a rare glimpse into the final moments of a planet's life.

WASP-18b, so-called because it is the eighteenth planet to be discovered by the Wide Angle Search for Planets (WASP), resides just three stellar radii from its 1.24 solar mass host. Hot jupiter planets like WASP-18b are born far from their parent star and over time spiral inwards to be consumed in the system's central stellar furnace.

WASP-18b is nearing the end of its life, soon to be consumed by its host star. Image: ESA/C.Carreau

For a planet as massive as WASP-18b orbiting so tantalizingly close to its parent star, tidal interactions should destroy the planet well within the next million years. Given that the star itself is around one billion years old, and since planets and stars form together, WASP-18b shares the same age and is now looking death straight in the face. The odds of observing a planet so close to the end of its life, say scientists lead by Coel Hellier of Keele University, is about one in one thousand, meaning that either WASP-18b is in a rare, exceptionally short lived state, or the interaction between star and planet in this system, and perhaps other hot-jupiter systems, is much weaker than in our own Solar System.

The theory of tidal interactions for hot jupiters in close orbits predicts that the tidal bulge on the star raised by the planet drains angular momentum from the planet, causing it to spiral inwards. This is a common outcome when a planet's orbit is shorter than the star's rotation, contrasting with the Earth-Moon system where the longer orbit of the Moon compared to the Earth's spin causes it to recede from the Earth. Hellier's team suggest that in this case, the star may be poor at dissipating tidal energy, which would increase the planet's lifetime, or that interactions with a second planet in the system in the past may have influenced today's state of affairs.

Despite the planet's unknown history, there is some good news. If the orbit of WASP-18b really is decaying rapidly inwards, the effects will become visible within a decade as a detectable change in the time it takes for the planet to transit the star. Eventually the planet will reach the Roche limit – the distance within which the planet's self gravity will be exceeded by the pull of the star – and material will feed onto the star. This act of cosmic cannibalism will spin up the star from a rotation rate of 5.6 days to 0.7 days, giving it a new lease of life as a rapidly rotating star. Heavy elements from the planet's core could contaminate the stellar atmosphere, an effect that has already been seen in the planet-hosting star HD 82943.

Continuous observation of the WASP-18 system, and others, should help reveal the secrets of these curious systems in the not-to-distant future.

The new discovery is published in this week's edition of the journal Nature.