Extrasolar Hot Jupiter: The Planet That 'Shouldn’t Exist'

The discovery, reported in this week’s Nature by Coel Hellier, of Keele University in the UK, and colleagues, poses a challenge to our understanding of tidal interactions in planetary systems.

The planet, called WASP-18b, belongs to a now-common class of extrasolar planets known as ‘hot Jupiters’ — massive planets that are thought to have formed far from their host stars, and migrated inwards over time. WASP-18b is so massive, and so close to its star — only about three stellar radii away — that tidal interactions between star and planet should have caused the planet to spiral inwards to its destruction in less than a million years.

Yet, as Hellier and colleagues show, the WASP-18 parent star is about a billion years old — making the likelihood of observing WASP-18b about one in a thousand.

How can this unlikely discovery be explained? One possibility is that the tidal dissipation in the WASP-18 system is a thousandtimes less than in our Solar System; this and other possible explanations are discussed by Douglas Hamilton in an accompanying News and Views article.

But if the planet’s remaining life is as short as predicted, the orbital decay should be measurable within a decade.