Because of the methods we use to identify extrasolar planets that orbit distant stars, our collection of known planets is biased towards what have been termed hot Jupiters, heavy planets orbiting close to their host stars. Yesterday, scientists announced the newest member of this class of planets, WASP-18b, and it's a monster: 10 times the mass of Jupiter, with an orbital period of less than a day. It's actually so close, that it will likely run into its host star within a fairly short period of time (at least in astronomical terms).

There's a small problem here, however. The star it's orbiting is roughly a billion years old, and the orbital decay should take WASP-18b into its surface in a fraction of a percent of that time. This means that we either got very lucky and caught the planet right in its death spiral, or the models we use to calculate its orbital decay are badly off.

First, some details about the system. It was discovered by the Wide Angle Search for Planets program, based in the Canary Islands. The host star, WASP-18, is only a fraction heavier than our Sun, but it appears to be substantially younger, somewhere between 500 million and 1.5 billion years old. That alone makes it a bit exceptional, as it's one of the youngest planet-hosting stars we've discovered.

The planet, WASP-18b, is big, heavy, and orbiting closely enough that it should be subjected to some significant tidal forces thanks to the gravity of the nearby star. The frictional resistance to these tidal forces should dissipate energy, slowing the planet and causing it to fall closer to the surface of the star. Based on estimates of the system's tidal dissipation factor, Q, we can estimate how long the planet has before it merges with its host.

Based on the solar system and binary stars, we've estimated that Q should be somewhere between 105 and 106. But, if you plug the higher value into the equations that give an estimate of how long the planet should survive, you get 650,000 years, a small fraction of the total life of the star. So, we've either gotten very lucky to catch things at this point in time, or we are way off when it comes to the value of Q.

There are a few reasons to opt for the lucky choice. These large planets don't actually form where we spot them—the host star ensures there's not enough material there to produce something of this size—but rather are pushed there from further out by interactions with other planets. So, given that we've found over 300 planetary systems, and our discoveries are biased towards hot Jupiters, maybe we were due to find one about to crash into the star.

Of course, that assumes that the 300-plus systems we've seen all have multiple large planets arranged such that one or more of them is launched into the inner reaches of the systems, which is hardly a safe bet. But the best alternative option, a value of Q that brings the orbital decay in line with the lifespan of its host star, would mean our measurements are off by three orders of magnitude.

In any case, the authors figure that the planet is now close enough that, were our values for Q reasonably accurate, we'll be able to see an orbital decay within a decade. If it does eventually crash into the star, they paint a compelling picture of its death. Parts of its atmosphere would be evaporated off, and another portion of its mass would be stripped off and drawn into the host star via the Lagrange point. The remains of the planet would spin up so that it rotated in under a day.

Because the planet probably carries some heavier elements, it's possible that they'll wind up smeared over the surface of the host star during this process. The authors point out that an earlier paper had identified a star, HD82943, that is host to a planet and seems to have an excess of a heavy isotope of lithium in its atmosphere, suggesting it obtained it by swallowing a planet.

Nature, 2009. DOI: 10.1038/nature08245

Listing image by The WASP cameras.