"Big things come in small packages," the saying goes, and there's no better example than an article by Coel Hellier (Keene University) and others in August 27th's Nature. Only 750 words long, this spare missive is already turning the study of massive, close-in exoplanets — known as "hot Jupiters" — on its head.
Hellier's team describes the scrutiny of a star designated WASP-18 and the hefty planet (WASP-18b) that circles it. The planet's existence came to light in 2006 during the United Kingdom's Wide Angle Search for Planets program.
WASP-18b orbits incredibly close to its host star, circling just 1.4 million miles (2.2 million km) from its surface. From such a ringside seat, the planet's star-facing hemisphere broils at an estimated 3,800°F (2,400K).
What has Hellier's team scratching its collective head is not that WASP-18b is so hot (it's too dense to simply evaporate), but instead why it exists at all!
Here's the conundrum: With roughly 10 times the mass of Jupiter, it's hefty enough and close enough to raise a significant tidal bulge on the star. And because it orbits in just 22½ hours, faster than any other confirmed hot Jupiter and much faster than the star spins, this tidal interaction should be robbing WASP-18b of angular momentum. In all likelihood, the planet is being dragged inward toward its doom — and fast.
The Earth-Moon system is a good example of such a tidal yin-yang. The Moon likewise raises tides in Earth's oceans, and their mutual attraction is also transferring angular momentum between them. However, because the Moon orbits much more slowly than Earth turns, the tradeoff is gradually pushing the Moon away from us (1½ inches, or 38 mm, per year) and our day is ever-so-slightly getting longer (0.2 second per century). But in the WASP-18 tidal dance, it's the other way around — the planet is spiraling inward, and the star's spin is speeding up.
So why hasn't WASP-18b already gone "poof"? Hellier's team figures that the solar-type star (spectral type F6) is about a billion years old. Yet tidal theory argues that the big planet will edge close enough to be torn apart in well under a million years. As theorist Douglas Hamilton notes in an accompanying perspective, the odds of finding WASP-18b just before its swan song are akin to "drawing two consecutive red aces from a well-shuffled deck of cards."
Conceivably, observers were indeed very lucky to have discovered WASP-18b when they did. But if that's the case, then lots of other hot Jupiters, like one dubbed OGLE-TR-56b, should likewise be wrapping up their death marches — and they're not. Perhaps there's an undiscovered second planet in the mix whose gravitational tugs are preventing WASP-18b's demise. More likely, something's amiss with our understanding of how tidal energy dissipates within the star's interior. If it's a far more frictionless process than believed — say, by a factor of 1,000 — then the planet can hang in there for another billion years.
Fortunately, observers can answer these questions relatively soon. If WASP-18b is quickly spiraling to its doom, then a decade from now its orbital period will be 28 seconds shorter — easily detectable by timing its transits. And if it stays put, then theorists will have to retool their ideas about the inner workings of stars.
Either way, the Hellier team's 750-word article is making some big waves in the extrasolar-planet community.