Why Does Exoplanet WASP-18b Exist?

"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-18 and its planet
The "hot Jupiter" known as WASP-18b orbits so close to its star that it should have spiraled to its doom long ago. Once the planet reaches its Roche limit, the star's gravity will start tearing it apart.
S&T: Casey Reed
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."

Broiling exoplanet
An artist's conception of the "hot Jupiter" OGLE-TR-56b, which takes just 1.2 days to orbit its star.
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.

4 thoughts on “Why Does Exoplanet WASP-18b Exist?

  1. Bjarne

    Mathematically Breakthrough
    It is now mathematically proven that the decelerating force that affected the Pioneer probes and the accelerating force that had caused many Fly-by anomalies:
    1.) Both affect the Earth (and the planets) as well, – and with full force.
    2.) Automatically equalize each other (when affecting the planets).
    More > science27. com read the chapter > The Pioneer Anomaly

    This explains the cause of the WASP-18b mystery (and much more)….

  2. Ross

    It is hard to believe that our understanding of how tidal forces work, even in the interiors of stars, could be off by a factor of 1000. Haven’t the effects of tidal interactions of our Sun with the planet Mercury been worked out? Wouldn’t a factor of 1000 inaccuracy here contradict the observed motions of that planet?

  3. Bjarne Lorenzen

    New calculations shows that WASP-18b probably not will be stable in its present orbit, but will be thrown away with from the star, with a velocity 3 times higher than the velocity astronomers today believe it “should” approach the star.

    This certainly will be a big chock, and something we should be able to confirm within 1 to 2 years.

    The idea that also planets are affected of the same accelerating force that space probes (by fly by) support this new theory and make it also possible to understand the cause of the 4 following mysteries:

    1.) Why gas-planets can be found very closed to starts ( with their atmosphere intact)
    2.) Why huge planets can be found more as 15 billion km. from their mother stars.
    3.) How Jupiter’s was created (longer away from the Sun) and what brought it closer to the Sun.
    4.) How water come to earth.

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