European Space Agency
Bon Voyage, WASP 18b!
Astronomers say they have detected a planet orbiting another star that appears to be on the verge of plunging into its sun (within the next million years).
The amount of time it has left in the land of the orbiting is far shorter than for any known planet, according to University of Maryland astronomer Douglas Hamilton. If the team reporting the discovery is correct about the planet's future, astronomers should begin to see evidence of a long, very lazy plunge into the star within the next decade, he estimates.
The planet -- WASP-18b -- has 10 times Jupiter's mass, squeezed into an orb about Jupiter's size. The planet is orbiting its star at a distance of just under 2 million miles (Earth is 93 million miles from the sun). That means it's orbiting at an enormous pace: It makes one swing around the star, WASP-18, every 22.6 hours. Surface temperature? Don't even ask! But if you did anyway, the team would tell you it's about 3,800 degrees Fahrenheit. Steel melts at around 2,500 degrees F.
As for the star, WASP-18, it is 326 light years away. It has roughly the same mass and diameter as the sun. But at 630 million years old, it's a young turk. The sun is 4.6 billion years old.
The planet was discovered by an international team of scientists led by Coel Hellier, an astrophysicist at Keele University in Britain. The members are part of the Wide Angle Survey for Planets, an effort that is hunting for planets orbiting other stars using the transit approach. Their network of telescopes looks for the changes in a star's light as a planet passes in front of it, as viewed from Earth. A formal report of their discovery appears in the current issue of the journal Nature.
Finding a planet like this, apparently on the verge of bidding the cosmos adieu, is rare, according to Dr. Hamilton. Given what scientists have learned so far about how the gravity between two closely orbiting objects interacts, and the million-year life expectancy of WASP-18b, the odds, of finding a planet at this stage of its evolution are about 1 in 1,000, he explains.
So either the team was really lucky, or something more scientific (in some cases scientifically unsettling) is going on.
Among the possibilities Hamilton identifies:
1. Perhaps the star is particularly poor at dissipating the tidal energy that builds as the planet orbits the star. In an email exchange, Dr. Hellier explains that as the planet swings around, it triggers tidal bulges on the star. The moon sets up similar bulges in Earth's oceans, giving us tides at the beach. In a star, those bulges set up sound waves that move through the star until they dissipate. If the star turns out to be very inefficient at dissipating that energy, the planet's lifetime would be prolonged.
Based on what scientists have learned about tidal interactions by looking at planets and moons in our solar system, as well as at binary-star systems (where two stars orbit each other), this option "would be a spectacular find," Hamilton writes in a commentary that accompanies the formal report of the discovery in the current issue of the journal Nature.
2. Maybe the star hasn't been slowly winding its way inward from a more distant birthplace, where enough raw material for planets would be available to build an orb as massive as WASP-18b. Maybe it was nudged out of a more distant orbit by a close encounter with another, as-yet undetected planet.
3. Something is keeping the planet in one piece -- some unknown bit of physics related to tidal interaction or to aspects of the star's behavior perhaps. Or it could be a kind of counter-tug from that pesky, undiscovered planet.
Whatever the explanation, this is one system astronomers will want to monitor over the next few decades for evidence of the planet's impending demise.