Were astronomers just lucky when they discovered the planet WASP-18b?
At first impression, the planet, described in the current issue of the journal Nature, fits a familiar profile for planets that have been discovered around other stars: big (about 10 times the mass of Jupiter), close to the parent star (about 1.9 million miles away, or just one-fiftieth of the distance between the Sun and Earth) and hot (3,800 degrees Fahrenheit). About one-quarter of the nearly 400 planets discovered so far have been such “hot Jupiters.”
But as an international team of astronomers looked more closely, they became more surprised that they had seen WASP-18b at all. The tidal forces between a star and a planet dissipate energy, and WASP-18b is so close that it should fall into its host star in less than a million years — an eye blink on the cosmic scale. (Andrew Collier Cameron, a professor of astronomy at the University of St. Andrew and a member of the team, noted that with the impending fiery fate of the planet, it seemed appropriate that it was located in the constellation Phoenix.)
The star system is about a billion years old, the astronomers reported, so the chances that they observed WASP-18b on the cusp of oblivion is about 1 in 1,000.
In an accompanying commentary in Nature, Douglas P. Hamilton, a professor of astronomy at the University of Maryland, noted that this was roughly the same unlikely probability as drawing two red aces in a row from a full deck of cards.
“Of those 400 objects, it’s unique,” Hamilton said. “It’s the only planet that’s going to be crashing into its star in one million years.”
But luck is not the only possibility. Ignorance could be another. It might be that astronomers do not understand the dynamics of stellar tides. The rate of energy dissipation depends on how well the star vibrates — ringing like a bell or thunking like a chunk of wood. (If the star is ringing, less energy is dissipated, and WASP-18b would not be falling as quickly.) This difficult-to-measure quantity, which depends on turbulence inside the star, is not known for individual stars, not even for the Sun.
The answer does not have to wait a million years. In fact, astronomers just have to wait 5 to 10 years. WASP-18b already whips around the star every 22 hours, 35 minutes, 41.5 seconds — a year in less than an Earth day. If it is falling inward as fast as predicted, its day will shorten noticeably in the coming years.