LITTLE ROCK — Scientists have discovered a planet that shouldn't exist. The finding, they say, could alter our understanding of orbital dynamics, a field considered pretty well settled since the time of astronomer Johannes Kepler 400 years ago.
The planet is known as a "hot Jupiter," a gas giant orbiting the star Wasp-18, located about 330 light-years away from Earth. The planet, Wasp-18b, is so close to the star that it completes a full orbit (its "year") in less than an Earth day, according to the research, which was published in the journal Nature.
Of the more than 370 exoplanets - planets orbiting stars other than our sun - discovered so far, this is just the second with such a close orbit.
The problem is that a planet that close should be consumed by its parent star in less than 1 million years, say the authors at Keele University in England. The star Wasp-18 is believed to be about a billion years old, and since stars and the planets around themare thought to form at the same time, Wasp-18b should have been reduced to cinders ages ago.
"This planet should spiral inwards on such a short time scale that the likelihood of seeing it is very low," said Coel Hellier, an astrophysicist at Keele University.
"That's a paradox," added Douglas P. Hamilton, an astronomer at the University of Maryland, who wrote a commentary accompanying the report. He said there are a variety of explanations, but none is satisfactory.
One possibility is that Wasp-18, which is a sunlike, medium-size star, is 1,000 times less energetic than expected. That would mean it would produce much less friction on the planet than would normally occur.
This orbital drag, which scientists call the "tidal dissipation factor," slows the planet each time it circles the star. Eventually, the planet no longer has enough energy to maintain its position and falls into the star.
But if the star's energy is 1,000 times less than expected, that would be a big surprise, Hamilton said. It would imply thatscience doesn't understand the composition and characteristics of sunlike stars as well as has been thought.
A second possibility is that the planet hasn't been in its current position very long, Hellier said. Wasp-18b could have started farther away and spiraled inward to its current position over millions of years. One way for this to happen is if the planet got bumped out of its orbit by another planet.
"However, that does not solve the problem," Hellier said, because the planet's lifetime should still be very short; it would be very unlikely for his team to have found it where they did.
In our solar system, the closest example of a similar mystery is Mars' moon, Phobos. It orbits Mars at a distance of only about 5,600 miles, 40 times closer than our moon is to the Earth. That orbit should cause it to crash into Mars in just 30 million years, a fraction of the 4.5-billion-year age of the solar system.
"Perhaps we really are missing some key bit of physics," Hamilton wrote in his commentary.
An answer could be coming in just a few years. According to Hellier, if the orbit of Wasp-18b really is decaying at the expected rate, the effects should be measurable within the next decade.
Front Section, Pages 7 on 08/30/2009