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Astrophysicists puzzle over planet that's too close to its sun

Completing an orbit in less than an Earth day, planet Wasp-18b should be burned up, according to accepted theory.

August 27, 2009|John Johnson Jr.

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, about 330 light-years 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 a million years, say the authors at Keele University in Britain. The star Wasp-18 is believed to be about a billion years old, and because stars and the planets around them are 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.

"That's a paradox," said Douglas P. Hamilton, an astronomer at the University of Maryland who wrote a commentary accompanying the report. He said there were a variety of possible explanations, none of them very satisfactory.

"It's like going to the scene of the crime and not finding the weapon," he said. "Something's happened, but a key piece of evidence is missing."

One possibility is that Wasp-18, a sunlike, medium-sized star, is a thousand times less energetic than would be expected. That would mean it produces much less friction on the planet than normal.

This orbital drag, which scientists call the "tidal dissipation factor," slows a planet each time it circles its star. Eventually, the planet no longer has enough energy to maintain its position, so it falls into the star and is engulfed.

But if the star's energy is a thousand times less than expected, that would be a big surprise, Hamilton said. It would imply that science doesn't understand the composition and characteristics of sunlike stars as well as it thought it did.

A second possibility is that the planet hasn't been in its current position very long, Hellier said. Wasp-18b could have spiraled inward to its current position over millions of years. It may have been bumped out of its original orbit by another planet, for example.

"However, that does not solve the problem," Hellier said, because the planet's lifetime should still be very short and it would be very unlikely for his team to find it where it did.

The final possibility is that "we're just missing something -- there is some property of stars or tides that we just don't understand," Hamilton said.

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. (Our moon orbits Earth at 40 times that distance.) Phobos' 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.

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john.johnson@latimes.com

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