The hot-blooded companion, named WASP-18b, has 10 times the mass of Jupiter, and is one of the heftiest planets to emerge from searches of nearby space.
So far, about 360 extra-solar planets have been detected, says Coel Hellier, a professor of astrophysics at Keele University in the United Kingdom. About 300 appeared because the planet regularly tugs its star, subtly changing the color of the starlight due to the Doppler effect.
Hellier's group, in contrast, looks for the slight dimming that occurs when a planet passes ("transits") in front of its star.
Although the transit method only "sees" planets that happen to orbit directly between the star and Earth, transits reveal the planet's size and mass, and that allows a density calculation. "With the transit method, we get a lot more information than with any other technique," says Hellier, who is first author of the new report on 18b.
18b is about 7 times as dense as Jupiter, and is likewise composed mainly of hydrogen.
Many of the previous discoveries from WASP -- the Wide-Angle Search for Planets -- orbit in less than four Earth days, but 18b is the first confirmed hot-Jupiter planet that orbits in less than one day.
And that means its orbital distance is about 2 percent of Earth's distance to the sun, which makes it the first planet ever seen with such a speedy orbit.
WASP-18 and 18b are, in a word, quite an item.
British astronomers reveal sizzling details of cosmic tryst
Just as a Hollywood babe can get burned by close proximity to a Brad Pitt, 18b endangers itself by nuzzling so close to a stellar biggie. And this big ball of hydrogen is more scorching than a Weekly World News romance: Hellier and Co. estimate its surface heat at 2111 °C.
For reasons of size and body heat, 18b is called a "hot Jupiter."
More important, a big planet in such a close orbit would create a huge tidal interaction and distort the star. "It's a bit like Earth's ocean being influenced by the moon, causing a bulge in the ocean," says Hellier. "In the same way, the planet is causing a bulge in the star."
Earth-bound paparazzis have snapped a dazzling new image of a hot young thing, a plus-size planet clinging perilously close to the star WASP-18, located about 320 light years from Earth in the southern sky.
The size of the bulge is difficult to estimate and impossible to see, but it could be tens of miles high, Hellier says.
And now we move beyond simple star-gazing. Creating that bulge steals energy from the planet's momentum, which is what causes it to orbit in the first place.
A brilliant item, but a brief one...
If the star is built like our closest stellar neighbor, then "one-millionth of the planet's tidal energy would be lost in each orbit," says Hellier, which would cause 18b to crash into its star within 1 million years.
Since we are less likely to detect brief events, it's highly unlikely that we would catch this system in this particular condition.
But if the star has less internal friction than our star, then the orbital decay could be much slower, which would raise the odds of viewing 18b.
Why Files dishes on how long 'til this twosome goes up in flames!
18b offers a "heads I win, tails you lose" situation. If the internal friction in WASP-18 is like that in our sun, then 18b will slow noticeably within 10 years, allowing astronomers to watch tidal interaction in action.
Otherwise, we may need to revise the assumption that our sun's innards reflect the situation throughout the universe. "There is a lot of theory, but not a lot of data, regarding the dynamics inside a star," says Hellier. "WASP-18 is telling us that the value for Q [a measure of internal friction] is probably not correct for this situation."
Even though almost 400 planets have been discovered, "There are very few that have a decay period shorter than a billion years, so this is spectacular," says Douglas Hamilton, a professor of astronomy at the University of Maryland. "If the decay is fast, we can measure it, and get a measure of Q based on evidence rather than theoretical arguments. Or if we watch for 10 years, and it's not moving in, that will show that Q is off by a spectacularly large amount, or something else is going on. It has potential to teach us something in 5 to 10 years, and none of the other planets will do that."
So which is it: is Q wrong, or is the planet as fleeting as Tara Reid's latest hunk? "Most likely it's a bit of both," says Hellier. "The Q value for this system is probably much higher than it is in our solar system, but the planet is probably a rare, short-lived object anyway."
David J. Tenenbaum
• An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b, Coel Hellier et al, Nature, 27 Aug. 2009 (see also News and Views by Douglas Hamilton, same issue).