Extrasolar orb is too close for comfort. (Planet's Slim-Fast Plan).

Planets beware! Get too close to your parent star and you will vaporize.

That's the message of a study that examines a planet residing within roasting distance of the star it orbits. The planet, dubbed HD209458b, circles a star at one-eight the distance that Mercury orbits our sun. Observations suggest that the close-in planet, blasted by the star's heat and radiation and tugged by the star's gravity, can't hold on to all its material. Every second, the star is stripping at least 10,000 tons of hydrogen from the planet, according to Alfred Vidal-Madjar of the Astrophysics Institute of Paris and his colleagues, who report their study in the March 13 Nature.

At that rate, the planet would have lost only 0.1 percent of its mass since its birth 5 billion years ago, and the orb would easily outlast its parent star. But the observations provide only a minimum rate of loss of hydrogen, the planet's most abundant element, so much more of the gas may be escaping. If so, the planet, which now weighs about 70 percent of Jupiter's mass and lies 150 light-years from Earth, maybe slimming down rapidly.

The findings suggest that some other planets, residing even closer to a stellar furnace HD209458b does, will simply evaporate. That could explain why among the roughly 100 extrasolar planets discovered to date, only 9 are so-called "hot Jupiters, orbiting their stars so closely that they complete one revolution in just a few days.

As with every other extrasolar planet known, astronomers discovered the extra-hot HD209458b indirectly by the tug the body exerts on the star it orbits. But the unseen planet has a special property: Its orbit is aligned so that when the body passes between its star and Earth, it periodically blocks a small amount of the starlight.

Each time the planet makes such a transit, some of the starlight must pass through the planet's atmosphere, which absorbs specific wavelengths of light according to the atmosphere's contents. Comparing the spectra of the star observed in and out of eclipse reveals the composition of the planet's atmosphere.

Two years ago, researchers used a spectrograph aboard the Hubble Space Telescope to detect trace amounts of sodium in the planet's atmosphere, the first time any constituent of an extrasolar planet was determined (SN: 12/1/01, p. 340).

Using the same spectrograph, another team, led by Vidal-Madjar, has now detected hydrogen. During three of the planet's transits, the team found that starlight was dimmed by 15 percent at an ultraviolet wave-length absorbed by hydrogen atoms. That's surprisingly large because the planet only blocks 1.5 percent of the star's area.

According to the team, the planet absorbs so much radiation because it has a bloated atmosphere that's more than twice as high as the radius of HD209458b. The planet's gravity can't hold all of the hydrogen atoms at the high altitudes. Succumbing to the heat and gravity of the star, some of the atoms escape into space. The gas expands and speeds away in a cometlike tail, the researchers speculate. The spectra confirm such a scenario, says Vidal-Madjar. His team now plans to look for heavier elements, such as carbon, oxygen, and nitrogen, leaving the planet's outer atmosphere.

"This data set is yet another milestone for an extrasolar planet, HD20948b, that can boast many," notes Adam S. Burrows of the University of Arizona in Tucson. "One hesitates to christen any astronomical object a Rosetta stone [for studying extrasolar planets], but in this case, one should not hesitate too long."