Distant star's planet displays atmosphere

Similarities to Jupiter in Hubble observations, scientists' study finds


The Hubble Space Telescope has detected an extensive atmosphere of hydrogen enveloping and escaping from a newfound planet of a distant star, scientists reported yesterday.

The discovery comes as no surprise, astronomers say, but it is important nonetheless as apparent confirmation that the extrasolar planets observed so far not only are much like the solar system's Jupiter in size but also are similarly huge gaseous bodies.

In an announcement by the European Space Agency and NASA, a French-led research team said three separate observations by the Hubble telescope had revealed a hot and puffed-up hydrogen atmosphere surrounding a planet orbiting the star HD 209458, in the constellation Pegasus 150 light-years from Earth. Details are described in today's issue of the journal Nature.

The most astonishing aspect, said the team leader, Alfred Vidal-Madjar of the Astrophysics Institute of Paris, is that the planet is so close to the searing heat of its parent star that the dense atmosphere reaches temperatures of about 18,000 degrees Fahrenheit and is boiling off and evaporating at a rate of perhaps 10,000 tons a second.

The escaping hydrogen was detected extending over the space of 125,000 miles, trailing the planet like a comet's tail.

In the journal report, the scientists said analysis of the observations showed that hydrogen atoms in the extended atmosphere had large velocities relative to the planet. Thus, they concluded, the hydrogen "must be escaping the planetary atmosphere."

As a result, astronomers said, the planet may already have lost a considerable amount of its mass. Much of it may eventually disappear, leaving only a dense core about 10 times the mass of Earth.

"The implication is that planets initially located even closer to their stars would not survive long," David Charbonneau of the California Institute of Technology said in an accompanying article. That, he added, "agrees with the observed paucity of extrasolar planets in such orbits."

The newfound planet, designated HD 209458b, is one of more than 100 extrasolar planets detected since 1995. None have been seen directly. In nearly all cases, their presence has been inferred from the wobbling effect of their gravitational tug on the star they orbit. From this, astronomers can estimate the planet's mass and orbital pattern.

Like several of these planets, HD 209458b is known as a "hot Jupiter," an object that orbits precariously close to its star. These objects presumably formed in the cold outer reaches of the star system and then spiraled into their close orbits.

This particular planet - with a diameter 1.3 times that of Jupiter, and two-thirds its mass - orbits its star at a distance of only 4 million miles, so close that it makes a complete circuit every 3 1/2 days. By comparison, Mercury, the planet closest to the Sun, orbits at a distance of 36 million miles, completing the orbit every 88 days. Jupiter, the closest gas giant in the solar system, is almost half a billion miles from the sun.

The atmosphere study was based on observations by the Hubble telescope's imaging spectrograph. As the planet passed across the face of its star, causing a slight dimming of the star's light, the spectrograph measured how the planet's atmosphere filters that light. During such a transit, the starlight is scattered and acquires a signature from the intervening atmospheric atoms.

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