Hubble sees key elements in atmosphere of Osiris

With its future in doubt, space telescope detects oxygen, carbon on planet

By Frank D. Roylance
Sun Staff, February 3, 2004

An international team of astronomers using the Hubble Space Telescope is reporting the first discovery of oxygen and carbon in the atmosphere of a planet circling another star.

Oxygen and carbon are two elements considered vital to the evolution of life as we know it. But scientists said the finding does not mean that the planet supports life. Far from it.

"This is oxygen in a very harsh environment," said University of Arizona astronomer Gilda Ballester, a member of the team.

The planet, tentatively named Osiris, is a "gas giant" nearly the size of Jupiter. It is orbiting just 4.3 million miles from its star - one-eighth the distance between our sun and its nearest planet, Mercury. So it's hot, an estimated 1,832 degrees Fahrenheit on the surface.

Worse, ultraviolet radiation streaming from the star appears to be ripping away the planet's mostly hydrogen atmosphere, blowing away the heavier carbon and oxygen in the process. And that's what the Hubble telescope has detected.

It is just the kind of discovery that the Hubble is uniquely suited for, said Steven V.W. Beckwith, director of the Space Telescope Science Institute in Baltimore.

"There is no other facility right now or planned, on the ground or in orbit, that would be able to do this kind of follow-up on extra-solar planets," he said.

The Hubble telescope's future was thrown in doubt last month after NASA Administrator Sean O'Keefe decided to scrap a final space shuttle mission to service the Hubble in orbit in 2006. The cancellation came in response to new priorities for manned space flight set by President Bush, and new shuttle safety guidelines established in the wake of the Columbia accident a year ago.

Protests by the scientific community, and by Maryland Sen. Barbara A. Mikulski, prompted O'Keefe to ask retired Adm. Harold W. Gehman Jr. last week to review the decision. Gehman was the chairman of the panel that reviewed safety concerns after the Columbia disaster.

The Hubble's latest discovery will appear in the forthcoming issue of Astrophysical Journal Letters, reported by a team of French, American, Canadian and Swiss astronomers, led by Alfred Vidal-Madjar, of the Institut d'Astrophysique de Paris.

Osiris was discovered in 1999, one of more than 100 planets that have been detected circling stars beyond our solar system. Its sun-like star is about 150 light-years from Earth, visible with binoculars in the constellation Pegasus.

Officially dubbed HD 209458b, the planet could only be detected because its 3 1/2 -day orbit carries it directly in front of the star as seen from Earth.

This periodic "eclipse" reduces the brightness of the star by 1.5 percent. That is what alerted astronomers to its existence, the first extra-solar planet detected that way.

It was also the first extra-solar planet discovered to have an atmosphere - hydrogen and a bit of sodium were the first two elements identified by scientists. And it is now also the first to be found to have oxygen and carbon in its atmosphere.

The Hubble Space Telescope is able to analyze the chemistry of Osiris' atmosphere by making spectrographic measurements of the star's light as the planet's atmosphere passes in front of it.

"It's been a gold mine," Ballester said. "The problem is, this is one example. There has been another ... planet identified that transits in front of its star, but the star is very far, and very difficult to observe."

Osiris' atmosphere was only visible to the Hubble telescope because it is being heated to 18,000 degrees and blown far into space by radiation from its star.

"We detected the outer parts of the atmosphere that are very, very extended," making it appear 10 times larger in the ultraviolet wavelengths visible to the Hubble, Ballester said.

The gas is being blown away at speeds approaching the speed of sound, she said.

The fact that carbon and oxygen - elements 10 times heavier than hydrogen - are being swept up in the solar wind means that the radiation blowing off the star is strong enough to overcome the planet's gravity and gather up the heavier elements along with the lighter hydrogen, like beach sand in a gale.

In a few billion years, said University of Pennsylvania astronomer David E. Trilling, who was not part of the latest study, the entire atmosphere will perhaps have eroded away, leaving only a small, rocky remnant of the planet's core.

Theorists have proposed that such skeletal objects be placed in a new category of evaporated gas giants called "chthonian" (pronounced "THO-nee-un") planets, named for a Greek god of the underworld, Khton.

Some have proposed that the early atmospheres of Venus, Earth and Mars were blown off in a similar gale from the young sun, which might account for the differences between the contents of the Earth's present atmosphere and the chemicals abundant on Jupiter.

But "that idea is not well thought of by the [scientific] community," said Trilling. A more modern view, he said, is that the Earth formed later than Jupiter, and its atmosphere is the product of falling comets and other processes.

Astronomers said they're not surprised to find oxygen and carbon in Osiris' atmosphere. The elements are known to be present in the atmospheres of nearby gas giants, such as Jupiter and Saturn.

Still, Trilling said, "it tells us the composition of this planet is more interesting than a big ball of hydrogen."