Hubble sees key elements in atmosphere of Osiris
With its future in doubt, space telescope detects oxygen, carbon on
By Frank D. Roylance
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
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,
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."