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Tuesday, February 3, 2004
Hubble sees key elements in atmosphere of Osiris
With its future in doubt, space telescope detects oxygen, carbon on planet
By Frank D. Roylance
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 Dr. 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.”
The Baltimore Sun • Tuesday, February 3, 2004 • HUBBLE SEES KEY ELEMENTS IN ATMOSTPHERE OF OSIRIS (cont’d.)
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.”