extrasolar_atmosphere_011127
Astronomers announced today the first detection of an atmosphere around a planet orbiting another star, a critical first step in understanding the skies above planets outside our solar system. The discovery of a small amount of sodium above a large planet 150 light-years away shows that planets outside our solar system do in fact use their gravity to trap gases and possibly other substances to form an atmosphere. Until now, scientists could only assume that such objects have atmospheres, as do the planets in our solar system.
More significant, the technique used in the investigation could eventually lead to the discovery of more interesting chemicals that might reveal the presence of extraterrestrial life.
"For the first time, we can begin to chart the atmosphere around other planets," said Timothy Brown of the National Center for Atmospheric Research.
Brown worked with Caltech astronomer David Charbonneau, who led the study, which was detailed at a NASA press conference.
The discovery was made using the Hubble Space Telescope. To optimize their precious telescope time, the researchers confined their search to sodium, an element they expect would exist around extrasolar planets, or exoplanets as astronomers call them.
"Sodium is kind of like skunk scent," Brown explained in a telephone interview. "It's easy to detect, even if there's only a little bit of it there."
Several research groups have been pursuing a similar goal with ground-based telescopes, but none have made a firm detection, said Andrew Collier Cameron, an exoplanet hunter at the University of St. Andrews in Scotland.
Cameron, who was not involved in the new study, said Hubble's stability and location above Earth's atmosphere gave Charbonneau and Brown "the edge that was needed to achieve this important milestone in exoplanetary science."
The research team also included Robert Noyes of the Harvard-Smithsonian Center for Astrophysics and Ronald Gilliland of the Space Telescope Science Institute, which operates Hubble.
Ongoing observations may reveal more details about the planet's atmosphere, the scientists said.
Science fiction to reality
Extrasolar planet discoveries have been dramatic in recent years. The first one was detected in 1995. In 1999, the first multi-planet system outside our own was discovered. Now there is evidence for more than six dozen planets around various other stars, with new discoveries being announced monthly.
"Only a decade ago, planets outside the solar system were still in the realm of science fiction," said Charbonneau. "Searching for a star's unseen planetary companion was crazy. Hoping to see its atmosphere was even crazier."
All of the exoplanets found so far are very large -- many are bigger than Jupiter. The bulk of them orbit very close to their host stars, some closer than Mercury is to our Sun.
The planet in the new study was discovered in 1999. It orbits a yellow, Sun-like star called HD 209458. It is relatively small as known exoplanets go, about 70 percent the size of Jupiter. Its year -- the time it takes to orbit the star -- is just 3.5 days. Temperatures likely soar to 2,000 degrees Fahrenheit (1,100 degrees Celsius).
Conventional theory can't explain how planets could form or evolve so large and close to a star, so understanding their atmospheres may shed light on their overall evolution.
Seeing the invisible
Like all exoplanets, this one has never been seen. Most are found by noting a slight wobble in their host star caused by the planet's tiny tug of gravity. The planet around HD 209458 was first found this way, but then its presence was confirmed when astronomers measured a dip in starlight each time the planet passed in front of the star.
In the new research, the planet's atmosphere was examined during these transits, as they are called. The starlight changed as it was filtered through the planet's atmosphere. An instrument aboard Hubble, called a spectrograph, broke the light into its component colors based on wavelength. Different elements and compounds created different light signatures.
Though the star HD 209458 also has sodium in its outer layers, the astronomers noted a slight additional amount of sodium -- less than one percent -- as the starlight passed through the planet's atmosphere.
The technique of using transits to study planetary atmospheres actually dates back nearly 300 years, when astronomers first noted a shimmer as Venus crossed the Sun. But the task is far more difficult when dealing with minor changes in light from very distant stars.
"It's hugely exciting to nail down anything at all about something as mysterious as planets outside our solar system," Brown said. "Is this planet's chemical personality unique or is it typical of a certain class of extrasolar planets? We have no clue. We hope to find out."
Brown said that there is a possibility that the new results could be proven wrong. "But I don't think it's likely," he said.
Finding air around an exoplanet was expected. But the ability to make the discovery now -- and to do it with the Hubble telescope -- caught experts off guard.
"It was amazing that the Hubble Space Telescope was able to do this, because Hubble was not designed specifically to detect extrasolar planets," said Alan Boss, a researcher at the Carnegie Institution. "We have entered into a new phase of extrasolar planet discovery and characterization."
The results will be published in the Astrophysical Journal.
Next Page: The real goal -- finding another Earth
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