Astronomers have made the first direct detection and chemical analysis of an atmosphere of a planet that exists outside our solar system.

The planet – HD 209458b – orbits a yellow, Sun-like star that lies 150 light-years away. The star HD 209458 is in the constellation Pegasus, and can be seen with an amateur telescope.

As the planet passes in front of – or “transits” – its parent star, light rays from the star pass through the planet’s atmosphere. Using NASA’s Hubble Space Telescope, the astronomers analyzed the spectrum of this light and detected the presence of sodium in the planet’s atmosphere.

The lead investigator of the discovery team is David Charbonneau of the California Institute of Technology and the Harvard- Smithsonian Center for Astrophysics. Other team members are Timothy Brown of the National Center for Atmospheric Research, Robert Noyes of the Harvard-Smithsonian Center for Astrophysics, and Ronald Gilliland of the Space Telescope Science Institute. The team used Hubble’s spectrometer (the Space Telescope Imaging Spectrograph, or STIS) to analyze the light spectrum. Their findings will be published in the Astrophysical Journal.

“This is just a remarkable result,” says Alan Boss of the Carnegie Institute of Washington. “The first detection of the atmosphere of an extrasolar planet means that we’ve entered into a new phase in the era of planet discovery and characterization.”

Hubble was never designed to search for extrasolar planets. But the use of this technique could allow scientists to use Hubble to measure the chemical signatures of the atmospheres of other extrasolar planets, so long as the planets can be seen transiting their star from Earth.

“I love this observation because it’s a surprise,” says Bruce Margon of the Space Telescope Science Institute. “It’s not a surprise that there are planets around other solar systems – I thought that we’d find that in my lifetime – and I guess I thought we’d find evidence for atmospheres around planets outside of our solar system in my lifetime, but I never thought that Hubble would be able to do it.”

Two years ago, scientists detected the planet transiting across the star HD 209458. As the planet passes in front of the star it blocks some of the starlight, and we observe a very slight dimming of the star for the duration of the transit.

The planet HD 209458b is a “hot Jupiter” – a planet that is about the same size and mass as Jupiter, but much closer to its star and therefore much more heated. Hot Jupiters experience temperatures in the thousands of degrees Centigrade.

Among all the approximately 80 extrasolar planets discovered to date, about are 15 hot Jupiters. But HD 209458b is unique because, unlike other hot Jupiters, the tilt of this planet’s orbit makes it pass in front of the star relative to our line of sight from Earth. While the other hot Jupiters no doubt pass in front of their stars in similar ways, we just don’t view them from the necessary angle to notice any dimming of their stars.

HD 209458b is estimated to be 70 percent the mass of Jupiter, or 220 times more massive than Earth. Transit observations by Hubble and ground-based telescopes confirmed that the planet is primarily gaseous, rather than liquid or solid. The planet therefore probably resembles the gas giants Jupiter and Saturn.

HD 209458b passes in front of its star every 3.5 days. This extremely short orbit is due to its proximity to the star – merely four million miles from the star’s surface. Because the planet orbits so close to its star, its atmosphere is heated to 1,100 degrees Celsius (2,000 degrees Fahrenheit).

“If you visited this planet the first thing that would happen is the change in your pockets would melt,” says Brown. “It’s not an abode for life.”

Although this planet’s vicinity near its Sun makes it inhospitable to life, the quick orbital period makes the planet an ideal target for repeat observations.

The scientists observed four separate transits of HD 209548b. During each transit, a small fraction of the star’s light on its way to Earth passed though the planet’s atmosphere. When the color of the light was analyzed by STIS, the telltale “fingerprint” of sodium was detected.

When light passes through a gas, some of the light wavelengths are often absorbed. This absorbed light shows up as black lines within the range of rainbow colors of the visible light spectrum. Different elements absorb light at different wavelengths. Sodium, for instance, absorbs two colors of light in the yellow-green part of the spectrum. Thus, by observing what parts of the spectrum don’t pass through the gas (i.e., appear as black lines), one can determine the composition of the gas.

In this study, there was one complication in that the star also has sodium in its outer layers. In order to overcome that complication, the scientists looked for changes in the sodium signature of the light as the planet transited. The scientists observed that when the planet passed in front of the star, the sodium lines on the spectrum deepened.

“It’s a very difficult observation,” says Margon. “This subtle contrast in the sodium signature when the planet blocks the star is only one part in ten thousand. So it’s just a whisper of a difference. And yet Hubble has managed to detect this difference.”

Sodium is not really very abundant in Jupiter’s atmosphere (only a few parts per million) and the scientists have no reason to believe that the atmosphere of HD 209458b is any different. The reason the scientists looked for sodium – as opposed to any other element – is because sodium has a very strong spectrographic signature.

“It’s the spectrum equivalent of a skunk,” says Brown. “You don’t need very much of it in the air before you notice it. That’s really the reason to look for it.”

“The planet as a whole is mostly hydrogen and helium, and sodium is just one element,” says Charbonneau. “The sodium just tells us that there really is an atmosphere.”

“We actually measured less sodium than we expected,” continues Charbonneau. “One reason for this may be that the planet has some high clouds that absorb light, and we just see the sodium at the top of the atmosphere. Or it could be that the sodium has reacted and formed more complicated molecules, and rained out of the atmosphere.”

The scientists think they may be able to use another technique to help solve this sodium puzzle, as well as study other aspects of the planet’s atmosphere.

“As the planet swings around the star, we know there are times when the planet will be fully illuminated; where it would appear as a full moon,” says Charbonneau. “It would pass behind the sun, and then reappear again as a full moon. By measuring that on-off-on signal, we could learn about the brightness, and therefore the chemical composition of the atmosphere. So by using these two techniques: transmission – when the planet’s in front – and reflection – when the planet’s behind – I think we could really begin to pin down this planetary atmosphere.”

The idea of using transits to probe atmospheres goes back 300 years. Astronomers watching the planet Venus as it passed in front of the Sun noticed a fuzzy shimmer around Venus. This shimmer was an indication of Venus’ atmosphere. But never before have scientists been able to detect atmospheres on planets outside of our solar system.

“This is the first measurement ever of any atom of any atmosphere of an extrasolar planet,” says Margon. One can imagine in just a few years we’ll be doing comparative [studies of] planetary atmospheres.”

“I would say in summary what we have here are two results,” says Charbonneau. “We’ve made the first detection of an atmosphere of a planet around another sun-like star. We’ve measured the sodium in that atmosphere, and we’ve learned about the clouds and the chemistry that are going on in that atmosphere. The other point is that we’ve demonstrated that by using this technique with the Hubble Space Telescope, we can achieve the sensitivity we need to study atmospheres in this way. Over the next few months and the next few years, we hope to gather more data very quickly on this system – and also on future transiting planet systems that are discovered – to look for other atoms and molecules in those atmospheres such as methane, carbon dioxide, and even water.”
What’s Next

“This is a preview of coming attractions,” says Boss. “There are going to be a number of succeeding space science updates in the next ten years that will herald the results from NASA telescopes that were designed specifically to look for extrasolar planets.”

Meanwhile, the discovery team will look at HD 209458 again with the Hubble Space Telescope. They plan to look in more colors of the star’s spectrum to try to determine other elements of the planet’s atmosphere. The solar system also could have additional planets that are much further away from the star, with orbits of a few years. If such planets are found, the team will compare chemical differences among the atmospheres of the planets.

“If we were to find systems of multiple planets around one star we would expect to find them on the same orbital plane,” says Charbonneau. “This system has only been looked at for a few years. As we look longer, we probably will discover additional planets. We’ll just have to wait a few years.”