From left to right, this illustration shows the star HD209458, the extrasolar planet named HD209458b, the spectrograph representing the STIS instrument on Hubble, and the resulting spectral signature of sodium. You can see that sodium's spectral signature (the two dark lines in the rainbow spectrum) are in the yellow band of visible light which means sodium can be measured using the STIS instrument. Click on image for full size (75K JPEG) Courtesy of STScI

HD209458b is a Jupiter-like planet that orbits around a star named HD209458. HD209458 is considered a near-by star at about 150 light years away from our solar system. HD209458b is a huge planet (0.7 times the mass of Jupiter) that orbits very, very close to its star (inside where the orbit of Mercury would be in our own solar system!). Because it is so close to its star, HD209458b has an atmosphere that is probably close to 1,000 degrees Celsius.

Seven hours of exposure time using the STIS instrument onboard the Hubble Space Telescope (HST) in April and May 2000, resulted in the detection of the atmosphere of HD209458b. The STIS instrument is the Space Telescope Imaging Spectrograph. It provides spectral signatures in the ultraviolet and visible wavelengths.

Specifically, the atmosphere was detected using the transit method and spectroscopy (see the image on this page). HD209458b takes about 3 hours to pass across the star and the light brightness we see from Earth dims for the period of the 3 hours (because the planet blocks some of the light). When HD209458b was transiting the star, the STIS looked at the light from the star as it passed through HD209458b's atmosphere. And so STIS could process a spectral signature from HD209458b's atmosphere. Spectroscopy depends on the fact that different chemical compounds have different spectral signatures. Scientists looked and found the prominent signature of sodium. Scientists only found half the amount of sodium that models predict, but they did find sodium! Scientists looked for the sodium spectral signature because it is so prominent (and because sodium’s spectral signature is in the visible region where the STIS instrument can measure).

The HST was not specifically designed for this type of measurement. In fact, the HST was launched before any extrasolar planets were known. So, it is impressive that we can look at the atmosphere of a planet that is so far away! Future space missions specifically designed for this type of measurement will no doubt discover many more exciting things about extrasolar planetary atmospheres, but HST has laid the groundwork.

In the next decade, scientists will likely want to:

From left to right, this illustration shows the star HD209458, the extrasolar planet named HD209458b, the spectrograph representing the STIS instrument on Hubble, and the resulting spectral signature of sodium. You can see that sodium's spectral signature (the two dark lines in the rainbow spectrum) are in the yellow band of visible light which means sodium can be measured using the STIS instrument. Click on image for full size (75K JPEG) Courtesy of STScI

HD209458b is a Jupiter-like planet that orbits around a star named HD209458. HD209458 is considered a near-by star at about 150 light years away from our solar system. HD209458b is a huge planet (0.7 times the mass of Jupiter) that orbits very, very close to its star.

Scientists used the STIS instrument onboard the Hubble Space Telescope (HST) to detect the atmosphere of HD209458b. The STIS instrument is the Space Telescope Imaging Spectrograph. It provides spectral signatures in the ultraviolet and visible wavelengths.

Specifically, when HD209458b was crossing in front of its star (see the image on this page), the STIS looked at the light from the star as it passed through HD209458b's atmosphere. And so STIS could process a spectral signature from HD209458b's atmosphere. Spectroscopy depends on the fact that different chemical compounds have different spectral signatures. Scientists looked and found the prominent signature of sodium. Scientists only found half the amount of sodium that models predict, but they did find sodium! In the future, scientists will look for many other molecules in the atmospheres of extrasolar planets. There will be future missions specifically designed to look at extrasolar planets (examples are SIM, TPF, NGST).

The HST was not specifically designed for this type of measurement. In fact, the HST was launched before any extrasolar planets were known. So, it is extra impressive that we can look at the atmosphere of a planet that is so far away!

From left to right, this picture shows the star HD209458, the extrasolar planet named HD209458b, the spectrograph which is one of the instruments on the Hubble Space Telescope. The rainbow part of the image is called a spectra. It is one way to see what makes up the atmosphere of a planet. Click on image for full size (75K JPEG) Courtesy of STScI

Planets that orbit other stars are called extrasolar planets. Just yesterday, scientists announced that they had used the Hubble Space Telescope (HST) to look at the first atmosphere of a planet outside our own solar system!

So, not only do we know of 76 extrasolar planets, we know one of them has an atmosphere. The planet that scientists looked at is called HD209458b. What would you name a extrasolar planet if you found one?

There will be future missions that will look at extrasolar planets (examples are SIM, TPF, NGST).