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!