Water on Extrasolar Planets
How Did Astronomers Detect the Composition of HD 209458b?
In early April 2007 astronomers announced the discovery of water on a planet orbiting the star, HD 209458. How is it possible to detect water on an extrasolar planet?
HD 209458
Too faint to be visible to the naked eye, the star HD 209458 (the star's name refers to star number 209458 listed in the Henry Draper catalog) is 150 light years away from us in the direction of the constellation Pegasus. This star is one of over 200 stars known to have at least one planet orbiting it. The planet, HD 209458b, is one of only 14 extrasolar planets known that transit its parent star, meaning that the planet passes in front of the star as seen from Earth. HD 209458b is a gas giant planet, like Jupiter, but is much closer to its parent star than the gas giants in our solar system. So it is often referred to as a hot Jupiter. HD 209458b was the first extrasolar planet observed to have an atmosphere and in April 2007 became the first observed to have water vapor in its atmosphere. How can astronomers detect water on a planet so far away?
Spectroscopy
The first key is spectroscopy, a tool that astronomers routinely use to find the compositions of astronomical objects. A rainbow is a very low resolution spectrum of the Sun. Just as water droplets separate sunlight into its component colors, or wavelengths, a spectroscope separates starlight into its component wavelengths, with much greater resolution.
Spectroscopy tells us the chemical compositions of astronomical objects because each type of atom or molecule has its own unique signature, which is a set of wavelengths, called lines, where the light is either brighter or fainter. The light is brighter, producing emission lines, when the atoms are in a hot transparent gas. The light is fainter, producing absorption lines, when light from another hotter source passes through a cool transparent gas. The wavelengths of these spectral lines, emission or absorption, are the signature that tells us which types of atoms or molecules are present. Lines for molecules, such as water, are often in the infrared rather than the visible part of the spectrum, so infrared spectroscopy is required. How do astronomers observe the spectral signature for the planet alone?
Transits
The other key is that HD 209458b is a transiting planet. Every 3.5 days the planet passes in front of, or transits, the star. Half its orbit away the planet passes behind, or is eclipsed by, the star. The eclipses, when the planet is completely hidden from view, allow astronomers to measure the spectrum of the star alone. When the planet is not eclipsed, the observed spectrum includes both the planet and the star. Subtracting the star's spectral features from the planet+star spectrum gives the planet's much fainter spectrum.
Applying this technique to find the spectrum of only the planet when the planet was not transiting the star, failed to detect water vapor in the atmosphere. To get water vapor absorption lines this way, requires that the interior of the planet be hotter that the outer atmospheric layers containing water. Without light originating from a hotter source a cool gas can not produce absorption lines. Therefore if HD 209458b has a uniform temperature, so its interior is not a hotter source, it will not have water vapor absorption lines despite large amounts of water.
To observe water on HD 209458b, Travis Barman of Lowell Observatory analyzed Hubble Space Telescope spectra of HD 209458 taken when the planet was transiting the star. Because the star is much hotter than the planet's outer atmosphere the gas in the atmosphere can produce spectral absorption lines in light from the star that passes through the outer layers of the planet's atmosphere. The analysis is complex, but these spectra taken during the planet's transit revealed water vapor in the planet's atmosphere that had, by the same technique, already been shown to contain sodium and hydrogen.
