An alien flying past the Earth could hardly fail to notice that our blue planet is largely covered by oceans. However, Earth is the only water-covered world in our Solar System. Since wet planets seem to be quite rare, it is not surprising that scientists are excited about the first discovery of a water-rich atmosphere on a planet in orbit around a distant star.
More than 200 of these ‘extrasolar’ planets are known, but very few have been imaged or directly observed. Fortunately, some of these ‘invisible’ planets can be detected when they transit, or pass in front of their stars. This is how the gas giant known as HD 189733b was found in 2005. The Jupiter-like world caused a slight dimming of the light from its parent star.
Since then, further studies of HD 189733b have been made by European and American scientists. Using NASA’s Spitzer Space Telescope, they measured the infrared light coming from the star. As the planet slowly moved across the star’s disk, its outer atmosphere blocked one wavelength of light more than other wavelengths. The only explanation for this pattern of dimming is the presence of water vapour in the planet’s atmosphere.
Unfortunately, the giant planet has no rocky surface where oceans can form. It also orbits extremely close to its star. With an atmospheric temperature over 700°C, it is far too hot for rain to fall. Steamy HD 189733b is almost certainly without life. However, these results are raising hopes of finding watery, Earthlike planets in the not-too-distant future.
