First Sunset Outside Our Solar System Glimpsed
By Dave Mosher
Staff Writer
posted: 11 December 2007
06:02 am ET

Traces of a distant extrasolar planet's hazy red sunset have been detected for the first time.
Astronomers pointed the Hubble Space Telescope HD 189733b, a gaseous Jupiter-like world about 63 light-years from Earth, as it passed in front of its parent star to catch a glimpse of the planet's atmosphere. Previous observations have not revealed much about the planet's atmosphere, other than that it has clouds.
"One of the long-term goals of studying extrasolar planets is to measure the atmosphere of an Earth-like planet [and] this present result is a step in this direction," said Frederic Pont, an astronomer at the Geneva University Observatory in Switzerland. Pont led the team of astronomers who made the new Hubble observations.
"HD 189733b is the first extrasolar planet for which we are piecing together a complete idea of what it really looks like," Pont said.
Starlight passing through a planet's outer atmosphere can take on different colors as it passes through different gases. In the case of HD 189733b, scientists said the light traveling through the planet's hazy atmosphere appeared red in front of its yellow star, which is about 76 percent of the diameter of the sun.
They expected to see the fingerprints of sodium, potassium and water in the red haze, but instead discovered iron, silicate and aluminum oxide (which sapphire gems are made of). The composition is similar to Venus and Saturn's moon Titan—both worlds with chokingly thick air.
So far, HD 189733b isn't thought to harbor any Earth-sized moons or Saturn-like rings, but more powerful telescopes of the future might detect them.