Visualize an alien sunset

Astronomers using the Hubble Space Telescope have spotted evidence of high-level haze in the air of a planet 63 light-years from Earth, by analyzing the starlight shining through the very edge of the alien atmosphere. Scientists even think they know what sunsets on this planet would look like: big and red.

The research team's leader, Frederic Pont of the Geneva University Observatory in Switzerland, says the hazy world is "the first extrasolar planet for which we are piecing together a complete idea of what it really looks like."

The planet that's the focus of the research revealed today, and written up for the Monthly Notices of the Royal Astronomical Society, is a "hot Jupiter" - actually, a planet slightly larger than Jupiter, whipping around its parent sun every 2.2 Earth days at a distance of just 3 million miles (5 million kilometers). Atmospheric temperatures would be around 1,300 degrees Fahrenheit (700 degrees Celsius), which would cook earthly smog right out of the air.

So how do astronomers know there are hazes covering the planet, known as HD 189733b? Pont told me that he and his colleagues relied on a clever "trick": Using Hubble's Advanced Camera for Surveys,  the astronomers looked for incredibly subtle variations in the light coming from the star that HD 189733b orbits. When the planet passes in front of the star's disk, the brightness dips ever so slightly. What's more, the spectrum of some of the light shifts, reflecting the chemical composition of the starlight shining through the very edges of the planet's atmosphere.

This transit trick has been done before - in fact, an earlier study led researchers to conclude that  HD 189733b's atmosphere contained sodium, potassium and water vapor. Pont's team expected to find those ingredients in their results as well, and they were surprised when they didn't. Instead, the spectral signature indicated that a different kind of air existed at altitudes of about 600 miles (1,000 kilometers).

Pont said water vapor may well exist at lower atmospheric levels, but his results imply that the planet is covered at its upper levels by a global haze - similar in broad terms to the sulfurous haze covering Venus, or the hydrocarbon haze covering Saturn's moon Titan.

"In narrower terms, the planet is so much hotter that the composition of the haze is certainly different," Pont said. Scientists suspect it consists of tiny particles of iron, silicates and aluminum oxide dust.

Pont can't predict precisely what color the planet would take on if you were to look at it from space. But based on the scattering characteristics of the haze, he and his fellow researchers predict that visitors looking up through the atmosphere from below would see a gorgeous red sun filling the sky - like an Athenian sunset writ large.

For Pont, the coolest thing about his research isn't that he can visualize an alien sunset, but that he can help fill out a complete picture of an alien atmosphere from hundreds of trillions of miles away. "For this planet, we are measuring the whole spectrum, little by little," he explained.

In fact, HD 189733b is the first planet beyond our solar system to have its temperature mapped, complete with a weather report. Scientists believe that the planet has one huge super-hurricane whipping around its sunlit side - which doesn't make it much of a romantic place for seeing a sunset.

Future telescopes could use even cleverer tricks to "sniff" the air of smaller and smaller alien planets. "It's one step toward studying the atmosphere of Earthlike planets," Pont said.

Someday, we may well find an Earth-sized planet that has just the right mix of oxygen, nitrogen, water vapor and other chemical markers for life as we know it. Then what? Do we start sending radio signals ... or spaceships? Leave your suggestions for the search for other Earths below.