WASHINGTON — For the first time, astronomers have spotted what looks like a sunset on a planet outside our solar system, they announced this week.
Using the Hubble Space Telescope, they detected traces of a red haze surrounding a Jupiter-like ball of hot gas circling a star in the northern sky 63 light-years — 370 trillion miles — from Earth.
The haze is similar to the thick atmospheres around Venus and Titan, Saturn's largest moon, according to Frederic Pont, an astronomer at the Geneva Observatory in Switzerland who led the discovery team.
The discovery is another step in the quest to find an Earth-like planet that could support life. NASA is planning space missions to detect such objects.
Since 1995, astronomers have detected 268 extrasolar planets orbiting 230 stars. Some of them form miniature solar systems that contain as many as five planets. So far none of them is a cool, rocky planet like Earth is, and none is thought to be capable of supporting life.
Pont's planet — with the klutzy scientific name HD 189733b — was discovered in 2005 during one of its frequent passes in front of its star. As it crossed the star's disk, it briefly dimmed the light reaching Earth. The dimming, about 3 percent, was repeated every 2.2 days as the planet whirled around its host.
The star is about three-quarters the size of our sun and can be seen with binoculars in the constellation Vulpecula ("little fox").
The haze was discovered because gases in a planet's atmosphere affect the color of starlight as it passes through on its way to Earth. The red light from this planet revealed traces of iron, silicate and aluminum oxide, the sources of rubies and sapphires.
Last July, British researchers using NASA's Spitzer Space Telescope reported finding signs of water vapor in the same planet's atmosphere, but Pont's group detected no such evidence, so it may have been a false alarm.
Last April, however, an American astronomer, Travis Barman of the Lowell Observatory in Flagstaff, Ariz., reported definite signs of water vapor in the atmosphere of a different planet 150 light-years — 880 trillion miles — away in the constellation Pegasus.
That planet, nicknamed Osiris after the Egyptian god of the dead, has oxygen, carbon and hydrogen as well as water in its atmosphere. It's about 1.3 times bigger than Jupiter and orbits its star every 3.5 days at a distance of 4 million miles, much closer than Mercury is to the sun. Its temperature, almost 2,000 degrees Fahrenheit, makes it uninhabitable for any form of life known on Earth.
The planet in Vulpecula is about 1.25 times bigger than Jupiter. It's only 3 million miles from its star, and its temperature, about 1,700 degrees Fahrenheit, is also far too hot for life as we know it.
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