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First direct detection of “confirmed” alien planets claimed

The findings were made using NASA’s Spitzer Space Telescope. They also utilized a new method that involves observing the infrared light that planets emit, a type of light slightly less energetic than visible light. 

This method makes the planets easier to see because stars outshine them less with this kind of light than with visible light.

It’s by no means the first time astronomers have found planets outside our solar system. But this finding is different, the astronomers said, because previous findings were either only tentative, or they used indirect detection methods that don't involve picking up light from the planet itself.

This is thus the first direct detection of light from “confirmed” planets, said Alan Boss of the Carnegie Institution of Washington, one of the researchers. It is confirmed, he added, because the objects sighted were already known as planets using other methods before this sighting.

The findings also lay the groundwork to enable astronomers to begin studying the temperature and atmosphere of these alien planets, researchers said. That is a necessary step to determining if life exists there.

Two teams of researchers observed two planets in our galaxy, the Milky Way. One, catalogued as HD 209458b, orbits a star 153 light years away, or 29 quadrillion miles, in the constellation Pegasus, the winged horse. The second planet, TrES-1, is even further — 489 light years away in the constellation Lyra.

At least 130 stars outside our solar system have been shown to have orbiting planets, based on a slight wobble in the stars’ positions or a variation in intensity as planets pass in front of them. Both HD 209458b and TrES-1 were initially detected by these indirect methods. However, “Our Spitzer observations are the first direct measurements of light from confirmed extrasolar planets,” says Joseph Harrington, a senior research associate in the Center for Radiophysics and Space Research at Cornell University, Ithaca, N.Y.

Harrington is a member of a team at NASA’s Goddard Space Flight Center, which measured infrared radiation from HD 209458b. Almost simultaneously, a team led by David Charbonneau at the Harvard-Smithsonian Center for Astrophysics measured infrared emissions from TrES-1. Both planets are gas giants similar to the planet Jupiter in our solar system. But they are “hot Jupiters” because they orbit very close to their stars and absorb so much heat that they radiate strongly in infrared light. 

Both teams used instruments on the Spitzer Space Telescope, which was launched in August 2003.

The two teams announced their discoveries at a press conference in Washington, D.C. on March 22. The journal Nature has posted a paper from the Deming group in its online edition and plans to publish the paper in its April 7, 2005, issue. A paper from the Charbonneau group is to appear in the June 20, 2005, issue of Astrophysical Journal.

Both teams used a similar technique: observing the change in intensity of light as the planet passed behind the star. Subtracting this from the intensity of star and planet system, measured prior to the transit of the planet behind the star, leaves a direct detection of the radiation from the hot planet. 

Even the nearest stars to Earth are so far away that most telescopes see them only as points of light, although future specialized NASA missions hope to obtain images showing a planet separated from a sunlike star. Meanwhile, astronomers can deduce a great deal by observing the variations in a star’s light and by observing its spectrum. Based on spectral type, HD 209458b is orbiting a star very much like our sun, researchers said, while TrES-1’s star is smaller and cooler.

Both planets are closer to their stars than is Mercury to our sun, and both complete one orbit about every three days, they added. Tidal forces cause their rotation to match that time, so that they always turn the same face to their stars, just as the moon always has the same face to Earth. Based on HD 209458b’s light output, Deming’s team put the planet’s temperature at 1,130 Kelvin (857 degrees Celsius or 1,574 degrees Fahrenheit), about double the temperature of a pizza oven. Charbonneau’s team estimates TrES-1’s temperature at 1,060 Kelvin (787 degrees Celsius or 1,448 degrees Fahrenheit).

Gas giants like Jupiter become very hot when they are formed, as the gas that makes them up is compressed by gravity. Jupiter cooled over time, and now has a temperature at its cloud tops of 123 Kelvin, or 150 degrees Celsius below zero. “Hot Jupiters,” by contrast, receive heat from their stars and remain hot. It is this heat that makes these planets a choice for infrared detection.

A significant finding by the Deming group is that HD 209458b’s eclipse behind its star occurs exactly halfway around its orbit from its transit in front, meaning that its orbit is likely circular rather than elliptical, researchers said. (It could be elliptical only if the long axis of the ellipse points directly toward Earth, which would be a huge coincidence, the researchers say.)

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