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Jeudi, 15 Septembre 2011 21:58

Exoplanet Portraits: Direct Images of Other Worlds

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While they may look like no more than fuzzy dots, these are real pictures of other worlds. Over the last few years, astronomers have been learning how to accomplish the difficult task of directly imaging extrasolar planets and have now captured about half a dozen subjects.

Direct imaging is one of the next big vistas in astronomy. Soon, researchers will have telescopes that can focus in greater detail on these exoplanets, providing better information about their atmospheric composition and possibly even detecting the signatures of life.

“The only way we’re going to really nail down an Earth-like planet around a star like our sun is with direct imaging,” said Bruce Macintosh, an astrophysicist at Lawrence Livermore National Laboratory.

Though more than 100 new extrasolar planets have been announced at the Extreme Solar Systems II meeting in Moran, Wyoming this week, all were discovered through indirect methods that detect planets as they dim the light of their parent star as they pass in front of it or gravitationally tug on it to cause a wobble. These techniques can discern a planet’s size and location — important clues to its habitability, but with direct imaging, light from a distant planet can be spread out into a spectrum, providing information about the molecules and compounds in its atmosphere. One day, researchers may find a directly imaged planet that harbors the ingredients of life, such as water and oxygen.

But direct imaging is a tricky business. Scientists need to figure out how to block out the light from a vivid star and resolve the much dimmer light reflected by a planet.

“It’s like trying to see a faint ember glowing next to a very bright searchlight,” said astronomer Ray Jayawardhana of the University of Toronto in Canada, author of the book Strange New Worlds.

To accomplish this task with ground-based telescopes, astronomers use adaptive optics to smooth out the blurring of the distant light due to the Earth’s atmosphere. They must also block out the central starlight, either by covering it with a small disk in the telescope or using computer software to subtract out the star’s light while retaining any glow from nearby objects such as planets.

With direct imaging, researchers have to focus their searches on young stellar systems, whose planets still retain a great deal of heat following their formation. Jupiter, an older planet, has cooled too much since it formed and now is about a billion times less bright than the sun. Directly imaged planets, still hot out of the oven, are a typically around 50,000 to 100,000 times less bright than their parent star.

With indirect techniques, astronomers are better at searching for planets that orbit close to their central star, where they cause a greater wobble or eclipsing effect. Indirect searchers, on the other hand, allow researchers to find planets much farther away from their central star, where the starlight is less overpowering. The two methods can complement one another, allowing scientists to study planets in systems at a wide variety of distances.


The first directly observed exoplanet is not quite a planet. That is, most planets we know about orbit stars. Spotted in 2004, the small red dot (the planet) is about 3 to 10 times more massive than Jupiter and is spinning around a brown dwarf, which is an object larger than a planet but without enough mass to ignite into a burning star. This odd pair is located approximately 172 light-years from Earth.

This system presents a great puzzle to astronomers, says astrophysicist Bruce Macintosh. While technically the smaller object counts as a planet, it probably didn’t form in a big dusty disk around a central star the way the planets of our solar system did, he says. Instead, it and the brown dwarf may have formed together, like a miniature version of two stars forming next to one another.

Image: ESO/VLT


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