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Searching for Another Earth

A new discovery advances the hunt for Earthlike planets beyond our solar system.

An international team of astronomers has discovered an exoplanet–one outside our solar system–that has a more Earthlike orbit than any alien planet discovered so far using the same technique.

Planet finder: The CoRot satellite is operated by the French Space Agency CNES, and its mission is to search for planets outside our solar system. Here it’s undergoing mechanical qualification tests prior to launch.

The planet, called CoRot-9b, was discovered by the French-operated satellite CoRot, which has been in orbit since 2006. The spacecraft detected CoRot-9b by measuring the dimming of its star’s brightness as the planet passed in front of it, a technique called “transit observation.” The small dip in brightness allows the planet’s size to be calculated. By measuring the amount of time it takes the planet to complete its orbit, researchers can determine the planet’s distance from its star.

The planet travels around its star every 95 Earth days, at a distance about half of that between Earth and the sun. This distance is still 10 times farther than any other planet discovered using this method. Its surface temperature is between -23 and 157 °C, so if CoRoT-9b was a terrestrial planet like Earth it could have liquid water on its surface, but it is made mostly of hydrogen and helium and therefore is unlikely to support life.

“This is the first time that we can probe the atmosphere of such a cool planet outside the solar system,” says Jason Wright, an assistant professor of astronomy at Pennsylvania State University, who was not involved in the discovery. Wright says that the finding is an important step forward in finding an Earthlike planet that could support life, one of the biggest goals in astronomy.

Astronomers used ground-based telescopes in Chile, Israel, and Tenerife, a Spanish Island off the coast of Africa, to confirm their finding, and to determine the planet’s mass by measuring the gravitational pull exerted on its parent star (this pull causes the star to wobble).

From the mass and radius measurements, the astronomers estimated the planet’s density, which is valuable in understanding its composition and structure, says Suzanne Aigrain, a lecturer in astrophysics at Oxford University and part of the team of astronomers that discovered the planet. The new planet may have a rocky core, but the rest is mainly made up of hydrogen and helium, so no form of life as we know it could exist, says Aigrain.

“Transit technology is state-of-the-art; it’s in its prime,” says Wright. “This is the first time we have detected a planet with such a long orbital period when it is so far from the star,” he says. Previously discovered exoplanets orbit much closer, making them both extremely hot and hard to study.

Jennifer Patience, a lecturer in astrophysics at the University of Exeter, in the U.K., says that there are two key technical challenges in imaging planets: overcoming the contrast ratio in the light from the star and the planet, and the fact that the planets are a typically quite close to the host star.

Star light, star bright: The transit method for detecting planets beyond our solar system works by looking for the drop in brightness of a star that’s caused by a planet passing in front of it. Astronomers use the data to calculate the size of the planet and its distance from its star.

The transit method employed by the CoRot satellite uses a telescope to collect light that is then fed to a two-part camera. One part of the camera is designed to look for stars; the other is optimized to detect the subtle variation in a star’s light. European researchers built a device to shield the camera from light coming from sources other than those objects at which the telescope is looking.

NASA’s Kepler satellite, which launched last year, is also using the transit method to look for Earthlike planets. Wright says the spacecraft will “revolutionize our search for habitable planets.” Kepler has 42 detectors that can measure the brightness of 150,000 stars and stores the data every six seconds. “No one has ever built a [space-based] instrument with this kind of precision,” says William Borucki, science principal investigator for the Kepler mission.

John Johnson, an assistant professor of astronomy at Caltech, says the new finding by CoRot foreshadows the work that astronomers expect Kepler to make over the next few years. Johnson predicts that Kepler will find the first inhabitable “Earth” outside our solar system in the next three to six years.

For astronomers to really understand the composition of exoplanets they discover, they will need to use an emerging method called direct detection. This technology uses an adaptive optics system and a device called a coronagraph, which blocks out as much light from the star as possible, to image the planet. The Hubble Space Telescope was able to image a planet around the star named Fomalhuat using a coronagraph, and only two other telescopes have this capability, the Keck Observatory in Hawaii and the Very Large Telescope in Chile.

Direct imaging instruments are being built for the Gemini observatory in Chile, the Subaru telescope in Japan, and the Very Large Telescope. NASA’s next space telescope, the James Webb Space Telescope, set to launch in 2013, will also use direct detection. As part of a Technology Development program, NASA recently issued a two-year $784,000 grant to researchers at Rochester Institute of Technology in New York and MIT Lincoln Laboratory, to modify one of Lincoln Lab’s photon detectors for imaging planets. The detector counts individual photons digitally, says Brian Aull, a technical staff member at MIT Lincoln Laboratory. “The advantage is that it is able to detect very faint objects with better signal-to-noise characteristics than conventional detector technology.”

“The main thing is finding planets, because in order to better understand our solar system, we have to understand how other planetary systems formed,” says Borucki. If astronomers can find lots of Earthlike planets, the next step is to develop instruments that can look for signatures of life. “We need to know: is there life out there, or are we alone?”

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