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Planetary News: Kepler (2009)Earth-Hunter Kepler Shines Light on "Hot Jupiter"By Amir AlexanderAugust 7, 2009
Five months into its mission the planet-hunting spacecraft Kepler is already proving its value to scientists. Even while its instruments were going through their calibration phases, Kepler detected and analyzed the atmosphere of a known "hot Jupiter" exoplanet orbiting a star 1000 light years away. In doing so Kepler proved that its camera is fully operational and equal to their main task: detecting Earth-like planets orbiting distant stars. Launched from Cape Canaveral on March 6, 2009, Kepler is equipped with a 95 megapixel camera, the largest ever launched into space. For the next three-and-a-half years this ultra-sensitive instruments will focus its sights continuously on a single batch of space near the constellations Cygnus and Lyra. This 100-square-degree region contains around 14 million stars, and 100,000 of these are the spacecraft's special focus, where it will search for orbiting exoplanets. To find these distant planets Kepler is relying on a method known as "transit photometry," which looks for the slight dip in a star's brightness when a planet passes between it and the Earth. While this method is responsible for the detection of several known exoplanets, its effectiveness is limited by the usual banes of Earth-bound astronomical observations: atmospheric interference, cloud coverage, and the regular alteration of night and day. These factors severely affect the precision of light measurements by Earth-bound telescopes, limiting the usefulness of the transit photometry technique. A space-based observatory such as Kepler changes all that. With no atmosphere to obstruct observations and no diurnal rotation Kepler can observe its selected stars continuously with unprecedented clarity and precision of measurement. A slight change in a star's luminosity, which would be lost in the "noise" of an Earth-based observation, will be clearly registered by Kepler, whose noise levels are 100 times lower. As a result Kepler will be able to detect planetary transits that could never be observed from a ground-based telescope.
Even though Kepler was specifically built to provide such accurate measurements, scientists were still amazed when they were first confronted with data from the spacecraft. "When the light curves from tens of thousands of stars were shown to the Kepler science team, everyone was awed" said Kepler's Principal Investigator William Borucki of NASA Ames. "No one had ever seen such exquisitely detailed measurements of the light variations of so many different types of stars." Detecting Earth-like planets orbiting distant stars is, but its nature, a slow process. To establish that a dip in a star's luminosity is indeed a planetary transit, scientists must first make sure that the dip is a regular event occurring at fixed intervals. This means that at least 3 separate transit events be observed for a star, and that the time lapse between the first and second suspected transits is equal to the time lapse between the second and the third. Since an Earth-like planet will presumably have an orbital period comparable to that of Earth, observing three transits will likely take years. That is why Kepler is slated to observe its patch of sky for three and a half years. Only once this long waiting period is complete will astronomers be able to announce the discovery of exo-Earth. But even if announcements of faraway Earths are still years away, valuable scientific data is already pouring in from Kepler. One of the stars targeted by Kepler called HAT-P-7 is about 1000 light-years from Earth and is already known to be home to a planet, which was previously detected by Earth-bound telescopes. The planet, known as HAT-P-7b, is a hot Jupiter 26 times closer to its star than Earth is to the Sun, completing each orbit in only 2.2 days. Kepler's high-precision measurements easily registered the transit of HAT-P-7b, but it also detected much more, providing scientists with an unprecedented wealth of information about the planet
As HAT-P-7b is observed moving around its star, it goes through "phases" just like the Moon when observed from Earth. In its "full" phase it contributes its reflected light to the light emanating directly from the star, while in its dark phase it only filters the star's light through its atmosphere. Finally, when the planet moves "behind" the star for a "secondary transit" (also known as an "occultation"), all light from the planet disappears altogether. Kepler's exquisite measurements were able to detect the planet's phases as well as its secondary transit. This enabled scientists to separate the light from the planet from the light emanating directly from the star. By analyzing the light from the planet scientists were able to conclude that the temperature on the planet's day side is about 2780 degrees Celsius (4310 degrees Fahrenheit).The temperatures on the night side, however, are a full 1000 degrees colder, indicating that there is little circulation taking place in the planet's atmosphere. This suggests that a dense layer of clouds in the planet's upper atmosphere blocks the star's radiation from penetrating deep into the atmosphere and instigating winds and storms. Even more importantly, Kepler's observations of HAT-P-7 demonstrate just how sensitive and reliable the spacecraft's sensors really are. The detection of the planet's phases and secondary transits requires a level of precision that is about 70% of what is required to detect an exo-Earth. "This early result shows the Kepler detection system is performing right on the mark" said David Koch of NASA Ames, who is the mission's deputy principal investigator. "It bodes well for Kepler's prospects to be able to detect Earth-like planets." Did you like this story? Send
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