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Hubble peers through Milky Way, spots 16 planets

October 06, 2006 11:00 IST

The National Aeronautics and Space Agency has discovered 16 extrasolar planets in the central region of the Milky Way with its Hubble Space Telescope, hinting the possible existence of approximately six billion Jupiter-sized planets in the galaxy.

Five of the newly discovered bodies, the US space agency said, represent a new extreme type of planet not found in any nearby searches. Dubbed Ultra-Short-Period Planets, these worlds whirl around their stars in less than one Earth day.

"Discovering the very short-period planets was a big surprise," said team leader Kailash Sahu of the Space Telescope Science Institute, Baltimore.

"Our discovery also gives very strong evidence that planets are as abundant in other parts of the galaxy as they are in our solar neighbourhood," he added.

The planet bonanza, NASA said, was uncovered during a Hubble survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS).

Hubble looked farther than has ever successfully been searched before for extrasolar planets. It peered at 180,000 stars in the crowded central bulge of Milky Way galaxy 26,000 light-years away. That is one-quarter the diameter of the Milky Way's spiral disk.

The tally is consistent with the number of planets expected to be uncovered from such a distant survey, based on previous exoplanet detections made in solar neighbourhood, NASA said.

Hubble's narrow view covered a swath of sky no bigger in angular size than two per cent of the moon's area. When extrapolated to the entire galaxy, the data provides strong evidence for the existence of approximately six billion Jupiter-sized planets in the Milky Way.

Hubble could not directly view the 16 newly found planet candidates. Astronomers used Hubble's Advanced Camera for Surveys to search for planets by measuring the slight dimming of a star due to the passage of a planet in front of it, an event called a transit.

The planet will have to be about the size of Jupiter to block enough starlight, about one to 10 percent, to be measurable by Hubble. The planets are, however, called candidates because astronomers could only obtain follow-up mass measurements for two of them due to the distance and faintness of these systems.

Following an exhaustive analysis, NASA said, the team ruled out alternative explanations such as a grazing transit by a stellar companion that could mimic the predicted signature of a true planet. The finding could more than double the number of planets spied with the transit technique to date.

There is a tendency for the planet candidates to revolve around stars more abundant in elements heavier than hydrogen and helium, such as carbon. This supports theories that stars rich in heavy elements have the necessary ingredients to form planets, NASA said.

The planet candidate with the shortest orbital period, named SWEEPS-10, swings around its star in 10 hours. Located only 740,000 miles from its star, the planet is among the hottest ever detected. It has an estimated temperature of approximately 3,000 degrees Fahrenheit.

"This star-hugging planet must be at least 1.6 times the mass of Jupiter, otherwise the star's gravitational muscle would pull it apart," said SWEEPS team member Mario Livio, adding: "The star's low temperature allows the planet to survive so near to the star."

Dharam Shourie in New York
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