By peering some 20,000 light years into the star-swarms of the Milky Way's central bulge, astronomers using the Hubble Space Telescope have turned up 16 possible new planets crossing the faces of their stars, two of which have been confirmed. Each one of the candidates is a roughly Jupiter-size body that orbits its host star in just 4 days or less. Five of the possible discoveries whip around their stars in less than an Earth day — causing the astronomers to group them into a new class of "ultra-short-period planets."
In results published in this week's edition of the journal Nature the team of astronomers, headed by Kailash Sahu (Space Telescope Science Institute), used Hubble's Advanced Camera for Surveys to monitor a portion of the galactic bulge in Sagittarius continuously for seven days, shooting one picture every six minutes. Over 180 planet candidates were identified, making the task of arriving at only 16 quite difficult. The biggest source of false alarms was transits by Jupiter-size red dwarf stars. Processing the data took an entire year. "This was the most complex project I have worked on," says Sahu.
This discovery now allows scientists to be relatively certain planets need elements heavier than hydrogen and helium to form. These are typically found in stars that formed after others died, creating a dependence on location and formation. A previous search in 2000 by Dr. Ron Gilliland turned up no planets, even though he assumed they would find as many as 17. The currently accepted reason is that Gilliland's study focused on a stellar nebula, where all of the stars had formed at the same time, and therefore there was a very low abundance of heavier elements.
Sahu knew some scientists would be skeptical of his findings and so he confirmed two of the planets by using the Very Large Telescope (VLT) in Chile to measure the movements of the stars caused by their orbiting partner, a technique known as measuring the radial velocity of the star. Although Sahu believes that at least fifty percent of the candidates have to be planets, he cannot be completely certain. "I think a lot of people will take issue with that," says Dr. Sara Seager, Staff Scientist of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, on Sahu's uncertainty. "There are lots of nasty tricks nature can pull on us."
More rigorous confirmation will have to wait for new technology, specifically NASA's James Webb Space Telescope, to be launched in 2013. Even so, Seager believes this is still "tremendously exciting. It doubles the amount of transiting planets found!"
With two other transiting extrasolar planets announced in September, the confirmed SWEEPS finds brings the number of known — and confirmed — transiting planets to 14.
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Hubble discovers 16 new planets