The four planets -- one circling the star Fomalhaut and three circling the star HR 8799-- are not the first extrasolar planets to be discovered; over the past 10 years scientists have found more than 300 orbiting distant stars.
But until now, astronomers generally inferred the planets' existence using the "wobble method," a technique that involves measuring the gravitational effect a planet has on its host star.
In this week's journal Science, a team led by University of California-Berkeley researcher Paul Kalas announced that they had found a planet in images taken by the Hubble Space Telescope. The images aren't all that different from ones taken using a digital camera, according to Kalas.
"What you're actually looking at is the star itself," he said.
At 25 light years from earth, the new planet known as Fomalhaut b is believed to orbit its star at a distance four times that between Neptune and the sun. The star is also surrounded by a dust belt. Gravity from the planet distorts the dust belt, so by studying the distortions, scientists were able to confirm that what they saw in the Hubble images was indeed a planet, and to roughly estimate its mass.
"Fomalhaut b is orbiting around the inside of this dust belt and dynamically sculpting and shaping it," said Mark Clampin, a project scientist at the NASA Goddard Space Flight Center.
Exactly what can be seen in the Fomalhaut photos is not entirely clear. The light could be the molten hot glow of Fomalhaut b itself, or it could be light reflected off the surrounding dust belt.
"It's a puzzle," Kalas said. "And it's actually very exciting. It's confounding in a way, and that's made it quite challenging for us to look at the data."
Kalas has had his eye on Fomalhaut for 15 years, when he studied the star's dust ring as a graduate student. But when he later he confirmed that what he was seeing was a planet, he said he nearly had a heart attack.
"No one had ever seen planets through such images orbiting a star," he said.
In a conference call with reporters, the researchers said they hope that with the refurbishment of the Hubble Space Telescope, scientists will be able to find more exoplanets, and one day, hopefully, an earthlike planet.
Friday's edition of Science also included a report from another team of scientists, led by astronomer Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia. In infrared images taken by two ground-based telescopes in Hawaii, Marois and his colleagues discovered three planets orbiting the star HR 8799, our relatively nearby neighbor at 25 million light years away.
"I would have been happy with one planet, but finding three is absolutely amazing. I searched for these planets for eight years," Marois told the Washington Post.
Marois and his colleagues estimate that the newly discovered planets are seven to ten times the size of Jupiter.
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