By Dennis Overbye
Thursday, November 13, 2008
SourceNEW YORK: In what might amount to an early Christmas present to the universe, two groups of astronomers have taken the first pictures of what they say - and other astronomers agree - are probably planets going around other stars.
The achievement, the result of years of effort on improved observational techniques and better data analysis, presages more such discoveries, the experts said, and will open the door to new investigations and discoveries of what planets are and how they came to be formed.
"It's the tip of iceberg. Now that we know they are there, there is going to be an explosion," said Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia. Marois is the leader of a team that recorded three planets circling a star - known as HR 8799 - 130 light years away in the constellation Pegasus.
The other team, led by Paul Kalas of the University of California, Berkeley, found a planet orbiting the star Fomalhaut, only 25 light-years from Earth, in the constellation Piscis Austrinus. In an interview by e-mail, Kalas said that when he finally confirmed his discovery last May, "I nearly had a heart attack."
In scratchy telescope pictures released to the world Thursday in Science Express, the online version of the journal Science, the new planets appear as fuzzy dots that move slightly around their star from exposure to exposure. Astronomers who have seen the new images agreed that these look like the real thing.
"I think Kepler himself would recognize these as planets orbiting a star following his laws of orbital motion," Mark Marley, of NASA's Ames Research Center in Mountain View, California, wrote in an e-mail message elaborating on HR 8799.
More than 300 so-called extrasolar planets have been found circling distant stars, making their discovery the hottest and fastest growing field in astronomy. But the observations have been made mostly indirectly, by dips in starlight as planets cross in front of their home star or by wobbles they induce going by it.
Astronomers being astronomers, they want to actually see these worlds, but a few recent claims of direct observations have been clouded by debates about whether the bodies were really planets or failed stars.
"Every extrasolar planet detected so far has been a wobble on a graph. These are the first pictures of an entire system," said Bruce Macintosh, an astrophysicist from Lawrence Livermore National Laboratory in California and a member of Marois's team.
The new planetary systems are anchored by young bright stars more massive than our own Sun and swaddled in large disks of dust, the raw material of worlds. The three planets orbiting HR 8799 are roughly 10, 9 and 6 times the mass of Jupiter, and orbit their star in periods of 450, 180 and 100 years respectively, all counterclockwise.
The Fomalhaut planet is about three times as massive as Jupiter, according to Kalas's calculations, and is on the inner edge of a huge band of dust, taking roughly 872 years to complete a revolution of its star.
Both systems appear to be scaled-up versions of our own solar system, with giant planets in the outer reaches, leaving plenty of room for smaller planets to lurk undetectable in the warmer inner regions. Dust rings lie even farther out, like the Kuiper belt of icy debris extending beyond the orbit of Neptune in our own solar system.
"This is a window into what our own solar system might have looked like when it was 60 million years old," Marois said. Our sun is about 4.5 billion years old.
Sara Seager, a planetary theorist the Massachusetts Institute of Technology, said it was significant that the planets in both cases seemed to be associated with disks of dust, particularly Fomalhaut, one of the brightest and closest stars and one of the "Fabulous Four," known to be host to a massive disk.
"Fomalhaut is like a Hollywood star to astronomers, so we have some personal excitement here," she said. "It feels like finding out that one of your four closest friends just won the lottery big time"
Alan Boss, a planetary theorist at the Carnegie Institution of Washington, said the triple-planet system in Pegasus was particularly convincing, "as we expect planets to form in systems in general, whereas spurious background interlopers will generally appear as single 'planets."' But he and others cautioned that much more study of these objects was necessary.
Being able to see planets directly opens the door to spectroscopic observations that can help determine the composition, temperature and other physical characteristics of planets and allow for comparisons with each other and with their parent stars. Macintosh said that he hoped to train a spectroscope on his new planets as early as Monday.
The new images are the first fruits of a long campaign by astronomers to see more and more of the unseeable. In particular, it is a triumph for the emerging technology of adaptive optics, in which telescope mirrors are jiggled and warped slightly many times a second to compensate for the atmospheric turbulence that blurs star images.
The problem in seeing other planets is picking them out of the glare of their parent stars, which are millions of times brighter, at least in visible light. As a result, planet hunters usually look for infrared, or heat radiation, which is emitted copiously by planets still shedding heat from the process of formation.
For their observations, Marois and his colleagues used the 8-meter diameter, or 26-foot diameter, Gemini North and the 10-meter Keck telescopes on Mauna Kea in Hawaii, both of which had been fitted with adaptive optics. Then they processed the images with a special computer program, which Marois described as "a software coronagraph," for processing the images.
A coronagraph is a telescope for observing things very close to the Sun. It uses a disk to block the Sun's bright surface.
Marois and his team had spent eight years looking for planets around Sun-like stars with no success. So they decided last year to look around younger and thus more massive and hotter stars, like HR 8799, which is estimated to be only 60 million years old, compared with the Sun, which is 4.5 billion years. The planets of such stars would be younger themselves and thus hotter and easier to see. More massive stars would also lead to more widespread disks and planets, making them easier to pick out, Marois explained.
They first spied a pair of dots about four billion miles and six billion miles out from HR 8799 last October. Following up, they discovered a third planet closer in, at about two billion miles. Subsequently, they discovered an old observation from 2004, which also showed the planets and how far they had moved around the star in three years. "Seeing the orbit is one of the coolest things," Macintosh said.
Kalas did his work with the Hubble Space Telescope, which is immune to atmospheric visual turbulence because it is in space. He used a coronagraph to block light from the actual star. He said he had been driven to look for a planet around Fomalhaut after Hubble photographs in October 2004 showed that a dust ring around the star had a suspiciously sharp inner edge, often a clue that the ring is being sculpted by the gravity of some body orbiting nearby.
Fomalhaut is also a young star, about 200 million years old, according to Kalas.
A second set of Hubble observations, in July 2006, revealed a dot moving counterclockwise around the star. "I basically held my breath for three days until I could confirm the existence of Fomalhaut in all of my data," Kalas recalled.
Because the outer edge of Fomalhaut's dust ring, which extends 11 billion to 20 billion miles from its planet, was not disturbed, the planet must be less than three times the mass of Jupiter, Kalas and his collaborators, including Eugene Chiang of the University of California, Berkeley, calculate in a separate paper to be published in the Astrophysical Journal.
In an e-mail message, Kalas pointed out that Fomalhaut was the closest exoplanet yet discovered, "close enough to contemplate sending spacecraft there."
Boss said the fact that Marois's group had found a triple planet system in the first batch of stars they searched meant either that they were lucky or that such systems were common. "Further observations will decide which is the case," he said.
A large consortium of astronomers and institutions, including Macintosh and Kalas, are building the Gemini Planet Imager, to be placed on the Gemini South telescope in Chile, a twin of the one on Mauna Kea, which will increase sensitivities tenfold to a hundredfold, enabling astronomers to see and study giant planets in systems the same size as our own.
It will take a new generation of space-based telescopes like the Terrestrial Planet Finders, now being studied by NASA, before astronomers can hope to discern rocky, Earth-size planets that they might want to examine for habitability and life.