海角大神

In the hunt for solar systems beyond our own, astronomers have crossed an important threshold 鈥� capturing from the ground, as well as from space, the first direct images of planets around bright, sun-like stars.

Since September, three teams say they have imaged planet-candidates. The most recent reports, published in Thursday鈥檚 issue of the journal Science, include a three-planet system around a variable star 128.5 light-years from Earth.

Until now, researchers have had to content themselves with shadowy, indirect approaches to finding planets 鈥� measuring a regular wobble in a star鈥檚 spectrum as gravity from its massive planets orbit and tug on it, or the cycle of brightening and dimming a star appears to experience as a planet crosses its face.

Over the past 20 years, such techniques have bagged more than 300 planets in solar systems in the Milky Way 鈥� some as distant as 17,000 light-years from Earth. Now, astronomers are beginning to spot the tiny specs of planets directly. What they can see, they can hope to follow up with more-detailed studies.

鈥淭his is the beginning of a new era,鈥� says Ray Jayawardhana, an astronomer at the University of Toronto and a member of one of the three teams.

The approaches he and others use are detecting planets that the other techniques can鈥檛, he explains. With three announcements in two months, 鈥渢he floodgates are beginning to open. We鈥檙e learning about a whole new population of quite massive companions that are in fairly far-out orbits from their star.鈥�

In many ways, the door to direct detection from the ground opened four years ago.

A team of astronomers led by Gael Chauvin, a researcher with the European Southern Observatory, found what appeared to be a giant companion to a brown dwarf 鈥� a dim star wannabe that never grew massive enough to ignite its fusion furnace.

The find still triggers some heated discussions. Is a brown dwarf really a star? If it isn鈥檛, can you call its companion a planet?

In the end, it doesn鈥檛 matter, says Travis Barman, an astronomer at the Lowell Observatory in Flagstaff, Ariz. It represents a valuable anchor for one end of a spectrum of objects around which planet-like companions form.

Whatever the outcome of those discussions, the discoveries published this week represent what astronomer Mark Marley calls 鈥渃ompelling images鈥� of companions 鈥渃learly orbiting stars.鈥� Dr. Marley is a research scientist at the National Aeronautics and Space Administration鈥檚 Ames Research Center at Moffett Field, Calif.

Exhibit A is the three-planet system found by a team led by 海角大神 Marois, with the Herzberg Institute of Astrophysics in Victoria, British Columbia. The planets orbit a young star in the constellation Pegasus, dubbed HR 8799. By the team鈥檚 estimate, one planet tips the cosmic scales at seven times Jupiter鈥檚 mass. The other two are 10 times as massive as Jupiter.

鈥淭he planets are a lot more massive than in our solar system, but they are at a comparable separation鈥� from each other and from their host star, Dr. Marois says. 鈥淪o you can believe that they formed in a way similar to the planets in our home solar system.鈥�

Exhibit B comes courtesy of the Hubble Space Telescope. A team led by Paul Kalas of the University of California at Berkeley spotted what they estimate to be a planet less than three times Jupiter鈥檚 mass orbiting Fomalhaut, a star 25 light-years from Earth.

And in September, a team led by David Lafreni猫re imaged a planet estimated to be six to 12 times Jupiter鈥檚 mass orbiting a star some 473 light-years away.

Two broad factors now make these observations possible, researchers say. One involves technology.

With large and growing hunks of glass being devoted to ground-based telescopes, and with more sensitive detectors, astronomers are better able to spot planets. They do this with hardware as well as software that in effect dims the star.

The other factor involves the targets they choose. By focusing on young solar systems, planets are still gathering material and contracting. So they give off heat. All this makes it easier for increasingly sensitive infrared detectors to pick up the planets鈥� heat signature and even analyze its spectrum for clues as to a planet鈥檚 composition. The holy grail, of course, is to find Earth-like planets at Earth-like distances from sun-like stars. That is likely to await a new generation of space-based telescopes, such as the Kepler mission scheduled for launch in March 2009, or the James Webb telescope, slated for launch in 2013.

Even then, many questions remain about solar-system evolution and the variety of configurations solar systems exhibit.
鈥淚f you really want to study planet formation and evolution, you need to look backwards in time 鈥� and finding planets around progressively younger stars is just the way to do it,鈥� says Lowell Observatory astronomer Dr. Barman.