In the hunt for solar systems beyond our own, astronomers have crossed an important threshold – 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’s 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 – measuring a regular wobble in a star’s 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 – 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.
The approaches he and others use are detecting planets that the other techniques can’t, he explains. With three announcements in two months, “the floodgates are beginning to open. We’re learning about a whole new population of quite massive companions that are in fairly far-out orbits from their star.”
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 – 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’t, can you call its companion a planet?
In the end, it doesn’t 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 “compelling images” of companions “clearly orbiting stars.” Dr. Marley is a research scientist at the National Aeronautics and Space Administration’s Ames Research Center at Moffett Field, Calif.
Exhibit A is the three-planet system found by a team led by Christian 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’s estimate, one planet tips the cosmic scales at seven times Jupiter’s mass. The other two are 10 times as massive as Jupiter.
“The planets are a lot more massive than in our solar system, but they are at a comparable separation” from each other and from their host star, Dr. Marois says. “So you can believe that they formed in a way similar to the planets in our home solar system.”
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’s mass orbiting Fomalhaut, a star 25 light-years from Earth.
And in September, a team led by David Lafrenière imaged a planet estimated to be six to 12 times Jupiter’s 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’ heat signature and even analyze its spectrum for clues as to a planet’s 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.
“If you really want to study planet formation and evolution, you need to look backwards in time – and finding planets around progressively younger stars is just the way to do it,” says Lowell Observatory astronomer Dr. Barman.