Separate teams today announced the discoveries of three planets outside our solar system, including one that is more than three times farther away than the previous record holder.
Tricks used to find the giant worlds are expected to also allow detection of Earth-sized planets by the end of the decade.
Previously the most distant known planet was 5,000 light-years away. One of the newly spotted worlds and its host star are 17,000 light-years distant. They inhabit a crowded region of space toward the center of our Milky Way Galaxy.
The far-off planet is estimated to be about 1.5 times the mass of Jupiter and presumed to be similarly gaseous. It orbits the star about three times farther than Earth is from the Sun, or 3 astronomical units (AU). Jupiter is 5.2 AU from the Sun. Its presence does not surprise astronomers, but they are thrilled to have developed a new technique that can locate such faraway objects.
Finding other Earths
The discovery involved a trick of light called gravitational microlensing. Planet hunters have been eager for years to prove it could work, and they expect it will soon enable discoveries of smaller planets.
"The real strength of microlensing is its ability to detect low-mass planets," said study leader Ian Bond of the Institute for Astronomy in Edinburgh, Scotland.
The combined gravity of the newfound planet and its host star act like a lens, focusing light from a more distant star that is 24,000 light-years away in an effect predicted by Albert Einstein. The predictable resulting image, a much brighter view of the more distant star, allows observers to infer the presence of a planet that is otherwise invisible to traditional telescopic observations.
"It's a very good measurement," said Dimitar Sasselov, an astronomy professor and planet hunter at the Harvard-Smithsonian Center for Astrophysics. Sasselov was not involved in the discovery but has reviewed the data. "We know exactly what the signature would be. It is a star with a planet."
The dim foreground star, called a red dwarf, had never been seen before, so the discovery really was of a star and a planet.
"In principle, this technique could detect a planet as small as Earth," Sasselov told SPACE.com, but he added that it could take five years to perfect the method to that level of sensitivity. Nearly all of the more than 120 known worlds beyond our solar system are gas giants, most of them at least as massive as Jupiter.
The discovery will be detailed in the May 10 issue of Astrophysical Journal Letters. It was presented to reporters today in a NASA-led teleconference.
"Rich gallery" in store
Researchers expect in coming years to make actual direct observations of extrasolar planets, using a new generation of space-based observatories. For now, all of the planets found outside our solar system have been detected by indirect means.
Microlensing is now one of three proven indirect methods. It is the most effective for finding planets that are very far from their stars, similar to the distances of Jupiter or even Neptune in relation to the Sun, astronomers say.
The first and by far most successful technique used by planet hunters is called the Doppler or wobble method. It notes the slight wobble of a star caused by the gravitational tug of an orbiting planet. But it works only for relatively nearby stars out to about 160 light-years. It easily finds planets that are close to stars, but struggles to detect those orbiting at great distances.
The wobble method has been employed over the past few years to find most of the known extrasolar planets.
Another recently developed method detects a planet's shadow when it transits in front of its host star. This transit method, as it is called, works only for the small percentage of planets whose orbits happen to be perfectly aligned from our vantagepoint. It also can be used, however, on very distant stars.
Sasselov led a team early last year that first used the transit method to find a planet. It was the distance record-holder until today. He said the transit and lensing approaches are very complementary and combined will provide a "rich gallery of planets at large distances."
Such a census will inform the development of future space-based planet-hunting missions, said Philippe Crane, a scientist with NASA's Origins program. Crane said mission planners need to know the pedigree of the many planet systems out there. "What shapes and sizes do they come in?"
Crane said the newfound star and planet "looks like the kind of system where we might expect to find a full planetary system." But he added that it is so far away that no mission on the drawing boards could be expected to find any Earth-like worlds that might be there. The planned Terrestrial Planet Finder and Space Interferometry Mission would both have to set their sites on more nearby systems.
Two more new planets
In a separate announcement today, a renowned European team of planet hunters based at the Geneva Observatory has found two giant planets using the transit method.
They are the first worlds discovered in this way since last year's finding by Sasselov's team. Both planets are called "hot Jupiters," each similar in mass to Jupiter yet orbiting its star so closely that it makes an annual trip in less than two days.
"It's an exciting day for planet hunters," Sasselov said.
Both the transit and microlensing methods work best when telescopes are pointed toward the center of the galaxy, where stars are highly concentrated. By monitoring many stars at once, researchers up the odds of making discoveries.
Sasselov said he thinks the transit method will lead to the first detection of an Earth-size planet, when it is employed by NASA's space-based Kepler observatory, slated to launch in 3 years.
Years of planning
Gravitational microlensing has an interesting history. In 1986, Bohdan Paczynski of Princeton University first proposed using it to look for mysterious dark matter, unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets.
Some groups have tried the approach but had not provided convincing evidence for other worlds. Meanwhile, the phenomenon of lensing has proved effective, with researchers in several instances noting a spike in a distant star's light caused by the focusing effect from a closer star.
The new lensing observation revealed extra spikes of brightness, indicating two objects in the foreground rather than just one. The research team, led by Bond, concluded that one object -- the presumed planet -- is just 0.4 percent as massive as the other.
"I'm thrilled to see the prediction come true with this first definite planet detection through gravitational microlensing," Paczynski said in a statement today.
The discovery involved important and timely cooperation between two international research teams, the Microlensing Observations in Astrophysics (MOA) group, and the Optical Gravitational Lensing Experiment (OGLE) project, for which Paczynski is the U.S. leader.
Tricky timing
Lensing events are brief, lasting just days or weeks, because the two stars and Earth are all moving in relation to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years.
But the spikes representing the newfound planet lasted just hours and occurred more than once within several days of observations, made last July and August. And unlike other methods, the lensing cannot be repeated because the chance alignment never occurs again.
"It's time-critical to catch stars while they are aligned, so we must share our data as quickly as possible," said Andrzej Udalski, who directs OGLE observations out of Poland's Warsaw University.
The initial brightening was spotted by the OGLE program, using the 51-inch (1.3-meter) Warsaw Telescope at the Las Campanas Observatory in Chile. The planet's spikes were observed by the Mt. John Observatory in New Zealand. This modest observatory is just 24-inches (0.6 meters) in diameter.
The OGLE team has observed millions of stars and, in 2002, released all of its initial into the public domain so astronomers worldwide could use it to glean discoveries. The other two planets announced today, by the Geneva Observatory team, were also found using OGLE data.
The OGLE survey is funded in part by NASA and the National Science Foundation.