For the first time, amateur and professional astronomers have teamed up to discover a new planet circling a distant star. The planet was detected by looking for the effect of its gravitational field on light from a more distant star, a technique known as microlensing. It is only the second world to be discovered using the microlensing technique. Gravitational microlensing offers the potential for detecting Earth-mass planets using existing or near-future technologies.

Astronomers discovered a new planet when it and its host star crossed in front of a very faint background star (marked by crosshairs) and magnified the distant star's light in a process called gravitational microlensing. Although the newfound planet is 3 times the size of Jupiter, microlensing offers the possibility of detecting earth-sized worlds using existing technologies. Credit: Optical Gravitational Lensing Experiment

"This discovery is the tip of the iceberg for microlensing searches," said astronomer Scott Gaudi (Harvard-Smithsonian Center for Astrophysics). "With improving technologies and techniques, the first Earth-sized planet may be found by microlensing."

"If an Earth-mass planet was in the same position [as the planet we found], we would have been able to detect it," agreed professor Andrew Gould (Ohio State University), a member of the team who made the discovery.

Microlensing searches scored their first discovery only last year. This second find confirms the power of this planet-hunting technique and heralds the beginning of routine planet detection by microlensing.

"This is the beginning of the age of microlensing. We expect many more discoveries in the coming years," said Gaudi.

The newfound planet weighs approximately 3 times as much as Jupiter and probably orbits a star similar to the sun. At the time of its discovery, it was located about 3 times the earth-sun distance from its host star. Although its orbit is uncertain, the possibility of a "hot Jupiter" that revolves very close to its star was ruled out.

Both the planet and its star are located about 15,000 light-years from the earth, making this world one of the most distant ever discovered. Gravitational microlensing offers unique advantages for astronomers hunting planets: not only can it find more distant worlds than more common techniques such as the radial velocity, or "wobble," method, but microlensing also is more sensitive to smaller worlds.

"Even the signal from an earth-mass planet can be relatively large - tens of percent, which is very detectable," said Gaudi.

Gaudi played a key role in confirming the new planet. Although a microlensing event can last days, the presence of a planet will affect the signal for only a day or so. Therefore, data must be analyzed as quickly as it is gathered to identify the events that merit close watching.

"I was in Korea when we saw that this event was doing something strange and different," said Gaudi. "When I first looked at the deviation, I thought it looked like a planet. As soon as I got home to Cambridge, I downloaded the data and worked on it for 12 hours straight to analyze it. I figured out that there was no other explanation. It had to be a planet."

Detecting the planet's signal required obtaining data from many observatories around the world so that the microlensing event could be monitored around-the-clock. Gaudi is a member of the Microlensing Follow Up Network (MicroFUN), which is headed by Gould.

"There was an old saying that the sun never set on the British Empire. You could say that the sun never rises on our collaboration!" said Gaudi.

Two New Zealand amateur astronomers belong to the MicroFUN collaboration-Grant Christie of Auckland, who used a 14-inch-diameter telescope, and Jennie McCormick of Pakuranga, who used a 10-inch telescope. Data from both telescopes proved crucial in detecting the planet, and both observers share co-authorship on the paper announcing the find.

"These amateur astronomers work all day, then go home and observe all night," said Gaudi. "Their contribution is a testament to how far amateur astronomers have come. It speaks highly of their dedication to the field."

The microlensing event was detected and monitored extensively by the Optical Gravitational Lensing Experiment, or OGLE, headed by Andrzej Udalski of Warsaw University. Two other collaborations-the Probing Lensing Anomalies NETwork (PLANET) and Microlensing Observations in Astrophysics (MOA) -also followed the event and contributed to the journal paper.

Gravitational microlensing occurs when a massive object in space, like a star or even a black hole, crosses in front of a star shining in the background. The object's strong gravitational pull bends the light rays from the more distant star and magnifies them like a lens. On earth, observers see the star get brighter as the lens crosses in front of it, and then fade as the lens gets farther away. The presence of a planet orbiting the nearby star will modify this otherwise smooth process in a predictable way.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.