To date, more than 150 planets have been found in orbit around distant stars using a technique that detects the gravitational tug the unseen planets exert on their parent stars.
The method favors detection of large planets that orbit close to their hosts.
But the newfound planet is the third to be revealed using a different technique known as gravitational microlensing, which is based on an effect predicted by Albert Einstein in 1915.
It works like this: When a star passes precisely in front of another star, the gravity of the star in front bends the light from the background star.
The bending effect acts like a lens that amplifies the light of the background star, making it brighter.
The extra radiance can last from a few days to a year. If the forward star happens to have a companion planet, it will produce a defect in the brightening that usually lasts from hours to days.
Scientists analyze this defect to calculate the mass and orbit of the companion planet.
To catch and characterize planets with this technique requires continuous round-the-clock monitoring of ongoing microlensing events. Such an effort demands cooperation of astronomers around the world.
The event that uncovered OGLE-2005-BLG-390Lb was first detected on July 11 by the Optical Gravitational Lensing Experiment (OGLE) search team, led by Andrzej Udalski of Warsaw University in Poland.
The team monitors the brightness of millions of Milky Way stars with a telescope in Chile. When the event was detected, the OGLE search team alerted astronomers around the world.
In turn, the newfound planet was first detected by astronomers on the Probing Lensing Anomalies Network (PLANET) team, led by Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris, on August 10. Other groups later verified the find.
To date, gravitational microlensing has yielded three planets in orbit around distant stars. The first two planets have masses a few times greater than Jupiter's.
"This [new] planet is actually the first and only planet that has been discovered so far that is in agreement with the theories for how our solar system formed," team member Uffe Gråe Jøgensen of the Niels Bohr Institute in Copenhagen, Denmark, said in a media statement.
According to those theories, the bits of dust swirling around a newborn star glom together and form rocks, asteroids, and ultimately small planets. A few planets go on to pull in gas, becoming giant planets.
Around a red dwarf, the type of star OGLE-2005-BLG-390Lb orbits, the model suggests there should be lots of small planets and few large ones.
Williams, the Perth Observatory astronomer, says the discovery of a planet using the microlensing technique confirms the belief that Earthlike planets may be common in the universe.
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