Stars and their planets orbit around a common center of mass, so a star with planets will move differently than a star without them.
Looking at how a star wobbles allows scientists to deduce how many and what types of planetary bodies are in orbit.
The wobble method has been especially successful at finding "hot Jupiters"—large Jupiter-mass planets that circle tightly around their stars.
(Related news: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)
But with current technology, astronomers using this method can study only systems that are tens to hundreds of light-years away.
By contrast, gravitational microlensing can study stars that are thousands of light-years away.
That's because the method uses the gravity of a nearby star and its planets to magnify the light from a much more distant background star.
"We wait for a system to pass in front of background star, and by monitoring the peaks and brightness we see we can figure out a lot about the planetary system," Gaudi said.
The better the alignment between the two stars, the better the magnification.
In the case of the two new siblings, the alignment between March and April of 2006 was so good that the background star was magnified by a factor of 500.
"This is the first time that we've found a Jupiter-size planet where the magnification was high enough that we could have detected a second, lower-mass system, and we found it the first time," Gaudi said.
"So that tells you either we're incredibly lucky or these things are actually quite common."
"The one thing about microlensing is you can't study the system in detail ever again," MIT's Seager said.
That's because the "lens" star and its planets will probably never pass in front of another background star with such good alignment again, she said.
But microlensing techniques do give scientists a way of taking a "population census" of the planets that are out there, study leader Gaudi said.
Jaymie Matthews, an astronomer and mission scientist for the MOST Space Telescope Project at the University of British Columbia in Vancouver, was not involved in the new study.
"The larger the sample that you can survey, the more reliable the results and the more likely that you will find the exceptional thing as well as the common ones," Matthews said.
"The next real milestone [for microlensing] that will truly excite the public and scientists will be the discovery of Earth-like planets in terms of Earth size and Earth mass."
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