Kepler employs the transit method, which watches for the tiny, telltale dips in a star's brightness caused when a planet crosses the star's face, blocking some of its light. Radial velocity looks for minuscule wobbles in a star's movement caused by the gravitational tugs of orbiting planets.
While these two methods have been highly productive, they're biased toward finding planets that orbit relatively close to their parent stars. In the new study, Cassan and his colleagues employed a different technique, known as gravitational microlensing, that feels this bias less strongly.
In gravitational microlensing, scientists watch what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object's gravitational field bends and magnifies the light from the distant star, acting like a lens.
This produces a light curve — a brightening and fading of the faraway star's light over time — whose characteristics tell astronomers a lot about the foreground object.
In many cases, this nearby body is a star. If it has any planets, even ones in relatively far-flung orbits, these can generate secondary light curves, alerting researchers to their presence.
Studying millions of stars
In the new study, the researchers looked at data gathered by a variety of Earth-based telescopes, which scanned millions of stars from 2002 to 2007 for microlensing events.