It wasn't until 2004 that astronomers released the first direct picture of an exoplanet, and that one was relatively easy to spot because it was circling a dim failed star known as a brown dwarf.
Study author Lafrenière was inspired to go on his retroactive planet hunt when he heard that scientists had taken the 1998 Hubble picture specifically to look for possible planets around HR 8799, but at the time had come up empty.
"You start with a set of several images of different stars obtained with the same instruments at the same wavelength," he said of the new starlight-removal method.
A computer program then combines the images to create a mock star that resembles the light from the real target star.
Subtracting that light from an image of the target star leaves behind any bright planetary dots that might be in orbit.
Astronomer Paul Kalas, also at UC Berkeley, found the first visible-light picture of an exoplanet in a 2005 Hubble image of the dusty disk around the star Fomalhaut.
Astronomers had suspected since the 1980s that a planet might exist there, but they needed more observations to confirm the find, which was announced in 2008.
"Proving that an object is a planet and not a faint background star requires evidence that the object moves along with the star in space," Kalas said.
"With archival [Hubble] data, we would be able to find planet candidates, but confirmation requires observations at later dates, after the star has moved a bit relative to the field of distant background stars."
Overall, Kalas said, a successful planet-finding program might combine archived telescope pictures with new data from ground-based observatories.
Improving techniques for direct imaging can offer more than just finding new planets, study author Lafrenière added.
Once astronomers can confirm that a given planet exists, pictures can tell us more about a planet's size, its chemical makeup, and its likely atmospheric and surface temperatures, he said.
Hubble's shot of the planet orbiting HR 8799, for example, supports theories that its atmosphere contains significant amounts of water vapor and is partially covered with dust clouds.
Also, most indirect planet-spotting techniques, such as the wobble method, aren't well equipped to find planets farther from their stars than Jupiter is from the sun, Lafrenière noted.
That's because they rely on a planet's gravitational influence on its host, an interaction that gets harder to detect farther away from the star.
But based on our own solar system, scientists know planets could exist in very distant orbits.
"So direct imaging is crucial," Lafrenière said, "to complete a census of the diversity of planetary systems that exist out there."
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