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Two teams of astronomers made headlines in November after announcing they had photographed planets orbiting regular stars other than our own sun. (Such bodies are known as extrasolar planets, or exoplanets.) One of those planets, Fomalhaut b, the companion to a star called Fomalhaut some 25 light-years away, was spotted by the Hubble Space Telescope; a star known as HR 8799, nearly 130 light-years distant, was found by ground-based observations to harbor a system of at least three planets.
Now reprocessed images taken by Hubble in October 1998 show that the space-based observatory had picked up the signal of the outermost of HR 8799's planets (white dot at lower right in image) 10 years prior to that announcement. Astronomer David Lafrenière, a postdoctoral fellow at the University of Toronto and a member of the team that found the planets around HR 8799 last year, led the archival work. In a statement, he credited an improved image-processing technique that he and colleagues developed for uncovering the hidden planet, saying it can now be used "to see planets that are one-tenth the brightness of what could be detected before with Hubble."
Exoplanets are usually found by observing the dimming in a star's brightness as a planet passes in front of it, the technique that will be used by the newly launched Kepler spacecraft, or by monitoring the telltale wobble in a star induced by an orbiting body. Direct observation is trickier, not least because a star's glare drowns out most of the dim objects—like planets, for instance—in its vicinity. Direct-imaging techniques use a coronagraphic spot to blot out the star's glow; Lafrenière said that the new technique allows astronomers to subtract even the glare "that spills over the coronagraph's edge."
The two other planets orbiting HR 8799 could not be seen in the archival Hubble image because their tighter orbits around the star brought them within the area obscured by the coronagraph.
Now that the technique has been proved on a star known to harbor exoplanets, the researchers plan to turn it loose in the Hubble archives. "During the past 10 years Hubble has been used to look at over 200 stars with coronagraphy," astronomer Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia, said in the statement. Marois was part of the exoplanet teams that published their findings in Science in November and contributed to the new work as well. "We plan to go back," he said, "and look at all of those archived images and see if anything can be detected that has gone undetected until now."
Image credit: NASA, ESA, and D. Lafrenière (University of Toronto, Canada)