Hubble Finds Hidden Exoplanet in Archival Data

A powerful, newly refined image-processing technique may allow astronomers to discover extrasolar planets that are possibly lurking in over a decade's worth of Hubble Space Telescope archival data.

David Lafrenière of the University of Toronto, Ontario, Canada, has successfully demonstrated this new strategy for planet hunting by identifying an exoplanet that went undetected in Hubble images taken in 1998 with its Near Infrared Camera and Multi-Object Spectrometer (NICMOS). In addition to illustrating the power of new data-processing techniques, this finding underscores the value of the Hubble data archive, on which those new techniques can be used.

The planet, estimated to be at least seven times Jupiter's mass, was originally discovered in images taken with the Keck and Gemini North telescopes in 2007 and 2008. It is the outermost of three massive planets known to orbit the dusty young star HR 8799, which is 130 light-years away. NICMOS could not see the other two planets because its coronagraphic spot -- a device which blots out the glare of the star -- also interferes with observing the two inner planets.

"We've shown that NICMOS is more powerful than previously thought for imaging planets," says Lafrenière. "Our new image-processing technique efficiently subtracts the glare from a star that spills over the coronagraph's edge, allowing us to see planets that are one-tenth the brightness of what could be detected before with Hubble." Lafrenière adapted an image reconstruction technique that was first developed for ground-based observatories.

Using the new technique, he recovered the planet in NICMOS observations taken 10 years before the Keck/Gemini discovery. The Hubble picture not only provides important confirmation of the planet's existence, it provides a longer baseline for demonstrating that the object is in an orbit about the star. "To get a good determination of the orbit we have to wait a very long time because the planet is moving so slowly (it has a 400-year period)," says Lafrenière. "The 10-year-old Hubble data take us that much closer to having a precise measure of the orbit."

NICMOS's view provided new insights into the physical characteristics of the planet, too. This was possible because NICMOS works at near-infrared wavelengths that are severely blocked by Earth's atmosphere due to absorption by water vapor.