An artistic illustration of the giant planet HR 8799b. The planet is young and hot, at a temperature of 815 degrees Celsius. It is slightly larger than Jupiter and might be 10 times more massive.
Hubble finds a hidden planet
3 Apr 2009
A powerful, newly refined image-processing technique may enable astronomers to discover 'exoplanets' that are possibly lurking in over a decade's worth of Hubble Space Telescope archival data. Exoplanets are ones that orbit stars outside our Solar System.
David Lafreniere of the University of Toronto, Ontario, Canada, has successfully demonstrated the 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 Hawaii in 2007 and 2008. It is the outermost of three massive planets known to orbit the dusty young star known as HR 8799, which is 130 light-years away.
Hubble's 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 Lafreniere. "Our new image-processing technique efficiently subtracts the glare from a star … allowing us to see planets that are one-tenth the brightness of what could be detected before with Hubble."
Using the new technique, Lafreniere 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 Lafreniere. "The 10-year-old Hubble data take us that much closer to having a precise measure of the orbit."
The archival Hubble image shows the planet as a small smudge in the lower right. The glare of the star has been blocked out in the centre.
NICMOS's view also provided new insights into the physical characteristics of the planet. This was possible because NICMOS works at near-infrared wavelengths that are severely blocked by Earth's atmosphere due to absorption by water vapour.
"The planet seems to be only partially cloud covered and we could be detecting the absorption of water vapour in the atmosphere," says Travis Barman of Lowell Observatory, Flagstaff, Arizona. "Measuring the water absorption properties will tell us a great deal about the temperatures and pressures in the atmospheres, in addition to the cloud coverage."
"If we can accurately measure the water absorption features for the outermost planet around HR 8799, we will learn a great deal about their atmospheric properties," Barman added. "Hubble, situated well above the Earth's atmosphere, is excellently located for such a study."
"During the past 10 years Hubble has been used to look at over 200 stars with coronagraphy, looking for planets … We plan to go back and look at all of those archived images and see if anything can be detected that has gone undetected until now," says Christian Marois of the Herzberg Institute of Astrophysics, Victoria, Canada.
If his team sees a companion object to a star in more than one NICMOS picture, and it appears to have moved along an orbit, follow-up observations will be made with ground-based telescopes. If they see something once but its brightness and separation from the star would be reasonable for a planet, they will also do follow-up observations with ground-based telescopes.
Adapted from information issued by STScI / NASA / ESA / G. Bacon (STScI) / Z. Levay (STScI) / D. Lafrenière (University of Toronto, Canada).
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