Hubble finds hidden exoplanet in archival data

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 02 April, 2009

A powerful image processing technique may allow astronomers to seek out exoplanets that could be lurking in over a decade’s worth of Hubble Space Telescope data.

This new planet hunting strategy was successfully demonstrated by David Lafreniere of the University of Toronto, who identified an exoplanet that went undetected in Hubble images taken in 1998 with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

Artist impression of the giant planet HR 8799b, which was identified in the NICMOS archival data in a follow-up search to see if Hubble had also serendipitously imaged it. Image: NASA, ESA, and G. Bacon (STScI).

The exoplanet was first identified in images taken with the Keck and Gemini North telescopes in 2007 and 2008, and is estimated to be at least seven times Jupiter’s mass. It orbits a dusty young star, HR 8799, which is located 130 light years away. The new planet is the outermost of three massive gas giants, but NICMOS was unable to observe the other two planets because its coronagraphic spot - a device which blots out the glare of the star - also interfered 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 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.”

The key to using the Hubble images to recover the planets is based on the stability of how light is scattered in the NICMOS camera, called the point spread function (PSF). This technique works by taking images of different stars and combining them to create a PSF of a star that closely resembles the star that is being studied for planets. For ground-based telescopes this is particularly challenging, since the technique relies on a reasonably steady PSF, which is hindered by atmospheric conditions that vary on a day-to-day basis. A space-based telescope, like Hubble, however, has the prime location above the Earth’s atmosphere, enabling unprecedented image stability over repeated visits to the same target.

The Hubble picture not only provides important confirmation of the planet’s existence, but also valuable data that proves it is orbiting a 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.”

NICMOS coronagraphic image of a planet orbiting HR 8799. The coronagraph has been used to block the light from the bright star (black circle) allowing the search for the dim glow of the planet HR 8799b. A special image-processing algorithm was used to suppress the starlight bleeding around the coronagraph to the point where the planet was detectable. Image: NASA, ESA, and D. Lafrenière (University of Toronto, Canada).

New insights into the physical characteristics of the planet were also gleaned from the processed image, suggesting that it is only partially cloud covered. “We could be detecting the absorption of water vapour in the atmosphere,” describes Travis Barman of Lowell Observatory in Flagstaff, Arizona. “The infrared light measured from the Hubble data is consistent with a spectrum showing a broad water absorption feature (at 1.4-1.49 microns), but the level of absorption seen is lower than it would be if the photosphere were completely devoid of dust.” Dust clouds can smooth out many spectral features, including water absorption bands. “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.”

Over the last decade, Hubble has been used to look at over 200
stars with coronagraphy. The team now plan to go back and examine all of the archived images to see if they can uncover any further planets that have previously gone undetected. “We’ll need a baseline of a few years for most objects to detect Keplerian motion and hence confirm their status as planets,” comments Christian Marois of the Herzberg Institute of Astrophysics in Canada. “The hardest part is to find them in the first place.”

If a companion object to a star is seen in more than one NICMOS picture and it appears to have moved along an orbit, or an object is seen once but its brightness and separation from the star would be reasonable for a planet, follow-up observations will be made with ground-based telescopes. As well as demonstrating the power of new data processing techniques, the new finding underscores the value of the Hubble data archive.