A powerful, newly refined image-processing technique may allow
astronomers to discover extra solar planets that are possibly lurking in
over a decade's worth of Hubble Space Telescope archival data.
David Lafreniere 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
"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." Lafreniere 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 Lafreniere. "The
10-year-old Hubble data take us that much closer to having a precise measure of
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.
"The planet seems to be only partially cloud covered and we could be
detecting the absorption of water vapor in the atmosphere," says Travis Barman
of Lowell Observatory, Flagstaff, Ariz. "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 of the spectral features that would otherwise be there—including
water absorption bands," Barman says. "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. 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 and disks. 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. "We'll need a baseline of a few years for most
objects to detect Keplerian motion and hence confirm their status as planets.
The hardest part is to find them in the first place."
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.
Taking the image of an exoplanet is not an easy task. Planets can be billions
of times fainter than the star around which they orbit and are typically located
at separations smaller than 1/2000th the angular size of the full moon from
their star. The planet recovered in the NICMOS data is about 100,000 times
fainter than the star when viewed in the near-infrared.
"Even when using the best telescopes available, with the best resolution, the
light from the bright star spills out in the area where the much fainter planets
are located, making them impossible to see. It is essential to subtract out this
bright glare of stellar light from the image to see faint dots, i.e., planets,
that could be hidden underneath," says Rene Doyon of the University of Montreal.
The stability of how light is scattered in the NICMOS camera, called the
point spread function (PSF), is key for using Hubble images to recover planets.
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. This requires a reasonably steady PSF because images of different stars
are taken on different days. Atmospheric conditions would vary from day-to-day
for ground-based telescopes, but not for a space telescope that enjoys
unprecedented image stability over repeated visits to a target.
Space Telescope Science Institute, Baltimore, Md.
University of Toronto, Ontario, Canada
ESA, and G. Bacon (STScI)
Planets in the Solar System and