planets are not sufficiently massive to sustain nuclear
sequence of observations. All the noise induced by
fusion, planets steadily cool with time from their
the telescope and instrument defects then stays fixed
initial hot state at formation. After about 100 million
with time relative to the instrument detector, while the
years, gas-giant planets are cool enough that many
image of planet revolves in the field of view around the
complex molecules can form in their atmospheres—a
bright star. The images are then analyzed to subtract
process that does not occur for ordinary stars. These
the static noise and retain the planet’s flux. ADI was
molecules include the formation of gaseous methane
first successfully used for science in the early part of
(CH4), which produces very strong features. One of
this decade by the two of us independently, M. Liu at
the strongest is at 1.6 microns, where the planet flux is
the W. M. Keck Observatory and C. Marois at Gemini
bright at wavelengths bluer than 1.6 microns and then
North (where ADI imaging was actually one of the
sharply drops to nearly zero at redder wavelengths. In
first science observations made with the Altair AO
contrast, stars have nearly blackbody spectra, smoothly
system). Recent improvements in the post-processing
continuous
acquires
reduction methods made by David Lafrenière have
simultaneous imaging at two (or more) wavelengths
further increased the power of this technique (the
adjacent to this sharp 1.6 micron drop to counteract the
“LOCI” algorithm). ADI was the key method used
time-variable images produced by the Earth’s turbulent
for the discovery of the HR 8799 multi-planet system,
atmosphere.
first detected in 2007 with the Gemini North telescope
at
these
wavelengths.
SDI
(Figure 1). Subtraction of the two spectral imaging channels removes the bright (methane-free) glare of the target stars and reveals any faint, ultracool (methane-bearing) planetary companions. Since the images at different wavelengths are acquired simultaneously, the images of the bright star suffer the same atmospheric AO residual and instrument aberrations at both wavelengths, producing much better subtraction than if the images were acquired non-simultaneously. The TRIDENT near-infrared camera was the first instrument to test
Up to now, three large-scale direct imaging surveys
this approach, installed at the Canada-France Hawai‘i
for exoplanets have been completed, or are ongoing, at
Telescope on Mauna Kea in 2001 as part of the Ph.D.
Gemini. The first one, the Gemini Deep Planet Survey
thesis work of one of us (C. Marois, under the direction
or GDPS (led by David Lafrenière, then at the University
of René Doyon, Daniel Nadeau, and René Racine at the
of Montreal), was carried out between 2004 and 2007.
University of Montreal). It has also been employed for
A total of 86 of mainly GKM stars have been acquired
the SDI camera at the Very Large Telescope and is one
with ADI for an hour with Altair/NIRI at Gemini
of the key features of Gemini’s new NICI instrument,
North. No planet was found, but the survey placed
described in more detail below.
very good constraints on the presence of Jupiter-like planets in wide (30-300 astronomical unit (AU)) orbits.
Angular Differential Imaging (ADI): ADI is an
GDPS concluded that less than 9% of GKM stars have a
observing strategy that uses Earth’s rotation to separate
3-12 Jupiter-mass planet between 50-250 AU.
the light of a planet from the light of a bright star. It thereby allows very accurate star light subtraction
An extension of the GDPS survey, called the
by post-processing in software. Large 8- to 10-meter
International Deep Planet Survey (IDPS), led by
telescopes are usually built such that the telescope
one of us (C. Marois) began in late 2007. The survey
rotation axes differ from the Earth’s axis, leading to
combines the use of the Gemini Telescopes, Keck, and
rotation of images as the telescope tracks an object in
VLT to acquire 1-2 hour ADI sequences of 100 nearby
the sky over time. To compensate for this field rotation,
young, high-mass (A- and F-type) stars. The main
telescopes usually employ an instrument rotator. With
goal is to remove the late-type bias of the GDPS and
ADI, the rotator is turned off, causing the instrument
study planet formation for a wide range of stellar host
to be perfectly aligned with the telescope for an entire
masses. This survey made a breakthrough discovery
December2009
Figure 1.
Before (left) and after (right) ADI processing of Altair/ NIRI HR 8799 data acquired in September 2008 at K-band. The left panel shows a 30-second exposure filtered to remove the smooth radially symmetric halo, while the image to the right is the combination of 60 ADI-processed 30-second exposures. The three 7- to 10-Jupiter mass planets are easily visible in the ADIprocessed images, while they were completely hidden by scattered light from the primary in the raw image. The solid white line in the right panel represents a one arcsecond angular separation. The concentric circles are the orbits of Jupiter, Saturn, Uranus, Neptune, and Pluto, viewed face-on at the distance of HR 8799 (39 parsecs or 127 light years). The central part of the images are masked out due to detector saturation. Both images are displayed with the same linear intensity range and have a 5.6 x 5.6 arcseconds field of view. North is up and east is left.
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