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Figure 2.

means. The near-infrared K-band (2.2-micron) image

for planets at separations and contrast levels around

of the planetary system (Figure 1) was obtained using

stars that were previously unreachable.

Left: the planets’ infrared color. Lower-mass objects, like 2M1207b (a 7 Jupiter-mass companion to a brown dwarf ) and two ~ 11 Jupitermass Pleiades candidate members exhibit similar characteristics as the HR 8799 planets. Right: HR 8799 planets plotted on exoplanet cooling tracks. For the estimated age of the system (60 million years), our derived luminosities are consistent with masses between 7 and 10 times that of Jupiter.

the Gemini North telescope, the Altair adaptive optics system, and the Near Infrared-imager and Spectrometer

For the past several years, our team has been


involved in two ADI surveys on large 8- to 10-meter telescopes. Our initial ADI survey—the Gemini Deep

The three planets were found using the “Angular

Planet Survey, led by David Lafrenière—involved the

Differential Imaging” or ADI observing technique,

observation of 85 nearby (< 25 parsecs) and young (less

which works as follows: after correcting for most

than 100 million year old) stars similar to the Sun with

of the turbulence in Earth’s atmosphere with an

the Gemini North telescope. No planets were found,

adaptive optics system (like the National Research

but good upper limits were derived: less than 8% of

Council Canada Herzberg Institute of Astrophysics’

Sun-like stars have a greater than five Jupiter-mass

(NRC-HIA) Altair system used on Gemini North),

planet in orbit between 30-300 AU. To remove the late-

the major component of residual “noise” in the image

type bias of the original survey I am currently leading

is stellar light scattered by surface irregularities from

a second survey that is focusing solely on massive

the telescope and instrument mirrors and lenses. This

young nearby stars—this time using the Gemini, Keck,

residual noise, because of its origin, has a fixed pattern

and Very Large Telescope facilities.

with respect to the telescope and camera orientation. To separate out any possible planets from scattered

Anatomy of the Discovery

starlight, the ADI technique relies on the slow rotation of the field of view (induced by the motion of the

The star HR 8799 was one of the first stars observed.

Earth) that occurs during tracking with an altitude/

The main reasons for its selection are simply its

azimuth telescope. While guiding on the star to keep

proximity (it lies 39 parsecs away), its estimated young

it registered at the detector’s center, the planets appear to slowly revolve around the star and against the

age (~60 million years), and its infrared excess— evidence that dust is orbiting the star and a possible

fixed pattern of stellar scattered light due to the field

indicator of planet formation. Two of the three planets

rotation. Computer software is then used to analyze

were initially found using data acquired at Gemini on

a sequence of many images and subtract the scattered

October 17, 2007. The system was then studied again in

stellar halo and reveal any previously undetected

follow-up observations using the Gemini and Keck II

nearby faint objects. The overall contrast gain with

telescopes. The outer two planets were also recovered

ADI is a factor of 10-100. This is a major improvement

in archival 2004 Keck data and the outermost one

that has opened a new regime, allowing us to search

found in Hubble Space Telescope data taken with the



Issue 38 - June 2009