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The Gemini Deep Planet Survey - First Results

October 8, 2007

A Canada-US-UK team led by David Lafrenière and René Doyon of the University of Montreal has released the first results from the Gemini Deep Planet Survey (GDPS), a near-infrared adaptive optics search for giant planets and brown dwarfs around 85 nearby young stars. The observations, made with the ALTAIR/NIRI adaptive optics system at the Gemini North telescope, are aimed at constraining the population of Jupiter-mass planets with orbits that have a semi-major axis in the range of 10-300 astronomical units (AU). This work constitutes a first step toward the detection of the population of “outer” giant planets around other stars.

Figure 1. Distribution of distance, spectral type, and age of the 85 target stars of the GDPS.

The use of angular differential imaging (see ADI "how-to" box below) in conjunction with NIRI/ALTAIR enabled the team to reach the best sensitivities to date for detecting giant exoplanets with projected separation above ~0.7 arcsecond. The typical detection limits (5 sigma) reached by the survey, in magnitude difference between an off-axis point source and the central star, are 9.5 at 0.5 arcsecond, 12.9 at 1 arcsecond, 15 at 2 arcseconds and 16.5 at 5 arcseconds, limits that are low enough to detect planets more massive than ~2 MJup with a projected separation of 40-200 AU around a typical target star. More than 300 faint candidate exoplanets were detected around 54 of the 85 stars observed, (see Figure 1) however, follow-up observations at a second epoch have revealed that all of the candidates were unrelated background objects. The observations also resolved three of the target stars into binaries for the first time.

A statistical analysis of the results indicates that the 95% credible upper limit to the fraction of stars harboring at least one planet more massive than 2 MJup with an orbit of semi-major axis in the range 25-420 AU or 50-295 AU is 0.23 or 0.12, respectively. These upper limits, the most precise to date, leave little room for the existence of a swarm of giant exoplanets orbiting their star at distances greater than the size of own planetary system.

More than 200 exoplanets have been discovered over the last decade through precise measurements of radial velocity (RV) variations of their primary star. Notwithstanding its great success in finding planets on small orbits around other stars, the RV technique cannot be used to search for and characterize planets with orbits larger than ~10 astronomical units (AU), i.e. larger than the orbit of Saturn in our solar system. A determination of the frequency of giant planets as a function of orbital separation to hundreds of AUs is necessary to elucidate the relative importance of various modes of planet formation and migration that are currently under scrutiny. This is why direct imaging searches for planets, such as GDPS, are crucial to improve our understanding of the formation and evolution of planetary systems.

For more details, see the article “The Gemini Deep Planet Survey – GDPS”, by David Lafrenière and 12 co-authors, scheduled for publication in the December 10, 2007, issue of The Astrophysical Journal.