Having started its activities in November 2014, the Gemini Planet Imager (GPI) celebrates one year of observations and discoveries. Installed at the 8-meter Gemini South Observatory in Cerro Pachon (Chile), the instrument, a coronagraph/spectrograph, is specifically designed for a three-year survey targeting 600 young stars in the search of sister planets to Jupiter as well as debris disks around them.
Within one month of activity, the Gemini Planet Imager Exoplanet Survey (GPIES) discovered 51 Eridani b, a young Jupiter-like exoplanet of about 20 million years of age with a methane-rich atmosphere and a surface temperature of approximately 425 °C. 51 Eri b orbital radius is situated 15 astronomical units* from its parent star, three times the distance between Jupiter and the Sun. “This is exactly what GPI was created for: to probe the scales and masses of objects similar to those in our Solar System,” says Julien Rameau, postdoctoral researcher at iREx and GPI user.
Also, along with Hubble Space Telescope observations, GPI has confirmed the ejection of a 13 Jupiter-mass extrasolar planet from its star system. Evidence even suggests that this planet, HD 106,906 b, separated by an impressive 700 AU from its star, has its own debris disk. “HD 106,906 is a uniquely complex and dynamic system,” states Mr Rameau.
Because of the currently strong El Niño, the weather allowed completion of only a quarter of the survey in the first length of the program. Nevertheless, the GPI team now expects to find merely a handful of planets within the next two years. “When GPIES started, we expected to detect 20 to 50 planets based on population statistics extrapolated from radial velocity data. After observing 150 stars, we have identified only one planet, and, among 20 other candidates, half turn out to be background objects in our line of sight,” explains Julien Rameau.
What does this discrepancy means? Are the population statistics inaccurate or has the age of the targeted stars been incorrectly evaluated? According to Rameau, “The population statistics are not wrong but different from the data derived from radial velocity and transit measurements for mature star hosts. In the end, this deceptively low result will be useful in order to better constrain the current statistics and models.”
On a positive note, GPI may turn out to be an exceptional tool to look for planetary disks. Initially designed to search for planets, the instrument can also resolve debris disks around young and red stars. Of the 150 stars observed so far, GPI has confirmed the existence of about 15 disks. This trove of data will help astronomers to better understand the physics and evolution or these structures.
The Gemini Planet Imager was designed by an international team of researchers among which are several iREx members. The instrument is optimized for direct observations of young gas giant with Jupiter to Neptune-like orbits as well as protoplanetary disks at near-infrared wavelengths (J, H and K bands, between 1.2 and 2.2 µm). The three-year exoplanet survey specifically targets young star systems of one million to one billion years of age with planets similar to our Solar System’s gas giants.
GPI can also be used to observe the surface of giant stars, the extended envelopes of Wolf-Rayet stars, the formation of new stars, the atmospheric activity of the Solar System’s gas giants as well as to identify previously unknown binary asteroids.
*One astronomical unit (1 AU) is defined as the average distance between the Earth and the Sun.