GeminiFocus 2015 Year in Review | Page 10

A Perfect Target 51 Eri is a nearby star 29.4 parsecs (96 light years) distant with a mass of 1.6 MSun. It belongs to the Beta Pictoris moving group, which has a well-determined age of about 20 million years (Myr). 51 Eri is also co-moving with the tight M-dwarf binary GJ 3305 at a distance of 2,000 astronomical units (AU). Studies of the star’s motion through space, and spectral characteristics of the GJ 3305 binary, provide further evidence of membership within the Beta Pictoris moving group, confirming the age of the 51 Eri system. Figure 1. Discovery image of 51 Eri b at H band (1.66 microns) with the Gemini Planet Imager in December 2014. The central star (indicated by a cross) has been subtracted as a part of the data reduction process, and the residuals are masked out to enhance the planet’s contrast in the image. The projected separation between the planet and the star is approximately 13 AU, with a contrast of ~ 10 6. Image credit: C. Marois (NRC-Herzberg), J. Rameau (University of Montréal). 8 The Power o f GPI GPI uses the most advanced technologies to achieve unprecedented performance in terms of angular resolution and contrast. Image quality is restored by up to 90 percent of the theoretical diffraction limit, and most of the starlight is blocked by a coronagraph to attenuate the primary star’s glare, revealing faint point sources close to the star. Instead of taking a single image, GPI obtains a lowresolution infrared spectrum for each pixel in the field-of-view, facilitating the detection of an exoplanet’s atmosphere and the most prominent chemicals found there. After a successful first light in November 2013, GPI started routine operations and became available to the wider astronomical community. Gemini Observatory also selected the GPI Exoplanet Survey (GPIES) team to conduct a 3-year, 890-hour campaign to search for, and characterize, new extrasolar systems around some 600 stars. In December 2014, after observing only 44 stars, the team identified a point source one million times fainter than one of the stars (51 Eridani; 51 Eri), at an angular separation of only 0.5 arcsecond; GPI’s first planet discovery: 51 Eri b (Figure 1). GeminiFocus Given 51 Eri’s young age, any planetary-mass companion will still be cooling from its recent formation, and will therefore be bright enough to be detected in the near-infrared via direct imaging. Additionally, the infrared excesses measured in the spectral energy distribution (SED) of 51 Eri by infrared satellites, such as WISE and Herschel, are indicative of a circumstellar debris disk — the residuals of putative planetary formation. Determination of the precise geometry of the debris disk will require further investigation. 51 Eri proved to be the perfect target to search for an exoplanet, but despite previous attempts to search for planetary-mass companions with a number of older instruments, 51 Eri b remained elusive until the GPIES team detected it in December 2014. At a projected separation from 51 Eri of only 13.4 AU, and an estimated mass from evolutionary models of 2 MJup, 51 Eri b is the first directly-imaged exoplanet most resembling the gas giants within our own Solar System (Figures 2 and 3). Methane, Methane, Methane As GPI uses an integral field spectrograph, we were able to extract a low-resolution spectrum of 51 Eri b, which, like Jupiter, shows strong methane absorption. We immediately recognized the significance of the discovery, as it provides a view of what Jupiter might 2015 Year in Review January 2016