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