As a complement to a survey
we were carrying out with
Gemini’s Near-Infrared Coronographic Imager (NICI) to
find planetary-mass companions to young, low-mass stars,
we stepped outside the box.
We decided to use GMOS to
verify if really massive planets
— much more massive than
Jupiter — exist in the most distant realms of stellar systems.
Figure 1.
Artist’s concept of GU
Psc b (foreground) and
its distant host star.
Illustration by Lucas
Granito
a mass ~11 times that of Jupiter, was found
through a survey carried out at Gemini South
with the Gemini Multi-Object Spectrograph
(GMOS) — without adaptive optics (AO) observations, nor with any sophisticated highcontrast instrument or special image analysis method.
Location, Location, Location
How then could we detect GU Psc b? Location. The key is that the planet is located
very far from its host — 42 arcseconds, to
be more precise, which translates to 2000
astronomical units (AU; the average distance
of the Earth from the Sun), at its distance of
about 48 parsecs (pc) or 157 light-years.
One reason we haven’t found this world before is simply because we were not searching for planets orbiting that far from stars;
probably because an anthropocentric bias
motivates us to search for exoplanets where
we find giant planets in our own Solar System. Also, from a theoretical point of view,
current mainstream formation theories for
exoplanets (core accretion, disk instability)
simply do not predict giant worlds to be that
far from their hosts. We discovered just the
opposite.
14
GeminiFocus
Our survey, sensitive to objects with masses ranging
from 5-7 Jupiter masses, was
based on a very simple fact:
that the Spectral Energy Distribution (SED)
of very cool, planetary-mass objects displays
a notable brightening from the red to the
infrared. Therefore, we decided to take two
images of the target star: one with an i filter, and the other with a z filter; cool objects
could then be identified via their distinctively red i - z color.
A Newly Identified Young Star
The star sample made with NICI and GMOS
at Gemini was one key to the survey’s success, because the environments around
young stars are ideal places to find planets
through direct imaging. These worlds a