JULY 2017
GPI Data Hint at Cold-Start
Giant Planet Formation
New research on the first exoplanet discov-
ered using the Gemini Planet Imager (GPI)
— 51 Eridani b — hints that it may have
formed by the the collapse of icy disk mate-
rials followed by the accretion of a thick gas
atmosphere, much like that described in the
cold-start model.
Figure 7.
GPI images in the K1,
K2, LP, and MS bands;
the emission of the host
star was blocked. The
exoplanet 51 Eri b is
indicated by an arrow.
Located about 100
light years from Earth,
Exoplanet 51 Eri b is
between 2–10 times the
mass of Jupiter.
Two main scenarios of giant planet forma-
tion exist: hot start and cold start. In the hot-
start model, gas giants form directly via the
rapid collapse of a gaseous protoplanetary
disk. In the cold-start scenario, a gas-giant
begins as a core that forms very early on
from planetesimal agglomerations before
collecting the plentiful gas around it.
Abhijith Rajan (School of Earth and Space
Exploration, Arizona State University), led
the international team that observed 51 Eri
b using GPI spectroscopy (Figure 7) as part
of the Gemini Planet Imager Exoplanet Sur-
vey (GPIES), combined with mid-infrared
photometry at the W.M. Keck Observatory.
These data were used to determine that
the planet — a young, cool object between
2-10 Jupiter masses — is redder than brown
dwarfs seen elsewhere. The enhanced red-
dening may be the result of clouds forming
as the planet transitions from at partially- to
partly-cloudy atmosphere, with lower mean
surface temperatures. If true, 51 Eri b ap-
pears to be one of the only directly imaged
planets that is consistent with the cold-start
scenario, resulting in a low temperature, low
luminosity planet.
The full results have been accepted for pub-
lication in The Astronomical Journal. A pre-
print is available here.
Gemini South Joins HST in Joint
Proper Motion Study
Tobias Fritz (University of Virginia) and col-
leagues used the wide-field Gemini Multi-
conjugate adaptive optics System (GeMS)
at Gemini South, combined with the Gemini
South Adaptive Optics Imager (GSAOI), to
study the proper motion of stars in the Ga-
lactic halo globular cluster known as Pyxis.
These data, together with those from the
Hubble Space Telescope, allowed the team
to set a lower limit for the Milky Way’s mass of
950 million Suns. This value is consistent with
most, but not all, previous determinations.
GeMS/GSAOI was crucial to the study, be-
cause traditional ground-based telescopes
are seeing limited and need a time baseline
of more than 15 years for the types of mea-
surements required in this survey. On the
other hand, GeMS/GSAOI has better spatial
resolution and can complete the project in
five years — about the same time required
for HST. Using GeMS/GSAOI, the team mea-
sured absolute proper motions of Pyxis to a
resolution of 0.08 arcsecond (Figure 8), and
combined these data with those from ar-
chival HST images, with a resolution of ~ 0.1
arcsecond.
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GeminiFocus
January 2018 / 2017 Year in Review