Figure 4.
Near-infrared SED of
51 Eri b measured by
GPI at J and H bands,
and by Keck/NIRC2
at L’ band (blue).
The main molecular
absorption bands are
labeled on the top
panel. The top panel
compares the spectrum
with the previously
known planetary
mass object 2M 1207
b (which has a very
dusty atmosphere) and
with a typical T6 field
brown dwarf. On the
bottom panel, the best
fitting low-temperature,
cloud free, and partly
cloudy models are
able to reproduce the
spectrum, but lead
to distinct physical
properties (see text).
6
Solar System-like?
51 Eri b is unique among the current population of directly imaged exoplanets in
many aspects:
Location and Mass: GPI is among the first instruments sensitive enough to image planets
in extrasolar systems at scales similar to our
own Solar System; and the discovery of 51 Eri
b demonstrates this. At 13.4 AU, the planet is
located at a separation between the orbit of
Saturn and Uranus, and with a model-dependent mass of 2 MJup, it would represent the
least-massive exoplanet imaged to date.
The exoplanets previously resolved with
direct imaging are typically found at a few
tens to hundreds of AU, with masses greater than 5 MJup, making the architecture of
these systems very different from that of our
own Solar System. With GPI, a new low-mass
population of exoplanets is now accessible
by direct imaging. Further discoveries will
better our understanding of the formation
and architecture of planetary systems, and
place the properties of our own Solar System into context.
GeminiFocus
Atmosphere: The presence of methane in the
atmosphere of 51 Eri b is by far the most important aspect of this discovery. Previous
directly-imaged exoplanets exhibit dusty
atmospheres, where thick clouds block the
light coming from the deep atmosphere and
prevent an investigation of its chemical composition. 51 Eri b is different, as the clouds are
more tenuous, allowing us to probe low altitude cloud layers and determine their chemical content.
51 Eri b’s methane-dominated spectrum is
similar to what models predict for a planet
of its mass. This newly discovered world may,
in fact, resemble what Jupiter looked like
soon after its formation. With this discovery,
astronomers now know how to differentiate
between L- and T-type young planetary-mass
objects, where atmospheres