massive than, a field dwarf.
A fit of the GPI JH spectrum,
and the L’ photometry of 51
Eri b (with cloud-free equilibrium chemistry atmosphere
models), gives it an effective
temperature of 750 K, consistent with the presence of
methane. But the data also
suggest an unphysical radius
of 0.76 RJup, and high surface
gravity, as seen in the spectra
of old brown dwarfs (Figure 4,
bottom panel).
as it provides a view of what Jupiter might
have looked like in its infancy, while offering
us a clue as to how it formed.
In January 2015 we obtained follow-up observations with GPI at J and H bands (1.24
and 1.66 microns, respectively) as well as L’
observations (3.78 microns) with the W. M.
Keck Observatory NIRC2 near-infrared imager
and the facility’s AO system. We then used the
data to construct the planet’s near-infrared
SED. The most significant property of 51 Eri
b was that, in addition to
water vapor absorption,
its spectrum exhibits
the strongest methane
absorption
measured
to date for a directly imaged exoplanet.
The results did not surprise
us, since the same models
had previously given similar
extreme properties for other
directly-imaged planets, such
as HR 8799 bcde. To fit the spectrum of 51
Eri b with more realistic properties, we used
a partly cloudy non-equilibrium chemistry
model (Figure 4, bottom panel); models of
this kind generally agree reasonably well with
the observations of other imaged exoplanets.
This new fit yielded a lower temperature (700
K) and a physical radius of (1 RJup) consistent
with evolutionary models for substellar objects; it also favored a lower surface gravity,
consistent with the planet’s young age.
While the spectrum of 51
Eri b resembles that of
a typical T6 field brown
dwarf several billion
years old (Figure 4, top
panel), its red H-L’ color
suggests that the object
is both younger, and less
October 2015
GeminiFocus
Figure 2.
Schematic diagram
comparing the 51 Eri
system with our own
Solar System. Both
systems harbor two
debris belts, assuming
a two-component fit to
the infrared excess of
51 Eri, with gas giants
in between. 51 Eri also
hosts a binary M-dwarf
at about 2000 AU, a
separation far too distant
to gravitationally perturb
the inner system. From
51 Eri b, each component
of the wide binary would
shine as brightly as
Venus, and they would
be separated by 17
arcminutes, roughly half
the angular diameter of
the Moon from Earth.
Figure 3.
The current population of
known extrasolar planets
classified by their detection
techniques. The mass and
semi-major axis
of the four gas
giants of our
Solar System
are overplotted
(letters). 51 Eri b
(large red star) is
the least-massive
directly-imaged
planet, found
at a separation
similar to the
scale of the Solar
System. Source:
exoplanets.eu,
retrieved August
20, 2015.
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