spectrum, and the L’ photometry of 51 Eri b (with cloudfree 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).
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 massive than, a field
dwarf. A fit of the GPI JH
January 2016
2015 Year in Review
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 2,000 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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