We present spectro-photometry spanning 1-5 μm of 51 Eridani b, a 2-10
MJup planet discovered by the Gemini Planet Imager Exoplanet Survey.
In this study, we present new K1 (1.90-2.19 μm) and K2 (2.10-2.40
μm) spectra taken with the Gemini Planet Imager as well as an updated LP
(3.76 μm) and new MS (4.67 μm) photometry from the NIRC2 Narrow
camera. The new data were combined with J (1.13-1.35 μm) and H
(1.50-1.80 μm) spectra from the discovery epoch with the goal of better
characterizing the planet properties. 51 Eri b photometry is redder than field
brown dwarfs as well as known young T-dwarfs with similar spectral type
(between T4-T8) and we propose that 51 Eri b might be in the process of
undergoing the transition from L-type to T-type. We used two complementary
atmosphere model grids including either deep iron/silicate clouds or
sulfide/salt clouds in the photosphere, spanning a range of cloud properties,
including fully cloudy, cloud free and patchy/intermediate opacity clouds.
Model fits suggest that 51 Eri b has an effective temperature ranging between
605-737 K, a solar metallicity, a surface gravity of log(g) = 3.5-4.0 dex,
and the atmosphere requires a patchy cloud atmosphere to model the SED. From
the model atmospheres, we infer a luminosity for the planet of -5.83 to -5.93
(logL/L⊙), leaving 51 Eri b in the unique position as being one of
the only directly imaged planet consistent with having formed via cold-start
scenario. Comparisons of the planet SED against warm-start models indicates
that the planet luminosity is best reproduced by a planet formed via core
accretion with a core mass between 15 and 127 M⊕.Comment: 27 pages, 19 figures, Accepted for publication in The Astronomical
Journa