We present spectro-photometry spanning 1–5 \mu m of 51 Eridani b , a 2–10 M _ { \text { Jup } } planet discovered by the Gemini Planet Imager Exoplanet Survey .
In this study , we present new K 1 ( 1.90–2.19 \mu m ) and K 2 ( 2.10–2.40 \mu m ) spectra taken with the Gemini Planet Imager as well as an updated L _ { P } ( 3.76 \mu m ) and new M _ { S } ( 4.67 \mu m ) photometry from the NIRC2 Narrow camera .
The new data were combined with J ( 1.13–1.35 \mu m ) and H ( 1.50–1.80 \mu m ) 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 ( \log L / L _ { \odot } ) , 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 _ { \oplus } .