51 Eri b - Jupiter-like planet discovered by GPI

Two Jupiter-masses, ~12 AU from its star.

http://www.scientificamerican.com/article/exclusive-astronomers-discover-the-most-jupiter-like-exoplanet-ever-seen/

Nice, very nice !

Good god. That's pretty comparable to 47 Ursae Majoris d in separation and mass; though this is obviously a comparitively young planet, it's striking that we're getting close to convergence on RV and imaging samples.

Anybody want to guess when we'll get an imaged exoplanet of ~1 Jupiter mass at ~5 AU around a fairly old, solar-type star?

Anyway… I'm looking forward to that paper.

Also known as HIP 21547 with a cool disc imaged my WISE.

http://www.space.com/30238-young-jupiter-exoplanet-51-eridani-b-explained.html
First planet of Gemini

http://news.sciencemag.org/space/2015/08/astronomers-discover-lowest-mass-exoplanet-seen-directly

Astronomers discover lowest mass exoplanet seen directly


http://quebec.huffingtonpost.ca/2015/08/13/-geante-gazeuse-51-eridan_n_7983714.html

Discovery paper now on Arxiv:

Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager

Quote :

Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric composition and luminosity, which is influenced by their formation mechanism. Using the Gemini Planet Imager, we discovered a planet orbiting the $sim$20 Myr-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water vapor absorption. Modeling of the spectra and photometry yields a luminosity of L/LS=1.6-4.0 x 10-6 and an effective temperature of 600-750 K. For this age and luminosity, "hot-start" formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the "cold- start" core accretion process that may have formed Jupiter.

Dynamical study of GJ 3305, the companion M+M binary to 51 Eridani.

Montet et al. "Dynamical Masses of Young M Dwarfs. I. Masses and Orbital Parameters of GJ 3305 AB, the Wide Binary Companion to the Imaged Exoplanet Host 51 Eri"
http://arxiv.org/abs/1508.05945

So extrasolar planets, even at large orbital separations, seem not so uncommon in binary/multiple systems.

http://arxiv.org/abs/1509.07514

Astrometric Confirmation and Preliminary Orbital Parameters of the Young Exoplanet 51 Eridani b with the Gemini Planet Imager

We present new GPI observations of the young exoplanet 51 Eridani b which provide further evidence that the companion is physically associated with 51 Eridani. Combining this new astrometric measurement with those reported in the literature, we significantly reduce the posterior probability that 51 Eridani b is an unbound foreground or background T-dwarf in a chance alignment with 51 Eridani to 2×10−7, an order of magnitude lower than previously reported. If 51 Eridani b is indeed a bound object, then we have detected orbital motion of the planet between the discovery epoch and the latest epoch. By implementing a computationally efficient Monte Carlo technique, preliminary constraints are placed on the orbital parameters of the system. The current set of astrometric measurements suggest an orbital semi-major axis of 14+7−3 AU, corresponding to a period of 41+35−12 yr (assuming a mass of 1.75 M⊙ for the central star), and an inclination of 138+15−13 deg. The remaining orbital elements are only marginally constrained by the current measurements. These preliminary values suggest an orbit which does not share the same inclination as the orbit of the distant M-dwarf binary, GJ 3305, which is a wide physically bound companion to 51 Eridani.

Wow, pretty good considering the short orbital arc.

http://www.seti.org/seti-institute/press-release/gemini-planet-imager-exoplanet-survey-one-year-in

Gemini Planet Imager Exoplanet Survey -- One Year Into The Survey

Spectral and atmospheric characterization of 51 Eridani b using VLT/SPHERE
https://arxiv.org/abs/1704.02987

The planet is coated in an optically thick cloud layer with small particle size. Temperature is > 700 K so probably no sulfur chemistry as described by Zahnle (2016). The planet has a much higher mass than thought, at ~9.1 M_J (give or take a few Jupiters).

Characterizing 51 Eri b from 1-5 μm: a partly-cloudy exoplanet
https://arxiv.org/abs/1705.03887

Quote :

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⊕.

Starting to get a sense of the orbit.

Hint of curvature in the orbital motion of the exoplanet 51 Eridani b using 3 years of VLT/SPHERE monitoring
https://arxiv.org/abs/1903.07620

Quote :

Context. The 51 Eridani system harbors a complex architecture with its primary star forming a hierarchical system with the binary GJ 3305AB at a projected separation of 2000 au, a giant planet orbiting the primary star at 13 au, and a low-mass debris disk around the primary star with possibly a cold component and a warm component inferred from the spectral energy distribution. Aims. We aim to better constrain the orbital parameters of the known giant planet. Methods. We monitored the system over three years from 2015 to 2018 with the VLT/SPHERE exoplanet imaging instrument. Results. We measure an orbital motion for the planet of ~130 mas with a slightly decreasing separation (~10 mas) and find a hint of curvature. This potential curvature is further supported at 3σ significance when including literature GPI astrometry corrected for calibration systematics. Fits of the SPHERE and GPI data using three complementary approaches provide broadly similar results. The data suggest an orbital period of 32+17−9 yr (i.e. 12+4−2 au in semi-major axis), an inclination of 133+14−7 deg, an eccentricity of 0.45+0.10−0.15, and an argument of periastron passage of 87+34−30 deg [mod 180 deg]. The time at periastron passage and the longitude of node exhibit bimodal distributions because we do not detect yet if the planet is accelerating or decelerating along its orbit. Given the inclinations of the planet's orbit and of the stellar rotation axis (134-144 deg), we infer alignment or misalignment within 18 deg for the star-planet spin-orbit. Further astrometric monitoring in the next 3-4 years is required to confirm at a higher significance the curvature in the planet's motion, determine if the planet is accelerating or decelerating on its orbit, and further constrain its orbital parameters and the star-planet spin-orbit.

De Rosa et al. "An updated visual orbit of the directly-imaged exoplanet 51 Eridani b and prospects for a dynamical mass measurement with Gaia"
https://arxiv.org/abs/1910.10169

Their fit indicates an eccentricity ~0.51, but there's a degeneracy between low-eccentricity inclined orbits and eccentric orbits and circular orbits are not necessarily ruled out.

Gaia mass determination requires the most optimistic predictions for accuracy, plus a high planetary mass.