GJ 504 b is a gas giant planet orbiting at a distance of 43.5 AU from a nearby Sun-like star. The relatively large separation of the planet from its host star means that the planet is not lost in the glare of its host star. As a result, the planet can be directly imaged with existing high-contrast imaging systems. GJ 504 b was observed as part of the LEECH exoplanet imaging survey using the Large Binocular Telescope (LBT) in Arizona.
With an estimated temperature of roughly 550 K, GJ 504 b is currently the coldest directly imaged exoplanet. Gas giant planets and brown dwarfs cool with time as they radiate away heat acquired during their formation. A more massive gas giant planet will cool at a slower rate than a less massive gas giant planet. Given two isolated gas giant planets that have the same age but have different masses, the more massive gas giant planet, with its slower rate of cooling, will have a higher temperature than the less massive gas giant planet.
The mass of GJ 504 b can be predicted by two models. For a young GJ 504 b (0.1 to 0.5 billion years old), it is expected to have 3 to 8 times the mass of Jupiter. For an old GJ 504 b (3 to 6.5 billion years old), it is expected to have 19 to 30 times the mass of Jupiter. Observations of GJ 504 b indicate that it has 0.95 ± 0.06 times the diameter of Jupiter and a low surface gravity. This favours the low-mass interpretation, suggesting that GJ 504 b is a young, low-mass gas giant planet.
Observations of GJ 504 b also reveal that the planet has 4.0 ± 1.3 times the metallicity of the Sun. The metallicity of an object refers to its total metal abundance (i.e. all elements heavier than helium). The metallicity of GJ 504 b is much higher than its host star which has 1.3 to 1.9 times the metallicity of the Sun. The super-stellar metallicity of GJ 504 b indicates that the object formed like a planet and not by binary fragmentation. This is because pairs of objects that formed by binary fragmentation should have similar metallicities. Such pairs of objects include star-star binary systems and star-brown dwarf binary systems.
Skemer et al. (2015), “The LEECH Exoplanet Imaging Survey: Characterization of the Coldest Directly Imaged Exoplanet, GJ 504 b, and Evidence for Super-Stellar Metallicity”, arXiv:1511.09183 [astro-ph.EP]