# Gliese 504 b

Gliese 504 b (or 59 Virginis b) is a jovian planet or a brown dwarf orbiting the solar analog 59 Virginis (Gliese 504), discovered by direct imaging using HiCIAO instrument and AO188 adaptive optics system on the Subaru Telescope of Mauna Kea Observatory, Hawaii by Kuzuhara et al.

## History of observation

The discovery images were taken in 2011 and common proper motion was confirmed in 2012 as part of the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) survey. The SEEDS survey aims to detect and characterize giant planets and circumstellar disks using the 8.2-meter Subaru Telescope.
In February 2013 Kuzuhara et al. submitted the discovery paper to The Astrophysical Journal, and in September it was published. A follow-up study published in the October 2013 edition of the Astrophysical Journal confirmed methane absorption in the infrared H band, the first time this has been done for a directly imaged planet that formed within a disk.
In January 2025 Mâlin et al. published a paper confirming the detection of ammonia, using the Mid-Infrared Instrument aboard the James Webb Space Telescope.

## Properties

GJ 504 b's spectral type was originally projected to be late T or early Y, and a follow-up study estimated that a T8 spectral type was the best fit. Its effective temperature is 512±10 К (238.9 ± 10.0 °C), much cooler than previously imaged exoplanets with a clear planetary origin. It is slightly larger than Jupiter, around 8%. The angular separation of the planet from its parent star is about 2.5 arcseconds, corresponding to a projected separation of 44.7 AU, which is nearly nine times the distance between Jupiter and the Sun, which poses a challenge to theoretical ideas of how giant planets form. Models such as core accretion or disk instability fail to reproduce the characteristics of this planet, such as its super-solar metallicity. This planet is seen as an excellent target for detailed spectroscopic characterization due to its proximity to Earth and its wide separation.

### Mass

The mass of Gliese 504 b is hard to measure, as it depends on the host star's age, which is poorly known. The discoverers adopted an age value 0.16$^{+0.35}$ $_{−0.06}$ Gyr and estimated mass as 4.0$^{+4.5}$ $_{−1.0}$ M$_{Jup}$. In 2015, other astronomers obtained age value 4.5$^{+2.0}$ $_{−1.5}$ Gyr, which corresponds to 20-30 M$_{Jup}$. In this case, the object is a brown dwarf rather than a planet.
In 2017, an intermediate age value 2.5$^{+1.0}$ $_{−0.7}$ Gyr was published, while in 2018 two ages of 21±2 Myr and 4±1.8 Gyr were published, corresponding to planetary masses of 1.3+0.6 −0.3 M$_J$ and 23+10 −9 M$_J$ respectively. Intermediate ages were proposed in 2025, ranging from 400 million to one billion years, which would imply a mass between one and 17 M$_{Jup}$, still not sufficient to confirm the nature of GJ 504 b. Measuring the abundance of ammonia in the planet's atmosphere could constrain its mass, current measurements suggest a mass likely within the planetary-mass regime, while the mid-infrared brightness seems to place the object at a higher age and mass. Indeed 59 Virginis appear to be older than a few million years, the properties which supported the very low age could also be explained by the engulfment of a planet. Ages between 360 million and 2.5 billion years were proposed in another 2025 study.