Gliese 504 b (or59 Virginis b) is ajovian planet or abrown dwarf orbiting thesolar analog59 Virginis (Gliese 504),^{[note 1]} discovered bydirect imaging using HiCIAO instrument and AO188adaptive optics system on theSubaru Telescope ofMauna Kea Observatory,Hawaii by Kuzuhara et al.^[4]

The discovery images were taken in 2011 and common proper motion was confirmed in 2012 as part of theStrategic 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-meterSubaru Telescope.

In February 2013 Kuzuhara et al. submitted the discovery paper toThe Astrophysical Journal, and in September it was published.^[4] 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.^[3]

In January 2025 Mâlin et al. published a paper confirming the detection ofammonia, using theMid-Infrared Instrument aboard theJames Webb Space Telescope.^[2]

GJ 504 b'sspectral 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.^[3] Itseffective temperature is512±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 2.5arcseconds, corresponding to aprojected separation of 44.7AU,^[2] which is nearly nine times the distance between Jupiter and the Sun, which poses a challenge to theoretical ideas of how giant planets form.^[5] Models such ascore accretion or disk instability fail to reproduce the characteristics of this planet, such as its super-solarmetallicity.^[2] This planet is seen as an excellent target for detailed spectroscopic characterization due to its proximity to Earth and its wide separation.

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
_−0.06Gyr and estimated mass as 4.0^+4.5
_−1.0M_Jup.^[4] 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 abrown dwarf rather than a planet.^[6]

In 2017, an intermediate age value 2.5^+1.0
_−0.7 Gyr was published,^[7] while in 2018 two ages of21±2 Myr and4±1.8 Gyr were published, corresponding to planetary masses of1.3+0.6
−0.3 M_J and23+10
−9 M_J respectively.^[8] Intermediate ages were proposed in 2025, ranging from 400 million to one billion years, which would imply a mass between one and 17M_Jup, still not sufficient to confirm the nature of GJ 504 b. Measuring the abundance ofammonia in the planet's atmosphere could constrain its mass, current measurements suggest a mass likely within theplanetary-mass regime, while the mid-infrared brightness seems to place the object at a higher age and mass.^[2] 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.^[9]

• "GJ 504 b".SIMBAD.Centre de données astronomiques de Strasbourg.

• Exoplanets detected by direct imaging
• Exoplanets discovered in 2013
• Giant planets
• Virgo (constellation)
• Exoplanets in the Gliese Catalog
• Brown dwarfs

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