51 Eridani b
First exoplanet discovered by the Gemini Planet Imager.
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51 Eridani b is a Jupiter-like gas giant orbiting the young F0 V star 51 Eridani, located 96 light-years away in the constellation Eridanus. It is approximately 20 million years old and was the first exoplanet discovered by the Gemini Planet Imager, announced in August 2015 after its detection in December 2014.
Lore & Background
51 Eridani b was discovered in December 2014 using the Gemini Planet Imager, an international project led by the Kavli Institute for Particle Astrophysics and Cosmology. The instrument was specifically designed to directly image dim, young planets around bright stars by using adaptive optics to sharpen resolution and then blocking the starlight. Prior to this discovery, all directly imaged exoplanets were gas giants many times the mass of Jupiter.
The planet is a gas giant more massive than Jupiter, with a radius about 1.36 times that of Jupiter and an effective temperature of 800 K—substantially hotter than Jupiter's 128 K. It orbits at 9.1 AU from its star with a period of roughly 23.5 years on an eccentric path. The higher-than-expected eccentricity suggests gravitational perturbation by another body.
Its atmosphere has a relatively low carbon-to-oxygen molar ratio and exhibits the second strongest methane signature of any exoplanet, after Gliese 504 b. Water vapor and ammonia have been detected in its spectrum, and atmospheric modeling favors low surface gravity and partly cloudy conditions. In 2025, the detection of water and carbon dioxide was announced, marking only the second time CO2 was directly detected in an exoplanet atmosphere.
Reader's Guide
51 Eridani b holds significance as the first exoplanet discovered by the Gemini Planet Imager, an instrument built specifically for direct imaging of young planets. Its discovery marked a shift from finding only massive gas giants to detecting planets closer in mass to Jupiter.
The planet's strong methane signature and low luminosity provide clues about its formation, while its higher-than-expected eccentricity hints at an unseen perturbing body. The direct detection of carbon dioxide in its atmosphere in 2025—only the second such detection—demonstrates the growing capability of direct imaging spectroscopy to characterize exoplanet atmospheres. The planet's relatively young age (20 million years) and hot temperature (800 K) make it a valuable laboratory for studying planetary evolution and atmospheric chemistry in a system still in its formative stages.
Frequently Asked Questions
What is 51 Eridani b?
It is a young, Jupiter-mass gas giant situated roughly 96 light-years away in the constellation Eridanus. At about 20 million years old, it is still in its early evolutionary stage and orbits the F0 V star 51 Eridani.
How was 51 Eridani b discovered?
The planet was detected in December 2014 by the Gemini Planet Imager, with the discovery made public in August 2015. It holds the notable distinction of being the first exoplanet ever found by that instrument.
What are 51 Eridani b's key physical properties?
The planet has an effective temperature of roughly 800 K and a radius about 1.36 times that of Jupiter. Its mass is expressed in Jupiter masses but remains model-dependent, with an astrometric upper limit established at the 3-sigma confidence level.
More in Exoplanets Found by Imaging and Microlensing
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: 51 Eridani b (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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