Gliese 12 b

Lore & Background

Discovered by TESS, Gliese 12 b was confirmed as a planet by two independent studies published in May 2024. Its mass was initially poorly constrained, with an upper limit of less than four Earth masses. Subsequent measurements in 2025 provided mass estimates of 0.71 and 0.95 Earth masses. The latter study also measured the planet's radius and found a density higher than that of the Solar System's terrestrial planets, suggesting a likely rocky composition similar to Venus. The former study used the discovery paper's radius and found a lower density, leaving scenarios such as an Earth-like composition, volatile-dominated composition, iron-poor composition, or a combination all plausible. Gliese 12 b orbits slightly closer than the inner edge of its star's habitable zone, receiving insolation between that of Earth and Venus. Its equilibrium temperature, assuming an albedo of zero, is 315 K; with an atmosphere the surface temperature would be higher. Assuming a Venus-like albedo, the equilibrium temperature is 218 K.

Reader's Guide

Gliese 12 b holds significant scientific interest as one of the nearest relatively temperate transiting exoplanets, alongside the planets of TRAPPIST-1 and LHS 1140 b. Its proximity and transiting nature make it a prime candidate for the James Webb Space Telescope to investigate whether it has retained an atmosphere. The planet's size, similar to Earth and Venus, and its orbital position receiving insolation between those of Earth and Venus, place it in a regime that could inform understanding of terrestrial planet atmospheres and habitability. The uncertainty in its mass and composition—ranging from Earth-like to volatile-rich or iron-poor—underscores the need for further observations. The 2025 mass measurements, while narrowing the range, still allow multiple compositional scenarios. If Gliese 12 b has an atmosphere, its surface temperature would exceed the 315 K equilibrium temperature (assuming zero albedo), potentially making it a Venus-like world. Its legacy will likely be shaped by future atmospheric characterization, which could reveal whether such planets around red dwarfs can sustain atmospheres and what conditions prevail at the inner edge of the habitable zone.

Did You Know?

• Gliese 12 b orbits its star every 12.8 days.
• Its host star, Gliese 12, is a red dwarf with about 24% the mass and 26% the radius of the Sun.
• The planet's equilibrium temperature ranges from 218 K (Venus-like albedo) to 315 K (zero albedo).
• Two independent studies confirming Gliese 12 b as a planet were published in May 2024.