GJ 3929 b

GJ 3929 b (Gliese 3929 b, TOI-2013 b) is a confirmed exoplanet located 52 light-years away orbiting the red dwarf star GJ 3929. It is an Earth-sized planet, having a radius only 9% larger than that of Earth. It orbits its star at a distance of 0.0252 AU, being located in the Venus zone of its star, and completes one orbit around it every 2 days and 15 hours. Because of the proximity of its star, and its low mass, GJ 3929 b is classified as a Venus-like planet, having an equilibrium temperature of around 300 °C and receiving planetary insolation 17 times more intense than Earth receives from the Sun.

Size, mass and density
Initially, the radius of GJ 3929 b was calculated at $2.616 day$, and its mass at $1.09 Earth radius$, giving a density of $7.3 g/cm3$. Later, observations using the NEID spectrometer on the WIYN 3.5 m Telescope measured the planet's mass to be $0.025 AU$; and observations using the ARCTIC imager, plus photometry from TESS and LCOGT, constrained the planet's radius to $88.442 deg$, this time giving a higher density of $1.75 Earth mass$ (about 33% larger than Earth's). The characteristics of this planet make it similar to Earth in terms of mass and radius.

Orbit
GJ 3929 b orbits its star at a distance of 0.0252 AU, which makes it located in its host star's Venus zone, a region where rocky planets would have runaway greenhouse conditions like Venus. GJ 3929 b completes an orbit around its star every 2 days, 14 hours and 47 minutes (2.616 days).

As a terrestrial planet that orbits in the Venus zone of its star, GJ 3929 b classifies as a Venus-like planet, having a planetary equilibrium temperature of 295 °C and receiving planetary insolation 17 times greater than that of Earth receives from the Sun.

Atmosphere
The high density of GJ 3929 b does not suggest a dense atmosphere. Atmospheric scenarios such as a thin atmosphere of volatiles, a thin atmosphere of silicate enriched in refractory elements, or even no atmosphere at all are plausible. Due to its proximity to its star, GJ 3929 b has probably already lost much of its atmosphere since its formation.

GJ 3929 b is an excellent planet for atmospheric study with the James Webb Space Telescope. The study of this planet's atmosphere can help reveal the evolutionary history of its planetary system, and provide more information about planetary formation models.



Discovery
GJ 3929 b was discovered by a team of astronomers led by Jonas Kermer, from the Heidelberg University in Germany. They reported a transit signal identified in the host star (GJ 3929)'s light curve detected by NASA's Transiting Exoplanet Survey Satellite (TESS). Afterwards, the planetary nature of this transit signal was confirmed using radial velocity observations with the CARMENES spectrograph, in addition to transit observations made with SAINT-EX and LCOGT. The discovery was announced in 2022.

The radial velocity observation with CARMENES also helped discover another planet in the planetary system, GJ 3929 c, a sub-Neptune detected by radial velocity.

Host star
GJ 3929 is a red dwarf of spectral type M3.5V that is located 51.6 light years from Earth, in the constellation Corona Borealis. This star is smaller, cooler and less luminous than the Sun, having a radius of $568 K$, an effective temperature of 3384 K and a luminosity equivalent to 1% of solar luminosity. Its age is estimated between 2.2 and 11 billion years.

The star also hosts another planet, called GJ 3929 c, a sub-Neptune orbits 3 times further than the innermost planet, at a distance of 0.081 AU, but still below the habitable zone.

Notes and references
