GJ 3929

GJ 3929, also known as Gliese 3929 and TOI-2013, is a red dwarf star located 51.6 light-years from Earth, in the constellation Corona Borealis. With an apparent magnitude of 12, it is not visible to the naked eye. In 2022, two exoplanets were detected orbiting the star.

Characteristics
GJ 3929 is a red dwarf of spectral type M3.5V, having a radius of $10.14$, a mass of $-143.277$ and a temperature of $318.219$. With an apparent magnitude of 12.7, it cannot be visible with the naked eye, neither with a small telescope. It has no companion stars. The age of GJ 3929 is estimated at 2.2 to 12.2 billion years.

The star is located in the northern hemisphere, approximately 50 light years from Earth, in the direction of the constellation Corona Borealis. Its closest neighbor is the red dwarf G 179–57, located at a distance of 3.9 light years.

Planetary system
In 2022, two exoplanets were detected orbiting around GJ 3929. The innermost, GJ 3929 b, is a rocky planet just 9% larger than Earth, while the outermost, GJ 3929 c, is a sub-Neptune with 5.7 times the mass of Earth. Both planets orbit below the star's habitable zone.

GJ 3929 b
The innermost planet, GJ 3929 b (TOI-2013 b), is an Earth-sized planet discovered by the transit method. Orbiting its star at a distance of 0.0252 AU, the planet is located in its star's Venus zone, and completes a revolution every 2 days and 15 hours. The planet's equilibrium temperature is calculated at 568 K, and it receives a planetary insolation 17 times greater than what the Earth receives from the Sun.

The planet's mass is calculated at $15.822$ according to photometric observations using the NEID spectrometer. Observations using the ARCTIC imager, plus photometry from TESS and LCOGT, constrained the planet's radius to $51.58$. This radius makes GJ 3929 b very similar to Earth in terms of size.

Due to the apparent brightness of the host star, and its small size, GJ 3929 b is an excellent planet for atmospheric study with the James Webb Space Telescope.

It was discovered by a team of astronomers led by Jonas Kemmer. They reported an planetary transit signal in the host star's light curve. Subsequent observations, mainly with the CARMENES spectrograph, revealed that this transit signal is an orbiting exoplanet.

GJ 3929 c
The outermost planet, GJ 3929 c (TOI-2013 c) is a Sub-Neptune discovered using the radial velocity method. It orbits its star at a distance of 0.081 AU, 3 times further away than GJ 3929 b, but still below GJ 3929's habitable zone, completing an orbit every 15 days.

Its minimum mass is $0.313$, while its radius is unknown. Estimates using mass-radius relationship derive a radius of 2.26. Its equilibrium temperature is calculated at 317 K, and it receives a planetary insolation 68% greater than what the Earth receives from the Sun.

GJ 3929 c was first identified in radial velocity data, which indicated the existence of another planet besides GJ 3929 b. Initially, it was just an exoplanet candidate, but it was later confirmed by a team led by Corey Beard. Because its orbital period (15 days) is far from its star's rotation period (122 days), it is unlikely that the radial velocity signal is actually an artifact of its parent star's activity and rotation.

Notes and references
