Teegarden's Star b

Teegarden's Star b (also known as Teegarden b) is an exoplanet found orbiting within the habitable zone of Teegarden's Star, an M-type red dwarf 12.5 light years away from the Solar System. It had the highest Earth Similarity Index (ESI) of any exoplanet, but in February 2024 a new study updated the parameters of the planet, thus reducing its ESI to 0.90, making it no longer the planet with the hightest ESI. Along with Teegarden's Star c, it is among the closest known potentially habitable exoplanets.

Discovery
In July 2019, a team of more than 150 scientists led by Mathias Zechmeister published a peer-reviewed article in Astronomy & Astrophysics as part of the CARMENES survey supporting the existence of two candidate exoplanets orbiting Teegarden's Star.

Because of the alignment and faintness of Teegarden's Star, Doppler spectroscopy (also known as the radial velocity method) was necessary to detect possible exoplanets. This method detects exoplanets indirectly by observing their effects on a host star's radial velocity, the speed at which it is moving towards or away from the Earth. These radial velocity anomalies in turn produce doppler shifts observable with a spectrograph-equipped telescope of sufficient power.

To accomplish this, the team used the CARMENES instrument on the 3.5-meter telescope of Spain's Calar Alto Observatory. After three years of observation, two periodic radial velocity signals emerged: one at 4.91 days (Teegarden's Star b) and another at 11.41 days (Teegarden's Star c).

Mass and orbit
Teegarden's Star b is the innermost known planet orbiting Teegarden's Star, with an orbital period of just 4.91 days. The planet's minimum mass is 1.05 Earth masses ; this value would be the true mass if the planet's orbit is not inclined from the Earth's perspective. Because of this, Teegarden's Star b is likely to be rocky. Astronomers estimate that Teegarden's Star b has a 60 percent chance of having liquid water, but only a 3 percent chance of having an atmosphere.

Host star
Teegarden’s Star is a low-mass red dwarf, with a mass of around 9 percent the mass of the Sun, and with a temperature of around 2,900 Kelvin (2,623 °C or 4,760 F). Due to the very low temperature and luminosity of Teegarden's Star, it was only discovered in 2003, since it has an apparent magnitude of only 15.1 (and an absolute magnitude of 17.22). Like most red and brown dwarfs, it emits most of its energy in the infrared spectrum. It is also older than the Sun, with an age of at least 8 billion years.

Habitability
Teegarden's Star b orbits within the habitable zone of its host star, meaning it is possible that its atmospheric composition could allow for stable liquid water on its surface, which could have also allowed the development of life.