Few known planets come closer to Earth's basic numbers than Teegarden's Star b. It gets about 1.08 times as much starlight as Earth does. Its equilibrium temperature, the temperature it would reach from starlight alone with no greenhouse warming, is about 277 K, a few degrees above freezing. And its minimum mass is about 1.16 Earths. A 2024 study gave it an Earth Similarity Index of 0.90, a rough score from 0 to 1 of how closely a planet's basic properties match Earth's.
Its star is one of the smallest known, about a tenth of the Sun's mass, with a surface near 2,700 °C, and it lies about 12.5 light-years away. In 2019 a team led by Mathias Zechmeister of the University of Göttingen reported two planets around it, after about three years of watching the star with the CARMENES instrument. They used the radial velocity method: an orbiting planet pulls its star slightly back and forth, and that wobble shifts the colour of the starlight. From more than 200 measurements they found two worlds, each of at least about 1.1 Earth masses, with orbits of 4.91 and 11.4 days. These were the first Earth-mass planets around such a small, cool star to have their masses measured this way.
A 2024 study led by Stefan Dreizler combined new measurements from CARMENES, ESPRESSO, MAROON-X and HPF with images from NASA's TESS satellite. It confirmed planets b and c, refined their orbits, and found a third planet, d, with a minimum mass of about 0.82 Earths on a 26-day orbit. Planet b orbits only 0.0259 astronomical units from its star, under 3 percent of Earth's distance from the Sun.
Much about planet b is still unknown. Radial velocity gives only a minimum mass, because the tilt of the orbit is unknown. Neither the 2019 nor the 2024 study found transits, the small dips in starlight when a planet crosses its star. Without them, nobody has measured b's size or examined its atmosphere, and whether it has an atmosphere at all is open. One curious twist: the Göttingen team noted that observers on these planets could, in turn, spot Earth crossing the Sun.
TerraForge's model, working only from b's mass, orbit and star, classifies it as a rocky terrestrial world at 288 K, slightly warmer than the 277 K equilibrium temperature. That matches the discovery teams' description of an Earth-like, potentially habitable planet. The caveat is that its rocky make-up, like its size, is inferred from a minimum mass rather than measured.