Teegarden's Star d is the third and outermost known planet of Teegarden's Star, a very small, dim red dwarf about 12.5 light-years away. It was announced in 2024, five years after its two inner siblings, Teegarden's Star b and c. It orbits every 26.1 days and has a minimum mass of about 0.82 Earth masses, a little less than Earth's.
A team led by Stefan Dreizler found it by combining a large number of new measurements from four instruments, CARMENES, ESPRESSO, MAROON-X and HPF, with the earlier data. They used the radial velocity method, which detects the slight back-and-forth motion a planet's gravity gives its star. Using several instruments matters for such a small signal: each has its own quirks, and a planet should appear in all of them. The same study traced a signal at 96 days to the star's rotation, and left the cause of another signal at 172 days open.
Teegarden's Star d receives far less light than its siblings: according to the NASA Exoplanet Archive, about an eighth of what Earth gets from the Sun, with an estimated equilibrium temperature of about 160 kelvin. The 2024 study describes b and c as the system's habitable-zone planets, the ones at distances where liquid water could last on a surface. Data from NASA's TESS satellite rule out transiting planets on short orbits down to about half Earth's size, and Teegarden's Star d has not been seen passing in front of the star, so its size is unknown. The authors note that this system is laid out quite differently from the tightly packed TRAPPIST-1 planets around a similar star.