Among the thousands of exoplanets discovered so far, only a handful combine three qualities that make astronomers pay special attention: they are nearby, terrestrial, and roughly Earth-sized. Teegarden's Star d belongs to that rare category.
Discovered in 2024, Teegarden’s Star d is the third confirmed planet orbiting one of the nearest stars to our Solar System. At first glance, it may sound like another entry in an ever-growing catalog of alien worlds. But its significance lies in something more subtle: it gives astronomers a nearby laboratory for understanding how Earth-sized planets form and survive around the smallest stars in the galaxy.
The host star itself is unusual. Teegarden's Star sits about 12.5 light-years from Earth in the constellation Aries, making it one of our close stellar neighbors. Yet despite that proximity, it escaped detection until 2003 because it is extraordinarily dim in visible light. Most of its energy is emitted in infrared wavelengths, making it difficult to spot with traditional sky surveys. The star is an ultracool red dwarf with only around one-tenth the Sun’s mass and an estimated age of roughly 8–10 billion years—potentially much older than our Solar System.
Teegarden’s Star first gained attention in 2019 when astronomers announced the discovery of two planets, Teegarden’s Star b and c, both receiving enough stellar energy to become candidates for habitability. Then, after additional observations using instruments including CARMENES, ESPRESSO, MAROON-X, HPF, and photometric data from TESS, researchers uncovered evidence for a third planet: Teegarden’s Star d.
Teegarden’s Star d is a terrestrial planet, meaning it is expected to be primarily rocky rather than gaseous. Current measurements indicate a mass of approximately 0.82 times that of Earth and an estimated radius of about 0.95 Earth radii. In other words, if you could stand on its surface, the world might feel broadly Earth-scaled rather than like a giant super-Earth or miniature Mercury.
Its orbit, however, immediately reveals that this is not another Earth.
The planet circles its star every 26.1 Earth days at a distance of only 0.079 astronomical units—less than one-tenth the Earth–Sun distance. Around a Sun-like star, that would place a planet in an intensely hot environment. Around Teegarden’s Star, things work differently because the star emits so little energy. Even so, current models place Teegarden’s Star d outside the system’s classical habitable zone, making it colder and less favorable for stable surface liquid water than its inner siblings.
That does not make the planet uninteresting.
Astronomers increasingly view nearby planetary systems as complete ecosystems rather than collections of isolated worlds. The architecture of the Teegarden system appears different from compact systems like TRAPPIST-1 and may resemble other low-mass star systems in important ways. Understanding where Teegarden’s Star d formed—and whether it migrated inward or outward over time—helps researchers reconstruct the history of the entire system.
There is also the question of atmosphere.
At present, no direct atmospheric observations exist for Teegarden’s Star d. Scientists cannot yet say whether it possesses a dense atmosphere, a thin envelope of gases, or almost none at all. That uncertainty matters enormously because atmospheric pressure and composition can radically alter a planet’s surface conditions. A world outside the nominal habitable zone could still maintain pockets of warmth under the right greenhouse conditions, while an airless planet would become far more hostile. Habitability studies increasingly emphasize that receiving the “correct” amount of starlight is only one variable among many.
Another reason Teegarden’s Star d matters is observational opportunity.
At only about 12.5 light-years away, the system is exceptionally close by exoplanet standards. Nearby systems offer stronger signals and better prospects for future measurements of atmospheric chemistry, orbital interactions, and planetary composition. Even if Teegarden’s Star d itself turns out to be cold and barren, it helps make its entire stellar neighborhood a prime target for next-generation observatories.
The broader lesson from Teegarden’s Star d is that modern exoplanet science is no longer focused only on finding a perfect “second Earth.” Researchers are building a statistical understanding of planetary diversity: how small rocky worlds emerge around stars unlike our own, how common they are, and which conditions produce environments where life could eventually appear.
Teegarden’s Star d may never become the most famous exoplanet in the sky. But as one of the nearest known Earth-sized worlds orbiting one of the smallest known stars, it represents something increasingly valuable in astronomy: not a final answer, but a nearby clue.
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