Tähtsaim

Tähtsaim is a star at the center of the Tähtsaim system, a nearby stellar system in the northern celestial hemisphere of the fictional region known as the Arcturan Borderlands. It is classified as a K-type main-sequence star (spectral type K2V) with a mass of about 0.8 solar masses, a radius around 0.8 R☉, and a luminosity near 0.4 L☉. Estimates place its age at roughly 6 to 7 billion years. The star exhibits modest magnetic activity and a rotation period on the order of several tens of days, typical for a star of its class.

The name Tähtsaim combines Estonian words tähis "star" and -saim, forming a designation used by early navigators who charted the system. Within the community, the name is sometimes translated as "the sought star" or "the star of significance."

The Tähtsaim system hosts at least two confirmed exoplanets in near-circular orbits. Tähtsaim b is a terrestrial world with a mass around 1.6–2.0 M⊕ and an orbital distance near 0.65 AU, placing it inside the inner edge of the system's habitable zone. Tähtsaim c is a water-rich super-Earth or mini-Neptune with a mass about 4–6 M⊕, on a ~1.25 AU orbit that lies near the outer boundary of the habitable zone. A gas-giant planet, Tähtsaim d, orbits further out at roughly 3.5–4.0 AU and influences the system's outer dynamics.

Discovery of the system occurred in the early 22nd century by the Lumen Survey using transit photometry and radial-velocity measurements. Follow-up observations have refined estimates of planetary masses and compositions. No Earth-sized moon has been confirmed yet; the planets' atmospheres remain subjects of speculative study in fictional or model contexts.

In fictional worlds, Tähtsaim is referenced as a case study of small-star planetary systems with compact configurations and potential habitability. The system is used as a setting for exploration narratives and comparative planetology discussions in the Estonian-language science fiction and worldbuilding communities.