8 Ursae Minoris

8 Ursae Minoris is a 7th-magnitude red clump star in Ursa Minor. The star is unusually rich in lithium, with an abundance of A = $1.51$.

In the 2019 NameExoWorlds competition, the star was assigned to contestants in South Korea. It was named Baekdu after Paektu Mountain, the tallest mountain in North Korea.

Planetary system
One exoplanet was discovered in 2015 by the Bohyunsan Optical Astronomy Observatory. It has a tight 93-day orbit at a distance of about 0.5 AU, which is unusually close for a giant host star — it should have been consumed during a previous expansion to 0.7 AU. It is officially named Halla after Hallasan, the tallest mountain in South Korea. There is also a stellar activity cycle of 65 days, and possibly a second companion orbiting at a distance of at least 5 AU.

A 2023 study suggests that 8 Ursae Minoris was initially a binary star with star masses of $10.73$. When the heavier star reached the end of the main sequence about $52.9$, it expanded until it dumped all of its mass onto the secondary and became a helium white dwarf. The other star eventually engulfed this white dwarf at around $4,847$, causing helium burning to start prematurely and forming the star that we know today. The planet may have survived the episode as a former circumbinary planet, or it may have formed from material ejected during the stellar merger. 8 Ursae Minoris is now in the red clump stage, and the planet will eventually be engulfed once it reaches the asymptotic giant branch.

Research published in May 2024 used three different methods to determine the age of 8 Ursae Minoris and came up with an estimate of 1.9–3.5 Gyr. This is much younger than the approximately 9 Gyr age required for the theory that the presence of the planet so close to 8 Ursae Minoris can be explained by a stellar merger. The researchers also estimated a new mass for the star of 1.7 solar masses, making it more compact. This would allow for the planet to orbit slightly further from the star, making its existence less of a mystery.