8 Ursae Minoris is a 7th-magnitude

red clump The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a denser region of ...

star in

Ursa Minor Ursa Minor (, contrasting with Ursa Major), also known as the Little Bear, is a constellation located in the far northern celestial hemisphere, northern sky. As with the Great Bear, the tail of the Little Bear may also be seen as the handle of ...

. The star is unusually rich in

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

, with an abundance of . In the 2019

NameExoWorlds NameExoWorlds (also known as IAU NameExoWorlds) were various projects managed by the International Astronomical Union (I.A.U.) to encourage names to be submitted for astronomical objects, notably exoplanets. The accepted names would later be cons ...

competition, the star was assigned to contestants in

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

. It was named Baekdu after

Paektu Mountain Paektu Mountain or Baekdu Mountain () is an active stratovolcano on the Chinese–North Korean border. In China, it is known as Changbai Mountain (). At , it is the tallest mountain in North Korea and Northeast China and the tallest mounta ...

, the tallest mountain in

North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia. It constitutes the northern half of the Korea, Korean Peninsula and borders China and Russia to the north at the Yalu River, Yalu (Amnok) an ...

Planetary system

One exoplanet was discovered in 2015 by the

Bohyunsan Optical Astronomy Observatory The list of asteroid-discovering Observatory, observatories contains a section for each observatory which has discovered one or more asteroids, along with a list of those asteroids. For each numbered asteroid, the Minor Planet Center lists one o ...

. 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 Hallasan () is a shield volcano comprising much of Jeju Island in South Korea. Its summit, at , is the highest point in the country. The area around the mountain is a designated national park, named Hallasan National Park. Hallasan is commonly ...

, 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 A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

with star masses of . When the heavier star reached the end of the main sequence about , 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 , causing

helium burning The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. In stars Helium accumulates in the cores of stars as a result of the proton–proton chain reaction a ...

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 A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understo ...

. 8 Ursae Minoris is now in the red clump stage, and the planet will eventually be engulfed once it reaches the

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

. 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.

References

{{Ursa Minor G-type giants Planetary systems with one confirmed planet Baekdu Ursae Minoris, 08 08 Ursae Minoris 133086 073136 BD+75 0547 J14564834+7454032 Horizontal-branch stars