海かい王おう星せい外がい天てん体たい

Meanings

1

noun

trans-Neptunian object; TNO

astronomy

Technical astronomy term for any minor planet or dwarf planet orbiting the Sun beyond Neptune. Often abbreviated as TNO in English.

outside

天heavens, sky, imperial

体body, substance, object

小しょう惑わく星せい
小惑星 (asteroid) generally refers to rocky bodies mainly in the asteroid belt, while 海王星外天体 are icy bodies beyond Neptune.
準じゅん惑わく星せい
準惑星 (dwarf planet) is a classification that includes some 海王星外天体 like Pluto, but not all TNOs are dwarf planets.

Compound of 海王星 (Neptune) + 外 (outside) + 天体 (celestial body), literally 'celestial body outside Neptune's orbit'.