原げん始し惑わく星せい系けい円えん盤ばん

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Meanings

protoplanetary disk

Technical term in astronomy referring to a rotating disk of dense gas and dust surrounding a young star, from which planets may form.

原げん始し惑わく星せい
原始惑星 (protoplanet) is a body in the process of becoming a planet, while 原始惑星系円盤 is the disk of material from which such bodies form.
降こう着ちゃく円えん盤ばん
降着円盤 (accretion disk) is a more general term for a disk of material accreting onto a central object; 原始惑星系円盤 specifically refers to the disk around a young star where planet formation occurs.

Compound of 原始 (primitive, original), 惑星系 (planetary system), and 円盤 (disk). A direct translation of the English term 'protoplanetary disk'.