In 2267, he was one of three who beamed to the surface of Gothos to search for the missing Captain Kirk. At the time the landing party was assembled, climactic conditions were thought to be extremely inhospitable, and his expertise in geophysics served the landing party well.
Trelane recognized Jaeger's name's national origin, gave a smart salute, and spoke to him in German, saying "Ein Deutsch Soldat, nein? Ein, zwei, drei, vier, gehen wir mit dem Schießgewehr!", while miming a military march. In response to Trelane's question ("A German soldier, no?"), Jaeger replied that his specialty was as a scientist, not as a military man. (TOS: "The Squire of Gothos")
"No detectable soil or vegetation, extremely hot toxic atmosphere swept by tornadic storms and continuous volcanic eruptions deadly to any lifeform as we know it without oxygen and life support systems." (TOS: "The Squire of Gothos")