Why did the then PhD student Didier Queloz get the Nobel prize?
He shared the 2019 Nobel Prize "for the discovery of an exoplanet orbiting a solar-type star".  It was shared with Michel Mayor. Maybe Queloz did much of the work and Mayor thought he rightfully deserved some of the credit. Queloz only got a fourth of the money because the prize was split between Mayor-Queloz and James Peebles. Peebles got his half for "for theoretical discoveries in physical cosmology."

This was a very unique case because it involved no new physics really just the discovery of what most astronomers knew existed.