Why was the discovery of 51 Pegasi b surprising to astronomers?

The discovery of 51 Pegasi b in 1995 was surprising to astronomers because it challenged existing theories of planetary system formation.1

Prior to this finding, scientific models were based exclusively on our own solar system, where gas giants orbit far from their host stars. In contrast, 51 Pegasi b is a "hot Jupiter"—a massive gas giant that orbits its sunlike star at an extremely close distance of only 7.8 million kilometers (4.8 million miles). This is significantly closer than Mercury orbits the Sun.12

Because such a large planet could not have formed in such a high-temperature environment, its existence forced astronomers to conclude that these planets likely formed much further away from their stars before migrating inward. This discovery by Michel Mayor and Didier Queloz fundamentally changed our understanding of how planetary systems evolve.12