 51 Pegasi b

51 Pegasi b is the giant exoplanet whose 1995 discovery established a planet around a Sun-like star. It is a hot Jupiter: its roughly 4.23-day orbital period and about 0.05 astronomical unit separation differ markedly from Jupiter's distant orbit. Earlier confirmed exoplanets had already been found around a pulsar; the host category is part of this discovery's significance.^[ref]

Michel Mayor and Didier Queloz used the ELODIE spectrograph to measure periodic Doppler shifts in the host star's spectrum. The star's motion around the shared centre of mass produces alternating approaching and receding velocities even though the planet is unresolved. For a nearly circular orbit and a planet much less massive than its host,The measured velocity amplitude and period therefore constrain $M_p\sin i$, originally about half Jupiter's mass, rather than independently determining inclination and true mass. Checking spectral-line changes also helps distinguish orbital motion from changing stellar line profiles.^[ref]