= Discovery of 51 Pegasi b
{c}
<Michel Mayor> and <Didier Queloz> began observing 51 Pegasi in September 1994 and revisited its repeating <Doppler spectroscopy> signal in 1995. Their circular fit used 35 measurements: <orbital period> $4.2293\pm0.0011$ days, <stellar radial-velocity semi-amplitude> $59\pm3\,\mathrm{m\,s^{-1}}$, and residual scatter $13\,\mathrm{m\,s^{-1}}$ after epoch-dependent <velocity> offsets. Apparently stable comparison <stars> checked the instrumental zero point. Low chromospheric <stellar activity> and an inferred roughly 30-day <stellar rotation> period argued against <starspots>; <light curves> and the <spectral line bisector> tested <stellar pulsation>. These checks favoured orbital motion but had finite sensitivity. The reported $M_p\sin i=0.47\pm0.02$ <Jupiter masses> was not a true <mass>. Stronger upper bounds used stellar-spin information and assumed spin–orbit alignment, limiting a nearly face-on <binary star> interpretation conditionally.<A Jupiter-mass companion to a solar-type star by Michel Mayor and Didier Queloz>[Mayor and Queloz (1995), pp. 356–358, figures 2–4, table 1, and alternative interpretations; scan: https://wwelsh.sdsu.edu/CLASSES/ASTR510/PAPERS/Mayor-Queloz_51Peg.pdf]{ref}
The <hot Jupiter> matters both for <planetary migration> and for how <exoplanets> are found. At fixed small companion <mass>, <orbital inclination> and <orbital eccentricity>, the <stellar radial-velocity semi-amplitude> scales as $K\propto P^{-1/3}$. Comparing chosen <orbital periods> of $4$ days and $12$ years gives an amplitude ratio of about $10.3$: a close companion can be much easier to detect and complete many <orbits> during one observing season. Finding a giant <planet> close to its <star> also asks how its present <orbit> relates to where it formed. <Doppler spectroscopy> supplies a dynamical constraint; it does not by itself establish a formation pathway or measure the <exoplanet>'s <radius>.
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