= Exoplanet
{wiki}
An <exoplanet> is a <planet> outside the <Solar system>; here the focus is on <planets> orbiting <stars> other than the <Sun>, including <pulsars>. Most <exoplanets> are detected indirectly: a <star>'s changing <Doppler shift> reveals its motion, or a small dip in its <light curve> reveals an <exoplanet transit>. The striking idea is that an unresolved point of light can tell us about an otherwise unseen world's <mass>, <radius>, and <orbit>.
The measurements answer different questions. <Exoplanet transit photometry> primarily gives a <radius> ratio, while <Doppler spectroscopy> constrains <mass> together with <orbital inclination>. Combining them with the host <star>'s properties gives a mean <mass density>, $\bar\rho_p=3M_p/(4\pi R_p^3)$, useful for <exoplanet interior> models. Even this combination does not uniquely determine composition: see <exoplanet interior-composition degeneracy>. <Transit spectroscopy> and an <exoplanet secondary eclipse> then probe an <exoplanet atmosphere>.
The discovery history has two distinct milestones. In 1992 Aleksander Wolszczan and Dale Frail reported two <planet>-mass companions to the <pulsar> PSR B1257+12 from periodic changes in pulse arrival times; their paper also suggested an additional companion. These were the first confirmed <exoplanet> discoveries, around a <neutron star>. https://www.nature.com/articles/355145a0[Wolszczan and Frail (1992)]{ref}. Wolszczan's contemporary conference account, archived by NASA, likewise distinguishes the two reported companions from a possible third. https://ntrs.nasa.gov/citations/19920021993[Wolszczan (1992), NASA archive]{ref}.
In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b through <Doppler spectroscopy>, the landmark first confirmed <exoplanet> around a <Sun>-like <star>. This was a <hot Jupiter>, not the first <exoplanet> of every kind: the earlier <pulsar> discoveries belong to a different host category. https://doi.org/10.1038/378355a0[Mayor and Queloz (1995)]{ref}. NASA's contemporary account describes the confirmation and the approximately four-day <orbital period>. https://science.nasa.gov/image-article/apod-1995-december-01-51-pegasi-a-new-planet-discovered/[NASA account (December 1995)]{ref}.
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