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SPIRou / Infrared SpectroPolarimeter

Velocimetry

To detect exoplanets, the most efficient technique is velocimetry.  This is illustrated on the animation shown below.

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ESO/L. Cal├žada

As the planet orbits its host star, the gravitational pull of the planet causes the parent star to move back and forth.  This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift generated by the Doppler effect   With super-sensitive spectrographs such as SPIRou the shifts can be measured and used to infer details of a planet mass and orbit.  In simple words, SPIRou works as a radar speed gun that would flash, not for an excess velocity of the observed star, but for the regular and periodic variations of its velocity that will betray the presence of an orbiting planet.