Exoplanet discoverers win Nobel prize

Nobel prize winners Didier Queloz (left) and Michel Mayor, in front of the ESO:3.6 m telescope in Chile. (Photo ESO).

In 1995 the historic discovery of the first planet orbiting another solar-type star was made. The Swiss astronomers Michel Mayor and Didier Queloz have now been awarded half of the 2019 Nobel prize in physics. 

Thus the term "exoplanet" was coined and became known. It simply describes a planet orbiting another star than our Sun. Like many other discoveries in astronomy it was a result of a combination between scientific curiosity and the latest technological advances. Mayor och Queloz could use a new very accurate spectrograph known as the Elodie. Connected to a large (1.93m) telescope at the observatory of Haute Provence it became possible using the Doppler effect to measure extremely small but regular motions of the star 51 Pegasi. They were caused by the gravitational influence from a planet orbiting the star. The Doppler effect shifts absorption lines in the spectrum of a star. All lines have the same displacements and by measuring many lines simultaneously a very high precision can be achieved – and thus of the velocity in the line of sight, the so called radial velocity. 

 Left: The Elodie spektrograph. Right: An echelle spectrum from the spectrograph. The position of the vertical dark lines can be measured with high precision.

The first discovered exoplanet around a solar-type star did not turn out very Earth-like.  Eventually named Dimidium, it became the first example of a so called "hot Jupiter". Its distance to its host star is almost 20 times smaller than the distance between Earth and the Sun. Thus its orbiting period is very small, only 4 days. The mass is larger than half of Jupiters and the temperature is estimated at almost 1300 degrees.  

The historic discovery led to further improvements in the technology for measuring the Doppler effect. Queloz and Mayor went on to develop an even more powerful spectrograph, called HARPS. It was connected to the large ESO 3.6m telescope in Chile from which a large number of discoveries were subsequently made.

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