The University of Geneva’s Michel Mayor and his graduate student Didier Queloz were the first to discover a planet orbiting a distant star much like our own Sun. Meticulously ruling out, one after the other, alternative interpretations of their measurements, in October 1995 they announced the discovery of the planet designated 51 Pegasi b, now known as Dimidium, orbiting the star 51 Pegasi, since named Helvetia. Michel Mayor presented the discovery to an international assembly of astrophysicists in Florence, Italy.
The hunt for these exoplanets was inspired by advances in understanding of the formation of stars: it was becoming clear that the gases that were contracting to form new stars were somehow shedding the bulk of their energy of rotation, while new observations were revealing disks full of gas and dust, spinning around such forming stars, and containing a lot of energy of rotation. That dust could in principle coagulate to make planets whose orbiting motions could relieve the rotation problem of forming stars. But did it?
Several teams around the world convinced their sponsors of the need for more sensitive instruments, not to see the tiny, faint exoplanets directly, but to study their effects on the stars that they orbit. The mutual gravitational tug between a star and an exoplanet would cause the star to move back and forth just enough that its light should alternate measurably in colour from red to blue and back as the exoplanet moved around it. Measuring that swing required unprecedented precision, a lot of observing time to cover enough of a planet’s orbital period, a large sample of stars (because no one knew how common exoplanets were), and a great deal of commitment of the team members to stick with it. Only a few groups pulled that off.
Mayor and Queloz from the Observatory of Geneva in Switzerland had joined forces with colleagues in France to develop a new advanced spectrograph, an instrument that unravels starlight into its constituent colors. They had as primary goal to look for the smallest stars in orbit around Sun-like stars. The potential to discover exoplanets was on their mind also, but they were not hopeful. They expected they could find only large, heavy exoplanets and these were thought to be on orbits that would take many years to complete, and therefore they would need to observe similarly long to discover them.
But once the spectrograph was connected to a telescope at the Observatoire de Haute-Provence in France, they were lucky. Among the 142 Sun-like stars that they were monitoring, they found the exoplanet known as 51 Peg b. By the ideas of the time, it should not have been there – and that is why their colleagues were skeptical at first. It is a heavy planet at half the size of the giant Jupiter, but so close to its star that its orbit lasts only 4.2 days. The theory of the day held no planet like that could form where it was.
The discovery was quickly confirmed by another team, however, and scientists subsequently found many other such “hot Jupiters”. We now estimate that there are over 100 billion planetary systems in our Galaxy alone. Our new understanding of the formation of such systems tells us how Dimidium most likely got to be where it is: it formed much further out from its star, but then its orbit contracted to end up close to its star.
Queloz checked his observations and computations many times over before the 1995 announcement. Then just entering the field of astrophysics, he realized that an erroneous discovery claim would abruptly end his career. Mayor was confident, however. His announcement in the meeting was met with a mix of skepticism and enthusiasm, but when he returned to his hotel room that same day, there was a pile of faxes already waiting for him from journalists around the world. Queloz and Mayor’s lives changed in that discovery, and the field of exoplanetary science rapidly took off. These discoveries help us understand how planetary systems form and evolve. In doing so, they also reveal what happened a long, long time ago when our own solar system formed, when the giant planets roamed around to find their final orbits, and how that affected everything else in the solar system, including Earth.
Featured image credit: “Artist impression of the exoplanet 51 Pegasi b” by European Southern Observatory. CC by 4.0 via Wikimedia Commons.