One of the great questions for science and humanity is: Are we the only solar system with planets, and the only one with planets that could support life? The discovery of planets around other stars makes it likely that other planets exist capable of supporting life.
Of great importance to astronomers, the discovery of other solar systems lets them test their theories on the origin of planets and solar systems. The discovery of distant planets has fundamentally changed how we perceive our place in the universe.
In the sixth century B.C., Greek scientist Anaximander was the first to theorize that other planets must exist. In 1600 Italian priest and astronomer Giordano Bruno was burned at the stake by the Catholic Church for professing the same belief. American astronomers were actively searching through giant telescopes for planets orbiting other stars by late the 1940s.
Michel Mayor was born in 1942 and even as a child was fascinated by stars and astronomy. With his collaborator, Antoine Duquennoy, he joined the many astronomers searching for small objects in the universe. But Mayor searched not for planets, but for brown dwarfs—cool, dim objects thought to form like stars, but which failed to grow massive enough to support hydrogen fusion and thus never lit up with starry furnace and fire. Too big for planets, too small to become stars, brown dwarfs were a galactic oddity.
Astronomers, however, had a problem: telescopes can’t see planets and brown dwarfs because they don’t give off light. Instead, astronomers searched for slight side-to-side wobbles in the motion of a star caused by the gravitational tug of a large planet (or brown dwarf).
Some tried to detect such wobble by carefully measuring the position of a star over the course of months or years. Others (Mayor included) looked for this wobble by using Doppler shift and measuring tiny shifts on a spectrograph in the color of the light coming from a star that would be the result of changes in the star’s motion toward or away from Earth.
Following the death of Duquennoy in 1993, Mayor teamed with graduate student Didier Queloz and developed a new, more sensitive spectrograph to search for brown dwarfs. Their new spectrograph was capable of measuring velocity changes as small as 13 meters per second, about the same as the wobble in our sun’s motion caused by Jupiter’s gravitational tug.
But everyone assumed that such massive planets would take years to orbit a star (as they do in our system). Thus the wobble from a planet’s tug would take years of data to notice. It never occurred to Mayor to use his new spectrograph and a few months’ worth of time on a telescope to search for a planet.
Beginning in April 1994, using the Haute-Provence Observatory in southern France, Mayor and Queloz tested their new spectrograph on 142 nearby stars, hoping to detect a wobble that would indicate a massive nearby object like a brown dwarf. In January 1995 one star, 51 Peg (the fifty-first brightest star in the constellation Pegasus) caught Queloz’s eye. It wobbled. It wobbled back and forth every 4.2 days.
They tested the star’s light to make sure it didn’t pulse. They tested to see if sun spots might create an apparent wobble. The tested to see if 51 Peg puffed up and contracted to create the appearance of wobble. Nothing could account for 51 Peg’s wobble except for a sizable orbiting object.
From the amount of 51 Peg’s wobble they calculated the mass of the object and knew it was too small to be a brown dwarf. It had to be a planet! They had discovered a planet outside our solar system.
By 2005, several hundred other planets had been located—gas giants speeding around Mercury-sized orbits; some rocky planets in cozy, not-too-hot-and-not-too-cold orbits; even some drifting free through space without a star to circle. Earth is certainly not alone. Mayor and Queloz were the first to discover proof of this spectacular reality.
If only one star in ten has planets (and current knowledge indicates that at least that many do), if the average star with planets has at least three, and if only one in every hundred are rocky planets in life-sustaining orbits (and recent discoveries indicate that to be the case), then there are at least 300,000 planets capable of supporting life in our galaxy alone.