Cobra Effect · Space

The first planet around a Sun-like star

How a star's tiny wobble revealed a giant planet where none was expected.

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A large bright star with a tiny dot beside it, almost lost in the glare

For centuries, people wondered whether other stars have planets of their own.

Planets are tiny and faint beside the stars they orbit. Seeing one directly, lost in its star’s glare, was far beyond the telescopes of the time. But a planet also tugs on its star, making the star wobble very slightly. That wobble might show up in the star’s light.

A wobbling star’s light shifts slightly back and forth in colour.

As a star moves toward us, its light shifts a tiny amount toward blue. As it moves away, the light shifts toward red. The shifts caused by a planet are minute, and need extremely precise instruments to measure. In Switzerland, Michel Mayor and his student Didier Queloz helped build such an instrument, called ELODIE. It was fitted to a telescope at the Haute-Provence Observatory, in the south of France.

In 1995 they found a star whose light swung back and forth every four days.

The star, called fifty one Pegasi, is much like our Sun. Its speed toward and away from us rose and fell in a smooth, regular rhythm. Something was orbiting it, pulling it round in a small circle. The rhythm showed that the unseen object went round its star in just over four days.

The planet was a giant, orbiting closer to its star than Mercury is to the Sun.

Its mass was at least about half that of Jupiter. Theories of how planets form said giants like that should be found far from their stars. In October 1995 Mayor and Queloz announced the discovery, and many astronomers were cautious. Within days, American astronomers Geoffrey Marcy and Paul Butler had confirmed it with their own measurements.

It was a new kind of world, a hot Jupiter.

So close to its star, the planet is scorched to searing temperatures. Other planets like it were soon found around other stars. Many astronomers now think such giants form farther out and then move inward. Our own solar system turned out not to be a pattern for all the others.

A few years earlier, planets had been found around a dead star.

In 1992 Aleksander Wolszczan and Dale Frail found planets orbiting a pulsar, the collapsed core left behind by an exploded star. But fifty one Pegasi b was the first planet found around an ordinary star like the Sun. In 2019 Mayor and Queloz shared half of the Nobel Prize in Physics for the discovery. By 2025, NASA’s count of confirmed planets beyond the solar system had passed six thousand.

So when theory says something should not be there, ask whether anyone has looked.

Nobody expected a giant planet so close to its star. The measurements showed one anyway, and the theories had to change. Planets turned out to be common, circling a great many of the stars in the sky. Some found since are about the size of the Earth, in orbits where liquid water could exist.

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