Earth, planets and orbits

1995

A planet around another sun

Michel Mayor and Didier Queloz

How do you spot a planet you cannot see?

The star 51 Pegasi wobbles back and forth every 4.2 days. Mayor and Queloz realised a giant planet was tugging it, orbiting closer to its star than Mercury is to the Sun. Thousands of planets have been found since.

Loading simulation…

How they figured it out

Planet hunters expected planets like Jupiter, taking years to orbit. Michel Mayor and Didier Queloz, using a new spectrograph in southern France, found the star 51 Pegasi wobbling every 4.2 days. A giant planet was orbiting closer to its star than Mercury is to the Sun.

Theory said such “hot Jupiters” should not exist, and many were sceptical. But they were confirmed within weeks, and they changed how we think planets form: planets can migrate. Mayor and Queloz shared the 2019 Nobel Prize in Physics.

The equation

T2=4π2G(M+m)a3T^2 = \frac{4\pi^2}{G(M+m)} a^3

Kepler’s third law, with Newton’s constant: measure the year, and you know the orbit’s size.

Everyday echoThe star’s wobble is like a hammer thrower leaning back against the hammer.

See it in real life

The hammer thrower

A hammer thrower spinning a heavy ball leans back and circles too, around a point between them. Stars do the same with their planets. The star’s tiny wobble shifts its light slightly blue, then red, then blue again.

Where engineers use it

  • Doppler sensing

    The star’s wobble is measured by tiny colour shifts, the same physics as Doppler ultrasound.

Where it came from