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Before 1995 nobody had found a planet orbiting a star like the Sun. Michel Mayor and Didier Queloz did it by noticing that a star was being tugged back and forth by something unseen. This post covers how they found it, works through the two calculations at the heart of the discovery, and ends with what their approach can teach you.
Who are Mayor and Queloz?
Michel Mayor was born in Lausanne, Switzerland, on 12 January 1942, earned his doctorate in astronomy in 1971 and became a professor at the University of Geneva in 1988, according to his Nobel facts page. Didier Queloz was born in Geneva on 23 February 1966. His facts page notes that he completed his doctorate at Geneva in 1995 with Mayor as his supervisor, and that their joint work led to the prize.
The 2019 press release records that they shared half of the Nobel Prize in Physics "for the discovery of an exoplanet orbiting a solar-type star". The other half went to James Peebles for work on cosmology.
How do you find a planet you cannot see?
A planet and its star both orbit their common centre of mass. So the star moves in a small circle, towards us and away from us. That motion shifts its spectral lines slightly by the Doppler effect.
The shifts are tiny. The Nobel Committee's popular science background notes that Jupiter makes the Sun move at about 12 m/s, and Earth adds just 0.09 m/s. Mayor's first spectrograph, mounted at the Haute-Provence Observatory in France in 1977, could detect speeds down to about 300 m/s. The new instrument built with Queloz and colleagues, finished in spring 1994, brought that limit down to about 10 to 15 m/s.
What did they find in 51 Pegasi?
The Nobel Committee's scientific background describes the search. Their ELODIE spectrograph had a precision of 13 m/s and let them follow 142 stars. As early as the autumn of 1994 they saw that 51 Pegasi wobbled with a period of about four days. The final values in their paper in Nature were a period of 4.23 days, a velocity half-amplitude of 59 m/s, and a minimum planet mass of 0.47 Jupiter masses.
A Jupiter-mass planet with a four-day year was a shock. In our solar system, Jupiter takes almost 12 years to go round once. In August 1995, the same month their paper reached Nature, a review in the journal Icarus concluded that no Jupiter-mass planets had been found around Sun-like stars. Mayor and Queloz announced their discovery in Florence on 6 October 1995, and other teams quickly confirmed it.
Worked example 1: how big is the Doppler shift?
For speeds much less than light, Δλ/λ = v/c.
With v = 59 m/s and c = 3.00 × 10⁸ m/s: v/c = 1.97 × 10⁻⁷.
For a spectral line at 550 nm: Δλ = 550 × 1.97 × 10⁻⁷ ≈ 1.1 × 10⁻⁴ nm.
That is about one ten-thousandth of a nanometre. ELODIE managed it by averaging over thousands of absorption lines at once (the scientific background says about 5,000).
Worked example 2: how close is the planet?
Kepler's third law, in units of AU, years and solar masses, is a³ = M T² for a planet much lighter than its star.
Period: T = 4.23 days = 4.23 / 365.25 = 0.01158 years
Star mass: M = 1.07 solar masses, from the NASA Exoplanet Archive
a³ = 1.07 × 0.01158² = 1.07 × 1.341 × 10⁻⁴ = 1.435 × 10⁻⁴. Taking the cube root, a ≈ 0.052 AU.
With 1 AU = 1.496 × 10⁸ km, that is about 7.8 million km, which matches the Nobel Committee's figure of roughly eight million kilometres.
Common mistakes:
Using days in a³ = M T². The formula needs years.
Forgetting the cube root. a³ is not a.
Treating 0.47 Jupiter masses as the true mass. The Doppler method gives only a minimum, because we do not know how tilted the orbit is.
For more on these tools, see our posts on Christian Doppler and the Doppler effect and binary star and two-body orbit problems.
What happened next?
The 2019 press release called the discovery the start of a revolution in astronomy. The NASA Exoplanet Archive now lists more than 6,000 confirmed exoplanets.
What can a student take from Mayor and Queloz?
Build the instrument your question needs. Years of work on spectrographs came before the discovery.
Do not throw away a surprising result, and do not rush it. They spent months refining the result before announcing it.
Supervisor and student can find something together. Queloz's doctoral project turned into a Nobel Prize.
When a formula like a³ = M T² keeps going wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can see where the units change.
Try it now: use a³ = M T² to find the orbit of a planet with a 10-day period around a star of one solar mass.
