Observatory · the wobble
Finding exoplanets
A planet round another star is a billion times fainter than its star and lost in its glare; almost none have been photographed. They are found by what they do to the star. A planet that passes in front of its star dims it a little, once every orbit: the transit. And a planet does not really go round its star: both go round their common centre of gravity, so the star wobbles towards us and away, and its light shifts blue and red, the Doppler effect. More than 6,000 planets have been found this way.
A star and its planet
Fig. I. A star and its planet from above and as we see them, with the dip and the wobble.
- One orbit
- The dip
- The wobble
- Could we find it?
The star is like the Sun; on the left both bodies circle the cross, their common centre, the star in a small circle because it is so much heavier. On the right is what a telescope gets. The brightness graph drops each time the planet crosses the star, by the fraction of the disc it covers: one percent for a Jupiter, 0.008% for an Earth. But only if the orbit is edge-on: tilt it a few degrees and, unless the planet is very close in, it passes above or below the star and there is no dip at all. The speed graph is the wobble, measured from the star's spectrum lines sliding back and forth, drawn here enormously exaggerated.
Big planets close in are the easiest: a deep dip every few days and a wobble of tens of metres a second. That is why the first planet found round a Sun-like star was a hot Jupiter, and why an Earth twin, with a dip of 84 millionths once a year and a wobble of 9 centimetres a second, is still out of reach. The two methods together give the planet's size and mass, so its density: rock, water or gas.
Who worked it out
Where it already matters
51 Pegasi b. In 1995 Michel Mayor and Didier Queloz in Geneva found a star 50 light years away wobbling at 56 m/s every 4.2 days: a planet half the mass of Jupiter, seven times closer to its star than Mercury is to the Sun. Nobody had expected giant planets there. It earned them the Nobel prize of 2019. The first planets of all, three years earlier, were found round a pulsar, by the delays in its ticks, as in the lighthouse.
Kepler and TESS. NASA's Kepler stared at one patch of sky from 2009 to 2018, watching 150,000 stars at once for dips, and found more than 2,600 planets; small rocky ones turned out to be common. TESS has been scanning nearly the whole sky since 2018. In 2025 the count of confirmed planets passed 6,000.
Reading their air. During a transit a little starlight shines through the planet's atmosphere, and the gases in it leave their own dark lines, the barcode of the spectroscope. In 2022 the James Webb Space Telescope found carbon dioxide this way in the air of WASP-39b, a hot Saturn 700 light years away.