What Are Exoplanets and How Are They Found?

A planet does not simply orbit its star; the two orbit their shared centre of mass, so the star moves in a small circle too. As it swings towards us and away, the light it emits is shifted slightly blue and then slightly red, the same Doppler effect that changes the pitch of a passing siren. Measure the star's spectrum precisely enough, over enough nights, and the rhythm of the shift gives the planet's orbital period and its minimum mass.

This radial velocity method found the first exoplanet around a Sun-like star. In 1995 Michel Mayor and Didier Queloz detected a planet at least half the mass of Jupiter orbiting 51 Pegasi every four days, so close to its star that its surface would be over a thousand degrees. Nobody had expected a giant planet there, and its existence forced a rethink of how planets form and move. The pair shared the Nobel Prize in 2019. The method favours heavy planets close to their stars, because those produce the largest wobble, which is why the early catalogue was full of hot Jupiters.

If a planet's orbit happens to lie edge-on from our point of view, the planet passes in front of its star once each orbit and blocks a tiny fraction of the light. A Jupiter crossing a Sun-like star dims it by about one percent; an Earth dims it by less than one part in ten thousand. The transit method watches for that dip repeating at regular intervals.

It needs a telescope that can stare at many stars at once for years with extraordinary steadiness, which is why the big discoveries came from space. NASA's Kepler telescope monitored 150,000 stars from 2009 to 2018 and found over 2,600 confirmed planets, and its successor TESS is surveying the whole sky for planets around nearby, brighter stars. The dip gives the planet's size, and combined with a radial velocity mass it gives density, which separates rocky worlds from gas balls. During a transit, starlight also filters through the planet's atmosphere, and the James Webb Space Telescope now reads the chemical fingerprints in that filtered light: water vapour, carbon dioxide and methane have all been detected.

A handful of planets have been photographed, by blocking the star's glare with a mask and using infrared, where a young hot planet glows on its own. The four giant planets of the star HR 8799 have been imaged moving along their orbits over more than a decade. Direct imaging only works for large planets far from their stars, the opposite of what the wobble finds.

Gravitational microlensing catches a planet when its star drifts in front of a more distant one and the gravity of both briefly magnifies the background star's light; a planet adds a short extra blip. It works at enormous distances and has found planets near the centre of the galaxy, but each event happens once and cannot be repeated. Astrometry, measuring a star's tiny sideways shift on the sky, is now being done by the Gaia mission and should add thousands more. Each method has its blind spots, which is why the catalogue is a patchwork:

• •Radial velocity: the star's colour shift, best for heavy planets close in
• •Transit: the star dimming, best for planets whose orbits are edge-on
• •Direct imaging: a picture, best for large young planets far out
• •Microlensing: a brief brightening, reaches across the galaxy but never repeats
• •Astrometry: the star's position shifting, best for massive planets on long orbits

The most common kind of planet in the galaxy appears to be one our solar system does not have at all: worlds between the size of Earth and Neptune, called super-Earths or mini-Neptunes depending on their density, on orbits shorter than Mercury's. Hot Jupiters exist but are rare. There are planets orbiting two stars at once, planets circling dead stars, and rogue planets drifting through space with no star. Systems are frequently packed tighter than ours, with several planets inside the orbit of Mercury.

The nearest known exoplanet orbits Proxima Centauri, the closest star to the Sun, 4.2 light years away; it is at least as massive as Earth and sits in the zone where liquid water could exist, though its star's flares make habitability doubtful. TRAPPIST-1, forty light years away, has seven Earth-sized planets, three of them in the habitable zone, and is the most studied system after our own.

Whether any of these worlds is inhabited is not yet answerable, and the honest position is that the tools to answer it are only now arriving. Webb can detect the atmospheres of rocky planets around small stars; the next generation of telescopes, planned for the 2030s and 2040s, aims to image an Earth-like planet around a Sun-like star directly and look for the oxygen and methane that living things on our own planet keep in the air. For now, what the exoplanet count has settled is the older question: planets are ordinary, and there are billions of them in this galaxy alone.

Exoplanets are planets orbiting other stars, found mostly by the wobble they give their star or the dip they cause when they cross it, and more than 5,500 are now confirmed. They come in kinds our solar system lacks, the nearest is four light years away, and the instruments able to test whether any of them is alive are only just being built.