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.

News of a planet in the habitable zone is easy to misread. The zone is simply the range of distances from a star at which a planet with a suitable atmosphere could keep liquid water on its surface. It says nothing about whether the planet actually has water, air or life. Our own solar system makes the point: by some estimates Venus lies near the inner edge of the Sun's zone and Mars near the outer edge, yet Venus is a furnace under thick carbon dioxide and Mars is a cold, dry desert. Being in the zone is a starting condition, not a result.

The atmosphere decides a great deal. Without its greenhouse gases, Earth's average surface temperature would be roughly minus 18 degrees rather than about 15. A planet in the zone with a thin atmosphere could be frozen, and one with a very thick one could boil, while a world outside the zone might hide liquid water under ice, as Jupiter's moon Europa appears to. Many of the most promising zone planets orbit small red dwarf stars, and those planets are likely to be tidally locked, with one side in permanent day and the other in endless night.

The search had a long history of claims that did not hold up. From the 1960s the astronomer Peter van de Kamp reported that Barnard's Star, one of the Sun's nearest neighbours, wobbled because of one or two unseen planets, based on decades of photographic plates. Other astronomers later found that a similar wobble showed up in other stars photographed with the same telescope, and traced it to adjustments made to the instrument. In 1991 a team at Jodrell Bank announced a planet orbiting a pulsar, then withdrew the claim within months after finding they had not fully corrected for the Earth's own motion around the Sun.

At the same meeting in January 1992 where that retraction was made, Aleksander Wolszczan presented planets around another pulsar, PSR B1257+12, found with Dale Frail using the Arecibo radio telescope. A pulsar is the spinning remnant of an exploded star, and it sends out radio pulses as regular as a clock. Planets tugging on it make the pulses arrive slightly early and then slightly late, a timing version of the wobble. Those planets, bodies a few times the mass of Earth bathed in radiation, were confirmed and are still accepted, which makes them the first exoplanets known, three years before 51 Pegasi b was found around an ordinary star.

Most exoplanets have catalogue names rather than proper names. The rule is simple: take the name of the host star and add a lowercase letter in order of discovery, starting with b, because the star itself is treated as a. The first planet found around a Sun-like star is therefore 51 Pegasi b, and planets in a system found later are c, d and so on.

The International Astronomical Union has also run public naming campaigns, inviting people around the world to propose names for selected stars and their planets. As a result, 51 Pegasi b is also called Dimidium, meaning half, a reference to its mass being roughly half that of Jupiter. Most of the thousands of known worlds, though, are still known only by their star's name and a letter.

A few numbers show why the work is so hard. Jupiter pulls the Sun around their shared centre of mass at about 12 metres per second, roughly a top sprinter's speed, and an alien astronomer would see that wobble repeat every 12 years. Earth moves the Sun by only about 9 centimetres per second, a slow crawl, repeating once a year. In the Sun's light, a speed of 9 centimetres per second shifts the spectral lines by about three parts in ten billion. The best spectrographs today can measure shifts of a few tens of centimetres per second, and a star's own boiling, spotted surface adds noise larger than that.

The transit method has a different handicap: geometry. A planet transits only if its orbit is lined up almost exactly edge-on to us, and the chance of that is roughly the star's radius divided by the planet's orbital distance. For an Earth-like planet one astronomical unit from a Sun-like star, that is about one in 200. For a hot Jupiter hugging its star at a twentieth of that distance, it is closer to one in ten. So every Earth-like transit a telescope catches stands for something like two hundred similar planets whose orbits are tilted out of view, which is how a few thousand detections become estimates of billions.

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.