Exoplanets and the search for life
Thirty years ago we knew exactly eight planets — all in our own solar system. Today, catalogues list more than 5,500 worlds around other stars, and statistically nearly every star in the night sky hosts at least one. Lava worlds, diamond planets, worlds with two suns: the variety outstrips any science fiction. The big question has shifted from "are there other planets?" to "which ones are alive?"
How do you find a planet you cannot see?
The transit method: blinking stars
When a planet crosses in front of its star, the starlight dips a fraction — often less than 0.01 percent. Space telescopes like Kepler and TESS monitored hundreds of thousands of stars for exactly such rhythmic dips. Most known exoplanets were found this way, revealing their size and orbital period.
The radial-velocity method: wobbling stars
A planet also tugs at its star, making it trace a tiny circle. That wobble shows up as a rhythmic red- and blueshift in the starlight. This method delivered the first exoplanet around a Sun-like star in 1995 — 51 Pegasi b, worth the 2019 Nobel Prize.
The habitable zone: not too hot, not too cold
Life as we know it needs liquid water. The habitable zone (or Goldilocks zone) is the ring around a star where temperatures allow it: closer in, oceans boil away; further out, everything freezes. Earth sits comfortably inside; Venus (too hot) and Mars (too cold, too small) flank us as reminders that the zone is no guarantee — atmosphere and geology matter just as much.
Around small, cool stars the zone lies much closer in. That is what makes systems like TRAPPIST-1 so exciting: seven Earth-sized planets, three in the zone — and because they orbit so tightly, they transit often, perfect for study.
From finding to understanding: reading atmospheres
The new phase has begun: not just counting planets, but dissecting their atmospheres. During a transit, the atmosphere filters a sliver of starlight, and every molecule leaves its fingerprint in it. The James Webb Space Telescope has already detected carbon dioxide, methane and water vapour on distant worlds — including the much-debated K2-18b, possibly a hydrogen-rich atmosphere over an ocean.
The strangest worlds so far
- 55 Cancri e — a lava world with a permanently molten surface; possibly carbon-rich (the "diamond planet").
- HD 189733 b — a striking blue, but hostile: 8,000 km/h winds and rain of molten glass.
- Kepler-16b — orbits two stars at once: a double sunset every evening, Tatooine-style.
- PSR B1257+12 b/c/d — the very first exoplanets ever found (1992), around a dead star: a pulsar.
- Proxima Centauri b — the nearest exoplanet, 4.2 light-years away, in the habitable zone of our neighbouring star.
Frequently asked questions
How many exoplanets have been discovered?
Over 5,500 confirmed, more every week. Nearly every star statistically hosts at least one planet.
What is the habitable zone?
The distance from a star where liquid water can exist on a planet's surface — necessary, but not sufficient, for life.
Has alien life been found?
No. Intriguing signals exist, but no evidence. James Webb is the first instrument that can seriously conduct the hunt for a biosignature.