Thirty years ago, no one knew whether any star besides the Sun had a planet. Today astronomers have confirmed thousands, and NASA's Exoplanet Archive lists more than 6,000. The next question is the one everyone asks: is there another Earth out there? To answer it, scientists start with a simple idea called the habitable zone.
The habitable zone is the range of distances from a star where a planet with an atmosphere could, in principle, keep liquid water on its surface. Too close and water boils away. Too far and it freezes. Liquid water matters because every form of life we know depends on it.
NASA is careful about the wording: being in the habitable zone does not mean a planet is habitable. It only means the temperature could be right, if the planet has the right atmosphere and enough water. Think of it as a first filter, not a verdict.
Our own Solar System shows why. Venus sits near the inner edge and has a crushing carbon dioxide atmosphere and surface temperatures hot enough to melt lead. Mars sits near the outer edge and lost most of its air. Earth is not special only because of where it orbits.
Most exoplanets are far too faint to photograph next to their glaring stars. Two indirect methods have found the majority:
Transits reveal a planet's size, while the wobble reveals its mass. Together they give density, which tells scientists whether a world is likely rocky or a gas giant. The same kind of patient measurement drives asteroid science; see how astronomers measure the size and speed of an asteroid.
Kepler-186f. Announced in 2014, it was the first Earth-size planet confirmed in the habitable zone of another star. It orbits a small red dwarf roughly 500 light-years away once every 130 days or so.
Proxima Centauri b. Found in 2016, it orbits the closest star to the Sun, about 4.2 light-years away. It has a minimum mass a little above Earth's. But red dwarfs can unleash powerful flares, so whether it keeps an atmosphere is unknown.
TRAPPIST-1. This tiny star, about 40 light-years away, hosts seven Earth-size planets, announced in 2017. Three of them, usually listed as e, f and g, orbit in the habitable zone. Because the star is so small and cool, the zone lies very close to it, and the planets are thought to be tidally locked, always showing one face to the star. The James Webb Space Telescope has been measuring the inner planets, and early results suggest at least some may lack thick atmospheres. The outer, cooler worlds are still being studied.
Red dwarfs are the most common stars in our galaxy, so many known habitable-zone planets orbit them. That is good news for statistics but uncertain news for life. Young red dwarfs flare often and can strip atmospheres. Tidal locking could create extreme temperature contrasts, though a thick atmosphere might spread heat around. Scientists do not yet know which effect wins, and that is why planets such as TRAPPIST-1e are such valuable test cases.
The next step after finding a planet is studying its atmosphere. When a planet transits, a sliver of starlight filters through its air, and different gases absorb different colors. Researchers look for combinations such as oxygen with methane that are hard to maintain without a living source. These are called biosignatures.
This work is difficult and sometimes contested. In 2023 and 2025, teams reported hints of gases in the atmosphere of the planet K2-18 b, but other astronomers questioned whether the evidence was strong enough, and no confirmed detection of life exists. The responsible reading is that the tools are improving quickly, while the claims need independent confirmation.
Future instruments aim to do better. ESA's PLATO mission is designed to find rocky planets in the habitable zones of Sun-like stars, and NASA has described a Habitable Worlds Observatory concept to image Earth-like planets directly. Launch plans and schedules can change, so check the agencies for current dates.
Finding many planets feeds directly into the debates we cover elsewhere. In the Drake equation, the fraction of stars with planets was once pure guesswork; exoplanet surveys have now shown it is high. The fraction of planets that are habitable is still uncertain. And the silence from the sky, discussed in the Fermi paradox, looks more puzzling the more potential homes we find. Still, the only radio signal that ever caused real excitement remains unexplained and unrepeated; read about the Wow! signal.
Our own neighborhood still has plenty to teach us too. Learn how astronomers watch for nearby rocks in near-Earth objects explained.