For most of human history, we could only study eight planets — the worlds of our own Solar System. Then, in 1992, astronomers confirmed the first exoplanets — planets orbiting a star other than our Sun. Today we have confirmed more than 5,800 exoplanets, and astronomers estimate that nearly every star in our galaxy hosts at least one planet. The galaxy contains roughly 100 to 400 billion stars, which means our Milky Way alone may hold trillions of worlds.

Detecting an exoplanet is incredibly difficult. Stars are roughly a billion times brighter than the planets that orbit them, so directly photographing a planet is like trying to spot a firefly next to a stadium spotlight from miles away. Instead, scientists use clever indirect methods that measure the effect a planet has on its parent star.

The transit method is the most successful technique to date. When a planet passes in front of its star (a "transit"), it blocks a tiny fraction of the star's light. This causes a small, periodic dip in the star's brightness — known as a light curve. By measuring how much the light dims and how often the dimming repeats, astronomers can calculate the planet's size and the length of its year. NASA's Kepler Space Telescope used the transit method to discover thousands of exoplanets between 2009 and 2018.

The radial velocity method (also called the Doppler wobble method) detects the gravitational tug a planet exerts on its star. As an unseen planet orbits, it pulls the star slightly back and forth. This wobble shifts the wavelengths of light coming from the star — toward red when the star moves away from us, and toward blue when it moves toward us. Larger planets in close orbits cause bigger wobbles and are easiest to detect.

Other methods include direct imaging (rare; works best for large, hot, young planets far from their stars), gravitational microlensing (using a foreground star's gravity as a lens to magnify the light of a distant star and reveal orbiting planets), and astrometry (precisely tracking tiny shifts in a star's position on the sky).

Finding a planet, however, is only the first step. The real prize is a planet inside the habitable zone — sometimes called the "Goldilocks zone" — where the temperature is not too hot and not too cold, but just right for liquid water to exist on the surface. Liquid water, as far as we know, is the universal requirement for life. The habitable zone of a cool red dwarf star sits very close to the star, while the habitable zone of a hot, bright star sits far away. Earth, of course, sits comfortably inside our Sun's habitable zone.

Even a planet in the habitable zone is not guaranteed to host life. Scientists also study a planet's atmosphere using spectroscopy — splitting the planet's light into a rainbow of colors. Different gases absorb specific wavelengths, leaving dark "fingerprints" in the spectrum. Some gases, when found together, are considered biosignatures — chemical clues of possible life. The strongest biosignature is the simultaneous presence of oxygen (O₂) and methane (CH₄). On their own these gases react quickly and disappear, so finding them together suggests something is constantly producing them — most likely, life.

1. Proxima Centauri b — Our Closest Neighbor (2016)
Just 4.24 light-years from Earth, Proxima Centauri b orbits the closest star to our Sun. The planet is roughly Earth-sized and sits inside the habitable zone of its red dwarf star. It completes one orbit in only 11 days because the habitable zone of a small, dim red dwarf is so close to the star. Proxima b may also be tidally locked, meaning the same side always faces its star — making one half scorching hot and the other freezing cold. Scientists are unsure whether intense radiation from the active red dwarf would strip away any atmosphere.

2. The TRAPPIST-1 System — Seven Earth-Sized Planets (2017)
In 2017, astronomers announced one of the most stunning discoveries in exoplanet science. The cool red dwarf TRAPPIST-1, only 40 light-years away, hosts seven Earth-sized rocky planets. Three of them — TRAPPIST-1e, f, and g — sit firmly inside the habitable zone. The planets orbit so close together that, if you stood on one, the others would appear larger than the Moon does in our sky. The James Webb Space Telescope is now studying their atmospheres for signs of water vapor, carbon dioxide, oxygen, and methane.

3. Kepler-452b — "Earth's Older Cousin" (2015)
Kepler-452b orbits a Sun-like star about 1,400 light-years away. Its year is 385 days — only twenty days longer than Earth's. It is roughly 60% larger than Earth and sits inside its star's habitable zone. The star is six billion years old — about 1.5 billion years older than our Sun — which means if Kepler-452b ever had life, that life would have had a long, long head start.

How do astronomers actually look for life?
We cannot fly to these planets — even Proxima b would take roughly 80,000 years to reach using current spacecraft. Instead, when a planet transits its star, a small amount of starlight passes through the planet's atmosphere on its way to our telescopes. Different gases absorb different wavelengths, leaving "fingerprints" in the spectrum. The James Webb Space Telescope, launched in 2021, is sensitive enough to detect these fingerprints on rocky exoplanets. In 2023 it tentatively detected dimethyl sulfide on K2-18b — a gas produced almost exclusively by living organisms here on Earth. The result is still being checked, but the search has clearly begun.