An exoplanet is any planet orbiting a star other than our Sun. For most of human history, we had no idea whether they existed. Other stars were too far away to see any planets — even the nearest star is over 4 light-years away. Then in 1992, the first confirmed exoplanet was discovered. By 2025, we've confirmed over 5,700, with thousands more candidates.
How do you find a planet you can't see?
You can't photograph most exoplanets directly — they're tiny and lost in the glare of their star. Instead, astronomers use clever indirect methods.
The transit method: When a planet passes in front of its star (from our viewpoint), it blocks a tiny fraction of the star's light — typically less than 1%. By measuring the regular dip in brightness, astronomers can detect the planet and work out its size and orbital period. NASA's Kepler telescope used this method to find thousands of planets.
Imagine watching a lamp across a field at night. You can't see the moth flying in front of it — but every time it passes, the lamp dims very slightly and then brightens again. By timing those dips precisely, you could work out the size of the moth and how often it circles the lamp. The transit method does exactly this, but with planets and stars instead of moths and lamps — and the "dimming" might be just 0.01% of the star's light.
What kinds of planets have been found?
All sorts. "Hot Jupiters" — gas giants orbiting incredibly close to their stars, completing an orbit in just a few days. "Super-Earths" — rocky planets larger than Earth but smaller than Neptune. "Mini-Neptunes." Some rocky planets orbiting in the "habitable zone" — the range of distances where liquid water could exist on the surface. A few bear a striking resemblance to Earth in size and temperature.
Have we found life?
Not yet, and not for lack of looking. The James Webb Space Telescope is analysing exoplanet atmospheres for chemical signatures that might indicate biological activity. Finding oxygen, methane, and water together in an atmosphere — gases that shouldn't co-exist without something continually replenishing them — would be a very strong hint. So far: no confirmed biosignatures. But we've only meaningfully analysed a tiny fraction of candidates.
An exoplanet is any planet going around a star that isn't our Sun. For most of human history, we didn't know if they existed. Other stars were too far away to see any planets around them. Even the nearest star is over 4 light-years away. Then in 1992, scientists found the first real exoplanet. By 2025, we've found over 5,700 of them. There are thousands more that might be planets too.
How do you find a planet you can't see?
You can't take photos of most exoplanets directly. They're tiny and get lost in their star's bright light. Instead, scientists use clever tricks to find them.
The transit method works like this. When a planet passes in front of its star, it blocks some light. It's only a tiny bit - less than 1% of the star's light. Scientists measure when the star gets dimmer and brighter again. This tells them there's a planet there. It also tells them how big the planet is. NASA's Kepler telescope used this method to find thousands of planets.
Think about watching a lamp across a playground at night. You can't see a butterfly flying in front of it. But every time it passes, the lamp gets slightly dimmer. Then it gets bright again. If you time those dim moments, you could work out things. You'd know how big the butterfly is. You'd know how often it flies around the lamp. The transit method does exactly this with planets and stars. But the dimming might be just 0.01% of the star's light.
What kinds of planets have been found?
All sorts of different ones. "Hot Jupiters" are huge gas planets like Jupiter. But they orbit very close to their stars. They go around their star in just a few days. "Super-Earths" are rocky planets bigger than Earth. But they're smaller than Neptune. There are also "Mini-Neptunes." Some rocky planets orbit in the "habitable zone." This is where liquid water could exist on the surface. A few planets are very similar to Earth in size and temperature.
Have we found life?
Not yet, even though scientists are looking hard. The James Webb Space Telescope looks at exoplanet atmospheres. It searches for chemicals that might show biological activity. Finding oxygen, methane, and water together would be exciting. These gases shouldn't exist together without something making them. This would be a very strong hint of life. So far, we haven't found any confirmed signs of life. But we've only properly studied a tiny number of planets.