In the last three decades, scientists have discovered around 4,000 exoplanets outside our solar system. What are these mysterious worlds, and will we find alien life on them?

What's an exoplanet? An exoplanet is any planet outside of our solar systemThe Sun and the objects whose movement it controls.. Exoplanets come in all shapes and sizes, from gas giants double the size of Jupiter to tiny, rocky, Moon-sized worlds.

Some planets orbit two stars. Other rogue planets don't orbit any star at all, but roam around the universe in darkness.

How many are there? NasaThe National Aeronautics and Space Administration, responsible for the US space programme. has discovered around 4,000 exoplanets since the search began in 1988. (The first two exoplanets to be discovered were announced in January 1992.)

But this total is just a drop in the ocean compared to what's probably out there.

Most stars in the Milky WayThe galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it. are orbited by at least one planet, and there are 400 billion stars in the
Milky Way. One known solar system, Kepler-90, has eight planets just as we do.

Across the vast universe, astronomers estimate there are one billion trillion stars (that's one with 21 zeros). So, the number of planets waiting to be found is truly unimaginable.

How do we find them? Scientists use many different telescopes and sensors on the ground and in space to find exoplanets.

The most famous of all is the Kepler telescope, which was retired in 2018 - nine years after it was launched into space. Over its career, Kepler detected more than 2,600 planets.

But it's not easy. Exoplanets are hundreds of times fainter and smaller than the bright suns they orbit.

Only rarely, can an exoplanet be seen directly through a telescope. Instead, most are detected by measuring when a star dims as a planet passes in front of it.

The next-generation planet-hunting telescope is currently being designed.

Will we find alien life on exoplanets? As the only example of life in the Universe, scientists are hunting for planets that resemble Earth, and liquid water is an essential ingredient.

So far, no aliens - but we have found dozens of planets that could potentially sustain life. And many, many more are waiting to be found: astronomers estimate there could be 40 billion Earth-sized, potentially habitable planets across the Universe.

To find them, scientists first look for planets in the habitableSuitable for life. region of their solar system, which is called the "Goldilocks zone".

...Like the fairy tale? Pretty much. Just like Goldilocks's porridge, most planets are too hot or too cold. If a planet is too close to its star, water will evaporate. Too far away, and it will freeze. Planets need to be just the right distance from their star, as does Earth.

But planets in the Goldilocks zone are not necessarily habitable. They need an atmosphere to protect them from space radiation, which also contains gases compatible with life.

So far, only one planet definitely has an Earth-like atmosphere: 55 Cancri A. But there's a catch. It's far too hot for life. In fact, its entire surface may be covered in lava.

The strongest contender for life found so far is Kepler 452b, nicknamed "Earth's cousin". But as it is 1,400 light years away, scientists can't yet observe it well enough to tell whether its atmosphere is compatible with life.

Will we ever visit an exoplanet? Due to the gigantic distances involved, not for a very long time - if ever. Our nearest exoplanet, Proxima Centauri b, is 4.24 light years away. NASA's super-fast New Horizons probe, which is currently approaching the very edge of our solar system after a 13-year journey, would take over 54,000 years to get there.

If we did attempt to visit an exoplanet, one of the biggest challenges would be keeping humans, or generations of humans, alive long enough to make the colossal journey. Besides, we still haven't visited our closest planetary neighbour in our solar system: Mars.

Solar System - The Sun and the objects whose movement it controls.

Nasa - The National Aeronautics and Space Administration, responsible for the US space programme.

Milky Way - The galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it.

Habitable - Suitable for life.