A planet is a world orbiting a star. An exoplanet is simply a planet that orbits a star beyond our Solar System.
Our familiar planetary neighborhood
The eight recognized planets in our Solar System orbit the Sun. They range from small rocky worlds such as Mercury and Earth to giant planets such as Jupiter and Saturn. Studying these nearby worlds gives astronomers detailed examples of how planets form and evolve.
What makes an exoplanet different?
The word “exo” means outside. Exoplanets are planetary worlds outside our Solar System. Because they are extremely far away compared with the planets orbiting the Sun, most cannot be photographed as clear disks with ordinary telescopes. Astronomers often detect them indirectly.
How astronomers detect distant planets
Transit method
If a planet crosses in front of its star from our viewpoint, the star can dim by a tiny, repeatable amount. Measuring these dips can reveal the planet's approximate size and orbital period.
Radial velocity
A planet's gravity also tugs on its star. That motion can create small shifts in the star's spectrum, helping astronomers estimate a planet's mass.
Direct imaging and other techniques
In some cases, instruments can separate faint planetary light from the much brighter star. Other methods use gravitational lensing or precise measurements of stellar position.
Why exoplanets matter
They show that planetary systems come in many forms. Some worlds orbit very close to their stars; others are larger or smaller than anything in our Solar System. By comparing many systems, astronomers can test ideas about how planets form and ask how common potentially temperate environments might be.
For an official overview, visit NASA Exoplanet Exploration. For more foundational reading on our own sky, see our Moon phase guide.
Key takeaway
Every exoplanet is a planet, but the label tells us it belongs to a planetary system beyond the Sun. These distant worlds help us understand both our cosmic neighborhood and the wider range of possible planetary systems.