An exoplanet is a planet that orbits a star other than our Sun. The word exoplanet comes from the term “extra-solar planet”.
The first exoplanets were discovered in 1992. They were found orbiting a pulsar called PSR 1257+12. Three years later, astronomers discovered a planet orbiting a Sun-like star called 51 Pegasi.
Since 1995, astronomers have discovered more than 5,000 exoplanets.
There are several ways to detect exoplanets. Most are found by studying small changes in a star. These changes happen because the planet’s gravity affects the star as it orbits. Exoplanets are difficult to see directly. They are much smaller than stars and do not produce their own light.
Scientists group exoplanets into four main types.
- Gas giant
- Neptunian
- Super-Earth
- Terrestrial
Around 30% of known exoplanets are gas giants. These planets are made mostly of gas and are similar in size to Jupiter or Saturn. Many gas giants orbit very close to their star. This makes them much hotter than the gas giants in our Solar System. Because of this, they are often called “hot Jupiters”.
About 35% of exoplanets are Neptunian. These worlds are similar in size to Neptune and Uranus.
Around 31% of exoplanets are known as super-Earths. Super-Earths are larger than Earth, but smaller than Neptune. There are no planets of this size in our Solar System. The name only describes their size. They may not be Earth-like in any other way.
About 4% of known exoplanets are terrestrial. These planets are rocky and similar in size to Earth. Some terrestrial exoplanets orbit at the right distance for liquid water to exist. This region around a star is called the habitable zone, or the “Goldilocks zone”.
At the moment, scientists do not know whether these planets have oceans, atmospheres, or life.
Many stars have more than one planet orbiting them.
In 2017, NASA announced the largest extra-solar system discovered so far. The star TRAPPIST-1 has seven terrestrial exoplanets. Some of these orbit within the habitable zone.
New telescopes, such as the James Webb Space Telescope , can study the light from exoplanets. This helps scientists learn how hot these planets are and what they are made of.