Quick answer: The most basic of all facts about exoplanets: an exoplanet is any planet that orbits a star other than the Sun. Astronomers have confirmed more than 6,000 exoplanets since the first discoveries in the 1990s, most of them found with the transit method, which spots the tiny dip in a star’s light as a planet passes in front. Studies suggest that, on average, every star in the Milky Way has at least one planet, which means our galaxy alone likely holds hundreds of billions of them.
Just over 30 years ago, nobody knew for sure whether planets existed around other stars. Today, exoplanets are found almost weekly, and some are stranger than anything science fiction imagined. Here are 15 facts about exoplanets, how scientists find them, and what they reveal about the chances of life beyond Earth.
Exoplanets in numbers
| Fact | Figure |
|---|---|
| Confirmed exoplanets | More than 6,000 (NASA Exoplanet Archive) |
| First planets found around another star | 1992, orbiting pulsar PSR B1257+12 |
| First planet around a Sun-like star | 1995, 51 Pegasi b |
| Closest known exoplanet | Proxima Centauri b, about 4.2 light-years away |
| Planets found by NASA’s Kepler mission | More than 2,600 confirmed |
| Most common known types | Super-Earths and mini-Neptunes |
The first exoplanet discoveries
In 1992, Aleksander Wolszczan and Dale Frail found planets orbiting a pulsar, the spinning remains of an exploded star. In 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b, a giant planet circling a Sun-like star every 4.2 days. It was so close to its star that it surprised everyone and created a new class: the “hot Jupiter”. Mayor and Queloz shared the 2019 Nobel Prize in Physics for the discovery.
How do scientists find exoplanets?
| Method | How it works | Best at finding |
|---|---|---|
| Transit | Measures the small dip in starlight when a planet crosses its star | Planet size; used by Kepler and TESS |
| Radial velocity | Detects the star wobbling as the planet’s gravity tugs on it | Planet mass |
| Direct imaging | Blocks the star’s glare to photograph the planet itself | Young, giant planets far from their stars |
| Gravitational microlensing | A star and planet briefly magnify a background star’s light | Planets far from their stars, and rogue planets |
| Timing | Measures tiny changes in pulsar signals or eclipse times | Planets around pulsars and binary stars |
15 amazing facts about exoplanets
- There may be more planets than stars. A 2012 microlensing study concluded that the average star in the Milky Way has at least one planet.
- Our solar system is unusual. The most common planets in the galaxy are between Earth and Neptune in size, a type our solar system doesn’t have.
- Some planets have years shorter than a day. Ultra-short-period planets orbit their stars in less than 24 hours.
- It may rain glass on HD 189733 b. This blue gas giant is thought to have silicate particles in its atmosphere and winds of several thousand kilometers per hour.
- WASP-76 b may rain iron. Its day side is so hot that iron vaporizes, then may condense on the cooler night side.
- Some planets orbit two suns. Kepler-16 b circles a pair of stars, like Tatooine in Star Wars.
- Rogue planets drift without a star. Microlensing surveys suggest free-floating planets wander through the galaxy, possibly in huge numbers.
- Some planets are covered in lava. 55 Cancri e orbits so close to its star that its surface may be partly molten.
- TRAPPIST-1 has seven Earth-sized planets. About 40 light-years away, this small red star has seven rocky worlds, several in its habitable zone.
- Our nearest neighbor star has a planet in its habitable zone. Proxima Centauri b, found in 2016, gets about the right energy for liquid water, though strong flares from its star may strip its atmosphere.
- Kepler-186 f was the first Earth-sized planet found in a habitable zone, announced in 2014.
- Hot Jupiters probably moved. Giant planets likely formed far from their stars and migrated inward, something scientists didn’t expect before 1995.
- Some planets are puffier than cotton candy. A few gas giants have extremely low densities because their atmospheres are swollen by heat.
- Webb can “taste” exoplanet air. The James Webb Space Telescope has detected water, carbon dioxide, methane and other molecules in exoplanet atmospheres.
- A possible sign of life is still unconfirmed. Webb detected methane and carbon dioxide on K2-18 b, a planet that may have an ocean. Claims of a possible biosignature gas there remain disputed and need more data.
Types of exoplanets
| Type | Description |
|---|---|
| Gas giants and hot Jupiters | Jupiter-like planets, some orbiting extremely close to their stars |
| Neptune-like | Ice giants similar in size to Neptune or Uranus |
| Super-Earths | Rocky planets larger than Earth but smaller than Neptune |
| Mini-Neptunes | Small planets with thick hydrogen or water-rich atmospheres |
| Terrestrial | Rocky planets around Earth’s size or smaller |
Could any exoplanets support life?
A planet in the habitable zone receives the right amount of starlight for liquid water to possibly exist on its surface. Being in that zone doesn’t guarantee life: the planet also needs the right atmosphere, chemistry and stability. Scientists are now looking for biosignatures, gases such as oxygen together with methane that are hard to explain without living things. Read more in the search for alien life.
Future exoplanet missions
- TESS (NASA) continues scanning nearby bright stars for transiting planets.
- PLATO (ESA) is designed to find Earth-like planets around Sun-like stars.
- Nancy Grace Roman Space Telescope (NASA) will search for planets with microlensing.
- Habitable Worlds Observatory is a proposed NASA telescope to image Earth-like planets directly and search their air for signs of life.
Frequently asked questions
What is an exoplanet?
An exoplanet, or extrasolar planet, is a planet that orbits a star other than our Sun, or that floats freely through space.
How many exoplanets have been discovered?
More than 6,000 have been confirmed, with thousands more candidates waiting for confirmation. The number grows every month.
What is the closest exoplanet to Earth?
Proxima Centauri b, which orbits the nearest star to the Sun, about 4.2 light-years away.
Can we travel to an exoplanet?
Not with current technology. Even the closest would take tens of thousands of years to reach with today’s spacecraft.
Has life been found on an exoplanet?
No. Some planets are in habitable zones, and scientists are studying their atmospheres, but no confirmed sign of life has been found.
Related articles
Sources
- Mayor, M. & Queloz, D. (1995). “A Jupiter-mass companion to a solar-type star.” Nature. doi:10.1038/378355a0
- Wolszczan, A. & Frail, D. A. (1992). “A planetary system around the millisecond pulsar PSR1257+12.” Nature.
- Cassan, A. et al. (2012). “One or more bound planets per Milky Way star from microlensing observations.” Nature.
- Gillon, M. et al. (2017). “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1.” Nature.
- NASA Exoplanet Archive. exoplanetarchive.ipac.caltech.edu