Other worlds
Exoplanets
An exoplanet is a planet that orbits a star other than the Sun. The first confirmation around a Sun-like star came in 1995, and astronomers have now catalogued more than 5,900 confirmed exoplanets in over 4,300 planetary systems. They range from scorching gas giants that skim their stars in a matter of hours to small rocky worlds in the habitable zones of cool red dwarfs, and even lonely rogue planets drifting between the stars.
This page introduces how astronomers detect exoplanets, the main types of worlds we have found, what the habitable zone actually means, and how the James Webb Space Telescope is starting to read the chemistry of their atmospheres.
How astronomers find exoplanets
No single technique captures every kind of planet. Each method is sensitive to a different combination of planet size, mass, distance and orbital geometry, so the full exoplanet catalog is a mosaic of all of them working together.
- Method
Transit method
A planet crossing between us and its star blocks a small fraction of the star's light, typically 0.01–1%. Repeat dips at a fixed period reveal an orbiting planet and measure its size relative to the star. NASA's Kepler and TESS missions found most confirmed exoplanets this way.
Explore the transit method →
- Method
Radial velocity
A planet's gravity tugs its star into a small, rhythmic wobble. High-resolution spectrographs measure the shift in stellar spectral lines to detect the tug, revealing the planet's orbital period and a minimum mass. This method works best for massive planets on short orbits.
Explore the radial velocity →
- Method
Direct imaging
Coronagraphs and starshades block the star's overwhelming glare so a nearby planet can be photographed. Direct imaging works best for young, hot, wide-orbit gas giants that still glow brightly in the infrared. It is how JWST and ground-based extreme-adaptive-optics instruments study young planetary systems.
Explore the direct imaging →
- Method
Gravitational microlensing
When a foreground star and its planet pass in front of a background star, their gravity bends and briefly magnifies the background light. The shape of the brightening curve reveals the planet's mass and separation. Microlensing is uniquely sensitive to planets at Jupiter-like distances and to free-floating worlds.
Explore the gravitational microlensing →
- Method
Astrometry
Precise measurements of a star's position on the sky can reveal the small looped path caused by an orbiting planet. ESA's Gaia mission is beginning to deliver astrometric planet detections, particularly for long-period giant planets around nearby stars.
Explore the astrometry →
For a step-by-step walkthrough of these techniques with worked examples, see the deep-dive article on How astronomers find exoplanets.
Types of exoplanets
The census of known exoplanets includes categories that do not exist in our Solar System. That variety is one of the strongest clues that planet formation is much more flexible than a single Sun-and-Earth story.
Hot Jupiters
Gas giants comparable to or larger than Jupiter orbiting extremely close to their stars, often in a few days. They were among the first exoplanets found and remain important laboratories for atmospheric physics.
Explore hot jupiters →
Warm Neptunes and sub-Neptunes
Planets roughly two to four times Earth's radius with substantial hydrogen–helium envelopes. They are the most common type discovered so far, with no direct analogue in our Solar System.
Explore warm neptunes and sub-neptunes →
Super-Earths
Rocky worlds larger than Earth but smaller than Neptune. Their bulk densities suggest a wide range of compositions from iron-rich to water-rich.
Explore super-earths →
Terrestrial worlds
Earth-sized rocky planets, sometimes in the habitable zones of small, cool stars. TRAPPIST-1 is the best-studied example, with seven such worlds around a single ultra-cool dwarf.
Explore terrestrial worlds →
Rogue planets
Planet-mass objects that drift through the galaxy unbound to any star. Microlensing surveys and infrared imaging suggest they are common; NASA's Roman Space Telescope is expected to find many more.
Explore rogue planets →
The habitable zone
The habitable zone is the range of distances from a star where a rocky planet could hold liquid water on its surface, assuming a suitable atmosphere. Its boundaries depend on the star's brightness and temperature. Small, cool red dwarfs have narrow habitable zones close to the star; hot, luminous stars push their habitable zones outward.
Being in the habitable zone does not guarantee that a planet is habitable. Surface conditions depend on atmospheric composition, cloud cover, magnetic field, plate tectonics and the age and activity of the host star. Venus and Mars both sit near the edges of the Sun's habitable zone, and neither is habitable today. The habitable zone is a starting point for searches, not an answer by itself.
Related reading: Venus, Mars and Earth — three rocky worlds in or near our own habitable zone with very different histories.
A tour of notable exoplanets
A small sample of the systems that have shaped how astronomers think about planets around other stars.
TRAPPIST-1 system
Seven Earth-sized rocky worlds orbit an ultra-cool dwarf 40 light-years away. Three sit in the habitable zone, and JWST is measuring their atmospheres one by one.
Explore TRAPPIST-1 system →
Proxima Centauri b
A roughly Earth-mass planet orbiting the nearest star to the Sun, discovered by radial velocity. It sits in the habitable zone but faces frequent stellar flares.
Explore Proxima Centauri b →
Kepler-452b
A super-Earth candidate in the habitable zone of a Sun-like star, about 1,400 light-years away. It helped popularise the search for true Earth analogues.
Explore Kepler-452b →
K2-18b
A sub-Neptune 124 light-years away. JWST detected methane and carbon dioxide in its atmosphere and tentative signatures of dimethyl sulfide that are still being tested.
Explore K2-18b →
WASP-39b
A hot, Saturn-mass giant chosen as one of JWST's first atmospheric targets. JWST found carbon dioxide, sulfur dioxide from photochemistry, water and clouds.
Explore WASP-39b →
51 Pegasi b
The first exoplanet confirmed around a Sun-like star, announced in 1995. Its discovery won the 2019 Nobel Prize in Physics.
Explore 51 Pegasi b →
Reading exoplanet atmospheres
When a transiting planet passes in front of its star, a small fraction of the starlight filters through the planet's atmosphere before reaching us. Different molecules absorb different wavelengths, so a spectrum of that filtered light acts as a fingerprint of the atmospheric composition. This technique is called transmission spectroscopy.
The James Webb Space Telescope has extended this method to smaller and cooler worlds than any previous facility. JWST has detected carbon dioxide, water, sulfur dioxide and cloud features on the hot giant WASP-39b, methane and carbon dioxide on the sub-Neptune K2-18b, and is now measuring atmospheres in the TRAPPIST-1 system to test whether small rocky planets around red dwarfs can hold onto air at all.
Upcoming facilities will push further. NASA's Nancy Grace Roman Space Telescope will conduct a wide-field microlensing survey of the galactic bulge and demonstrate coronagraphic imaging of nearby planets. Extremely large ground-based telescopes and future missions like the Habitable Worlds Observatory aim to search Earth-like planets around Sun-like stars for atmospheric biosignatures.
Frequently asked questions
- What is an exoplanet?
- An exoplanet, or extrasolar planet, is a planet that orbits a star other than the Sun. Some exoplanets are free-floating and are not bound to any star.
- How many exoplanets have been discovered?
- NASA's Exoplanet Archive lists more than 5,900 confirmed exoplanets in over 4,300 planetary systems, with several thousand additional candidates still under review. The catalog grows every few weeks.
- How do astronomers find exoplanets?
- The most productive methods are the transit method, radial velocity, direct imaging, gravitational microlensing and astrometry. Missions like Kepler, TESS, JWST and Gaia each contribute to different parts of this catalog.
- What is the habitable zone?
- The habitable zone is the range of orbital distances where a rocky planet could hold liquid water on its surface, given a suitable atmosphere. Being in the habitable zone does not guarantee life; it is a starting point for searches.
- Has life been found on any exoplanet?
- No confirmed evidence of life has been found on any exoplanet. Instruments like JWST are measuring atmospheric compositions and searching for potential biosignatures, but any single detection needs independent confirmation.
- What is the closest exoplanet to Earth?
- Proxima Centauri b, about 4.24 light-years away, is the closest known exoplanet. It orbits the red dwarf Proxima Centauri in the habitable zone.
Related resources
How astronomers find exoplanets
Detection methods explained in depth, from transit light curves to microlensing.
Nancy Grace Roman Space Telescope
The next NASA flagship, built to survey exoplanets by microlensing and demonstrate coronagraphic imaging.
Compare exoplanets with the eight familiar worlds that formed around our own star.
The stellar environments that shape whether an exoplanet can be temperate, irradiated or torn apart.