Exoplanets
Exoplanet
Planets orbiting other stars — over 5,500 confirmed; some potentially habitable
An exoplanet is a planet outside our Solar System, orbiting another star. First confirmed exoplanet around main-sequence star: 51 Pegasi b (1995, Mayor and Queloz, Nobel 2019). Now: 5,500+ confirmed, growing rapidly. Detection methods: transit (Kepler, TESS), radial velocity, direct imaging, microlensing, astrometry. Diversity remarkable — hot Jupiters, super-Earths, mini-Neptunes, ocean worlds. Some in habitable zones. JWST analyzing exoplanet atmospheres directly. Implications for life, planetary formation, cosmic biology.
- Confirmed exoplanets~5,500+ (as of 2024)
- First confirmed51 Pegasi b, 1995 (Mayor & Queloz)
- Detection methodsTransit, radial velocity, imaging, microlensing
- Closest exoplanetProxima Centauri b (4.24 ly)
- Habitable zone candidatesHundreds of potentially habitable
- Mass range~0.5× Mercury to >10× Jupiter
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Why exoplanets matter
- Habitability. Search for life beyond Earth.
- Planet formation. Diversity reveals processes.
- Cosmology. Earth in context of galaxy.
- Astrobiology. Direct atmospheric studies.
- Mission targets. JWST, future LUVOIR, HabEx.
- Stellar physics. Star-planet interactions.
- Public engagement. Compelling science.
Common misconceptions
- Exoplanets are rare. Hundreds of billions estimated.
- Earth is unique. Statistics suggest commonality.
- Habitable zone = habitable. Many other factors needed.
- Transit method gives mass. Gives radius; mass needs RV.
- Direct imaging easy. Very difficult; few confirmed.
- All exoplanets are like our solar system. Highly diverse.
Frequently asked questions
How are exoplanets detected?
(1) Transit — planet crosses star face; star dims slightly. Most exoplanets discovered. Kepler and TESS missions. (2) Radial velocity — gravitational tug shifts star spectrum. Original detection method. (3) Direct imaging — hardest; requires bright planets at large separations. JWST capable. (4) Microlensing — planet enhances stellar lensing event. (5) Astrometry — precision position measurements.
What types exist?
(1) Hot Jupiters — gas giants very close to star (<0.1 AU). Surprised everyone (1995). (2) Super-Earths — 1-10 Earth masses. Common. (3) Mini-Neptunes — slightly larger; gas envelope. (4) Earth-sized — fewer detected; harder. (5) Ocean worlds — partially water. (6) Tidally locked — same face to star.
Are any habitable?
Hundreds of candidates — in habitable zone (where liquid water possible). Notable: Proxima Centauri b, TRAPPIST-1 system (7 planets, 3 in HZ), Kepler-22b. None confirmed habitable; most around M dwarfs (issues with atmospheric stripping). Habitability depends on atmosphere, geology, magnetic field — not just distance from star.
How does JWST help?
(1) Detects atmospheric composition through spectroscopy. (2) Measures temperatures of exoplanet atmospheres. (3) Searches for biosignature gases (methane + oxygen, etc.). (4) Discovers new exoplanets through direct imaging. JWST observations of WASP-39b (2022) revealed CO₂ — first direct detection. Atmospheric science of exoplanets is now a thing.
How common are exoplanets?
Estimates: most stars have planets. ~17% of M dwarf habitable zones have planets. Earth-sized: about 22% of Sun-like stars in habitable zone. Total in galaxy: ~10¹¹+ exoplanets. Statistically, life-supporting conditions might be common.
What's the closest exoplanet?
Proxima Centauri b — orbits Proxima Centauri (4.24 ly away). Mass ~1.27 Earth, in habitable zone of star. M dwarf flare activity may strip atmosphere. Possible to send probes (Breakthrough Starshot proposal). Studied with JWST.
How does Earth compare?
Earth is small, rocky, in habitable zone of G-type star. Few exoplanets exactly like this. Most known: hot, large, around M dwarfs. Earth's combination is rare in current sample (selection bias — easier to detect closer/larger). True statistics unclear yet.