What Are Exoplanets?
An exoplanet is any planet orbiting a star other than our Sun. As of 2024, we've confirmed over 5,500 exoplanets — with thousands more candidates awaiting verification.
How We Find Them
Exoplanets are too dim to see directly (usually). We detect them indirectly:
#### Transit Method
When a planet passes in front of its star, the star dims slightly. The Kepler Space Telescope found thousands this way.
What we learn: Planet size, orbital period, atmospheric composition
#### Radial Velocity
A planet's gravity tugs its star, causing tiny wobbles. We detect these as shifts in the star's light spectrum.
What we learn: Planet mass, orbital period
#### Direct Imaging
For large, young planets far from their stars, we can sometimes photograph them directly using coronagraphs to block starlight.
Types of Exoplanets
- Hot Jupiters: Gas giants orbiting very close to their stars
- Super-Earths: Rocky planets larger than Earth but smaller than Neptune
- Mini-Neptunes: Small gas planets
- Earth-like: Rocky planets in the habitable zone
The Habitable Zone
The "Goldilocks zone" where liquid water could exist — not too hot, not too cold.
- TRAPPIST-1 system: Seven Earth-sized planets, three in habitable zone
- Proxima Centauri b: In the habitable zone of our nearest stellar neighbor (4.2 light-years)
- Kepler-442b: Potentially rocky, in habitable zone
Could They Have Life?
- Atmosphere
- Liquid water
- Magnetic field
- Stable climate
The James Webb Space Telescope is now analyzing exoplanet atmospheres for biosignatures like oxygen and methane.
Related Reading
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