Exoplanet
An exoplanet (extrasolar planet) is any planet that orbits a star outside our solar system. First confirmed in 1992 around a pulsar and in 1995 around a main-sequence star (51 Pegasi b), exoplanets are detected primarily through the transit method (measuring periodic dips in stellar brightness as the planet passes in front of its star) and the radial velocity method (detecting Doppler shifts in starlight caused by the planet's gravitational tug). As of 2024, over 5,600 exoplanets have been confirmed, ranging from hot Jupiters and super-Earths to Earth-sized worlds in the habitable zones of their stars — with the search for biosignatures on these worlds being a central goal of astrobiology.
Key Formula
ΔF / F = (R_p / R_*)²
LaTeX: \frac{\Delta F}{F} = \left(\frac{R_p}{R_*}\right)^2
| Symbol | Meaning | Unit |
|---|---|---|
| ΔF / F | Fractional dip in stellar flux during transit | dimensionless |
| R_p | Radius of the exoplanet | meters (m) or Earth radii |
| R_* | Radius of the host star | meters (m) or Solar radii |
Worked Example
Problem
A transit light curve shows a flux dip of 1.5% (ΔF/F = 0.015) in a star identical to the Sun (R_* = 6.957 × 10⁸ m). What is the radius of the exoplanet in Earth radii? (Earth radius R_E = 6.371 × 10⁶ m)
Solution
Step 1: Use the transit formula: (R_p / R_*)² = ΔF/F = 0.015. Step 2: Take the square root: R_p / R_* = sqrt(0.015) = 0.1225. Step 3: Calculate R_p: R_p = 0.1225 × 6.957 × 10⁸ = 8.52 × 10⁷ m. Step 4: Convert to Earth radii: R_p = 8.52 × 10⁷ / 6.371 × 10⁶ = 13.4 R_E.
Answer
Exoplanet radius ≈ 13.4 Earth radii (approximately a super-Jupiter or sub-Saturn size planet)
Exoplanet Detection Methods Compared
| Method | What Is Measured | Best For | Key Mission / Instrument |
|---|---|---|---|
| Transit photometry | Stellar flux dip | Close-in planets, planet size | Kepler, TESS, CHEOPS |
| Radial velocity | Stellar Doppler shift | Massive planets, minimum mass | HARPS, ESPRESSO |
| Direct imaging | Planet's own/reflected light | Wide-orbit young planets | GPI, SPHERE, JWST |
| Gravitational microlensing | Flux magnification | Distant/free-floating planets | OGLE, Roman Space Telescope |
| Astrometry | Stellar position wobble | Long-period planets | Gaia |
Interactive Tools
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Related Terms
From the Greek prefix exo- (outside, external) combined with "planet" (from Greek planetes, meaning "wanderer"). The term came into widespread use in the 1990s following the first confirmed detections. An older synonym, "extrasolar planet," uses the Latin prefix extra- (outside) and solaris (of the Sun).