Exoplanets
Transit Method
Detecting exoplanets by the dimming of starlight as planets pass in front
The transit method detects exoplanets by observing the dimming of a star's brightness when a planet passes in front of it (transit). Tiny dip — Earth-sized planet around Sun-like star: 0.01% dim. Repeats every orbital period. Sensitive to small planets in close orbits. Kepler space telescope (2009-2018) discovered most exoplanets via this method. TESS continues. Provides: planet radius, orbital period, sometimes atmospheric composition through transit spectroscopy.
- Dim signal (Earth-Sun)~0.01% (10⁻⁴)
- Dim signal (Jupiter-Sun)~1% (10⁻²)
- Required precision10⁻⁴ photometric for Earth-Sun
- Kepler mission2009-2018; ~2,700 confirmed planets
- TESS2018+; surveying brighter stars
- ProvidesRadius, period, transit timing
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why transits matter
- Most exoplanet discoveries. >70% of confirmed.
- Atmospheric science. JWST exoplanet atmospheres.
- Planet radius. Direct measurement.
- Habitable zone. Test small planets in HZ.
- Stellar physics. Spot crossings during transit.
- Time domain. Continuous monitoring for variability.
- Education. Visual concept easy to understand.
Common misconceptions
- Transits give mass. Give radius; mass needs RV.
- All planets transit. Need edge-on orbit; <1%.
- Detected by amateurs easily. Some big planets, yes; Earth-sized needs space telescopes.
- Transit dim is noticeable. Tiny — 10⁻⁴ for Earth.
- Transit method shows planet color. Just brightness; spectroscopy needed.
- Method only works for nearby stars. Works to thousands of light-years.
Frequently asked questions
How does the transit method work?
Star's brightness measured continuously. When planet passes in front (transit), star dims briefly. Magnitude depends on planet/star area ratio: ΔF/F = (R_p/R_s)². Earth-Sun: 10⁻⁴. Jupiter-Sun: 10⁻². Detected by space-based telescopes (atmospheric transparency limits ground-based precision). Confirmed by repeating signal over time.
What does Kepler do?
NASA mission, 2009-2018. Continuous photometric monitoring of ~150,000 stars. Designed for transits of Earth-sized planets. ~2,700 confirmed exoplanets discovered. Results: planets common; small planets common; Earth-sized in habitable zone exists. Major science driver of modern exoplanet research.
What's TESS?
Transiting Exoplanet Survey Satellite — NASA, launched 2018. Surveys ~85% of sky for transiting exoplanets around brighter stars (closer to Earth). Better targets for follow-up with JWST/ground-based RV. Discovered many planets; complementary to Kepler's deeper but smaller field.
What information does transit give?
(1) Planet radius — directly from depth. (2) Orbital period — from transit recurrence. (3) Inclination — must be edge-on (probability ~R_s/a). (4) Transit timing variations — gravitational interaction with other planets. (5) Atmospheric composition — through transit spectroscopy. JWST's main mode for exoplanet atmospheres.
What's transit spectroscopy?
Spectrum of starlight passing through planet's atmosphere during transit. Atmospheric absorption features in spectrum. Reveals composition, temperature, pressure of atmosphere. JWST has spectacular results — WASP-39b (CO₂, SO₂), TRAPPIST-1 atmospheric measurements. Pioneering atmospheric exoplanet science.
What's the bias?
Method is biased toward (1) close-in planets — short period, more chances for transit. (2) Edge-on systems — high inclination needed. (3) Larger planets — bigger signals. So: Earth-sized in 1-yr orbits hard to detect (low transit probability + small signal). Improvements in telescope precision (PLATO, ARIEL) will help.
How is it confirmed?
(1) Multiple transits — periodic. (2) Transit shape correct (V-shaped vs flat-bottomed). (3) Color independence (planet not color-selective). (4) Radial velocity follow-up — measures mass. (5) Direct imaging (rare). False positives: eclipsing binaries, background stars. Detective work needed.