Exoplanet Systems

When a planet passes in front of its star as seen from Earth, it blocks a tiny fraction of the starlight. For a Jupiter-sized planet orbiting a Sun-like star, this causes about a 1% dip. For an Earth-sized planet, the dip is only 0.01% - one part in ten thousand.

The depth of the transit tells us the ratio of the planet's cross-sectional area to the star's: deeper dips mean bigger planets. The duration tells us how fast the planet is moving - which, combined with the orbital period, reveals the distance from the star.

Toggle "Show Raw Data" on any card to see what astronomers actually measure: the clean model curve buried in noise from stellar variability, instrumental effects, and photon counting statistics. Small planets require folding dozens of transits together before the signal emerges from the noise.

Switch to the "Radius vs Period" view to see the exoplanet population structure. Hot Jupiters cluster in the upper-left: large planets on short orbits, easy to detect because they create deep, frequent transits. The Kepler mission revealed they're actually quite rare - only about 1% of Sun-like stars host one.

The most common type of planet is the "sub-Neptune" - between 2 and 4 Earth radii, with no analogue in our Solar System. Notice the "radius gap" around 1.8 Earth radii: planets seem to cluster either above or below this line, likely because atmospheric mass loss strips some planets of their envelopes.

The habitable zone - where liquid water could exist - sits at longer orbital periods. These planets transit less frequently and show shallower dips, making them the hardest to find. The TRAPPIST-1 system is exceptional: seven Earth-sized planets, three in the habitable zone, all transiting a small, dim star.