The transit method
When a planet passes in front of its star as seen from Earth, it blocks a tiny fraction of the starlight. Space telescopes such as Kepler and TESS have found thousands of exoplanets by watching for these regular dips in brightness.
Transit depth
The fraction of light blocked is roughly the ratio of the two disks’ areas:
Depth ≈ (Rp ÷ R)²*
where Rp is the planet’s radius and R* the star’s. Depth is often expressed in parts per million (ppm).
- Earth crossing the Sun: about 84 ppm (0.0084%).
- Jupiter crossing the Sun: about 1%.
- An Earth-size planet around a small red dwarf: several thousand ppm, much easier to see.
Transit duration
For a planet crossing the center of the star’s disk, the transit lasts about:
T ≈ (P ÷ π) × arcsin((R + Rp) ÷ a)*
where P is the orbital period and a the orbital distance, which Kepler’s third law gives from the period and the star’s mass: a (AU) = (M × P²)^(1/3), with P in years. Off-center transits are shorter.
Example: Earth seen from afar
An alien astronomer watching the Sun from the right direction would see Earth:
- dim the Sun by 84 ppm;
- for about 13 hours;
- once a year.
That is why detecting a true Earth twin needs extremely precise, long space-based observations.
Transit probability
A transit only happens if the orbit is nearly edge-on to us. For random orientations, the probability is about R ÷ a*:
- Earth around the Sun: about 0.47%;
- a hot Jupiter at 0.05 AU: about 10%.
So for every transiting planet found, many more around nearby stars go unseen.
What else transits reveal
- Combined with radial-velocity measurements of the planet’s mass, the radius gives its density: rocky, water-rich or gaseous.
- Starlight filtered through the planet’s atmosphere during transit reveals its composition, as JWST does for water, carbon dioxide and other molecules.
- Variations in transit timing can reveal other, unseen planets in the system.
Real light curves also depend on limb darkening, star spots and the impact parameter, which professional fits take into account.