Planets around other stars
An exoplanet is a planet that orbits a star other than the Sun. Thousands have now been confirmed, and most of them were found with the transit method. Exoplanets are extremely hard to see directly: they are tiny, faint and lost in the glare of their stars. Instead, astronomers look for their effects on the star.
How a transit works
If a planet’s orbit is lined up so that it passes between its star and us, it blocks a small part of the star’s disc once every orbit. A telescope cannot see this happening, but it can measure the star’s total brightness very precisely. A graph of brightness against time is a light curve, and a transit appears as a small, repeating dip. To confirm a planet, astronomers look for at least three evenly spaced dips with the same depth and duration, and rule out other causes such as a pair of stars eclipsing each other.
Transit depth and planet size
The fraction of light blocked is roughly the planet’s disc area divided by the star’s:depth ≈ (planet radius ÷ star radius)². Jupiter crossing the Sun would dim it by about 1%; Earth by only about 0.008%, or 84 parts per million. So if the star’s size is known, the depth gives the planet’s size. The same planet produces a deeper dip around a smaller star, which is why red dwarfs are good places to look for Earth-sized planets.
Timing: period and duration
The time between dips is the planet’s orbital period. Using Kepler’s third law and the star’s mass, the period gives the size of the orbit, which tells astronomers how much heat the planet receives. Theduration of each transit depends on how fast the planet moves and how much of the star it crosses: close-in “hot Jupiters” transit for a few hours every few days, while an Earth twin would transit for about 13 hours once a year.
Why most planets never transit
Orbits are tilted at random relative to our line of sight. A transit only happens if the orbit is almost exactlyedge-on, so that the planet passes within about one star radius of the star’s centre as seen from Earth. The chance of this is roughly the star’s radius divided by the orbit’s radius: about 10% for a hot Jupiter but only about 0.5% for an Earth twin and 0.1% for a Jupiter twin. For every planet we catch transiting, many more are there but unseen. Survey telescopes therefore watch tens or hundreds of thousands of stars at once.
Finding tiny dips
Every measurement has some random scatter, or noise. From the ground, Earth’s shimmering atmosphere adds a lot of it, so small dips are hidden. Space telescopes avoid the atmosphere and can measure brightness to tens of parts per million. Combining many transits beats the noise down further, which is how planets as small as Earth have been found.
Formulas
depth ≈ (R_planet ÷ R_star)²P² = a³ ÷ M (P in years, a in AU, M in solar masses)transit possible if a × cos i < R_star + R_planetchance of a transit ≈ R_star ÷ aDiscoveries and missions
- HD 209458 b was the first exoplanet seen to transit, in 1999.
- Kepler (2009–2018) stared at about 150,000 stars and found thousands of planets, showing that small planets are common.
- TESS (launched 2018) surveys almost the whole sky, focusing on bright, nearby stars.
- TRAPPIST-1, a small, cool star, has seven Earth-sized transiting planets.
- Atmospheres: during a transit, starlight filters through the planet’s atmosphere; telescopes such as JWST read the gases in it from the light’s spectrum.
Common misconceptions
- “Astronomers see the planet as a black dot.” They measure only the star’s total light.
- “A big dip means a heavy planet.” The depth measures size, not mass.
- “If no transit is seen, there is no planet.” Most planets’ orbits are simply not lined up with us.
- “The planet blocks all the light while it crosses.” Even Jupiter blocks only about 1%.
What the model simplifies
Orbits are circular, and only one planet orbits each star. The star’s brightness profile uses a simple limb-darkening law with a fixed value, and the dip is calculated by sampling the planet’s disc at 400 points. The three star types and the two noise levels are illustrative round numbers, not specific stars or instruments. Real light curves also include starspots, flares, the faint light reflected or emitted by the planet, and dips from eclipsing binary stars that must be ruled out. Time is speeded up between transits and slowed down during them.
Key takeaways
- A transit is a small, repeating dip in a star’s light as a planet passes in front of it.
- Depth ≈ (planet radius ÷ star radius)², so the dip reveals the planet’s size.
- The time between dips gives the orbital period; Kepler’s third law then gives the orbit’s size.
- Only orbits that are almost edge-on to us produce transits, so most planets are missed.
- Tiny dips need very precise, low-noise measurements, usually from space.