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Planetary & Solar-System Phenomena

Exoplanet Transits

A transit happens when a planet passes in front of its star and blocks a tiny fraction of its light.

The transit method has found most known exoplanets, including those seen by Kepler and TESS.

  1. Observe

    Starlight dips regularly and briefly.

  2. Measure

    A light curve of brightness against time.

  3. Model

    Fit orbital period, size ratio and inclination.

  4. Explain

    A planet passes in front of its star.

What is it?

When a planet passes in front of its star from our point of view, the star dims slightly. Repeated dips reveal the orbital period, and the depth of the dip shows the planet's size relative to the star. A planet like Jupiter blocks about one percent of a Sun-like star; an Earth-sized planet blocks less than one hundredth of a percent.

Combining transit data with measurements of the star's wobble gives the planet's mass and density. Spectra taken during a transit can show what its atmosphere is made of.

How does it happen?

  1. Stage 1
    Planet crosses its star's face
  2. Stage 2
    Small decrease in brightness
  3. Stage 3
    Repeated measurements
    Same dip each orbit
  4. Stage 4
    Orbital period and planet size derived

What do we actually observe?

Light curve
A flat line with a small, regular dip.
Depth and duration
Depth gives the size ratio; duration gives clues about the orbit.
Atmosphere signal
Tiny wavelength-dependent changes reveal gases in the planet's atmosphere.

How do scientists detect it?

  • Kepler and K2. Found thousands of planets between 2009 and 2018.
  • TESS. Surveys bright nearby stars across almost the whole sky.
  • JWST and Hubble. Measure atmospheres during transits.
How we know

A transit only occurs if the orbit is edge-on from our viewpoint, which for an Earth-like orbit around a Sun-like star is only about half a percent likely, so surveys must watch many stars. Signals must also be checked against false alarms such as eclipsing binary stars and starspots.

Why does it matter?

  • It gives the population statistics of planets in the galaxy.
  • Atmospheric measurements show which worlds might be habitable.

The SpaceWeatherTracker connection

Stellar flares and starspots, familiar from our Sun, can mimic or distort transit signals, so understanding stellar activity is central to reading them.

Related phenomena

Primary family: Planetary & Solar-System Phenomena

Exoplanet Transits: common questions

What can a transit tell us about a planet?

Its size relative to its star, orbital period, and, with more data, mass, density and atmosphere.

What can a transit not tell us?

It cannot detect planets whose orbits don't line up, and on its own it doesn't measure mass or show a planet's surface.

Do planets in our Solar System transit the Sun?

Yes. Venus last transited in 2012 and will not again until December 2117. The next Mercury transit is in November 2032.

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