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SESK: Stuff Everyone Should Know · sesk.info/astro-transit-method-size

What does the transit method of finding exoplanets tell us about the planet?

Its size, from how much starlight it blocks.

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  1. Its color
  2. Its size ✔
  3. Its exact mass

Why: When a planet crosses in front of its star, it dims the star slightly. The dip depends on the planet's area compared to the star's, so if we know the star's size we get the planet's size.

Huh, didn't know that: A Jupiter-size planet crossing a Sun-like star blocks about 1% of its light.

The dad joke
Exoplanet hunters are always looking for a dip. Sounds like a snack party.
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References

Answer: Its size. When a planet crosses in front of its star, it dims the star slightly. The dip depends on the planet's area compared to the star's, so if we know the star's size we get the planet's size.

  1. Astronomy 2e, 21.4 Planets beyond the Solar System: Search and Discovery openstax.org/books/astronomy-2e/pages/21-4-planets-beyond-the-solar-system-search-and-discovery
    If we can determine the size of the star, the transit method tells us the size of the planet.
    Checked 2026-10-10.

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