Exoplanet Detective: Find Planets from a Dip in Starlight
Learn the transit method: a planet crossing its star blocks a fraction of light equal to (planet radius ÷ star radius)², and the dips repeat once per orbit.
Before you try
What if the planet’s orbit were tilted so it never passed in front of the star? Would we ever know it was there?
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What happened?
Move a control to see what changes and why.
The orbit is circular round a Sun-mass star and the planet crosses the star’s middle; real light curves are noisier and the star is darker at its edges.
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Try these ideas
- Set the planet to 1 Earth and watch how small the dip becomes. Then set 11 (Jupiter).
- Halve the orbital period and see the dips bunch up and each transit get shorter.
- Switch off “edge-on” and explain why the curve goes flat.
Why it works
We cannot see planets round other stars directly; they are too faint and too close to their star. But when a planet passes in front of its star, the star dims very slightly. A Jupiter-sized planet hides about 1% of a Sun-like star; an Earth-sized one hides only 0.01%. Space telescopes such as Kepler and TESS measured these tiny dips for hundreds of thousands of stars and found thousands of planets.
The depth of the dip gives the planet’s size, because the blocked fraction is the ratio of the two areas. The time between dips is the planet’s year, and Kepler’s third law turns that into the distance from the star. The Detective challenge records a hidden planet’s light curve and asks you to match its size and period by reading the graph.
Check your understanding
Question 1 of 4
A transit dims the star by 1%. Roughly how big is the planet compared with the star?
Dips repeat every 10 days. What is 10 days?
Why does the transit method miss most planets?
Two planets make dips of equal depth round two different stars. Which is the bigger planet?
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Cover a torch partly with a coin and describe how the brightness changes.
- Ask: could this method find a planet that orbits every 100 years?
- Look up TESS and how many planets it has confirmed.