How do we find planets around other stars?
A tiny change in brightness may reveal a planet crossing in front of a distant star.
The short answer
An exoplanet orbits a star other than our Sun. If it passes in front of its star as seen from Earth, it can block a little starlight: the star briefly looks dimmer. A repeated dip is a clue researchers investigate further.
Equal-sized Wusels study a small planet model in front of a glowing model star. Museum model; sizes and distances are not to scale. The brightness line is simplified, not research data. AI-created illustration, not a photograph of nature.
What the diagram shows
The diagram simplifies the process. First compare its shapes and arrows. You can read its labels in larger text below.
Read the picture labels
These are the words and sentences from the diagram above. The full explanation follows below.
- A transit changes measured brightness
- before
- during transit
- after
- View from the same side
- Brightness
- Time →
- Part of the starlight is blocked.
- Diagram: simplified viewpoint and curve shape; not actual measurements.
An imagined Wusel scene
Puschel and Flauschi investigate
In the safe table model, Flauschi moves a small paper circle across a lit area. Puschel sees only a little being covered. They mark bright and slightly dimmer. Then they compare how repeated dimming in starlight could suggest a planet.
Look closely
Find the small dark planet near the edge of the bright star model. The nearby line schematically shows a temporary brightness dip. Without axes or numbers it is not an actual measurement graph.
An example to help you understand
An adult safely positions a cool lamp. Hold a small paper circle between the lamp and a white wall without touching the lamp. Less light reaches the part of the wall where the circle blocks it. Move it sideways and the shadow disappears. This explains blocking light; actual transit measurements observe a distant star without a wall behind its planet.
Try explaining it
How can a distant planet reveal itself?
Choose a), b) or c), then explain your choice.
- a)It can brighten its star's light when passing in front instead of dimming it.
- b)Its shadow must fall on our Earth for this to happen.
- c)It can block a little of its star's light.
Show the explanation
Correct: c): It can block a little of its star's light.
An exoplanet orbits a star other than our Sun. If it passes in front of its star as seen from Earth, it can block a little starlight: the star briefly looks dimmer. A repeated dip is a clue researchers investigate further.
The planet is not brighter than its star here: it blocks a small part of its light. Its shadow need not fall on Earth.
Word help
- atmosphere
- The gas layer around a world; on Earth it is the air above us.
- orbit
- The path of one body around another.
- exoplanet
- A planet outside our Solar System.
Explore in more detail
Here you will find longer explanations and further details. You can explore them together.
1. Planets far beyond our system
An exoplanet is a planet outside our solar system. Many orbit other stars. A star produces its own light; a nearby planet usually appears much fainter. That makes photographing these planets as separate spheres difficult. Researchers can instead look for measurable effects.
Checked against: [1]
2. A little light is missing
If a planet passes in front of its star from our viewpoint, it covers part of the bright area. An instrument briefly receives less starlight. This crossing is called a transit. The measured brightness then rises again. Repeated dips can fit an orbit. Not every orbit lines up for us to see a transit.
3. Clues are checked
A brightness dip alone does not establish a discovery. Researchers check further observations and other explanations. Another method measures a star’s slight back-and-forth motion: the planet’s gravity pulls on the star too. Different measurements can strengthen an explanation. Some suspected planets are not confirmed.
4. A planet is not automatically another Earth
Exoplanets vary in size and temperature. For some, researchers study light passing through the atmosphere, their surrounding gases. Particular substances leave measurable patterns in that light. These indicate gases, not necessarily inhabitants. A habitable zone is a distance from a star where liquid surface water could be possible under suitable conditions.
An optional question for further exploration
You can use the longer explanation above. This is extra, not needed for the first quiz.
An extra question
Why might a planet exist even though we do not observe a transit?
Discover the answer
From our viewpoint its orbit might pass beside the star instead of covering it. Researchers therefore use other methods as well.
Pictures and sources for adults
Sources for further reading
Facts checked against the specialist sources below on 12 October 2026. The Wusel scene is fictional; comparisons and simplified models are explicitly explained. Sources are in English.