How Do You Discover a Planet You Cannot See?

Detecting Exoplanets by Watching a Star Blink

Astronomy has a rather wonderful problem.

The objects we most want to investigate are often unimaginably far away, extremely faint and sitting beside something enormously brighter.

An exoplanet may be hundreds of light-years from Earth. It does not conveniently appear in a telescope photograph as a neat little sphere next to its star. In many cases, we discover that the planet is there without ever seeing the planet itself.

Instead, we watch the star.

And occasionally, almost imperceptibly, the star becomes slightly dimmer.

That tiny dip in brightness can be enough to reveal an entire world.

This makes exoplanet detection an excellent example of science beyond the normal school syllabus because it combines astronomy, physics, data analysis, graph interpretation and experimental design with one of the most important ideas in science:

You do not always have to see something directly to discover that it exists. You can measure the effect it has on something else.

NASA describes the transit method in essentially these terms: when a planet passes between its star and us, it blocks a small fraction of the star's light. Plotting the measured brightness against time produces a light curve, and a planetary transit appears as a dip in that curve.

And we can reproduce the basic idea on a laboratory bench.

A Star That Apparently Blinks

Imagine watching a distant star continuously.

For most of the time its measured brightness remains approximately constant.

Then this happens:

Normal brightness -> slight fall -> minimum brightness -> rise -> normal brightness

Nothing necessarily happened to the star itself.

Instead, a planet may have crossed the face of it.

From Earth, we see something rather like a very small eclipse.

The crucial word is small.

A planet is normally considerably smaller than its parent star, so only a fraction of the star's light is removed. Detecting exoplanets therefore depends on making extremely precise measurements and deciding whether a tiny change is genuine or merely noise.

That immediately makes this much more interesting than simply moving a ball in front of a lamp.

The real experiment is about measurement.

Can we detect the change?

Can we distinguish it from random fluctuations?

Can we extract information about our "planet" from the graph?

Building a Model Exoplanet System

The simplest version needs surprisingly little equipment.

You need:

a bright lamp or LED source;
preferably a translucent diffuser to create a circular illuminated "star";
several opaque balls or discs of different diameters;
a light sensor, lux sensor or data logger;
some way of moving the model planet steadily across the star;
software capable of recording light intensity against time.

A data logger is particularly useful because it turns the demonstration into something very close to the way astronomical observations are actually treated: a sequence of brightness measurements taken over time.

Why I Would Not Use a Bare LED

There is a useful experimental-design point here.

A bare LED is very nearly a small point source. Put an opaque object directly in front of it and you may simply block most or all of its light.

That is not a very good model of a planet crossing a star.

A better arrangement is to illuminate a circular translucent screen from behind. The whole circle then becomes the visible surface of our model star.

Now a small disc passing across it blocks only part of the illuminated area.

That gives us something much closer to a genuine transit.

First Experiment: Find the Planet

Begin with the detector recording a steady brightness.

Do nothing for perhaps five or ten seconds.

Then move the model planet steadily across the illuminated disc.

Continue recording for another five or ten seconds after it has left.

When the data are plotted, students should see something resembling:


Light
intensity

100 |____________             ____________
 98 |            \           /
 96 |             \_________/
 94 |
    +------------------------------------> time

We have created our first transit light curve.

The flat section before the transit represents the normal brightness of the star.

The falling section represents the planet beginning to move across the stellar disc.

The lower section occurs while much of the planet is in front of the star.

The brightness then rises again as the planet moves away.

The planet itself has never been detected by the light sensor.

We detected its shadow.

That is a deceptively profound scientific idea.

Can the Light Curve Tell Us How Big the Planet Is?

Now we can start doing some mathematics.

Suppose the star has radius Rs and the planet has radius Rp.

Ignoring complications such as the star being brighter in its centre than around its edge, the approximate fraction of light blocked is:

Transit depth = (Rp / Rs)^2

This occurs because the amount of light blocked depends approximately on the ratio of the areas, not simply the diameters.

Area is proportional to radius squared.

For example, suppose our model star has a diameter of 15 cm and our model planet has a diameter of 3 cm.

The radius ratio is:

3 / 15 = 0.20

So:

Transit depth = 0.20^2

Transit depth = 0.04

The expected brightness decrease is therefore about:

If the normal sensor reading were 1,000 arbitrary units, we might expect it to fall to roughly 960 during the central part of the transit.

Suddenly a small dip on a graph contains physical information about an object we cannot see.

NASA uses exactly this principle with real transit observations: if astronomers know the size of the star, the depth of the transit helps them determine the radius of the planet.

Investigation 1: Bigger Planet, Bigger Dip

Now repeat the experiment with different-sized balls or discs.

Perhaps use:

1 cm;
2 cm;
3 cm;
4 cm;
5 cm.

Keep everything else approximately constant.

Students can record:

Planet diameter	Minimum brightness	Percentage brightness decrease
1 cm
2 cm
3 cm
4 cm
5 cm

They should discover that increasing the planet's diameter does not produce a simply proportional increase in the light lost.

Doubling the radius means approximately four times the area.

That gives a lovely connection between familiar school mathematics and modern observational astronomy.

Investigation 2: What Does Orbital Speed Do?

Use the same planet but move it across the star at different speeds.

Importantly, if the planet follows the same path, the depth of the transit should remain broadly similar.

What changes is its duration.

A slowly moving planet produces a wider dip.

A fast-moving planet produces a narrower one.

This introduces another important feature of astronomical light curves:

The shape of a graph can tell us more than the minimum value does.

Astronomers use the timing of transits to learn about planetary systems. Repeated transits reveal a planet's orbital period, while transit duration and shape contribute further information about the system's geometry.

Investigation 3: Central or Grazing Transit?

This is one of my favourite variations because it shows why the graph needs interpreting rather than simply reading.

First send the planet straight across the centre of the star.

Then repeat the experiment with the planet just clipping the upper edge.

The second is a grazing transit.

The planet never completely crosses the stellar disc, so it never blocks as much light.

The resulting light curve may therefore be:

shallower;
shorter;
differently shaped.

Now ask:

Did we use a smaller planet?

No.

But if we looked only at the depth of the graph without considering the geometry, we might draw the wrong conclusion.

This is real science.

Measurements are rarely interpreted in isolation. Scientists construct models and ask which combination of variables could have generated the data.

Investigation 4: Add Measurement Noise

Real astronomical data do not form beautifully smooth textbook curves.

So perhaps ours should not either.

Try introducing small disturbances.

Move somebody near the apparatus.

Allow a little ambient light into the room.

Introduce a tiny variation in lamp brightness.

Move the detector slightly.

The graph becomes noisier.

Now hide a transit somewhere within the results and ask students to identify it.

This changes the question from:

"Can you see the dip?"

to:

"Are you sufficiently confident that this dip represents a planet?"

That is much closer to the real problem.

NASA's own citizen-science projects invite people to examine actual stellar light curves for the tell-tale patterns of planetary transits.

One Dip Is Not Necessarily a Planet

This is an important addition to the experiment.

Suppose our star becomes slightly dimmer once.

Have we discovered a planet?

Not necessarily.

There could be other explanations.

Astronomers therefore look for evidence that supports the planetary interpretation — particularly repeated transits occurring at regular intervals.

If a similar dip appears every 5.2 days, for example, that becomes much more interesting.

The interval gives us the orbital period.

Our laboratory version could mimic this by mounting the planet on a rotating arm so that it repeatedly passes in front of the star.

Students could be given a long data trace containing several transits and asked:

What is the orbital period of this planet?

Measure the time from one transit centre to the next.

If dips occur at:

10 s, 25 s, 40 s, 55 s...

the model orbital period is approximately:

15 seconds

The same reasoning can be applied to astronomical observations collected over days, months or years.

Could There Be More Than One Planet?

Now things become considerably more entertaining.

Introduce two different-sized planets travelling with different periods.

One produces a deep dip every 20 seconds.

The other produces a shallower dip every 13 seconds.

Record for long enough and the light curve becomes much more complicated.

Students then have to identify two repeating patterns.

NASA notes that light curves become more complicated when several planets transit the same star, but astronomers can disentangle the different signals.

You have effectively turned a lamp, two balls and a light sensor into a simplified planetary-system discovery problem.

From a School Experiment to TESS

This is where I think demonstrations like this become especially valuable.

We have not merely constructed an analogy for something astronomers used to do.

The basic technique remains enormously important.

NASA's TESS — the Transiting Exoplanet Survey Satellite — searches stars for periodic changes in brightness associated with planetary transits. NASA reported in May 2026 that TESS had identified more than 7,900 candidates and 885 confirmed exoplanets at that point.

There is something rather satisfying about showing a student a graph generated using a ball and a light sensor and then explaining:

Space telescopes are looking for essentially the same signature.

The instrumentation is vastly more sophisticated.

The mathematics is much more sophisticated.

The data processing is vastly more sophisticated.

But the underlying observation is recognisable.

Something crossed the star.

The star became dimmer.

Measure that change carefully enough and you may have discovered another world.

And a Transit Can Tell Us Even More

The story does not end with finding the planet.

Modern astronomers can study starlight passing through a planet's atmosphere during a transit.

Different gases absorb particular wavelengths of light.

Instead of measuring only:

How much light disappeared?

astronomers can ask:

Which wavelengths disappeared slightly more than others?

That opens the door to studying exoplanet atmospheres.

NASA's James Webb Space Telescope, for example, records extremely detailed transit observations. Its measurements of LHS 475 b included more than a thousand individual brightness measurements over an observation lasting almost three hours.

Our laboratory experiment has therefore taken us from a simple shadow all the way to spectroscopy of the atmospheres of planets orbiting other stars.

Can Students Work With Real Data?

Yes — and this would make an excellent extension.

Once students understand the model experiment, show them a genuine exoplanet light curve and ask them to identify:

normal stellar brightness;
start of transit;
minimum brightness;
end of transit;
transit depth;
transit duration;
uncertainty and scatter.

They can then compare the real curve with the one obtained experimentally.

NASA's Planet Hunters TESS citizen-science project goes a stage further: participants can examine actual TESS light curves looking for possible transits. No specialist astronomy knowledge is required to begin.

NASA also runs Exoplanet Watch, where observers can collect telescope images and turn them into transit light curves using its EXOTIC analysis software.

That creates an extraordinary progression:

Model planet -> model light curve -> real astronomical data -> citizen science.

The Experiment Is Really About Evidence

There is a much broader lesson here than exoplanets.

We often teach science using objects that can conveniently be seen.

Here is the cell.

Here is the circuit.

Here is the spring.

Here is the reaction.

But much of science deals with things that cannot be observed directly.

We discovered the internal structure of atoms from scattering.

We infer the presence of dark matter from gravitational effects.

We determine the composition of distant stars from their spectra.

We study Earth's interior using seismic waves.

And we discover planets by watching stars become fractionally dimmer.

The ability to reason from an effect to an unseen cause is one of the most powerful forms of scientific thinking.

A Small Shadow From Another World

What I particularly like about the exoplanet transit experiment is that it begins with equipment that looks almost trivial.

A lamp.

A ball.

A sensor.

A graph.

But the question behind it is enormous:

Are there planets orbiting other stars?

For centuries that was largely speculation.

Today we can measure them.

A tiny repeated decrease in a distant star's brightness can tell us that a planet exists, estimate how large it is, determine how frequently it orbits and, with considerably more sophisticated observations, begin investigating its atmosphere.

So perhaps the most important lesson is not really about exoplanets at all.

It is about what scientists mean by evidence.

Sometimes discovery does not begin by seeing the thing you are searching for.

Sometimes it begins by noticing that something else has changed.

And asking why.

Practical challenge

Try building your own transit experiment.

Start with one planet and see whether you can produce a convincing light curve.

Then make it progressively harder:

different planet sizes -> different speeds -> grazing transits -> measurement noise -> repeated transits -> two planets

Finally, compare your graph with a genuine exoplanet light curve.

You may be surprised by how recognisable it looks.

03 September 2026