Space · Living in space and watching the sky

Planets of other stars

Planets around other stars

Only spacecraft can see it

Planets of other stars — A planet passing in front of its star: the light that reaches us dips a tiny bit, then comes back.

The numbers

What it is: A planet around another star
Known: More than eight thousandThe count goes up every month
Found how: Almost always indirectlyBy watching the star
Main method: The transit in front of the star

To get an idea

Almost no exoplanet has been seen directly: they are discovered by watching the star, not the planet.

During a transit the star's light drops by a very small amount, and that drop repeats, always the same.

Fun facts

These are not in the book: there was no room on paper.

It works only if the planet's orbit is lined up the right way as seen from us.
Today more than eight thousand are known, spread across thousands of different planetary systems.
More than a thousand of those systems have more than one known planet around the same star.
There are also thousands of candidates waiting for confirmation or still considered uncertain.
The count of known exoplanets keeps growing, so any number goes out of date fast.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What is an exoplanet?

Question 2 of 5

Why are they hard to see?

Question 3 of 5

How are they almost always discovered?

Question 4 of 5

What is the transit method?

Question 5 of 5

How many are known today?

Riddles

I go around a star that is not yours, and almost none of us has ever been photographed: you find me by watching my star and noticing that something does not add up. What am I?

When I pass in front of my star I cover a crumb of it, and its light drops by a tiny amount. If that drop repeats, always the same, you have found me. What am I?

Until a few decades ago nobody had managed to find a single one of us, and now there are more than eight thousand on your lists. What am I?

Try it yourself

Activity 1

Try to see a coin resting next to a car's lit headlight: that is roughly how hard it is to photograph an exoplanet.

Activity 2

Look up how many exoplanets are known today: the number will already be different from the one printed on this page.

The words

transit: A planet passing in front of the disk of its star.

Sources: Wikidata (Q44559), Wikipedia (Pianeta extrasolare).