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What we know about the innumerable planets outside of our solar system

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected].

Are there any planets outside of our solar system? – Eli W., age 8, Baton Rouge, Louisiana


This is a question that human beings have wondered about for thousands of years.

Here’s how the ancient Greek mathematician Metrodorus (400-350 B.C.) put it: A universe where Earth is “the only world,” he said, is about as believable as a “large field containing a single stalk.”

About 2,000 years later, in the 16th century, the Italian philosopher Giordano Bruno suggested something similar.

“Countless suns and countless earths” existed elsewhere, he said, all rotating “round their suns in exactly the same way as the planets of our system.”

Scientists now know that both Metrodorus and Bruno were essentially correct. Today, astronomers like me are still exploring this question, using new tools.

The exoplanets

There is now evidence that demonstrates the existence of “exoplanets” – that is, planets orbiting stars other than our Sun.

That evidence is based on the discoveries made by the Kepler space telescope, launched by NASA in 2009.

For four years, the telescope stared continuously at a single region of space within the constellation Cygnus.

Looking from Earth, it’s an area that takes up less than 1% of your view of the sky.