How do we know other planetary systems are out there?

DW (English)

Our solar system is just one of many in the universe. Over 4000 other exoplanetary systems have already been discovered in our galaxy alone. How do we detect them? This viewer question was sent in by Wayne K. in the US.

Transcript

00:00How do we know there are other star systems, and are they like ours?

00:10Our solar system is just one of countless others in the universe.

00:15More than 4,000 planetary systems have already been discovered in our galaxy alone.

00:22Most are so far away that telescopes can see the stars directly, but not the planets, so

00:28how do we know they're there?

00:31Many were detected using what's called the transit method, which can be illustrated by

00:35the example of a solar eclipse here on Earth.

00:39When the Moon's orbit takes it between us and the Sun, it casts a shadow here on Earth.

00:45When it covers the Sun completely, it grows dark here.

00:49When a distant planet crosses between us and its star, the light from it also appears to

00:54us to dim just a little.

00:57In a telescope, the star flickers very slightly.

01:00The transit method can even be used to determine whether distant planets have atmospheres.

01:08An exoplanet can also be detected via the movement of the star it orbits.

01:13As the planet moves around the star, its gravity causes the much larger body to wobble a little.

01:20That changes its light spectrum, and we can see that.

01:24If the planet is moving towards us, the light from its star shifts towards the blue part

01:30of the spectrum.

01:31If it's moving away from us, the star's light grows more reddish.

01:37The first exoplanetary system discovered has at least six planets.

01:44Eight planets circle our own Sun, four rocky worlds close in, and four gas giants on more

01:51distant orbits.

01:54Another planetary system identical to ours has yet to be discovered, but one around 2,500

02:00light years away looks at least somewhat similar.

02:04Kepler-90 also has eight planets, with rocky worlds on the inside tracks and gas giants

02:11farther out.

02:12However, all of them orbit closer to their star than the Earth is to the Sun.

02:18So that's where the similarities end.

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