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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