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Outer Space

Rogue Planets Wander The Galaxy Without A Star To Call Home

Billions of stars orbit the center of our galaxy, and many of these stars have at least one planet orbiting them. But not all stars belong to a galaxy, and not all planets orbit a star. Rogue planets are worlds that drift untethered to any star, and experts estimate that there are billions of them in our galaxy alone.

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Usually, astronomers detect exoplanets—that is, planets outside of our solar system—in relation to their home stars. Since they don't have a home star, rogue planets present a unique challenge. Astronomers overcome this challenge in one of two ways. If the planet is young enough, researchers can use infrared technology to detect the heat it gives off, just like a SWAT team would use night-vision goggles to catch a suspect. If the rogue planet is large enough, its mass can actually bend the light of the stars we see behind it. Its gravity basically acts as a massive lens, which is why astronomers call this method gravitational microlensing. Neither of these methods is perfect, since they can't detect anything smaller than Jupiter, but so far the discoveries they've enabled have led researchers to estimate that in the Milky Way, at least, rogue planets outnumber stars two to one. A Nature study in 2011 suggested that our galaxy contains at least 400 billion rogue planets. Yet another galactic mystery uncovered. Learn more about rogue planets in the videos below.

What Are Rogue Planets?

Explore how we go about discovering these lonely worlds.

How Planets Go Rogue

Did rogue planets leave their home stars, or were they never tethered to one in the first place?

Exoplanets Explained

Discover the different methods astronomers use to find planets outside our solar system.

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Key Facts In This Video

  1. The way we discovered the first planets outside our solar system was with the Doppler Technique. 00:25

  2. Actually trying to see a planet would be like standing on Hawaii and looking at flame flickering near a lighthouse in California. 02:26

  3. Every 30 minutes, Kepler takes a measurement of the brightnesses of 160,000 stars. 03:41

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