What is an exoplanet? Learn about planets that circle distant stars

Planets that circle distant stars are called exoplanets. While we have eight planets in our solar system, astronomers have discovered thousands of these far-flung worlds around other stars.

Exoplanets are so far away, but so close to their bright stars, that they are extremely difficult to see in a telescope. Early on, astronomers looked for stars that “wobbled,” which meant a big planet might be tugging on them. And many of the first discovered exoplanets were larger than the size of Jupiter.

Now the most successful way to find exoplanets is the transit method: noticing when a planet regularly goes in front of the star and blocks some of its starlight. With satellites like Kepler and TESS, astronomers can detect much smaller objects – even those the size of Earth – from trillions of miles away.

51 Pegasi b. This giant planet, which is about half the mass of Jupiter and orbits its star every four days, was the first confirmed exoplanet around a sun-like star, a discovery that launched a whole new field of exploration.

Weird worlds

Exoplanets around the size of Jupiter that orbit very close to their stars are called “Hot Jupiters.” One such world named 51 Pegasi b is so near to its star that it circles it once every four days.

An exoplanet named HD 189733b has bright blue skies. But winds there gust at 4,500 miles an hour and it rains glass instead of water.

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