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.
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.
Kepler-16b was the first exoplanet found to circle a double star. That means if you lived there, you would have two suns in your sky.
Exoplanet 55 Cancri e circles so close to its star that it may be covered in lava. The surface of this world could be about 4,900 F (2,704 C).
The star Kepler-90 has eight planets, like our solar system. The planets closer to the star are smaller and rocky, while the ones farther out are large and gaseous.
A new batch of discoveries is making astronomers take a closer look at Hycean planets. These exoplanets are much larger than Earth but not as big as Jupiter or even Neptune. They could have atmospheres similar to Earth and their surfaces are most likely entirely covered by liquid with very little land.
Despite the high temperatures (usually more than 200 C), and their high levels of methane (instead of water), Hycean planets could actually be places to look for life.
Astronomers call exoplanets that are not too hot and not too cold “in the Goldilocks Zone” (or Habitable Zone). These worlds are at just the right distance from their star to potentially have liquid water and a slim chance to support life.
Kepler-452b is the first planet found that is similar in size to Earth and circles its star at a similar distance. The most Earth-size exoplanet found so far may be Kepler-1649c. It is only slightly larger than Earth and orbits in the habitable zone of a red dwarf star.
Exoplanet Kepler-22b may have a liquid ocean covering its surface. If so, the surface temperature could be a mild 60 F (16 C).
Closer to home
Proxima Centauri is the smallest member of a three-star system called Alpha Centauri. Two large yellow suns orbit each other while the red dwarf star Proxima hangs nearby. So if you lived on a planet circling any of these stars, you could see one, two or three suns in the sky at one time. Imagine watching a double sunset or a triple sunrise. You may even live on a planet where one sun is always in the sky – it may never get completely dark!
Astronomers detected a planet in the Habitable Zone around Proxima Centauri. Not only does the closest star to us have an Earth-like planet, but it shows that small stars (which represent the vast majority of stars in our galaxy) can have planets.
It is possible that 1 in every 5 stars has a planet circling it. And with 300 billion stars in our Milky Way galaxy alone, there could be lots of places to search for alien life.
Dean Regas is the Astronomer for the Cincinnati Observatory and author of Facts From Space and 100 Things to See in the Night Sky. He can be reached at [email protected]
Take an Online Astronomy Class
What: Take a fast, fun, online class with astronomer Dean Regas and zoom around the universe. Perfect for the beginning stargazer.
When: Sept. 28, 7 p.m., Venus. Sept. 30, 7 p.m., The Zodiac.
Cost: $10 per household.