Astronomers working with the Kepler telescope, led by Elisa Quintara of the SETI Institute, have announced the discovery of the first planet orbiting another star that meets the two characteristics we have been particularly waiting for. The planet is Earth-sized AND it orbits its star in the zone where water is likely to be liquid (called the “habitable zone.”)
The planet and star have no name, but only a catalog number – Kepler 186. Located in the constellation of Virgo, about 500 light years away, the star is “red dwarf” – smaller and cooler than the Sun. So a planet has to be closer to it to have the right temperature for liquid water. But every star has its own habitable zone and Kepler 186 is no exception.
We have actually found five planets around Kepler 186 so far, but the other four planets are very close to the star and much too hot for life as we know it. Called Kepler 186f, the newly found planet takes about 130 days to go around its star, and its distance is in the zone where water could be liquid (if the planet has a significant atmosphere.)
Astronomers actually know of some 1,800 planets around other stars so far, orbiting at a wide range of distances and showing a wide range of sizes. In the past, we have found a number of planets that were the same size as Earth but all of these were too hot – orbiting too close to their stars. We have also found a number of planets that were in the habitable zone of their stars, but these were bigger than Earth. Most likely they were so big they would look more like Neptune or Jupiter, made mostly of gas and liquid (or at least having a huge shell of gas and liquid before you could touch solid ground.)
Kepler 186f (sorry planets don’t get names yet) is the first planet that is both the right size and the right distance from its star. Today, at their national press conference, the scientists who discovered it made sure not to call it an “Earth twin,” however. Instead, they used the term “Earth cousin” to describe their discovery. That’s because Kepler 186 is a cool red star, about half the size and mass of the Sun. So the light of this star will look different on the newly discovered planet – instead of the yellow sunlight we are used to, anyone standing on the surface of Kepler 186f would see reddish-orange sunlight. The scientists speculated whether any plants on the new planet would receive enough energy to do photosynthesis, and their first conclusion was a tentative yes.
What’s especially important about this discovery is that roughly 80% of all the stars in our Galaxy are red dwarf stars. If, as our observations are starting to show, MOST stars will have planets, then most planets are likely to orbit red dwarfs. So places like Earth and our Sun may be the exception. And Kepler 186f may be a mainstream kind of world in the vastness of the Milky Way.
On our diagram, above, you can see the orbit of the Kepler 186 system’s planets to the same scale as the inner planets in our own solar system. The fuzzy green region in each case is the habitable zone of each star. You can see the Earth comfortably in the habitable zone of our Sun, and 186f in the habitable zone of 186. The painting of 186f in this picture is just from the artist’s imagination. We really have no idea what the planet looks like.
(By the way, for those of you who like to count, you may wonder why the fifth planet in the Kepler 186 system gets the letter f, when f is the sixth number of the alphabet. That’s because in this pretty awkward naming system we are using, the star is called Kepler 186a, and the letters of the planets start with b. I don’t endorse this system, folks, I just explain it.)