We've discovered many rocky planets around distant stars, but they're mostly super-Earths, much larger and more massive than our planet. Plus, most of these "super Earths" orbit stars vastly different from our Sun. Now we've discovered two planets that really could be Earth's twins.
Kepler-20e and Kepler-20f are part of the five-planet solar system called Kepler-20, named for the exoplanet-hunting NASA probe that discovered it. They are located about a thousand light-years away in the constellation Lyra. Unfortunately, these two planets are much too close to their parent star to be in their solar system's habitable zone, which means in that rather crucial way they're not exact duplicates of Earth. But otherwise, they're astonishingly close.
In particular, Kepler-20f is nearly identical to Earth in terms of size, with a radius that's just 1.03 times larger than that of our planet. Kepler-20e is smaller, at just 0.87 times Earth's radius, which means it's slightly smaller than Venus. They represent the very first Earth-sized exoplanets ever found. Until now, Kepler-10b, the first rocky exoplanet ever found, was the smallest known exoplanet, at 1.4 times Earth's radius.
What really clinches the physical similarity between these two planets and Earth is their star. Kepler-20 is a class G star, just like the Sun, and it's only slightly smaller and cooler than ours. However, while the dimensions of the planets and stars may be similar, their positioning is vastly different. Kepler-20e orbits the star every 6.1 days and has an average temperature of 1,400 degrees Fahrenheit, while Kepler-20f clocks in at 19.6 days and 800 degrees.
While these Earth-sized exoplanets naturally attract the most attention, the entire Kepler-20 system is fascinating. The other planets - Kepler-20b, Kepler-20c, and Kepler-20d - are all between the size of Earth and Neptune, making them super-Earths. They orbit every 3.7, 10.9, and 77.6 days, respectively, which means the smaller rocky planets slot in between those orbits. We've never seen such alternation between bigger and smaller planets before. In our solar system, for instance, there's clear segregation between the small rocky planets and the big gas giants.
This announcement comes hot on the heels of the discovery of Kepler-22b, a planet in another solar system that is in the habitable zone but likely too large to have a rocky surface. As Kepler deputy science team lead Natalie Batalha observes, we keep finding exoplanets with some, but not all, of the qualities needed to support life. With this latest find, it seems the discovery of a true Earth-2 really is just a matter of time.