NASA’s Kepler spacecraft, which is looking for planets outside our solar system, has found three of the smallest exoplanets yet. They’re all smaller than Earth — their radii are 0.78, 0.73 and 0.57 times that of Earth’s — and the smallest is about the size of Mars.
They’re very close to their star, too close to be good candidates for life because liquid water can’t exist on them, and their star is a red dwarf. But what makes them special is that they are so small.
The techniques we use for finding exoplanets work best with large, massive planets, as I’ve mentioned before. It’s only been recently that we’ve been able to find planets of around Earth’s size, and especially those that are likely to be rocky, terrestrial planets like Earth. The three that Kepler’s found fall into that small-rocky territory. That’s crucial — we’ve mainly found gas giants, which made astronomers wonder if our solar system was an unusual one because it has so many rocky planets in it. As we find more rocky planets, we learn more about how solar systems form and help us understand if life here on Earth is a fluke or likely to be repeated across the galaxy.
Scientific America has a good discussion of why Kepler was able to find these three planets, if you’d like to know more. What’s really exciting is that Kepler may be able to discover planets as small as our Moon, and given the firehose of data coming out of Kepler, there are likely many more discoveries where these three came from.