NASA reports that the Kepler mission has discovered the first Earth-sized exoplanets ever discovered. What’s even wilder: they found the pair of them in the same damned system.
Size comparison nicked from Bad Astronomy. An artist’s impression of the relative size differences of the two planets and ours. Note that Kepler was actually able to detect a planet smaller than Venus. That’s something!
The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days. All five planets have orbits lying roughly within Mercury’s orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.
The system has an unexpected arrangement. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.
This throws a bunch of things we thought we understood about planetary formation on their ear. Either our idea that rocky worlds tend to be inner planets with the gas giants further out is wrong, or we’ve found an outlier. More data needed!
This comes hot on the heels of Kepler’s last landmark find: a super-Earth in a star’s habitable zone.
Why should you care? you ask, impertinently. Well, the very existence of exoplanets pretty much proves the weak anthropic principle, which serves as a very convenient and very devastating club with which to whack theists who use the fine-tuning argument. Not to mention what it does to astrologers’ ideas about planets governing and/or predicting people’s lives!