One of the most exciting fields in astronomy—really, in all of science—is the search for alien worlds. The first planet around another star was found in 1992 (though the star was the remnant of a supernova, so not terribly Sun-like), and the first planet around a Sun-like star just three years later. Fast forward two decades, and we now know of hundredsof such planets, and have thousands more detected that have to be confirmed (the data look good, but we still call them candidates until confirmation).

In fact, there are enough that the field of exoplanets is in the next step of the scientific process past discovery: categorization. We have enough known planets orbiting other stars that we can start to plop different labels on them: massive, big, small, orbiting hot stars, orbiting cool ones, having tight orbits or wide-sweeping ones. And once you can do that some very, very interesting things start to fall in place.

For example, you can use some statistics to extrapolate how many planets there must be in our galaxy. A new study has done just that, and the number they get is stunning: they calculate there may be a hundred billion planets in the Milky Way, with 17 billion of them the size of Earth!

First, let me point out that previous studies have gotten roughly the same numbers (for example, recently my pal John Johnson of Caltech was part of an announcement that the number of planets in the Milky Way was around a hundred billion, using one star and its system of five planets as a basis; it’s solid work). This new study is important because it uses more data and analyzes it more robustly than most previous work.

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