60-Second Science

Milky Way May Contain 100 Billion Planets

Based on exoplanet observations made using the Kepler space telescope, astronomers estimate that our galaxy's ratio of stars to planets is at least one to one. Karen Hopkin reports

[Carl Sagan speaking:] “There are in fact 100 billion galaxies, each of which contains something like a 100 billion stars.” Well, Carl Sagan would have loved to get the latest estimate of our galaxy’s planet count.

Caltech astronomers set their sites on a star called Kepler 32. It’s an M dwarf star, a class that’s smaller and cooler than our sun, and accounts for about three quarters of the stars in the Milky Way. But what’s really cool about this particular star, from the astronomers’ point of view, is that its five planets orbit in a plane that the Kepler telescope sees edge on. So the star’s light dims each time a planet passes between it and the scope, which makes the planets easier to detect.

Now, taking into account the percentage of M dwarf systems that lie in a similar edge on orientation, and the number of planetary systems the Kepler telescope has already detected, the researchers figure our galaxy is host to at least 100 billion planets. Their calculations are served up in The Astrophysical Journal. [Link to come.]

Many of those planets may be the size of Earth. But we’re the only planet that produced Carl Sagan.

—Karen Hopkin

[The above text is a transcript of this podcast.]

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