Statistics Suggest Billions of Earths in the Milky Way


Statistical studies of solar systems are giving us encouraging results about the frequency of Earth-sized planets in our Galaxy.  But what does it mean?

A survey of 166 sun-like stars using the Keck Telescope in Hawaii has shown an interesting trend (graph below).  As we look at planets of lower and lower masses, a higher percentage of the stars observed possess them.

Let’s note a few things about this graph.  Firstly, the scales are logarithmic: this means that the tick marks are multiples of 10.  This also means that the “bin size” (i.e. what range of masses fit in every bar) is quite big.  The second thing is the big question mark on the lowest mass bar.  We can’t really detect planets effectively at these masses yet (although the Kepler Space Telescope will change things by the end of the year!).  The last bar is an extrapolation – it is an assumption based on a model which fits the rest of the data.  This is not cheating in any sense – it’s an appropriate measure to take in the absence of reliable data (made doubly appropriate by the large question mark they ingenuously stamp on the bar).  If this extrapolation is correct, then out of the 200 billion stars in our galaxy, at least 46 billion Earth-size (ish) planets exist in orbit around them.  While it’s been commonly thought among astronomers that lower mass planets are more common than higher mass planets, this is an unusual attempt to do a statistically accurate survey of our local neighbourhood to study this.  Plus, there are details in this study which are in conflict with theory (in particular, an overabundance of warm Super Earths) which beg important questions that need answering.

This study might not let us directly see these countless Earth analogues, but they’re pointing us towards them.  We are on the cusp of a new revolution of exoplanet science, where the Earth mass regime is waiting to show itself.  We might catch the first glimpses as soon as December, when the Kepler Space Telescope announces its first science results.

I, for one, cannot wait!

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