News of new extrasolar planet discoveries keeps coming, fast and furious. It’s hard to keep track of the latest. There are some very strange planets out there. But what really interests most people, it seems, is how many planets out there are like Earth. Apparently there’s this deep-seated desire to to find places in the universe that seem familiar.
Unfortunately, that’s not easy. In the first place, Earth is a relatively small planet, as are most rocky planets, in contrast to larger “gas giant” planets. It is these small, rocky planets that should be most like Earth, especially in terms of allowing for “life as we know it”. Although there are several different methods of detecting extrasolar planets, they’re all more likely to notice larger planets than smaller ones.
Secondly, only planets that orbit in a narrow range of distances from their host star are neither too hot nor too cold to permit the existence of liquid water and a decent atmosphere on the planet’s surface. The size of the range depends on the size and (relatedly) the brightness of the star. For stars that are reasonably close in size to our own, it’s easier to detect any kind of planet closer to the star than farther out. Usually too close to be in the habitable zone.
For these two reasons alone, present extrasolar planet searches are likely to seriously undercount Earthlike planets in a star’s habitable zone. However, by analyzing the data already accumulated on extrasolar planets around stars similar in size to the Sun, and making a reasonable assumption, the data can be extrapolated to suggest roughly how many planets similar to Earth are out there, even though we can’t yet detect most of them.
The assumption is that the pattern of existence and location for smaller planets is similar to the pattern for larger planets we can detect. With that assumption, the conclusion stated in the title here is at least plausible.
What interests most astronomers is how many exoplanets orbit at a greater distance, inside the habitable zone. Most of these planets are too far away from their stars to have been picked up by Kepler yet. But Traub says his data analysis provides a way to work out how many their ought to be.
That’s because he’s found a power law that describes how the number of stars with a given orbital period. So all he has to do is assume a longer orbital period equivalent to being in the habitable zone to work out how many planets there ought to be at this distance.
Here’s the answer: “About one-third of FGK stars are predicted to have at least one terrestrial, habitable-zone planet,” he says.