All Your Planets Are Belong to Us

Planets, planets everywhere
and on some there will be drops to drink.

An interesting confluence of research occurred over the last few months, leading to:

"A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around Kepler M-dwarfs" by Ravi kumar Kopparapu PSU (arXiv), ApJLetters in press

Bottom line: about half of low mass stars in the Solar neighbourhood are estimated to have earth size planets in their habitable zone!

The implications are that habitable planets may be very common and the closest one within a few light years.
We'll know for sure very soon.

Schematic of the low star mass habitable zone and current planetary candidates

From PSU Press Release

Last month, two papers were released:

"The Occurrence Rate of Small Planets around Small Stars" by Courtney Dressing and David Charbonneau (arXiv) ApJ in press

This is a very nice analysis of Kepler data looking at the occurrence rate of small (Earth size-ish) planets, and hence a statistical analysis of how many such planets we might expect to find around nearby stars.
The data is restricted to short orbital period planets around low mass stars, because that is where the observations are currently sensitive.
The extrapolations of the data to sun like stars are interesting to say the least, and will be constrained by actual data in the very near future.

The press release quoted an occurrence rate in the Habitable Zone of 6% which is impressive.
The paper has since been modified to predict a higher occurrence rate of 15%.

The change in the prediction came from Ravi Kopparapu, who noticed a discrepancy in the original data which implied a higher occurrence rate.

Ravi, in turn, had a paper out at about the same time: "Habitable Zones Around Main-Sequence Stars: New Estimates" by Kopparapu et al (arXiv) ApJ in press - which updated the classic habitable zone calculations of Kasting and collaborators, based on improved atmospheric opacities and stellar spectral energy distributions.
The new habitable zone estimates, which are conservative underestimates because they do not include all known feedback effects that can stabilize habitable zones, are broader than the previous estimates.

Which leads to Ravi's current paper.

Taking Dressing and Chardonneau's methodology, with the revised occurrence estimate, and applying it to the new improved habitable zone calculations, the occurrence rate of Earth size planets in the habitable zones of low mass stars is about 50%!

ie if we look at the 10 or so known low mass stars within about 10 light years of the Sun, we expect essentially all of them to have low mass planets, and there should be about 5 roughly Earth size planets within the habitable zone of their parent star.
Note that this includes the possibility that some stars may have 2 or 3 small planets within its habitable zone.

This is a fabulous result, and has huge implications for the possibilities of life in the universe, and observing strategies for targeting biosignatures on exoplanets in the near future.

The occurrence numbers are converging on this relatively high fraction, and becoming consistent when different detection techniques are compared.
Further, both the estimates and the definitions of what is habitable try to stick to the conservative side.
There could be more habitable planets than this estimate.

Again, the extrapolation to the occurrence rate for Earth size planets around Sun like stars is also very interesting.
Interestingly high.

PSU Press Release

Handy-dandy updated habitable zone calculator