In the unlikely event that you hadn’t heard, the extra Kepler results are out. I’d just like to highlight one bit:
- The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size – up to twice the size of Earth – to larger than Jupiter.
54 planets inside stellar habitable zones! Woo!
Also, in addition, the Kepler-11 system is pretty cool too. (Although these planets are probably less good candidates for life … Kepler-11 is pretty much as big as our Sun, and all of these planets orbit inside less than 0.5 AU, a combination which would tend to imply runaway greenhouse effects.)
In fact, while we’re on the subject, let’s do a quick bit of maths. 5 near Earth-sized planet-candidates in the habitable zones, and Kepler observes 156,000 stars. So, that’s one close Earth-analogue per 31,200 stars. Now, let’s make some hay with this ratio. Let’s assume this is perfectly representative of the entire galactic population (haha! – yes, that’s an enormous assumption*).
There are something like 100 billion or so stars in the galactic disk. So, these numbers imply 100,000,000,000 * (1/31,200) = 3,205,128 Earth-analogues across the whole galaxy.
Three million. That’s a lot – a lot! – more than I would have expected. Wow!
In fact, let’s make a bit more hay with it.
Let’s assume the galactic disk is about 100 parsecs deep and 100,000 pc across. (Round numbers, folks, round numbers!) That gives us a volume of 7.8 x 10^11 cubic parsecs (I’m ignoring the bulge and the halo here – you wouldn’t expect to find many inhabitable planets in either). Now, 3.2 million planets means an average of 245,000 cubic parsecs per planet. That sounds huge, but that’s a volume, not a distance. If we assume that these are spherical volumes, then we get a surprise.
The average separation would be about 39 parsecs, or ~127 light years.
Again, that’s a lot closer than I would have expected. If you’d asked me yesterday what distance the next inhabitable planet was, I’d have guessed hundreds or even thousands of parsecs.
*Also, you’ll note I’m neglecting Earth and Glieses 581 g, c and d here.