Kepler, Again

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.


2 Responses to “Kepler, Again”

  1. That’s potentialy a lot of habitable and habited planets. Considering how tenacious life is on this planet I wouldn’t be at all suprised if there weren’t at least bacteria out there some where clinging grimly hanging on to life.

    On another note I’m looking at things like exoplanets as potential subject matter for the major project which I’ll be having to put forward a proposal for in the next few weeks.

    • In fact, I realised after posting this that the numbers here are actually a *lower* limit on the total, because of course Kepler is only sensitive to things that transit! (And that means lie on a line of sight with us, which the vast majority of exoplanets won’t.)

      This is a very exciting time for exoplanets work right now.

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