The Milky Way estimate on stars may not be fully adjusted for the discovery in 2009 that the Milky way is 15% wider and 50% heavier than earlier estimates.
An extrapolation from the Kepler data from 6 months ago indicated that there was over 100 million planets in the habitable zone in the milky way galaxy
Goatguy had helped with an analysis that Kepler is still only detecting a small fraction of the planets and the farther out a planet is then the lower the odds of detection by the Kepler space telescope.
Kepler is still scheduled to operate for another year and a half and the data analysis will continue for several years. Kepler will be finding more planets among the 156,000 stars that it is observing.
The estimates for the number of stars in the milky way galaxy has not been adjusted for the more red dwarf stars and more brown dwarf stars.
I think over several years it will be confirmed that there are planets in the habitable zone of 10-80% of stars and planets around almost all stars. I think there will be a lot more brown dwarf stars discovered and the number of stars in the Milky Way will be more like one trillion stars. Even at that point we will still not be seeing or detecting everything in our galaxy.
AU DISCOVERY P PERIOD(DAYS) PLANET EXAMPLE 0.026 1 : 20 2 0.033 1 : 25 2 0.041 1 : 31 3 0.051 1 : 40 4 0.064 1 : 50 6 0.080 1 : 64 8 0.100 1 : 82 12 Inner Kepler 0.125 1 : 104 16 0.156 1 : 132 23 0.195 1 : 173 32 0.244 1 : 236 44 Outer Kepler 0.305 1 : 313 62 0.381 1 : 428 86 Mercury 0.477 1 : 590 120 0.596 1 : 846 168 0.745 1 : 1,250 235 Venus 0.931 1 : 1,900 328 1.000 1 : 2,280 365 Earth 1.164 1 : 3,000 458 1.455 1 : 5,200 641 Mars 1.819 1 : 9,200 895 2.274 1 : 18,000 1,251 2.842 1 : 40,000 1,749 Ceres 3.553 1 : 90,000 2,444 4.441 1 : 250,000 3,416 5.551 1 : 800,000 4,774 Jupiter
Point is - the probability of detection is very heavily weighted toward planets that are closer to their parent stars. For a planet at 0.1 AU (with an orbital period of about 12 days), the probability of detection exceeds 1%. Since the Kepler study has so far turned up a LOT of planets that are "shockingly close" to their parent star, one might reasonably ask, "so is our system unusual then?"
I quite easily can now see: "no". It just wouldn't be an easily detected system, is all. There's a 1:400 chance of detecting Mercury, 1:1,200 for Venus and 1:2,300 for Earth. It gets much less probable for the outer planets. Essentially have a 1:1,000,000 chance of detecting Jupiter.
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