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N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone); and N ∗ = Number of stars in the Milky Way Galaxy; f p = the fraction of those stars that have planets. n e = the average number of planets that can potentially support life per star that has planets.
Yellow dwarfs comprise the G-type stars of the main sequence, with masses between 0.9 and 1.1 M☉ and surface temperatures between 5000 and 6000 K, like the Sun. [2] [3] They are the third most common in the Milky Way Galaxy and the only ones in which the habitable zone coincides completely with the ultraviolet habitable zone. [2] [4]
The Milky Way [c] is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
As many as 15% of them could have Earth-sized planets in the habitable zones. [ 6 ] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way galaxy.
In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. [123] NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F , G , and K are expected to have ...
Listed below are some notable galaxies under 700,000 light-years in diameter, for the purpose of comparison. All links to NED are available, except for the Milky Way, which is linked to the relevant paper detailing its size.
The naked eye planets, which include Mercury, Mars, Jupiter, and Saturn, will not all become visible in Tennessee until around 5 a.m. Central Time, since Mercury and Jupiter are very low in the sky.
[5] [7] [8] A 2012 study of gravitational microlensing data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5 and 10 AU per star in the Milky Way.