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In November 2013, it was announced that 22±8% of Sun-like [a] stars have an Earth-sized [b] planet in the habitable [c] zone. [9] [10] Regardless of the proportion of stars with planets, the total number of exoplanets must be very large. Since the Milky Way has at least 100 billion stars, it should also contain tens or hundreds of billions of ...
The Milky Way contains between 100 and 400 billion stars [12] [13] and at least that many planets. [150] An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than 300 ly (90 pc) from the Sun.
The stars with the most confirmed planets are the Sun (the Solar System's star) and Kepler-90, with 8 confirmed planets each, followed by TRAPPIST-1 with 7 planets. The 1007 multiplanetary systems are listed below according to the star's distance from Earth. Proxima Centauri, the closest star to the Solar System, has three planets (b, c and d).
A 2013 study by Ravi Kumar Kopparapu put η e, the fraction of stars with planets in the HZ, at 0.48, [4] meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. [120] However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered. [121]
This star, known as HD 110067, may have even more planets. The six found so far are roughly two to three times the size of Earth, but with densities closer to the gas giants in our own solar system.
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. [107] Most of the information we have about dwarf galaxies come from observations of the local group, containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies.
The precise nature of many of the other objects – and, perhaps, even planets – orbiting Sagittarius A*, as well as how they could have formed so close to the supermassive black hole, remain a ...
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.