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Exoplanets around main sequence stars are being discovered in large numbers
An increasing number of extrasolar planet discoveries are being made with 3,621 planets in 2,712 planetary systems known as of 1 July 2017. Rare Earth proponents argue life cannot arise outside Sun-like systems. However, some exobiologists have suggested that stars outside this range may give rise to life under the right circumstances; this possibility is a central point of contention to the theory because these late-K and M category stars make up about 82% of all hydrogen-burning stars.
Current technology limits the testing of important Rare Earth Criteria: surface water, tectonic plates, a large moon and biosignatures are currently undetectable. Though planets the size of Earth are difficult to detect and classify, scientists now think that rocky planets are common around Sun-like stars. The Earth Similarity Index (ESI) of mass, radius and temperature provides a means of measurement, but falls short of the full Rare Earth criteria.
Rocky planets orbiting within habitable zones may not be rare
Some argue that Rare Earth's estimates of rocky planets in habitable zones are too restrictive. James Kasting cites the Titius-Bode law to contend that it is a misnomer to describe habitable zones as narrow when there is a 50% chance of at least one planet orbiting within one. In 2013 a study that was published in the journal Proceedings of the National Academy of Sciences calculated that about "one in five" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars"; 8.8 billion of them therefore exist in the Milky Way galaxy alone. On 4 November 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. 11 billion of these estimated planets may be orbiting sun-like stars.
In the past, astronomers have found that the cosmos is hierarchically assembled, with galaxies being arranged in clusters, superclusters, sheets, walls and filaments. These are separated by immense cosmic voids, which together create the vast “Cosmic Web” structure of the Universe.
The cosmic void that contains the Milky Way's is dubbed the Keenan, Barger and Cowie (KBC) void, after the three astronomers who identified it in the 2013 study. It is the largest cosmic void ever observed — about seven times larger than the average void, with a radius of about 1 billion light-years, according to the study.