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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.
originally posted by: Vratyas
"Astronomers have added 219 candidates to the growing list of planets beyond our solar system, 10 of which may be about the same size and temperature as Earth, boosting their chances of hosting life.
Scientists found the candidates in a final batch of Nasa’s Kepler Space Telescope observations of 200,000 sample stars in the constellation Cygnus.
They include 10 newly discovered rocky worlds that are the right distance from their parent stars for water, if it exists there, to pool on the surface. Scientists believe liquid water is a key ingredient for life.
An important question for us is, ‘Are we alone?’” Kepler program scientist Mario Perez said in a conference call with reporters. “Maybe Kepler today is telling us indirectly ... that we are not alone.”
As of January 2015, Kepler and its follow-up observations had found 1,013 confirmed exoplanets in about 440 star systems, along with a further 3,199 unconfirmed planet candidates.[B] Four planets have been confirmed through Kepler's K2 mission. In November 2013, astronomers estimated, based on Kepler space mission data, that there could be as many as 40 billion rocky, Earth-size exoplanets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. It is estimated that 11 billion of these planets may be orbiting Sun-like stars. The nearest such planet may be 3.7 parsecs (12 ly) away, according to the scientists.
originally posted by: EasyPleaseMe
a reply to: eriktheawful
There is another factor in the Rare Earth hypothesis - the protection afforded by the gas giants, sweeping up potentially deadly asteroids which lowers the chance of impact with the earth.
A planet without this sort of protection, even if ideal in every other way, may receive too many extinction level impacts to develop intelligent life.
originally posted by: JanAmosComenius
In my opinion there is another factor bounded to size and composition of Earth like planet: Plate tectonics.
Live in relatively stable environments tends to create stable ecosystems. Stable ecosystems means lower rate of evolutionary changes. Too fast and vast changes of environment are on other hand also bad. Other factor is depletion of nutrients from environment by biological and sedimentary processes.
Steady pace of environmental changes is on Earth guaranteed by plate tectonics and subsequent volcanizm. Not only is surface environment replenished with material from Earth's mantle but new sedimentary material is constantly forged on tectonic boundaries, spitted out by volcanoes or slowly squeezed out to surface hundreds of millions of years later. This (with life itself) creates vast diversity of constantly changing ecosystems. It is constant pressure needed for evolution to really kick in.
Plate tectonics is probably detrimental for rise of technological civilization: accessible deposits of almost all raw materials are created by this process.