Alien Worlds Beyond Our Solar System - Full Documentary HD<br />Alien Earth | Life Beyond Earth <br />One of eight new planets spied in distant solar systems has usurped the title of "most Earth-like alien world", astronomers have said.<br />All eight were picked out by Nasa's Kepler space telescope, taking its tally of such "exoplanets" past 1,000.<br />But only three sit safely within the "habitable zone" of their host star - and one in particular is rocky, like Earth, as well as only slightly warmer.<br />The find was revealed at a meeting of the American Astronomical Society.<br />Red sky<br />The three potentially habitable planets join Kepler's "hall of fame", which now boasts eight fascinating planetary prospects.<br />And researchers say the most Earth-like of the new arrivals, known as Kepler 438b, is probably even more similar to our home than Kepler 186f - which previously looked to be our most likely twin.<br />At 12% larger than Earth, the new claimant is bigger than 186f but it is closer to our temperature, probably receiving just 40% more heat from its sun than we do from ours.<br />So if we could stand on the surface of 438b it may well be warmer than here, according to Dr Doug Caldwell from the Seti (Search for Extra-Terrestrial Intelligence) Institute in California.<br />"And it's around a cooler [red dwarf] star... so your sky would look redder than ours does to us," Dr Caldwell said.<br />That first-person encounter, however, is unlikely - both because the planet is 475 light-years away and because we still have essentially no idea what it's made of.<br />Images from the Kepler telescope, which trails behind the Earth and peers far into the distance as we orbit our own sun, are used to identify far-off planets by observing "transits".<br />This refers to the dimming of a star's light when a planet passes in front of it. A large team of researchers then uses additional data from Earth-bound telescopes to further explore these unfamiliar solar systems.<br />They try to calculate how big the planets are, and how closely they orbit their host stars.<br />Not everything that causes such a dimming eventually turns out to be a planet, however.<br />At the same time as the eight confirmed new exoplanets were announced by a 26-strong team spanning Nasa and multiple US institutions, the Kepler mission's own scientists released another tranche of more than 500 "candidate" planets.<br />"With further observation, some of these candidates may turn out not to be planets," said Kepler science officer, Fergal Mullally.<br />"Or as we understand their properties better, they may move around in, or even outside, the habitable zone."<br />'Star Trek' scenario<br />Even once scientists have anointed a candidate as a confirmed exoplanet, the question of whether or not it is "Earth-like" is a fraught one, with fuzzy boundaries.<br />The size of the habitable, or "Goldilocks" zone, where a planet is far enough from its sun to hold water but not so distant that it freezes, depends on how confident scientists want to be with their guess-work.<br />According to Dr Cardwell, just three of the eight new exoplanets can be confidently placed in that zone - and only two of those are probably rocky like the Earth.<br />More detailed description is very difficult. <br />"From the Kepler measurements and the other measurements we made, we don't know if these planets have oceans with fish and continents with trees," Dr Caldwell told BBC News.<br />"All we know is their size and the energy they're receiving from their star.<br />"So we can say: Well, they're of a size that they're likely to be rocky, and the energy they're getting is comparable to what the Earth is getting.<br />"As we fill in these gaps in our solar system that we don't have, we learn more about what it means to be Earth-like, in some sense."<br />Speaking at a related event at the conference, Prof Debra Fischer from Yale University said she remembered a time before the first exoplanet was discovered, more than two decades ago.<br />"I remember astronomers before that point being very worried," she said.<br />"We really had to
