There may be up to 20 billion habitable planets in our galaxy, scientists have said.
About one in five stars which researchers have been able to study for evidence of planets so far has an Earth-sized world within the so-called 'habitable zone'.
The conclusion? Planets like ours are a "relatively common" occurrence in the universe.
The 'habitable zone' is described as a distance far enough from the sun to have a surface temperature which could support liquid water.
So far Nasa's Kepler space telescope, which has scoured the skies for planets since its launch in 2009, has found evidence of these Earth-like planets in orbit around 22% of stars.
The team admits that not all of these planets will be able to hold life - variables such as their size, density and the age of the star all affect how possible it is that life could be sustained.
Artists' Conceptions Of Extrasolar Planets
NASA's Kepler Mission Discovers Planet(01 of05)
Open Image ModalIn this handout illustration made available on December 5, 2011 by NASA, the Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star is digitally illustrated. For the first time NASA's Kepler mission has confirmed a planet to orbit in a star's habitable zone; the region around a star, where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habit. Clouds could exist in this earth's atmosphere, as the artist's interpretive illustration depicts. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images) (credit:Getty)
NASA's Kepler Mission Discovers Planet(02 of05)
Open Image Modal In this handout illustration made available on December 5, 2011 by NASA, a diagram compares our own solar system to Kepler-22, a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. The diagram displays an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star and is about 2.4 times the size of Earth. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images) (credit:Getty)
Extrasolar Planet HD 209458 b, Osiris(03 of05)
Open Image ModalArtist's conception released by NASA of extrasolar planet HD 209458 b, also known as Osiris, orbiting its star in the constellation Pegasus, some 150 light years from Earth's solar system. Scientists have used an infrared spectrum -- the first ever obtained for an extrasolar planet -- to analyze Osiris' atmosphere, which is said to contain dust but no water. The planet's surface temperature is more than 700 Celsius (1330 Fahrenheit).' (credit:Getty)
Planet & Its Parent Star(04 of05)
Open Image ModalPicture released 04 October 2006 by the European Space Agency shows an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period. Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called 'hot Jupiters,' which are so close to their stars they complete an orbit within days, or even hours. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM (credit:Getty)
Hot Jupiter(05 of05)
Open Image ModalPicture released 04 October 2006 by the European Space Agency shows an artist's impression of a unique type of exoplanet discovered with the Hubble Space Telescope. This image presents a purely speculative view of what such a 'hot Jupiter' (word dedicated to planets so close to their stars with such short orbital periods) might look like. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM (credit:Getty)
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms," said Geoffrey Marcy, UC Berkeley professor of astronomy.
"We don't know what range of planet types and their environments are suitable for life."
But if their rough calculation holds true across the galaxy, the nearest Earth-like planet capable of sustaining life might be just 12 light years away.
"Human beings have been looking at the stars for thousands of years," study researcher Erik Petigura, a graduate student at the University of California, Berkeley, told Space.com.
"How many of those stars have planets that are in some way like Earth? We're very excited today to start to answer that question.
About 1,000 alien worlds have been confirmed so far using data from Kepler and other telescopes. While the Kepler telescope is now at the end of its mission after suffering damage to its rotator wheels, there is still mountains of data from the satellite with which scientists can find more planets.