There are around 300 million planets that exist outside our Solar System but within the Milky Way which could potentially harbour life, according to NASA.
Four of them are within just 30 light-years from Earth, with the closest just 20 light-years away, the space agency states.
Researchers arrived at the 300 million figure based on the conservative assumption that seven per cent of Sun-like stars have a habitable world orbiting them.
However, they warn the true figure could be as high as 75 per cent, which would see the 300 million figure jump to around three billion.
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Pictured, Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone of a Sun-like star
Data from Kepler, the deep-space telescope which was retired in October 2018 when it finally ran out of fuel, led to the discovery.
Kepler’s mission was originally earmarked to last 3.5 years but stringent use of fuel enabled it to remain operational for 9 years, 7 months and 23 days.
Its mission was to scour the skies, both near and far, in the search for planets orbiting other stars.
Kepler discovered more than 2,600 worlds beyond our solar system and statistically proved that the Milky Way is home to more planets than stars.
It was made of 42 image sensors called charged coupled devices (CCDs) and each one had a resolution of 1,024 by 2,200 pixels.
The data from Kepler is so vast that astronomers are still trawling through its reserves to this day and publishing new findings.
NASA astronomers did just this, with their work recently appearing in the The Astronomical Journal.
‘Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable,’ said the lead author Steve Bryson, a researcher at NASA’s Ames Research Center in California.
‘Though this result is far from a final value, and water on a planet’s surface is only one of many factors to support life, it’s extremely exciting that we calculated these worlds are this common with such high confidence and precision.’
The researchers set some parameters for the data, narrowing their field of research to planets similar in size to Earth, either half the diameter of 50 per cent as large.
By doing this it ensures the planets observed will be rocky as it is the much larger planets that tend to be gaseous, just like the Solar System’s Jupiter and Saturn.
Kepler’s mission was originally earmarked to last 3.5 years but stringent use of fuel enabled it to remain operational for 9 years, 7 months and 23 days. Its mission was to scour the skies, both near and far, in the search for planets orbiting other stars. Pictured, the telescope before it launched
There are around 300 million planets that exist outside our Solar System but within the Milky Way which could potentially harbour life. According to research from NASA, four of them are within just 30 light-years from Earth, with the closest just 20 light-years away, NASA claims
They went in search of the Holy grail of astronomical research, scouring for the ‘Goldilock’s Zone’ where life may thrive.
One prerequisite of this is for the star to resemble the Sun, so the researchers looked for distant stars that resemble it in age and temperature.
But this simplified approach is no guarantee life will thrive and makes many assumptions, researchers acknowledge, so the astronomers cross-referenced this data with that of the ESA’s Gaia telescope.
Gaia measures the energy output of individual stars, which provides valuable insight into if it is emitting too much harmful radiation or too little thermal energy for water to survive a a liquid, for example.
‘We always knew defining habitability simply in terms of a planet’s physical distance from a star, so that it’s not too hot or cold, left us making a lot of assumptions,’ said Ravi Kopparapu, an author on the paper and a NASA scientist.
‘Gaia’s data on stars allowed us to look at these planets and their stars in an entirely new way.’
Gaia allowed astronomers to look at how a planet’s atmosphere would impact on its habitability.
This analysis led to the figure of 50 per cent of Sun-like stars having habitable planets.
A conservative estimate sinks as low as seven per cent, whereas an optimistic valuation can go as high as 75 per cent.
Kepler was the first spacecraft to survey the planets in our own galaxy, and over the years its observations confirmed the existence of more than 2,600 exoplanets – many of which could be key targets in the search for alien life
RIP Kepler: NASA ‘retires’ planet-hunting spacecraft
NASA’s planet-hunting Kepler spacecraft died in orbit in October 2018 after a historic nine-year mission that first opened our eyes to the existence of planets beyond our solar system.
The space agency revealed the grim news in a teleconference this afternoon after the veteran craft put itself into ‘sleep mode’ earlier this month.
The ageing telescope was expected to run out of fuel sometime in the near future – but, exactly when this would happen was unclear.
According to NASA, the now-retired spacecraft is in a ‘safe orbit, away from Earth.’
Kepler, which launched back in 2009, came to be known by its team as ‘the little spacecraft that could,’ going above and beyond the expectations NASA had for it.
It was the first spacecraft to survey the planets in our own galaxy, and over the years its observations confirmed the existence of more than 2,600 exoplanets – many of which could be key targets in the search for alien life.
Before Kepler, we’d never found any planets outside of our solar system.
‘As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond,’ said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.