There may be as many as six BILLION Earth-like planets in our galaxy capable of hosting alien life, new astronomical model reveals
- Scientists provide new estimates based on data from the Kepler space telescope
- There may be one Earth-like planet for every five Sun-like stars in our Milky Way
- Our galaxy has 400 billion stars of which 7 per cent have masses like our Sun
Scientists have estimated that there are as many as six billion Earth-like planets in our galaxy capable of hosting alien life.
Canadian astronomers claim there’s likely one Earth-like planet – with rocky terrain capable of hosting water and life – for every five Sun-like stars in the Milky Way.
The exciting new estimates come from data from NASA’s now retired Kepler telescope, which spent nearly a decade in space looking for Earth-size planets orbiting other stars.
To be considered Earth-like, a planet must be rocky, roughly Earth-sized and orbiting a Sun-like stars, know as a ‘G-star’.
‘G-stars’ have between 0.84 and 1.15 solar masses (one solar mass is the mass of our Sun) and surface temperatures of between 5,000 and 6,000 kelvins.
There may be as many as one Earth-like planet for every five Sun-like stars in the Milky Way galaxy, according to new estimates by University of British Columbia astronomers using data from NASA's Kepler mission. Pictured is an artist’s conception of Kepler telescope observing planets transiting a distant star
Our own existence is evidence that there is almost certainly life on other planets in our galaxy, as the likelihood of Earth being a ‘one-off’ is almost zero.
But distinguishing rocky planets that could support life from those that are simply freezing balls of gas is no mean feat.
As of the start of this month, there are 4,268 confirmed exoplanets – planets that are outside our Solar System – in 3,154 star systems, 698 of which have more than one planet.
However, the majority of these exoplanets are gaseous, like Jupiter or Neptune, rather than terrestrial, according to NASA's online database.
To be considered Earth-like, planets also have to orbit in the 'habitable zones' of their host star – at an optimal distance to support liquid water and host life.
Most of the known exoplanets were found by Kepler, which identifies planets by analysing minuscule brightness changes that occur when a planet transits across the face of its host star.
This is an artist impression of the Kepler Space Telescope that was decommissioned by NASA in 2018 after nearly a decade of service
‘My calculations place an upper limit of 0.18 Earth-like planets per G-type star,’ said study co-author Michelle Kunimoto at the University of British Columbia (UBC) in Canada, who published her research in The Astronomical Journal.
'Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets.’
Our Milky Way has as many as 400 billion stars, 7 per cent of which are classified as G-type, according to the research team.
'This means less than six billion stars may have Earth-like planets in our galaxy,’ said UBC astronomer and study co-author Jaymie Matthews.
Previous estimates vary from around 0.02 to more than one potentially habitable planets per Sun-like star.
Typically, planets like Earth are more likely to be missed by a planet search that other types of celestial bodies, because they are relatively small and orbit so far from their stars.
That means that observable planets represent only a small subset of the planets that are actually in orbit.
To overcome this issue and provide more accurate estimates, Kunimoto used an analytical technique known as 'forward modelling'.
‘I started by simulating the full population of exoplanets around the stars Kepler searched,’ she said.
‘I marked each planet as “detected” or “missed” depending on how likely it was my planet search algorithm would have found them, then I compared the detected planets to my actual catalogue of planets.
‘If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars.’
The majority of exoplanets found so far are gaseous, like Jupiter or Neptune, rather than terrestrial, which are exciting to scientists due to their potential to be home to alien life. Pictured is an artist's impression of a rocky exoplanet much like Earth
In February, UBC revealed Kunimoto had discovered an impressive 17 exoplanets, including a potentially habitable, Earth-sized world, also based on data from Kepler, which was decommissioned by NASA in 2018.
Officially named KIC-7340288 b, this planet is just one and a half times the size of Earth – small enough to be considered rocky, instead of gaseous like the giant planets of the Solar System – and in the habitable zone of its star.
Its year is 142.5 days long, orbiting its star at 0.444 astronomical units (AU, the distance between Earth and our Sun) – just bigger than Mercury’s orbit in our Solar System.
'This planet is about a thousand light years away, so we’re not getting there anytime soon!’ said Kunimoto at the time.
‘But this is a really exciting find, since there have only been 15 small, confirmed planets in the Habitable Zone found in Kepler data so far.’
Sizes of the 17 new planet candidates, compared to Mars, Earth, and Neptune. The planet in green is KIC-7340288 b, a rare rocky planet in the Habitable Zone, also discovered by UBC astronomer Kunimoto
Earlier this year, another team of scientists reported a second planet orbiting the star closest to our Solar System, Proxima Centauri.
Observations of light readings from Proxima Centauri revealed a low-mass candidate planet orbiting around it, approximately half the size of Neptune.
Data suggests this planet candidate – named 'Proxima c' – completes an orbit of Proxima Centauri every 5.2 years.
WHAT IS THE KEPLER TELESCOPE?
The Kepler mission has spotted thousands of exoplanets since 2014, with 30 planets less than twice the size of Earth now known to orbit within the habitable zones of their stars.
Launched from Cape Canaveral on March 7th 2009, the Kepler telescope has helped in the search for planets outside of the solar system.
It captured its last ever image on September 25 2018 and ran out of fuel five days later.
When it was launched it weighed 2,320 lbs (1,052 kg) and is 15.4 feet long by 8.9 feet wide (4.7 m × 2.7 m).
The satellite typically looks for 'Earth-like' planets, meaning they are rocky and orbit within the that orbit within the habitable or 'Goldilocks' zone of a star.
In total, Kepler has found around 5,000 unconfirmed 'candidate' exoplanets, with a further 2,500 'confirmed' exoplanets that scientists have since shown to be real.
Kepler is currently on the 'K2' mission to discover more exoplanets.
K2 is the second mission for the spacecraft and was implemented by necessity over desire as two reaction wheels on the spacecraft failed.
These wheels control direction and altitude of the spacecraft and help point it in the right direction.
The modified mission looks at exoplanets around dim red dwarf stars.
While the planet has found thousands of exoplanets during its eight-year mission, five in particular have stuck out.
Kepler-452b, dubbed 'Earth 2.0', shares many characteristics with our planet despite sitting 1,400 light years away. It was found by Nasa's Kepler telescope in 2014
1) 'Earth 2.0'
In 2014 the telescope made one of its biggest discoveries when it spotted exoplanet Kepler-452b, dubbed 'Earth 2.0'.
The object shares many characteristics with our planet despite sitting 1,400 light years away.
It has a similar size orbit to Earth, receives roughly the same amount of sun light and has same length of year.
Experts still aren't sure whether the planet hosts life, but say if plants were transferred there, they would likely survive.
2) The first planet found to orbit two stars
Kepler found a planet that orbits two stars, known as a binary star system, in 2011.
The system, known as Kepler-16b, is roughly 200 light years from Earth.
Experts compared the system to the famous 'double-sunset' pictured on Luke Skywalker's home planet Tatooine in 'Star Wars: A New Hope'.
3) Finding the first habitable planet outside of the solar system
Scientists found Kepler-22b in 2011, the first habitable planet found by astronomers outside of the solar system.
The habitable super-Earth appears to be a large, rocky planet with a surface temperature of about 72°F (22°C), similar to a spring day on Earth.
4) Discovering a 'super-Earth'
The telescope found its first 'super-Earth' in April 2017, a huge planet called LHS 1140b.
It orbits a red dwarf star around 40 million light years away, and scientists think it holds giant oceans of magma.
5) Finding the 'Trappist-1' star system
The Trappist-1 star system, which hosts a record seven Earth-like planets, was one of the biggest discoveries of 2017.
Each of the planets, which orbit a dwarf star just 39 million light years, likely holds water at its surface.
Three of the planets have such good conditions that scientists say life may have already evolved on them.
Kepler spotted the system in 2016, but scientists revealed the discovery in a series of papers released in February this year.
Kepler is a telescope that has an incredibly sensitive instrument known as a photometer that detects the slightest changes in light emitted from stars
How does Kepler discover planets?
The telescope has an incredibly sensitive instrument known as a photometer that detects the slightest changes in light emitted from stars.
It tracks 100,000 stars simultaneously, looking for telltale drops in light intensity that indicate an orbiting planet passing between the satellite and its distant target.
When a planet passes in front of a star as viewed from Earth, the event is called a 'transit'.
Tiny dips in the brightness of a star during a transit can help scientists determine the orbit and size of the planet, as well as the size of the star.
Based on these calculations, scientists can determine whether the planet sits in the star's 'habitable zone', and therefore whether it might host the conditions for alien life to grow.
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
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