Alien life could be much more common than previously thought: Scientists claim nine out of ten 'Twin Earths' in distant star systems could harbour complex life Aliens could be more common than thought, as nine out of ten Earth-like planets that orbit two distant stars could have the right conditions for life, a study has found. Known as binary stars, these systems make up about one in two solar systems — bringing the discovery of extra-terrestrial life a step closer. These binary systems are more likely to have Earth-like worlds with a titled planetary axis that undergoes only mild changes with time — a setup agreeable to life. Having a relatively stable axis helps to ensure that a planet's climate also remains stable. In contrast, planets like the Earth that have a stable axis and orbit a single star are much rarer, US researchers found. Astrophysicist Billy Quarles of the Georgia Institute of Technology and colleagues found that 87 per cent of so-called 'exo-Earths' — distant worlds similar in size to our planet — should have axial tilts as steady as the Earth's. 'Multiple-star systems are common and about 50 per cent of stars have binary companion stars,' said paper author and astrophysicist Gongjie Li. 'So, this study can be applied to a large number of solar systems.' In contrast, systems like our own with a single star and multiple planets appear to more rare across the universe, she noted. The mild changes in the Earth's axial tilt — which experts refer to as its 'obliquity' — have allowed for a climate that could support life and its evolution. On the other hand, the wild shift of Mars' axis may have helped wreck the red planet's atmosphere, the researchers said. The researchers used a model to see how the Earth's axis might behave if our planet was orbiting in the habitable — or 'Goldilocks' — zones that are just right for life around various star systems from across the universe. Professor Quarles and colleagues started with a neighbouring star system — Alpha Centauri AB — which lies four light years from the Earth, and has one star called 'A' and the other 'B.' 'We simulated what it would be like around other binaries with multiple variations of the stars' masses, orbital qualities, and so on,' explained Professor Quarles. 'The overall message was positive — but not for our nearest neighbour.' The outlook for an Earth-like planet orbiting Alpha Centauri A was not that bad, but the likelihood finding mild axis dynamics on a planet around star B was poor. This may douse the hopes of Breakthrough Initiatives' planned Starshot mission, which had planned to launch a solar-sail-driven probe to travel to the neighbouring star system and hunt for signs of life there. Astrophysicist Billy Quarles of the Georgia Institute of Technology and colleagues found that 87 per cent of so-called 'exo-Earths' — distant worlds similar in size to our planet — should have axial tilts as steady as the Earth's. Pictured, a diagram of the Earth highlighting the axial tilt Binary stars make up about one in two solar systems - bringing the discovery of extra-terrestrial life a step closer. Pictured, an artist's impression of a binary star system No exoplanets have been confirmed around A or B. There is one around the nearby red dwarf star, Proxima Centauri, but it is very likely to be uninhabitable. Despite its ice ages and heatwaves, Earth's overall climate has been relatively calm for hundreds of millions of years thanks to its axis-tilt, allowing life to take hold. This is the angle between the plane of its orbit and the equator. In contrast, these oscillations change wildly on Mars, which experts believe would regularly kill off any advanced life that might arise there. Earth's orientation only changes between 22.1 and 24.5 degrees over the course of 41,000 years. Our large moon stabilises the planet, otherwise it would be jolted out of sync by the gravity of Mercury, Venus, Mars and Jupiter. 'If we didn't have the moon, Earth's tilt could vary by about 60 degrees,' explained Dr Quarles. 'We would look maybe like Mars, and the precession of its axis appears to have helped deplete its atmosphere.' Binary systems are more likely to have Earth-like worlds with a titled planetary axis that undergoes only mild changes with time — a setup agreeable to life. In contrast, planets like the Earth that have a relatively stable axis and orbit a single star are much rarer. Pictured, an artist's impression of two planets orbiting a binary star system Mars' axis alters between 10–60 degrees every two million years. The former level of change leads to polar caps that lock up much of the atmosphere in ice. The latter, however, is severe enough to grow an ice belt around the equator. In Alpha Centauri AB, star B — about the size of our sun — and the larger star A orbit very close to one another. 'Around Alpha Centauri B, if you don't have a moon, you have a more stable axis than if you do have a moon. If you have a moon, it is pretty much bad news.' Even without a moon and with mild axis variability, complex life would have a hard time on the modelled exo-Earth around B. The biggest effect you would see is differences in the climate cycles related to how elongated the orbit is,' Dr Quarles said. 'Instead of having ice ages every 100,000 years — like on Earth — they may come every 1 million years, be worse and last much longer.' But when the researchers expanded the model to include binary star systems , the likelihood of gentle obliquity variations increased significantly. 'In general, the separation between the stars is larger in binary systems, and then the second star has less of an effect on the model of Earth,' said Professor Li. 'The planet's own motion dynamics dominate other influences, and obliquity usually has a smaller variation. So, this is quite optimistic.' The full findings of the study were published in The Astrophysical Journal. .