One third of planets circling red dwarf stars in our galaxy could be in the ‘habitable zone’ – and may host alien life, study claims

Finding life on planets outside our own is one of the biggest quests for astronomers.

Now, a new study suggests the Milky Way galaxy has hundreds of millions of promising targets to probe for signs of life outside our solar system. 

Using NASA’s Kepler telescope, researchers studied a small sample of planets that orbit red dwarfs – low mass stars that are common in our galaxy. 

They found that a third of the planets – equating to hundreds of millions in the Milky Way overall – likely have the right conditions for hosting life.  

The new study was conducted by experts at University of Florida and published in the journal the Proceedings of the National Academy of Sciences. 

‘I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,’ said study author Sarah Sagear.

‘These stars are excellent targets to look for small planets in an orbit where it’s conceivable that water might be liquid and therefore the planet might be habitable.’

Our Milky Way galaxy is estimated to contain anywhere from 100 billion to 400 billion stars – and at least that number of planets. 

Interestingly, only around 20 per cent of stars in the Milky Way galaxy are like our sun, making it a ‘relative rarity’, according to the researchers.

By far the most common stars are red dwarfs, which are considerably smaller and cooler and have just half the mass of our sun at most. 

Red dwarfs make up the bulk of the Milky Way’s population of stars – nearly 75 per cent, according to estimates – and are about the size of Jupiter. 

For their study, the researchers used new data from NASA’s Kepler telescope, which would capture information about exoplanets while moving in front of their host stars – known as ‘transiting’. This is an artist impression of the Kepler Space Telescope that was decommissioned by NASA in 2018 after nearly a decade of service

What’s more, billions of planets orbit these common dwarf stars in our galaxy, making them a particular target for astronomers. 

For their study, the Florida team measured the ‘eccentricity’ of orbits of a sample of more than 150 planets around red dwarf stars in the Milky Way.  

To measure the planets’ orbits, the researchers focused especially on how long the planets took to move across the face of the stars – known as ‘transiting’. 

Their study also relied on new data from the European Space Agency’s Gaia telescope, which measured the distance to billions of stars in the galaxy.

‘The distance is really the key piece of information we were missing before that allows us to do this analysis now,’ said Sagear. 

To capture enough warmth to be habitable, planets need to huddle very close to their red dwarf stars, which leaves them susceptible to extreme tidal forces, the astronomers say. 

In their analysis based on the telescope data, the team discovered that two thirds of the planets around red dwarfs stars could be roasted by these tidal extremes, sterilising them. 

But that leaves one-third of the planets – equating to hundreds of millions across the galaxy – that could be in a ‘goldilocks’ orbit close enough, and gentle enough, to hold onto liquid water and possibly host life.

They also found that stars with multiple planets were the most likely to have the kind of circular orbits that allow them to retain liquid water.

Meanwhile, stars with only one planet were the most likely to see tidal extremes that would sterilise the surface.

The researchers say their findings have ‘implications for planetary formation and follow-up observations’. 

New telescopes such as the James Webb will now be key to identifying water – a key sign of life – on ‘exoplanets’ (planets outside our solar system).