Red dwarf stars - the stellar runts of the galaxy - probably aren't so great for nurturing Earth-like worlds, say scientists running new simulations of the formation of planets around a variety of stars.
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Astronomers have discovered a huge variety of alien worlds orbiting all types of stars, but one type of star, the M dwarf, has stuck out as one location where Earth-like exoplanets could be nurtured. Red dwarfs are plentiful in our galaxy and many nearby red dwarfs are known to play host to small and (likely) rocky exoplanets. Red dwarfs are long-lived and should there be a suitable rocky world orbiting within the star system's habitable zone, surely there's a good chance for life to evolve?
Unfortunately, there's some limitations with this thinking. As the habitable zone around M dwarfs is extremely compact (these stars are smaller and therefore dimmer than our sun, for example), any potentially habitable world would need to orbit a red dwarf very closely. In these cases, the rocky world will likely become "tidally locked" with the star, forcing one hemisphere to endure an eternal day, while the other freezes in an eternal night - certainly not very "Earth-like" in the classical sense.
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Also, many of these dwarfs are thought to be tumultuous little stars, erupting with powerful flares that could extinguish any form of biology (as we know it) before it can get a foothold. Already red dwarfs are looking a little unlikely as hosts for bona fide Earth-like worlds.
But as the vast majority of stars in our galaxy are red dwarfs, surely, just because there are so many, a few of these red dwarf star systems have formed with the right balance of planetary material and perfectly-located orbits that worlds similar to Earth have been able to coalesce?
According to new research from Tokyo Institute of Technology and Tsinghua University, not so much.
The researchers, Shigeru Ida (Tokyo Tech) and Feng Tian (Tsinghua), first defined what "Earth-like" means. For an alien world to meet this classification, it needs to be of the approximate mass and physical size as Earth. It also needs to have a similar water:land mass ratio. Exoplanets with too little water are considered "dune worlds" - with too little water for Earth-like biology to take hold - and those with too much water are known as "water worlds" - with chaotic climates and poor nutrient supply.
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Ida and Tian found that, for an Earth-like exoplanet, with just the right water:land ratio to form around an M dwarf, there needs to be an extremely unlikely balance of planetary material very early in that world's formative years.
As red dwarfs form pre-main sequence, they are extremely luminous, blasting out vast quantities of energy. They then quickly settle down into a more quiescent, cooler state. For any would-be Earth-like exoplanet forming in a habitable orbit, the early years of a red dwarf's life are hard. Due to their close proximity, these worlds would be cooked, with extreme implications for their future habitability.
After running their simulation for 1000 stars 30 percent the mass of our sun (the approximate mass of M dwarf stars), 5,000 exoplanets with Earth-like masses formed. However, only 55 of those planets formed within the stars' habitable zones and only 1 formed with the "perfect" Earth-like water:land ratio. 31 habitable zone planets turned into "dune worlds" and 23 turned into "water worlds."
So far, things are looking bleak for red dwarfs hosting Earth-like planets, but the story changes as the stars become more massive.
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For example, for 1,000 stars 50 percent the mass of our sun, 75,000 exoplanets formed and 9,000 of them had Earth-like masses. 292 of those worlds formed inside the habitable zone and 12 were truly Earth-like.
But after simulating 1,000 sun-like stars, 38,000 exoplanets formed, 8,000 of them of Earth-mass. Although this may be a lower number, 407 formed inside the habitable zone and 271 of those formed with just the right water:land ratio, making sun-like stars the most likely place to form Earth-like exoplanets.
Although these are simulations, they take their lead from established planetary formation theories and will ultimately help guide future exoplanet-hunting missions. But things aren't looking good for red dwarfs being particularly good locations for Earth-like planets, and therefore life, to form.
"We suggest that stars close to the size of the sun should be the primary targets for detecting Earth-like planets," the researchers conclude.