There's something very strange about Earth, aside from all the organisms crawling all over it. It's our star, the Sun, that's unusual: It's a yellow dwarf.
Sun-like stars are a minority in the massive Milky Way. It's calculated that fewer than 10 percent of the stars in our galaxy are G-type stars, like the Sun.
The largest stars are those we can't even see with the naked eye: red dwarfs. They're only up to about half the mass of the Sun, cool, dim, and with the longest duration of time of any stars.
These stellar lightweights consider for up to 75 percent of all the stars in the Milky Way. One would therefore think, statistically, that if life were to appear anywhere, it would be on a planet around a red dwarf.
Yet here we are, with our bright yellow Sun. This difference between expectation and reality is known as the Red Sky Paradox, and physicists have yet to figure it out.
Basically, it seems like it might be much difficult for life as we know it to get started in red dwarf planetary systems – because they lack the asteroid and gas massive architecture to deliver the ingredients for life to Earth-like worlds.
The outcomes could have suggestions for our search for life outside the Solar System, especially since exoplanets defined as "potentially habitable" are often observed in orbit around red dwarf stars.
Red dwarfs, in some respects, are some of the most favorable targets in our search for habitable worlds. Because they are too small, they burn through their hydrogen fuel much more slowly than Sun-like stars do.
Red dwarfs also represent an opportunity for our current detection techniques.
Because they burn so slowly, they are much cooler and fainter than the Sun. This means that the habitable zone – the distance extend from the star in which habitable temperatures can be found – is much closer. Recently, astronomers detected an exoplanet in the habitable zone of a red dwarf star with an orbit of just 8.4 days.
But it seems like life's emergence and continued existence might be a difficult thing.
Models suggest that the creation of a stable asteroid belt, and late asteroid bombardment, needs the presence of a gas giant beyond a distance from the star known as the snow line, beyond which explosive compounds condense into solid ice.
This is because such a gas giant can gravitationally have interaction with the asteroid belt, causing instabilities that pelt asteroids inwards towards the habitable zone.
So, the researchers looked at red dwarf systems to see if they could detect one of these gas giants.
There are currently 48 red dwarf stars with detected, rocky exoplanets revolving in the habitable zone. Of these dwarf stars, 27 have more than one exoplanet. Of that group, 16 dwarf stars have mass measurements for the exoplanets in the system.
However, based on our latest information and understanding of life, things aren't looking well for red dwarf planets.
And, in turn, that might be at least partially why our home planet isn't revolving one of these cranky small red stars.
The research has been approved into The Astrophysical Journal Letters and is available on arXiv.
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