There is something very strange about Earth, apart from all the living things that are crawling all over it. It’s our star, the Sun, that’s weird: it’s a yellow dwarf.
Sun-like stars are a minority in the Milky Way. It is estimated that less than 10 percent of the stars in our galaxy are G-type, like the Sun.
The most abundant stars are those we can’t see with the naked eye: red dwarfs. It’s only about half the mass of the Sun, cold, dimmer, and has the longest lifespan of any star.
These stellar light weights account for up to 75 percent of all stars in the Milky Way. So one might think, statistically, that if life appeared anywhere, it would be on a planet around a red dwarf.
And here we are with our yellow sun. This discrepancy between expectation and reality is known as the Red Sky Paradox, and scientists have yet to discover it.
New paper, accepted in Astrophysical Journal Letters And uploaded to the arXiv server before printing while it is undergoing peer review and publication, it can have proof.
Basically, it looks like it might be much more difficult for life as we know it to get started in red dwarf planetary systems – because they lack the gigantic architecture of asteroids and gases to deliver the ingredients for life to Earth-like worlds.
The findings could have implications for our search for life outside the solar system, especially since exoplanets that have been defined as “habitable” are often found in orbit around red dwarf stars.
Red dwarfs are, in some ways, some of the most promising targets in our search for habitable worlds. Because they are so small, they burn through hydrogen fuel much more slowly than Sun-like stars.
It can hang around it for trillions of years—much longer than the Sun’s estimated 10 billion year lifespan and even the universe’s 13.8 billion year age. This means there is more time available for life and you are likely to thrive.
Red dwarfs also represent an opportunity for our current detection methods. Because it burns very slowly, it is cooler and fainter than the sun. This means that the habitable zone – the distance from the star at which habitable temperatures can exist – is much closer. Astronomers recently discovered an exoplanet in the habitable zone of a red dwarf star with an orbit of only 8.4 days.
But it seems that the emergence and continued existence of life can be difficult.
Previous studies have suggested that red dwarfs may not represent the most hospitable environment. For example, these stars tend to be very active, erupting frequently with flares that would strike any nearby planets with radiation.
The authors of the new research — astronomers Anna Childs, Rebecca Martin, and Mario Livio of the University of Nevada, Las Vegas — wanted to determine if red dwarf systems contained enough of the components that we think started life on Earth.
Current studies indicate that the bombardment of asteroids and comets relatively late in the youth of the solar system changed the Earth’s crust in ways that made it more habitable and provided many of the chemical components needed for it.
Therefore, without the asteroid belt, recovery systems and lifelong chemical delivery are significantly reduced.
Models suggest that the formation of the stable asteroid belt, and the late bombardment of asteroids, require the presence of a gas giant far from the star known as the snow streak, after which the volatile compounds condense into solid ice. This is because such a gas giant can interact gravitationally with the asteroid belt, causing instability that flings asteroids inward toward the habitable zone.
So researchers looked at red dwarf systems to see if they could find one of these gas giant planets.
There are currently 48 red dwarf stars with confirmed rocky exoplanets orbiting in the habitable zone. Of these, 27 have more than one exoplanet. Of that group, 16 have mass measurements of the exoplanets in the system.
The team identified the gas giant as a planet between 0.3 and 60 times the mass of Jupiter and calculated the location of the snow line for those systems, and the team went looking for the gas giants.
They found that none of the systems with a rocky Earth-like planet in the habitable zone has a known gas giant as well.
Statistically, the team calculated, there is a group of giant exoplanets orbiting red dwarf stars beyond the snow line. This means, in theory, that red dwarf stars could have asteroid belts.
It’s just that none of the known red dwarf systems with rocky worlds in the habitable zone are likely to fall into this category, suggesting that the structure of the red dwarf planetary system could be very different from the Solar System we know and love.
There are a lot of assumptions in the game. For example, asteroid impacts are probably not that important. Perhaps life on red dwarf exoplanets does not look like life on Earth. Perhaps we overestimate the importance of the habitable area.
However, based on our current knowledge and understanding of life, things don’t look great for red dwarf planets.
“The absence of giant planets in (to date) the observed systems containing exoplanets in the habitable zone indicates that these systems are unlikely to contain the asteroid belt and the mechanism required to deliver late-stage asteroids to the habitable zone,” the researchers wrote.
“Therefore, if asteroid impacts are indeed necessary for life, the planets observed in the habitable zone are unlikely to harbor life.”
And that, in turn, may be at least partly why our home planet doesn’t orbit one of these strange little red stars.
The search has been accepted Astrophysical Journal Letters It is available on arXiv.
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