This trait of red dwarf star systems could help us solve the red sky paradox: ScienceAlert

There is something very special about the Earth, apart from all the organisms crawling on it. It’s our star, the Sun, it’s weird: it’s a yellow dwarf.

Sun-like stars are a minority in the Milky Way. We think that less than 10 percent stars in our galaxy are G-type stars, like the Sun.

The most abundant stars are those that we cannot even see with the naked eye: the red dwarfs. They are only about half the mass of the Sun, are cold, dark, and have the longest lifespan of any star.

These stellar lightweights represent up to 75 percent of all the stars in the Milky Way. One could therefore think, statistically, that if life were to emerge somewhere, it would be on a planet around a red dwarf.

Yet here we are, with our yellow Sun. This gap between expectation and reality is known as the Red Sky Paradox, and scientists have yet to figure it out.

A new paper, accepted in Letters from the Astrophysical Journal and uploaded to arXiv preprint server as it undergoes peer review and publication, might have a clue.

Basically, it looks like it might be a lot harder for life as we know it to get started in red dwarf planetary systems – because they don’t have the architecture of asteroids and gas giants to provide 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 defined as “potentially habitable” are often found orbiting red dwarf stars.

Red dwarfs, in some ways, are among the most promising targets in our search for habitable worlds. Because they are so small, they burn their hydrogen fuel much slower than Sun-like stars.

They can hang around for potentially trillions of years – far longer than the estimated 10 billion year lifespan of the Sun and even the 13.8 billion year age of the Universe. This means there is more time available for life to emerge and thrive.

Red dwarfs also represent an opportunity for our current detection methods. Because they burn so slowly, they are cooler and less luminous than the Sun. This means that the habitable zone – the range of distance from the star in which habitable temperatures can be found – is much closer. Recently, astronomers discovered an exoplanet in the habitable zone of a red dwarf star with an orbit of just 8.4 days.

But it seems that the emergence and continued existence of life might be a tricky thing.

Previous studies suggested that red dwarfs might not present the most hospitable environment. For example, these stars tend to be very active, frequently emitting flares that radiate to all nearby planets.

The authors of the new paper – astronomers Anna Childs, Rebecca Martin and Mario Livio of the University of Nevada, Las Vegas – wanted to find out if red dwarf systems had enough of the ingredients we believe sparked life on Earth. .

Current studies suggest that the bombardment of asteroids and comets relatively late in the solar system’s youth altered the earth’s crust in ways that made it more hospitable to life and provided many necessary chemical ingredients.

Without an asteroid belt, therefore, systems for terraforming and delivering chemicals for life are greatly reduced.

The models suggest that the formation of a stable asteroid belt and the late asteroid bombardment require the presence of a gas giant beyond a distance from the star known as the snow line, at the beyond which the volatile compounds condense into solid ice. Indeed, such a gas giant can gravitationally interact with the asteroid belt, causing instabilities that throw asteroids toward the habitable zone.

So the researchers looked at red dwarf systems to see if they could find any of these gas giants.

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 this group, 16 have mass measurements for exoplanets in the system.

Defining a gas giant as a planet whose mass is between 0.3 and 60 times the mass of Jupiter and calculating the position of the snow line for these systems, the team went in search of gas giants.

They found that none of the systems with a rocky Earth-like planet in the habitable zone also had a known gas giant.

Statistically, the team calculated that there is a population of giant exoplanets orbiting red dwarf stars beyond the snow line. This means that, theoretically, red dwarf stars can have asteroid belts.

It’s just that none of the known red dwarf systems with rocky habitable zone worlds are likely to fall into this category, suggesting that the architecture of the red dwarf planetary system may be very different from the solar system that we know and love.

There are a lot of assumptions involved. For example, maybe asteroid impacts aren’t that big. Perhaps life on red dwarf exoplanets doesn’t resemble life on Earth at all. Perhaps we overestimate the importance of the habitable zone.

However, based on our current knowledge and understanding of life, things are not looking very good for the red dwarf planets.

“The lack of giant planets in observed systems (so far) containing habitable zone exoplanets suggests that these systems are unlikely to harbor an asteroid belt and the mechanism required for asteroid delivery to an advanced stage in the habitable zone”, researchers write.

“Therefore, if asteroid impacts are indeed necessary for life, the planets observed in the habitable zone are unlikely to support life.”

And, in turn, that could be at least part of the reason why our home planet doesn’t orbit one of those pesky little red stars.

The search was accepted in Letters from the Astrophysical Journal and is available at arXiv.

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