There is something very peculiar about the earth, apart from all the organisms crawling all over it. It is our star, the Sun, that is strange: It is a yellow dwarf.
Sun-like stars are a minority in the Milky Way. It is estimated that fewer than 10 percent of the stars in our galaxy are G-type stars, like the Sun.
The most numerous stars are those that we cannot even see with the naked eye: red dwarfs. They are only up to about half the mass of the Sun, cool, faint and with the longest lifetimes of any star.
These starlights account for up to 75 percent of all the stars in the Milky Way. One would therefore, statistically speaking, believe that if life were to appear anywhere, it would be on a planet around a red dwarf.
But 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 figure it out.
A new paper, accepted in The Astrophysical Journal Letters and uploaded to the preprint server arXiv while it undergoes peer review and publication, may have a clue.
Initially, it appears that it may be much more difficult for life as we know it to get started on red dwarf planetary systems – because they lack the asteroid and gas giant architecture to deliver the ingredients for life to Earth-like worlds.
The results may 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 are in some ways some of the most promising targets in our search for habitable worlds. Because they are so small, they burn through their hydrogen fuel much more slowly than sun-like stars do.
They can hang around for potentially trillions of years—much longer than the estimated 10 billion-year lifespan of the sun and even the universe’s 13.8 billion years. This means that there is more time available for life to emerge and potentially thrive.
Red dwarfs also represent an opportunity for our current detection methods. Because they burn so slowly, they are cooler and weaker than the Sun. This means that the habitable zone – the range of distance from the star where 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 only 8.4 days.
But it seems that the emergence and continued existence of life can be a difficult thing.
Previous studies have suggested that red dwarfs may not be the most hospitable environment. For example, such stars tend to be very active, often erupting with flares that will blast any nearby planets with radiation.
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 that we think started life on Earth.
Current studies suggest that asteroid and comet bombardment relatively late in the solar system’s youth altered the Earth’s crust in ways that made it more hospitable to life and supplied many of the chemical ingredients it needed.
Without an asteroid belt, therefore, the terraforming and chemical delivery system for life is significantly reduced.
Models suggest that the formation of a stable asteroid belt, and late asteroid bombardment, requires the presence of a gas giant beyond a distance from the star known as the snowline, beyond which volatile compounds condense into solid ice. This is because such a gas giant can gravitationally interact with the asteroid belt, causing instabilities that throw asteroids inward towards the habitable zone.
So the researchers looked at red dwarf systems to see if they could find one 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 that group, 16 have mass measurements for the exoplanets in the system.
By defining a gas giant as a planet between 0.3 and 60 times the mass of Jupiter and calculating the location 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 red dwarf stars could theoretically have asteroid belts.
It’s just that none of the known red dwarf systems with rocky worlds in habitable zones are likely to be in that category, suggesting that the architecture of the red dwarf planet system may be very different from the solar system we know and love.
There are many assumptions that come into play. For example, asteroid impacts may not be that important. Maybe life on red dwarf exoplanets doesn’t look like 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 good for red dwarf planets.
“The lack of giant planets in the (so far) observed systems containing exoplanets in habitable zones suggests that these systems are unlikely to have an asteroid belt and the mechanism required for late-stage asteroid delivery to the habitable zone,” the researchers write.
“Therefore, if asteroid impacts are indeed necessary for life, the observed planets in the habitable zone are unlikely to have life.”
And in turn, that may be at least partially the reason our home planet doesn’t orbit one of these tiny little red stars.
The research is accepted The Astrophysical Journal Letters and is available on arXiv.