They’re the most common star in the universe. We know of hundreds of rocky exoplanets orbiting them. And as a result, red dwarf stars are prime targets for scientists’ ongoing search for life in the universe.
But David Kipping, an astronomy professor at Columbia University, is not convinced red dwarfs are quite so ripe for alien life.
“It’s just a question that has always perplexed me,” Kipping tells Inverse. “If they’re so numerous, so long-lived, potentially trillions of years, and so they really seem to have everything going for them ... it's kind of odd then that we don’t live around a red dwarf.”
In a new paper, Kipping presented the “red sky paradox,” the curious question where red dwarf stars rank the highest for potentially habitable star systems, and yet here we are, orbiting a larger yellow star.
The paper, published Monday in the journal Proceedings of the National Academy of Sciences, presents resolutions to the paradox and explores whether life on Earth is an outlier or a common occurrence based on the habitability of the worlds that orbit around red dwarf stars.
WHAT’S NEW — The “red sky paradox” refers to the idea that red dwarf stars are the most common type of star in the universe, outnumbering stars like the Sun by five times, and could potentially sustain life on the planets that orbit them. But so far, our searches for life around them are coming up empty.
“Then when we looked up into the sky, we would see a red dwarf disk and we would also see more of a red tinge to the sky itself,” Kipping says. “It's a little bit poetic but it's sort of a play on the idea that we should expect to live around these stars.”
In order to resolve the red sky paradox, Kipping and his team came up with a few resolutions.
- Chance — Humans are the “freaks of the universe” with a one in 100 probability of being born around a yellow star
- Circumstance — Something inherent to red dwarf stars inhibits the development of complex life
- Lifespan — Red dwarf stars last trillions of years; perhaps they have prolonged adolescences, taking far longer for them to develop conditions that would allow for life to flourish on their orbiting planets
The first resolution, Kipping argues, is not very satisfying.
“It kind of runs into tension with this famous thing called the cosmological principle which essentially argues that our view of the universe is typical, and where we live is a completely ordinary place in the universe,” Kipping says. “But now we're saying, actually we live in a highly irregular place in the universe.”
The paper leans more towards the idea that red dwarf stars do not make for habitable star systems.
A number of factors play into this:
- Red dwarf stars tend to have highly energetic flareups
- The planets that orbit around them tend to be tidally locked, meaning that one side of the planet is constantly facing the star (and hence getting the brunt of those flares)
- Gas giant planets such as Jupiter rarely form around red dwarf stars — and in our solar system, Jupiter’s massive gravity keeps large asteroids from bullying their way into the inner solar system
If the resolution of the red sky paradox is that red dwarf stars do not make for habitable planets, then that has implications for the Fermi paradox. The Fermi paradox, named after Italian-American physicist Enrico Fermi, is the contradiction between the high probability that life exists elsewhere in the universe and the lack of evidence of said extraterrestrial life.
Since red dwarf stars are the most common type of star in the universe, their non-habitability would mean that life is much less common throughout the universe than scientists may have initially believed.
If scientists were to assume that red dwarfs could sustain life on their orbiting planets, then it would exacerbate the Fermi paradox.
“Now there are basically five times more stars which can have life that you wouldn’t have had previously,” Kipping says. “So you have to now have five times better answers to why aliens haven’t visited us.”
HERE’S THE BACKGROUND — Red dwarf stars make up the largest group of stars in the universe, and they are also the longest living stars out there. These stars tend to be dim and have lower temperatures, therefore burn through their hydrogen slowly. This gives them a lifespan several magnitudes longer than any other type of star.
The nearest star to our star system is a red dwarf, Proxima Centauri. The star is located around 4.25 light years away from Earth and is orbited by two known exoplanets. One of the exoplanets, Proxima Centauri b, orbits within the star’s habitable zone, and some scientists believe it’s our best bet on finding habitability beyond Earth.
But if Kipping is right, then our search for life around Proxima Centauri will likely turn up empty.
WHY IT MATTERS — As NASA prepares to launch the James Webb Telescope in late October, some of its prime targets will be red dwarf stars.
“And so if these stars are basically incapable of having life around them, that seems like something we should probably figure out before we start investing huge amounts of resources into the assumption that they do have life,” Kipping says.
Understanding the potential habitability of these red dwarf stars also helps scientists better understand our own star, and whether the Sun is a typical star throughout the universe or if it possesses some special ingredient that made it possible for life to exist on Earth.
Abstract: Most stars in the Universe are red dwarfs. They outnumber stars like our Sun by a factor of 5 and outlive them by another factor of 20 (population-weighted mean). When combined with recent observations uncovering an abundance of temperate, rocky planets around these diminutive stars, we are faced with an apparent logical contradiction—Why do we not see a red dwarf in our sky? To address this “red sky paradox,” we formulate a Bayesian probability function concerning the odds of finding oneself around an F/G/K-spectral type (Sun-like) star. If the development of intelligent life from prebiotic chemistry is a universally rapid and ensured process, the temporal advantage of red dwarfs dissolves, softening the red sky paradox, but exacerbating the classic Fermi paradox. Otherwise, we find that humanity appears to be a 1-in-100 outlier. While this could be random chance (resolution I), we outline three other nonmutually exclusive resolutions (II to IV) that broadly act as filters to attenuate the suitability of red dwarfs for complex life. Future observations may be able to provide support for some of these. Notably, if surveys reveal a paucity of temperate rocky planets around the smallest (and most numerous) red dwarfs, then this would support resolution II. As another example, if future characterization efforts were to find that red dwarf worlds have limited windows for complex life due to stellar evolution, this would support resolution III. Solving this paradox would reveal guidance for the targeting of future remote life sensing experiments and the limits of life in the cosmos.