Why We Don’t Live Around a Red Star: Understanding Why M-Dwarfs May Be Unlikely Homes for Observers

David Kipping investigates why humanity exists around a Sun-like star, rather than one of the much more common, smaller, and longer-lived M-dwarfs. Despite making up about 75% of all stars and having many planets in their habitable zones, M-dwarfs appear not to host observers like us, a mystery known as the “Red Sky Paradox.” Kipping argues that this is not a coincidence and uses a statistical model to explore possible reasons why.

Setting Up the Problem

Kipping begins by noting two peculiar facts about our existence. First, life on Earth arose very early in the Universe’s long “stelliferous era,” the time when stars are still burning, which is expected to last trillions of years. Second, despite the overwhelming number of M-dwarfs, our Sun is a G-type star, uncommon compared to the majority. To explain this, Kipping explores two possibilities: (1) that stars below a certain mass threshold, are unable to produce observers (the “desolate M-dwarf hypothesis”), or (2) that planets only remain suitable for observers for a limited time, regardless of their star’s long lifetime (the “truncated window hypothesis”).

The Bayesian Framework

To test these ideas, Kipping builds a Bayesian model, an approach that allows him to compare how well different hypotheses explain our single data point: the existence of observers around a Sun-like star at this time in cosmic history. The model simulates the formation and evolution of a million stars, drawing their birth times and masses from well-established astrophysical distributions. By tracking which stars could host observers over time, Kipping calculates how likely it would be to find ourselves orbiting a Sun-like star under different assumptions.

For the simulation, he assumes that life takes about 3 billion years to evolve from the birth of a star (as it did on Earth) and that stars more massive than the Sun die too quickly to host observers. He then evaluates how many potential observer-hosting stars exist at different cosmic times for various cutoff masses and lifetimes, allowing the probabilities to be compared across scenarios.

Results: The Role of Star Mass and Time Windows

The model reveals that simply shortening the time window for life cannot explain why we live so early in the Universe and around a G-type star. Only by introducing a lower mass cutoff, excluding M-dwarfs, does the model match our observed situation. In the combined (“joint”) model, Kipping finds that stars below about (0.45 M⊙​) are unlikely to host observers, with an even stricter cutoff of (0.34 M⊙​) when assuming a 10-billion-year observer window (comparable to Earth’s expected geophysical lifespan). This excludes roughly two-thirds of all stars as potential homes for intelligent life.

Kipping compares several models statistically and finds that the “desolate M-dwarf” hypothesis explains the data much better than random chance. The Bayesian evidence ratio, known as the Bayes factor, shows that pure luck is about 1,600 times less likely to explain our situation. Models relying only on time truncation are also strongly disfavored.

Beyond Chance: The Implications

The conclusion is striking: it is probably not a coincidence that we live around a Sun-like star. If M-dwarfs rarely produce observers, then our Sun’s characteristics may be essential for complex life. Kipping suggests that conditions around M-dwarfs, such as their intense radiation, tendency to strip planetary atmospheres, and prolonged early flaring, might make them inhospitable even for simple life. Observations from the James Webb Space Telescope of planets like TRAPPIST-1b, c, and e, which seem to lack atmospheres, support this idea.

This work also carries implications for the search for extraterrestrial intelligence (SETI). Since M-dwarfs may not host observers, focusing on stars between about 0.7 and 1.6 times the Sun’s mass could be a more effective strategy.

Final Reflections

Kipping closes by reflecting on the limits of statistical reasoning in such a unique problem. Bayesian inference can show how our beliefs should change given data, but it cannot determine prior assumptions about how common observers should be. Still, his analysis provides strong evidence that our Sun’s properties are not accidental, and that, at least for beings like us, solar-type stars may indeed hold a special place in the cosmos.

Source: Kipping