Does Intelligence Depend on Star Color?

Our civilization on Earth resides around a yellow dwarf star, but orange and red dwarf stars are much more numerous in the galaxy. Does this suggest that we should orient the search for extraterrestrial intelligence (SETI) toward these smaller and redder star systems? Or are certain star types better abodes for intelligent life than others?

Myself and Ravi Kopparapu address these questions in a chapter titled, “The Drake equation as a function of spectral type and time,” which appears in the book Habitability of the Universe Before Earth. Our approach invokes the Drake equation, which is a probabilistic device for estimating the number of communicative civilizations in the galaxy. The conventional terms of the Drake equation are the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, the probability of life, the probability of intelligence, the probability of communicative technology, and the average lifetime of such civilizations. The product of all these terms yields the number of civilizations in the galaxy that SETI could conceivably discover.

Our book chapter analyzes calculations of the liquid water habitable zone for plants orbiting yellow, orange, and red dwarf stars. These habitable zone calculations provide constraint on the terms of the Drake equation. In particular we consider the dependence of each term of the Drake equation on both the stellar type (color) of the star, as well as the change in each parameter over time since the galaxy formed. We suggest that the habitability of red dwarf systems may peak in the far future, while the present time is optimal for habitability around yellow and orange dwarf stars.

Water Loss on Planets Orbiting Low-Mass Stars

An Earth-like planet tends to increase its water vapor content as its mean temperature increases. The inner edge of the habitable zone is defined by the point at which such a planet begins to lose its water, thus rendering it uninhabitable. A “moist greenhouse” occurs when the (usually dry) upper atmosphere becomes wet, which results in the destruction of water molecules by incoming sunlight. Another process knows as a “runaway greenhouse” occurs due to the increased greenhouse effect of water vapor in the lower atmosphere, which further drives evaporation and more warming. Either of these processes could cause a planet at the inner edge of the habitable zone to lose its oceans entirely.

In a recent paper published in The Astrophysical Journal, titled “Habitable Moist Atmospheres On Terrestrial Planets Near the Inner Edge Of the Habitable Zone Around M-dwarfs,” my co-authors and I conduct three-dimensional climate simulations of planets orbiting low-mass stars. We show that planets near the inner edge of the habitable zone should generally first enter a moist greenhouse state, although planets around the coolest stars we analyzed should directly transition into a runaway greenhouse state instead. Some of these planets orbiting low-mass stars could experience very slow water loss that could last up to the lifetime of the star, which could allow habitable conditions to persist even during a moist or runaway greenhouse.

Snowball and Greenhouse Atmospheres Around Other Stars

Earth’s climate has been shown by a wide range of climate models to be bistable, which means that it can exist in both a frozen state or a warm, ice-free state with the same amount of solar energy. Earth today resides in a warm state with small ice caps, but geologic evidence suggests that the Neoproterozoic Earth, about 500 Myr ago, may have been frozen all the way down to the equator.

Other terrestrial planets orbiting other stars should behave similarly, although the transition point between climate states may differ for stars that are brighter and dimmer than the sun. In a recent paper published in The Astrophysical Journal, titled “Constraints on climate and habitability for Earth-like exoplanets determined from a general circulation model,” my co-authors and I analyze three-dimensional climate simulations of planets orbiting a range of stars. We show that planets can exist around such stars as a frozen snowball, partial melt with an equatorial waterbelt, temperature conditions everywhere, and a hothouse with gradual water loss. Stars slightly cooler than the sun could maintain habitable conditions for longer periods of time, due to the slower rate of water loss.

Why do we live around a yellow star?

Red dwarf stars outnumber yellow dwarf stars like our sun by over a factor of ten. Observations of exoplanets have also shown that rocky, and potentially habitable, planets are just as common around red dwarfs as yellow dwarfs. But if these much smaller stars are more commonplace, then why do we find ourselves around a yellow star like the sun, instead of a red dwarf?

My co-authors and I attempt to address this question of selection bias in a recent paper titled “Why do we find ourselves around a yellow star instead of a red star?” and published in International Journal of Astrobiology. We take a statistical approach to thinking about the region around all stars where life is most likely to develop. The liquid water habitable zone provides the best observational constraint on where we would expect to find planets that could support conscious observers like us, and this study examines the probability of finding oneself on a planet in the habitable zone of a yellow dwarf star, compared to a red dwarf. The results show that even though red dwarfs are much more numerous, they have a narrower habitable zone than yellow dwarfs, so our existence around a star like the sun is actually to be expected.

This study also considers that red dwarf stars will be even more numerous in the distant future of the universe, due to their much longer lifetimes than other stars. If these red dwarf stars will eventually become the predominant place for conscious observers to develop, then why do we not instead find ourselves around a red dwarf star billions or trillions of years into the future? The statistics for this aspect of the problem suggest that our existence around a yellow dwarf star today, compared to a red dwarf star in the future, might be a slight statistical anomaly—perhaps on the order of finding oneself born ambidextrous or with perfect pitch. But this statistical unlikelihood might also suggest that life is wholly impossibly around red dwarf stars, or else any type of conscious observers that do develop around such stars will be drastically different from our type of conscious life.