Hypothesis: Does the Evolution of Complex Life Depend on the Star Type?

Earth is the only known example of a planet with life, so the history of life on Earth provides the only information regarding the timescale required for microscopic, single-celled life to evolve into bigger and more complex forms similar to plants, animals, or fungi. This process took about four billion years from the cooling of early Earth through today. But does this four billion year timescale apply when thinking about life on other planets?

It is certainly possible that complexity, on average, takes about four billion years to develop on any planet that already has life. If this were the case, then astronomers should search a wide range of stars (yellow dwarfs like our sun as well as cooler orange and red dwarf stars) because any of them might already have complex life. Although we really have no idea at all, this idea of “equal evolutionary time” is sometimes invoked by scientists as a default assumption: since we don’t know anything else, why not assume an average timescale of four billion years?

I suggest an alternative viewpoint to this assumption in a paper entitled “Does the evolution of complex life depend on the stellar spectral energy distribution?” and published in Astrobiology. In this paper, I present the hypothesis that the evolutionary timescale is constrained by the total energy that falls upon a planet and could actually be harnessed by life. Instead of assuming a fixed four billion year timescale, I calculate the amount of time it would take planets around different star types to accumulate the same amount of free energy that Earth received over its history. This assumption of “proportional evolutionary time” suggests that complex life on planets orbiting yellow dwarf stars like our sun might also take about four billion years to develop; however, planets around orange dwarf stars would take much longer, closer to five or ten billion years. And, following this hypothesis, planets orbiting red dwarfs would need a hundred billion years (longer than the present age of the universe) before they accumulated enough energy for complex life.

This idea remains a hypothesis until astronomers are able to search for signs of life around extrasolar planets. But this idea of proportional evolutionary time suggests that the coolest stars might not be the best places to look for complex life today.

Inferring the Climates of Red Dwarf Planets

Planets orbiting red dwarf stars are unique compared to other star systems because such planets are prone to falling into synchronous rotation, so that one side experiences perpetual day and the opposite side resides in permanent night. Such planets could still be habitable, sustaining liquid water and perhaps even life, so such systems continue to be targeted in the search for signs of life on exoplanets.

One starting point to looking for life on such worlds is to infer properties of an exoplanet climate from astronomical data. Eric Wolf, Ravi Kopparapu, and myself examine this problem in a paper titled “Simulated phase-dependent spectra of terrestrial aquaplanets in M dwarf systems” and published in The Astrophysical Journal. Infrared emission and reflected stellar light from a planet changes as it orbits its host star. We should that observations of these orbital changes in thermal energy could provide important information on the circulation state of the planet, the location of major cloud decks, and the abundance of water vapor. As the next generation of space telescope are designed and launched, methods such as these will become important tools for understanding the potential of M-dwarf systems to support life.

Greenhouse Warming on Earth’s Past, Present, and Future

Carbon dioxide in Earth’s atmosphere provides an important regulator of climate. Without it, or with too little, Earth would be completely frozen. But the rapid rise in carbon dioxide in recent times due to fossil fuel consumption and changes in land use has caused unprecedented warming with consequences to human civilization.

Understanding how Earth’s climate responds to atmospheric carbon dioxide is an important problem not only for anthropogenic climate change today but also for understanding Earth’s distant past (when the sun was fainter than today) and distant future (when the sun becomes brighter than today). In a paper by Eric Wolf, Brian Toon, and myself titled “Evaluating climate Sensitivity to CO2 across Earth’s history” and published in Journal of Geophysical Research – Atmospheres, we calculate the expected warming for early-, modern-, and future-Earth scenarios across a much wider range of carbon dioxide levels than typically considered for present-day climate change. We show that a doubling of atmospheric carbon dioxide would have caused a greater amount of warming on early Earth (when the carbon dioxide fraction of the atmosphere was high) compared to today (when carbon dioxide is a trace constituent). In general the amount of warming to be expected from such a carbon dioxide doubling (known as the “climate sensitivity”) depends upon the amount of solar energy received, the starting carbon dioxide budget, and the mean temperature of the planet.

Paying for Space Settlement

Bold visions for the human settlement of Mars, and perhaps beyond, would require unprecedented technical management over successive generations. Space settlement can also only succeed with an uninterrupted and sufficient supply of resources, as the unique dangers of space would make food, water, and even breathable air scarce and expensive resources. Gradually building the infrastructure of a permanent space settlement would also be costly, and unlikely to provide any direct financial benefits to investors in a timely manner.

I recently published a chapter titled “Can deep altruism sustain space settlement?” in the book The Human Factor in a Mission to Mars. In this chapter I explore the idea of “deep altruism,” where a donor is concerned not with benefits to themselves or kin but to distant future descendants for whom they may have no direct connection. Similar to a time capsule or other grand construction or big science projects in human history, space settlement may provide benefits to the distant future that are not easily identifiable today. Wealthy individuals, foundations, and corporations interested in the permanent human settlement of space may find the idea of investing in humanity’s long-term future to be valuable and worthwhile, even if the return on investment is not immediately obvious.

Commercial interests will likely be an important factor as space industries expand. But investment in space infrastructure out of deep altruism can complement market forces in establishing a sustainable human presence in space.