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.

Limits to Growth

Human population continues to grow, with recent United Nation projections estimating over 11 billion people by 2100. Likewise, global energy use continues to grow at an exponential rate as we all seek higher standards of living. Technology continues to increase resources and reduce costs for everyone, but can this growth in population and technology continue indefinitely?

One of the first scientists to examine this question was Sebastian von Hoerner, a radio astronomer who conducted most of his research at the Green Bank Observatory in West Virginia. Von Hoerner argued in 1975 that continued growth of energy consumption on Earth would eventually start to contribute direct heating to the planet. (This is a consequence of the conservation of energy and is a separate issue from the emission of fossil fuels.) Even if technology is able to continually lower costs, we will eventually reach a limit to growth where our technology itself starts to warm the planet.

In a paper titled “Population growth, energy use, and the implications for the search for extraterrestrial intelligence,” part of the Futures special issue on the Detectability of Future Earth, Brendan Mullan and I update von Hoerner’s approach to calculate limits to population and energy growth. We demonstrate that Earth could conceivably support up to 20 billion people by optimizing current farmland or up to 100 billion people if all available land were dedicated to agriculture. These limits would require everyone to adopt a strict vegetarian diet and a life of poverty, so increasing the average standard of living would decrease the total carrying capacity. We also show that direct thermal heating of the planet from increased energy use could occur in the 2300’s to 2400’s if energy growth continues at a rate of about 2% per year.

If our civilization ever reaches this point, then our energy consumption as a civilization will equal the total energy Earth receives from the Sun. If such an endpoint is possible and sustainable, then any advanced extraterrestrial civilizations may already have achieved such an energy-intensive state. If we do eventually discover that energy-intensive civilizations are commonplace enough in the galaxy, then we can have greater confidence that our own future will survive any transitions as we approach limits to growth. But if energy-intensive civilizations are rare, or if we are the only ones, then our challenge for the future is even greater. The long-term success of civilization on Earth depends upon how we manage our population and energy growth over subsequent generations.

The Risk of Transmitting to Space

The idea of messaging to extraterrestrial intelligence (METI) suggests that a possible way to establish contact with civilizations on other planets is to first send transmissions ourselves. The search for extraterrestrial intelligence (SETI) has traditionally followed a passive listen-only mode to detect any alien transmissions headed our way. If everyone is listening and nobody is transmitting, then METI might be the way to attract attention.

But is attracting attention from extraterrestrial civilizations necessarily good? We have no idea if contact with extraterrestrial beings would benefit or harm humanity, or even be completely neutral in its impact. Some scientists are unconcerned about possible risks and suggest that METI transmissions should occur whenever they are viable. Others worry that METI transmissions could expose Earth to significant risk and argue in favor of a moratorium on METI activities.

I recently published a paper titled “Policy options for the radio detectability of Earth” in the Futures special issue on the Detectability of Future Earth. In this paper, I argue that the METI risk problem cannot be conclusively decided until contact with extraterrestrial intelligence actually occurs. This implies that any moratorium on METI activities cannot be based on the requirement for new information, as the only new information that would actually suffice is the actual discovery of alien life. Following from this conclusion, there are three possible policy options for proceeding with SETI and METI:

  1. Precautionary malevolence – alien contact is likely to be harmful, so we should not engage in METI until SETI succeeds.
  2. Assumed benevolence – alien contact is likely to be helpful, so we should engage in METI along with SETI.
  3. Preliminary neutrality – alien contact is unlikely to occur at all, so we may as well do SETI and METI if funds are available.

All three of these policies remain viable options until we actually discover extraterrestrial intelligence and learn the actual risks to humanity. Precautionary malevolence would imply that human civilization should reduce all of its transmission activities so as to minimize its detectability by alien observers. Likewise, assumed benevolence implies that greater transmissions from Earth would increase the chances of contact. But both of these policies are optimistic about the likelihood of contact with alien life. Perhaps a more pragmatic approach is preliminary neutrality, which would remain consistent with business-as-usual on Earth and would not recommend any significant changes to Earth’s future detectability.