American Scientist

To the Editors:

Concerning the Perspective column “The Imprecise Search for Extraterrestrial Habitability,” by Kevin Heng, in the May–June issue, it seems to me that the focus on surface temperature as a habitability criterion for exoplanets considers only one aspect, and perhaps it is not even the most important prerequisite for life. What is life but a heat engine, producing a local reduction in entropy?

Thermodynamics suggests that the key element to an efficient cycle is the availability of a “hot” reservoir and a “cold” one. For example, on Earth we receive blackbody radiation from the Sun at 5,780 kelvin and radiate blackbody radiation out to space at 255 kelvin. Net energy gain or loss is roughly zero because we are in thermal equilibrium, but life builds ordered systems using the difference in entropy between the two reservoirs. It follows that a planet orbiting a redder star would be more limited in capacity to support life, even with a surface temperature similar to Earth’s, and conversely a bluer star could provide a more encouraging environment. I am curious why the current habitability criteria do not take this point into account, even though stellar spectral data are more readily available than surface temperature data for exoplanets. I have even seen articles speculating that rogue planets with only internal heating could support life if they are at the correct temperature, but my understanding of thermodynamics argues against that likelihood (but doesn’t rule it out; some life here exploits thermal gradients near underwater geothermal vents).

Russ Howard
Frederick, CO

Dr. Heng responds:

Dr. Howard touches on a point that has been explored extensively in the astrophysical literature: the climatic effects arising from variations in the incident flux associated with different types of stars. (For a recent example, see the June 2015 article by S. Rugheimer and colleagues in The Astrophysical Journal.) Sunlike stars radiate roughly with a blackbody spectrum that peaks in the visible (at 0.5 microns). Red dwarfs (M stars), on the other hand, have blackbody spectra that peak in the near infrared (around 1 micron), and there has been constructive speculation on the consequences of the difference for photosynthesis.

Because of a temperature inversion in Earth’s atmosphere, where cold air is trapped under warmer air at higher altitudes, water is confined near the surface. But because ozone is a good absorber of ultraviolet and optical radiation but a poorer absorber of near-infrared radiation, this so-called cold trap would not operate in the atmosphere of an Earth analogue irradiated by a red dwarf. Generally, the situation is more complicated than Earth radiating as a 255-kelvin blackbody, because sources of absorption and scattering from both gaseous and aerosol species need to be taken into account to properly compute the atmospheric temperatures. It is probably an oversimplification to imagine that life is more likely in environments that have larger temperature gradients. A counterargument is that blue stars (for example, the short-lived O stars) live only for millions—rather than billions—of years, and one may speculate about whether this time frame is long enough for life to develop and evolve.

Much has also been said about the stability of these environments to flares and outbursts from the star (linked to its magnetic activity). Such flares and outbursts together have negligible energies compared to the star’s continuous energy output, but they may have outsized effects on any biology residing on the surface of the exoplanet. (For an example, see the October 2010 article in Astrobiology by A. Segura and others.) All of these thoughts remain speculative until one can actually construct a working model of how life forms and exploits energy from its surroundings, robustly generalizable to environments beyond that of Earth. It is perhaps premature to construct “habitability criteria.”

At the end of the day, I would say that data are king: We, as astrophysicists, would mostly like to think about ideas that may be falsified by astronomical data. Some thoughts, although plausible, may remain outside of the realm of falsifiability, at least in the foreseeable future. Rogue exoplanets, adrift from their stars, may harbor internal heat sources that are warm enough to support life. (See Dorian Abbot’s and Eric Switzer’s 2011 article in The Astrophysical Journal Letters.) Another variation on this idea is the “deep hot biosphere” (as per Thomas Gold), which suggests that a hidden biosphere of microbial life exists many kilometers beneath our feet, with a biomass that rivals its surface counterpart. Even if such deep hot biospheres are prevalent among exoplanets, they would essentially be invisible to the telescopes of astronomers and thus would fall outside the realm of ideas that are testable by current scientific methods.