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Physics, Earth & Space Icon Physics, Earth & Space

Search for Habitable Planets Is a Design Detection Exercise

David Coppedge
Image credit: M. Mizera / PTA / IAU100, CC BY 4.0 , via Wikimedia Commons.

Astrobiology was born in 1996 when NASA held a lively press conference for excited journalists to show possible life forms in a Martian meteorite named ALH 84001. The worm-like structures in the meteorite, appearing as if ready to crawl out onto a lab table, were reproduced in newspapers around the world. They turned out to have inorganic explanations. 

Hardly any scientist believes anymore that the Mars meteorite had anything to do with life. The event served its purpose, though, and soon NASA was flush with funds to begin a new Astrobiology Institute which continues to this day — a science without a subject to study. I sometimes joke that astrobiology has no “bio” in it (reducing it to “astrology”) but the new science has not been entirely in vain. It is bringing clarity to requirements for life. Since 1996, thousands of exoplanets have been discovered, primarily by the Kepler mission (2009-2018). Expectations are high that the James Webb Space Telescope will find more exoplanets, and possibly detect biosignatures — indications of non-natural processes that could indicate life.

Expanding Real Estate

Philosophers and novelists have long imagined life in outer space on mythical worlds, but now we have some data on which to build sound hypotheses. The real estate at habitable planets has expanded enormously — and with thousands of exoplanets in the catalog now, astrobiologists are focusing on their properties. Simultaneously, information on physical conditions at potentially habitable bodies in our own solar system has grown. But as the realty has expanded, has the habitability kept apace? 

By analogy, we can compare this question with Douglas Axe’s work on functional protein space within the set of sequence space. Only polypeptides with the ability to fold and perform work are “interesting” with respect to life. Dr. Axe’s research demonstrated that functional space is a minuscule subset of sequence space. Expanding sequence space, therefore, can only go so far to extend probabilistic resources for an origin of life by chance. If that life must emerge on a single planet, where the lucky accidents can coexist, chance can be eliminated on probabilistic grounds.

Similarly, an abundance of exoplanets only supports astrobiology if the subset of “functional” planets (habitable ones) comprises a significant fraction of all planetary real estate in the universe. (We are assuming life is not to be found on or inside stars, agreeing with most astrobiologists that life will be carbon-based with liquid water. We are also assuming the universe is not physically infinite or infinitely old.) Constraining the conditions for habitability in the set of all exoplanets, a factor that Frank Drake labeled ne (the “mean number of planets that could support life per star with planets”), turns astrobiology into a design detection exercise. 

Entertaining Speculations

A few types of exoplanets can be ruled out of serious consideration. The late Carl Sagan gave wings to his imagination at times, imagining life forms floating in the cloud tops of Jupiter. Star Trek portrayed creatures of silicon that ate rock. While entertaining as speculations, such tales are hardly useful. Scientists need to stay realistic, respecting the periodic table, the four fundamental forces, and the laws of physics. Thus, most of them do not include lava worlds or any world too hot or cold to sustain liquid water somewhere, either on a solid surface or in a subcrustal ocean. Those minimal requirements can be further constrained by reasonable exigencies about types of host stars (e.g., deadly flares, tidal locking), orbital parameters, availability of essential elements, and so forth. Casey Luskin on ID the Future recently made the case for magnetic fields and plate tectonics as requirements. Michael Denton’s Privileged Species books have amassed numerous constraints for complex life as well as for any conceivable life. Intelligent design advocates have narrowed the field since The Privileged Planet. But what do evolutionary astrobiologists say? 

Here Are a Few Recent Statements

“How do we know distant planets are Earth-like?” appeared on an “Ask an Expert” episode of CORDIS from the European Commission. Their list of requirements included a solid surface — eliminating lava worlds and gas giants — a temperate climate that supports liquid water, and a “suitable atmosphere” (not specified).

On, Robert Lea reported that a “scorching super-Earth” named GJ 1252b, with a surface hot enough to melt gold, has probably lost its atmosphere. Not good. Eliminate that planet and any others like it.

The Harvard Gazette agreed that plate tectonics combined with a stable magnetic field laid the “geological groundwork for life on Earth.” Researchers believe they have found evidence of an early magnetic reversal in Australia. Geological factors combined to facilitate habitability:

The reversal tells a great deal about the planet’s magnetic field 3.2 billion years ago. Key among the implicationsis that the magnetic field was likely stable and strong enough to keep solar winds from eroding the atmosphere. This insight, combined with the results on plate tectonics, offers clues to the conditions under which the earliest forms of life developed. [Emphasis added.]

At The Conversation, Joanna Barstow of the Open University offered four biosignatures the James Webb Space Telescope might detect for clues to alien life: (1) oxygen and ozone, (2) phosphine and ammonia, (3) methane plus carbon dioxide, and (4) chemical imbalances. She cautions against jumping to conclusions, though, suggesting that “rocky planets with mild temperatures and atmospheres dominated by nitrogen or carbon dioxide” might still be experiencing a “runaway greenhouse effect.” Her words hint at several requirements for habitability.

Liquid water may not be enough to infer life. A thought-provoking discovery from Hokkaido University revealed nuances of ocean currents that might be necessary to sustain a life-giving water cycle. They found evidence for an “ocean conveyor belt” initiated by melting sea ice, called frazil ice, that sinks in the North Atlantic. The dense freshwater ice flows under the salt water all the way to the Antarctic! Consider how this aids life:

“It is important to learn that such a major process is occurring underwater, revealing an aspect of the circulation system that has been at least partially obscured from view,” Kay says.

The researchers also suggest that the frazil ice could incorporate the sediment at the sea bottom and release it as the ice melts. This may yield new understanding of the circulation of nutrients that fertilize plankton to influence the general biological productivity of Antarctic waters.

SETI and UFO-logy go beyond astrobiology, but reported on a design-related story with the title, “NASA announces 16 people who will study UFOs to see what’s natural — and what isn’t.” If it isn’t natural, what is it? Designed!

These articles suggest ways that astrobiologists and design scientists can bring data to bear on longstanding questions about life in space. It’s possible, of course, to go overboard on requirements for habitability, such as insisting there must be molybdenum on the surface. But by reasonably narrowing down the subset of habitable real estate in the set of all planetary surfaces in a universe of finite dimensions and limited time, astrobiology can help philosophers and theologians stand on more solid ground when representing the likelihood of life beyond the earth, and thereby argue whether the probabilistic resources available are sufficient to rule out chance. If so, design would prevail as the best explanation for life’s presence or absence.