How many habitable exoplanets are there, really?

The TRAPPIST-1 system of exoplanets, approximately 40 light-years away

Exoplanets galore

As of the present date (August 2019), more than 4000 exoplanets have been discovered orbiting other stars, and by the time you read this even more will have been logged. Several hundred exoplanets were announced in a July 2019 paper (although these await independent confirmation). All of this is a remarkable advance, given that the first confirmed exoplanet discovery did not occur until 1992.

Most of the discoveries mentioned above are planets that are either too large or too close to their sun to possess liquid water, much less complex carbon-based compounds (see this analysis), and thus there is no conceivable chance that they harbor life even vaguely analogous to that on Earth. Thus researchers have been on the lookout for exoplanets in the circumstellar habitable zone around a star, which is loosely defined as an exoplanet that has a temperature regime capable of supporting liquid water, given sufficient atmospheric pressure, based on its distance from its host star. See this Wikipedia page, which lists more than 40 such potentially habitable exoplanets.

Along this line, an August 2019 study estimated that there are between 5 billion and 10 billion exoplanets in the Milky Way that reside in the habitable zone about their respective stars.

Among other things, researchers have focused microwave antennas and other receptors at these exoplanets, on the off chance that something might be heard at one of these locations. So far, nothing…

The public is clearly excited and fascinated by such reports. After reading some of these press reports, one might think that we are on the verge of discovering Earth 2.0, complete with little green men and women (or that we already have discovered Earth 2.0, but that “elites” are hiding the fact…). But is this type of enthusiasm really warranted, either in scientific literature or in the public arena?

The abiogenesis criterion

Unfortunately, there are many reasons to hold the champagne. To begin with, just because an exoplanet is in a “habitable zone” about its star certainly does not mean that it actually has water, much less biological organisms. Many other factors need to be considered.

For example, Harvard researcher Laura Kreidberg has noted that the recently discovered exoplanet K2-18b, which has generated considerable excitement because its atmosphere has been confirmed to contain water, has a diameter about 2.7 times the size of Earth, making it more similar to Neptune than to Earth. What’s more, the atmospheric pressure near the rocky surface of this planet is bound to be thousands of times higher than on Earth, and the resulting temperature may exceed 2800 Celsius or 5000 Fahrenheit. There is no possible way any complex carbon-based molecule such as DNA could survive under such conditions.

In fact, as a recent New Scientist article points out, most likely none of the current list of 4000 exoplanets is capable of hosting life. This is because life needs much more than a water-friendly temperature regime. For example, a leading scenario for the emergence of life on Earth crucially involves ultraviolet light with a certain moderate energy level to enable simple molecules to combine to form more complex compounds.

To that end, Marcos Jusino-Maldonado and Abel Méndez, of the University of Puerto Rico at Arecibo, have defined an “abiogenesis” criterion, meaning that sufficient UV light of an appropriate energy level for abiogenesis (the origin of life from nonliving molecules) would be available. When they applied their criterion to a list of 40 known exoplanets in the habitable zone, only eight of these matched their abiogenesis condition, and most of these eight are not likely to harbor life because they have a large radius (and thus are probably not rocky planets but instead are gas giants). Only the single planet named Kepler-452b, orbiting a star 1400 light years away, remained a viable candidate. Its radius is 1.63 times that of the Earth, and it marginally meets the abiogenesis and habitability criteria.

Red dwarfs, radiation, loss of atmosphere and plate tectonics

Another major problem is that most of the “habitable” planets identified so far are planets orbiting red dwarf stars. Red dwarf stars are in the fact the most abundant and longest-living stars. Some researchers have championed such stars as likely places to hunt for exoplanets harboring life.

But as an August 2019 Scientific American article points out, red dwarf stars are notorious for frequent flares with x-rays and high-energy UV radiation that almost certainly would sterilize any planet in the “habitable” zone. In other words, if an exoplanet is close enough to a red dwarf for the star’s feeble light to permit water to exist, then it is also dangerously close for lethal radiation from stellar flares. What’s more, high-energy stellar winds would very likely strip away any protective atmosphere that any such planet might possess or develop.

Bolstering this conclusion is an August 2019 study by a team of researchers led by Laura Kreidberg of Harvard and Daniel D. B. Koll of MIT. They examined the exoplanet LHS3844b using a new astronomical technique, and showed that it lacks any significant atmosphere, very likely because its host star (a red dwarf) has stripped it away. They conclude that “hot terrestrial planets orbiting small stars may not retain substantial atmosphere.”

Other studies have found even more restrictive conditions on true life-hosting exoplanets. For instance, a team of researchers led by Paul Byrne at North Carolina State University recently found that many exoplanets, even those that are not gas giants but instead have solid crusts, might well be “toffee planets,” with surface rocks that are hot enough to slowly stretch and deform like toffee candy — see this technical paper for details. Such planets most likely would not exhibit plate tectonics, as on Earth, and thus are unlikely to enjoy the benefits of plate tectonics.

Plate tectonics and the Earth’s underlying geophysical features are now thought to be crucial to life on Earth. Among other things, plate tectonics acts as a global thermostat, regulating CO2 levels in the atmosphere to yield a moderate, long-term temperature regime. In addition, one major hazard to life on Earth is streams of high-energy particles emanating from the Sun and elsewhere, which radiation is lethally hazardous to most life. But here on Earth, almost all of this cosmic radiation is deflected by Earth’s magnetic field, which is generated by the same movement of molten iron in the Earth’s core that is the dynamo behind plate tectonics [Ward2000]. This magnetic field also significantly reduces the loss of the atmosphere to outer space.

How special is the Solar System?

In addition to Earth being special, the Sun and Solar System are also unusual in many ways. For example, an October 2018 Scientific American article noted that in most of the recently discovered exoplanet systems, planets tend to be of the same size — if one planet, is, say, 1.5 times the radius of Earth, the other planets in the same system are likely to be of roughly this same size also. This is in stark contrast to our Solar System, which features tiny planets such as Mercury and huge planets such as Jupiter, with roughly 20 times the radius (and 8000 times the volume) of Earth. The existence of a large planet such as Jupiter is now thought to be crucial to clearing out debris from the inner planets in the Solar System’s early life, so that, as a result, Venus, Earth and Mars have been relatively undisturbed by asteroid collisions over the past 3.8 billion years or so, allowing life to form and develop, at least on Earth [Ward2000].

Additionally, our system’s position in the Milky Way is also quite favorable: at roughly 27,000 light-years from the galactic center, our Solar System strikes a good balance between being close enough to the center to have a critical concentration of heavier elements for complex chemistry, and yet not so close as to be bathed in sterilizing radiation — only about 7% of the galaxy is in a “galactic habitable zone” by these criteria [Gribbin2018]. Along this line, roughly 85% of stellar systems in the Milky Way are binary systems (with two or more stars). Exoplanets in such systems typically have very irregular orbital patterns, almost certainly destroying any hope for a stable, long-term, life-friendly temperature/radiation regime.

A 2012 study, published in the Royal Astronomical Society of Canada, after surveying numerous criteria and other studies, found that, contrary to popular opinion, the Sun is a very special star: “[I]f one picked a star at random within our galaxy, then there is a 99.99% chance that it will not have the same intrinsic characteristics as our Sun and (basic) Solar System.”

See this 2018 Scientific American article by John Gribbin for additional facts and discussion.

Fermi’s paradox

In previous blogs (see Blog A and Blog B), we discussed the nagging puzzle known as Fermi’s paradox: If the universe (or even just the Milky Way) is teeming with life, why do we not see evidence of even a single other technological civilization? After all, if such a civilization exists at all, very likely it is thousands or millions of years more advanced, and thus exploring and even communicating with habitable planets in the Milky Way would be a relatively simple and inexpensive undertaking, even for a small group of individuals.

Numerous solutions have been proposed to Fermi’s paradox, but almost all of them have devastating rejoinders. Arguments such as “ETs are under a strict global command not to disturb Earth,” or “ETs have lost interest in space research and exploration,” or “ETs are not interested in a primitive planet such as Earth,” or “ETs have moved on to more advanced communication technologies,” all collapse under the principle of diversity, a fundamental feature of evolution. In particular, it is hardly credible that in a vast, diverse ET society (and much less credible if there are numerous such societies) that not a single individual or group of individuals has ever attempted to contact Earth, using a means of communication that an emerging technological society such as ours could quickly and easily recognize. And note that once such a signal has been sent to Earth, it cannot be called back, according to known laws of physics.

Some (see this PBS show for instance) have claimed that since only 50 years or so have elapsed since radio/TV and radio telescope transmissions began on Earth, this means that only ETs within 50 light-years of Earth (if any such exist) would even know of our existence. But this is clearly groundless, because networks of lights have been visible on Earth for hundreds of years, other evidence of civilization has been visible for thousands of years, large animal species (including early hominins) have been visible for millions of years, and atmospheric signatures of life have been evident for billions of years.

Arguments that exploration and/or communication are technologically “too difficult” for an ET society immediately founder on the fact that human society is on the verge of launching such technologies today, and ET societies, as mentioned above, are almost certainly thousands or millions of years more advanced. As a single example, since we now have rapidly improving exoplanet detection and analysis facilities, as mentioned above, surely any ET society has a far superior facility that can observe Earth. Within a few decades it will be possible to launch “von Neumann probes” that land on distant planets or asteroids, construct extra copies of themselves (with the latest software beamed from the home planet), and then launch these probes to other stars, thus exploring the entire galaxy if desired [Nicholson2013]. Such probes could then beam details of their discoveries back to the home planet and, importantly, even initiate communication with promising planets. Along this line, gravitational lenses, which utilize a star’s gravitational field as an enormously magnifying telescope, could be used to view images of distant planets such as Earth and to initiate communication with these planets [Landis2016].

So why have we not seen any such probes or communications? There is no easy answer. See this Math Scholar blog for more discussion of proposed solutions and rejoinders to Fermi’s paradox.

The Copernican principle

One cogent solution to Fermi’s paradox is the following: Perhaps the reason the heavens are silent is that Earth is an extraordinarily unique home for intelligent life, according to the criteria mentioned above and perhaps even other criteria that we do not yet understand, so that the closest Earth 2.0, if it exists at all, is exceedingly distant from our Earth. If so, this means that Earth is far more singular than anyone dreamed even a few years ago, and human society has a far greater obligation not to destroy, overheat or otherwise foul our nest — our biosphere in general, and our race in particular, are of cosmic importance.

Just as significantly, we may have to rethink the Copernican principle, namely the notion that there is nothing particularly special about human society, Earth or our position in the universe, a principle that has guided scientific research for decades if not centuries. To the contrary, it is increasingly clear that the Earth is rather special — at the very least, there does not appear to be any equivalent to Earth, complete with an advanced technological civilization, within hundreds of light-years of Earth. If the Copernican principle is overturned, even partially, this will mark a very significant juncture in the history of science.

On the other hand, we could hear an announcement tomorrow that not only has life been detected elsewhere, but even intelligent life, with which we can communicate. That would also be an event of incalculable significance, certainly among the most important scientific discoveries of all time.

Such considerations underscore why research into exoplanets is so important and so exciting. However this turns out, we eagerly await the new experimental findings!

Comments are closed.