The Copernican Principle, named in honor of Nicolaus Copernicus (who proposed the heliocentric model of the Universe), states that Earth and humans do not occupy a special or privileged place in the Universe. In cosmological terms, this essentially means that Earth is representative of the norm, and life is likely to exist throughout the cosmos. While our efforts to find extraterrestrial life, a field of study known as astrobiology, have yielded no results so far, these efforts have been limited in scope. As a result, scientists are forced to speculate based on the only planet known to support life—i.e., Earth.
Thanks to the huge spate of exoplanet discoveries, multiple rocky planets have been found orbiting within the habitable zones (HZs) of red dwarf stars. For decades, there has been an ongoing debate as to whether these systems could be our best bet for finding evidence of life beyond Earth. In a recent study, Professor David Kipping addresses two key facts that could mean humanity is an outlier. Based on the age of the Universe and the relatively rare nature of our Sun, he concludes that astrobiologists examining red dwarf planets may be looking in the wrong place.
Professor Kipping is an associate professor of astronomy at Columbia University, a former Carl Sagan and Idina Menzel Fellow at Harvard, and the leader of Columbia’s Cool Worlds Laboratory. This group is dedicated to the exploration of extrasolar planetary systems with a focus on potentially habitable (or “cool”) planets and the development of novel methodologies and techniques for identifying signs of potential technological activity (aka. technosignatures). The paper, “Solar Hegemony: M-Dwarfs Are Unlikely to Host Observers Such as Ourselves,”
As Kipping notes, the idea that Earth is “humdrum” and a typical example of planets throughout the Universe has become deeply ingrained in the public psyche. This can be attributed to the influence of Carl Sagan and cosmologists, dating back to Konstantin Tsiolkovsky (1857-1935), whose writings on spaceflight and the possibility of extraterrestrial civilizations had a significant impact on scientists and engineers in the 20th century. But as Kipping told Universe Today via email, the roots go deeper:
There’s a long history to this, borne from the Copernican Revolution. Theology has historically often presented humanity (and by association, the Earth) as being of central importance, but modern science has sequentially demoted our privileged position, such as realizing that the Earth orbits the Sun, the Sun is one of billions of stars in our galaxy, and our galaxy is one of billions too. So there is a tendency to assume everything about us is typical, since it seems to have been the recurring theme of the last four centuries of astronomy.
By removing the Earth as the center of the Universe, Copernicus triggered a revolution in astronomy and the way humans perceive their place in the Universe. Carl Sagan highlighted this accomplishment in his seminal paper, “The Solipsist Approach to Extraterrestrial Intelligence,” written in response to Hart and Tipler’s conjecture that extraterrestrials did not exist. “One of the distinctions and triumphs of the advance of science has been the deprovincialization of our world view,” he wrote, citing multiple scientific revolutions that indicated that neither humanity nor Earth is unique or exceptional in the Universe.
Addressing the absence of proof for extraterrestrial life, Sagan famously replied, “But absence of evidence is not evidence of absence.” This thinking has informed astrobiology studies and every effort in the Search for Extraterrestrial Intelligence (SETI) over the past sixty years. However, there are two salient issues that pose a problem for this view, as highlighted by Kipping in his latest study. As he explained:
My paper looks at two puzzles that are undeniably unusual. Some 80% of stars are M-dwarfs, stars which apparently often harbor rocky planets in their habitable zones, yet we do not live around one, something which I called the Red Sky Paradox in a previous paper. Second, the stelliferous period of the Universe extends until 10,000 Gyr from now, yet here we are living in the first 0.1% of that window, when the Universe I just 13.8 Gyr old.
For those who are optimistic that extraterrestrial intelligence (ETI) exists and humanity may establish contact with it someday, there is no shortage of favorable arguments. For starters, the Milky Way is home to between 100 and 200 billion stars (though some estimates place that number higher), which translates to endless opportunities for life to emerge. Second, there is the age of the Universe itself (13.8 billion years old), which makes our Solar System a relative newcomer to the cosmos, as it formed roughly 4.6 billion years ago. Between these two undeniable facts, the statistical likelihood of advanced life existing in our galaxy is very high.
However, Kipping notes that these two points have their flaws and that modern astronomy has revealed more detailed information about astronomical objects, which is moving the needle in the opposite direction. “Yes, the Sun is one of billions of stars, but several properties clearly make it unusual amongst that sample,” he said. “For example, G-dwarf stars only make up a few percent of the total population, and even amongst those, the Sun is somewhat odd in being a fairly quiescent, single star system accompanied by two Jupiter-sized planets (only about 10% of Solar analogs have Jupiters).”
The presence of Jupiter and other giant planets in the outer Solar System is considered by many scientists to be a prerequisite for the existence of life. Thanks to their gravitational pull, objects that are bound for the inner Solar System are often captured and even impact these giants, as demonstrated by the Shoemaker–Levy 9 comet that struck Jupiter in 1994 (which constituted the first direct observation of a collision between Solar System objects. Then there’s the timeline of the Universe to consider and the Solar System’s relatively “late entry to the party.”
While it is almost certain that the conditions and building blocks for life existed billions of years before life emerged on Earth (ca. 4 billion years ago), it will be trillions of years before all stars in the Universe exhaust their fuel and die off. While stars like our Sun will die sooner, red dwarf stars are expected to remain in their main sequence phase for up to ten trillion years. Considering that extended timeline, the “stelliferous period” Kipping mentioned, humanity may actually be early to the party – a possibly previously explored by Harvard Professor Avi Loeb.
Alas, the question of whether rocky planets orbiting within the HZs of M-type red dwarf stars could support life is an open one. While some research has shown that tidally locked terrestrial planets could receive enough heat on their sun-facing side to maintain liquid water and conditions favorable to life, other research has indicated that the nature of M-type stars is unfavorable to habitability. This includes their unstable nature (relative to stars like the Sun), their tendency to form massive sunspots, and the way they are prone to flare activity.
This includes “superflares” that release enough electromagnetic energy to strip away planetary atmospheres, though observations have shown that these events are largely confined to the poles. To assess the likelihood of M-type stars being a good place for astrobiologists to focus their efforts (and the possibility that Earth may be an outlier), Kipping conducted a Bayesian statistical analysis of the two points he raised: the rarity of G-type stars and the “stelliferous period” of the Universe. As he explained, his analysis showed that humanity’s existence cannot be attributed to “luck or happenstance”:
My paper finds the odds of this being the case to be 1600:1 against. In science, we usually say anything above 10:1 is strong evidence and 100:1 is “decisive”, so 1600:1 is truly enormous odds for a luck-proponent to sit comfortably with. I explore two possible solutions. One is that planets have finite lifetimes for observers like us to emerge, and the second is that stars below a certain mass do not produce observers. The second one does a much better job of explaining the data, favored by odds of about 30:1. The result is a cutoff that stars below 0.34 Solar masses don’t develop observers to 95% confidence, which encompasses about 2/3 of all stars in the Universe.
This could be bad news for those hoping to get a look at the many rocky planets that orbit nearby red dwarf stars. Within 50 light-years of Earth, there are 30 systems where rocky exoplanets have been confirmed. Of those, 28 are found within red dwarf systems, including the closest rocky exoplanet beyond our Solar System (Proxima b), located about 4.25 light-years from us. While Breakthrough Starshot appears to have stalled in the research and development phase, there are other efforts to develop lightsail craft that could travel to Proxima Centauri within a human lifetime – like the Swarming Proxima Centauri concept.
Nevertheless, these findings do not dismiss the possibility of there being life on planets orbiting M-type stars, but they do question that prospect with a healthy dose of skepticism. In the meantime, Kipping stresses that astrobiology efforts should expand their focus to keep looking for Earth analogs that orbit Sun-like stars. These efforts will be bolstered immensely once the proposed Habitable Worlds Observatory (HWO) takes to space, which is expected to happen by the mid-2040s. As Kipping summarized:
We have good reasons to be skeptical of low mass stars harboring complex life, such as their intense flaring behavior, for example. But this is still largely speculation. My paper doesn’t include any such speculations about mechanism; it’s purely an analysis of our existence and the population/evolution of stars. So it’s reassuring that the same result emerges from a completely different argument, and together, I think this raises serious doubts about SETI looking at M-dwarfs too intensely. I certainly wouldn’t suggest they abandon looking at M-dwarfs, but I would encourage future programs to strongly prioritize G-dwarfs (like HWO will).
Further Reading: arXiv