In the latest issue of Astrophysical Journal Letters three astrophysicists claim that there are habitable planets near the center of our galaxy and that these planets are exchanging life with one another.1 Life exchange between planets is known as panspermia, based on the Greek words pan (meaning all) and sperma (meaning seed).
Panspermia proposes that extraterrestrial bodies such as asteroids, comets, meteorites, and/or cosmic dust transported microscopic life-forms from one planet to another. Panspermia is an old idea.
The first known description of the hypothesis was by the Greek philosopher Anaxagoras (510–428 BC).2 Nineteenth century physicists Hermann Richter and Hermann von Helmholtz suggested that the origin of life on Earth may have come about through panspermia. Lord Kelvin (William Thomson), in the 1871 inaugural address to the British Association at Edinburgh said, “We must regard it as probable in the highest degree that there are countless seed-bearing meteoritic stones moving through space.”3 However, it was the Swedish chemist Svante Arrhenius in 1903 who elevated panspermia to a detailed scientific hypothesis and popularized the idea in his famous book, Worlds in the Making: The Evolution of the Universe, published in 1908.4
In the book I cowrote with Fazale Rana, Origins of Life, I devoted the latter part of chapter 15 in explaining why panspermia cannot possibly deliver a viable life-form, no matter how small and how primitive, from any conceivable extrasolar planetary system to ours.5 Briefly, given the minimum travel distances and the harsh radiation and vacuum conditions of interstellar space, any microbe not shielded by at least a meter thickness of solid rock would be killed and destroyed beyond recognition. As for interstellar rock delivery, astronomer H. Jay Melosh calculated the probability of Earth receiving a rock from beyond the solar system larger than a human fist as being much less than 1 part in 100,000 over the entire history of the solar system! It was these considerations made public at an origins of life conference in 2002 that killed panspermia as a conceivable explanation for life’s origin on Earth.
Revival of Panspermia Hypothesis
The three authors of the paper recently published in Astrophysical Journal Letters have attempted to revive the panspermia hypothesis by greatly shortening the interstellar travel distances. While acknowledging that interstellar travel distances in the vicinity of the Sun make panspermia untenable, they suggest that panspermia may be possible in the crowded stellar neighborhood near the center of our galaxy. The featured image shows how tightly packed together are the stars residing near the center of our Milky Way Galaxy. The European Southern Observatory has produced a combination visible light/infrared light video scan of the galactic center that you can watch here.
There are about 10 million stars within 3 light-years of the galactic center. The nearest star to the Sun is 4.25 light-years away. Thus, the three astrophysicists point out that what is a way-too-long interstellar travel time for a microbe in the Sun’s vicinity is an orders of magnitude shorter travel time for a microbe near the galactic center.
The three astrophysicists spend the bulk of their paper attempting to show that habitable planets exist near the galactic center. Ironically, they appeal to the intense x-ray and ultraviolet radiation emitted from the region immediately outside of the event horizon of the supermassive black hole at the galactic center to argue that there is a much higher percentage of habitable planets near the galactic center than near the Sun. This radiation, they correctly point out, would evaporate all of the hydrogen and helium gases, nearly all of the heavier gases in the initial very thick atmospheres, and nearly all the liquid water enveloping Neptune-sized and sub-Neptune-sized planets that happen to orbit their host stars at a distance conducive for water being in the liquid state. Within relatively short time periods, the three astrophysicists demonstrate, Neptune- and sub-Neptune-sized planets would be reduced to rocky cores with just a thin layer of liquid water and a thin atmosphere.
Near the Sun, the planet categories with the highest population numbers are Neptune- and sub-Neptune-sized planets. Assuming that these kinds of planets are just as numerous per unit star near the galactic center as they are near the Sun, and recognizing that the density of stars near the galactic center is about a million times greater than near the Sun, the three astrophysicists conclude that there are vastly more habitable planets near the galactic center than near the Sun.
Implicit, though not explicitly stated, in their paper is the assumption that a large fraction of habitable planets indeed will possess life. This assumption, common among astronomers, stems from how early and how rapidly life arose on Earth. Given that life appeared on Earth as early as the laws of physics and the physical conditions of the universe would permit and given that the time window during which life arose was less than five million years, many astronomers have concluded that the origin of life must be a straightforwardly easy step. Based on this conclusion, the three astrophysicists posit that there must be a huge number of life-bearing planets near the galactic center.
The three astrophysicists finish their paper by showing that panspermia could raise the number of life-bearing planets near the galactic center to a significantly higher number. They show that with planetary systems separated from one another by an average of a mere 5,000 times the distance Earth is from the Sun (for comparison, the nearest Oort Cloud object is 6,000 times Earth’s distance from the Sun and the nearest star is 270,000 times Earth’s distance from the Sun), inhabited planets near the galactic center, via panspermia, could relatively rapidly infect nearly all the habitable planets around them.
The three authors of the paper even suggest that with such a high density of inhabited planets, many of these planets will have evolved intelligent life.6 Given how close planetary systems are to one another near the galactic center, the authors speculate that intelligent life may have relatively rapidly colonized nearly all the habitable planets near the galactic center and that these colonies are communicating with one another using electromagnetic signals. The authors end their paper suggesting a search targeting the vicinity of the galactic center for signals from extraterrestrial intelligent life.
Critique of Revived Panspermia Hypothesis
There are many critiques of the revived panspermia hypothesis described above that I believe are catastrophic for the hypothesis. Here, I will briefly mention just five:
- The authors point out in their paper that the x-ray and ultraviolet radiation from outside the supermassive black hole’s event horizon is highly variable. However, even during its quieter episodes the x-ray and ultraviolet emissions are far too intense for either the origin of life or the long-term survival of life on planets near the galactic center.7
- The origin of life is not a straightforwardly easy step. Even with all the wealth and technology at their disposal, the world’s best biochemists employed in the world’s most sophisticated biosynthesis laboratories have been not been able to build a protein, RNA, or DNA molecule from scratch, let alone assemble a living organism.8 Furthermore, the limited laboratory successes have been performed under conditions not found anywhere in the natural or cosmic realm. What astronomers overlooked in presuming that the very early and extremely rapid origin of life on Earth implies that the origin of life is an easy step is that human life on Earth would be impossible unless (1) the origin of life occurred on Earth as early as the laws of physics and the conditions of the universe would permit, and (2) the origin of life on Earth occurred within a time window shorter than a few million years.
- The authors considered only one condition for habitability, namely whether the planet’s orbital distance from its host star would permit the possible existence of liquid water on its surface. In addition to the liquid water habitable zone there are eight other known habitable zones.9 Furthermore, several of these known habitable zones are now acknowledged to be much narrower than what astronomers previously thought. For a planet to be truly habitable it must simultaneously reside in all nine known habitable zones. Of the 3,763 planets discovered to date10 only one resides in all nine known habitable zones (three guesses on which one, and the first two guesses don’t count).
- Planetary systems near the galactic center are so close to one another that the passing of nearby stars inevitably will dynamically disturb the orbits of planets to outside one or more habitable zones. The authors of the paper acknowledged this problem and suggested that for dim stars the liquid water habitable zones would be so close to the host stars that the gravity of the host stars almost always would sufficiently supersede the gravity of nearby stars. However, for dim stars it is impossible for all the known habitable zones to overlap. Furthermore, the known flaring activity of these stars rules out life on planets orbiting them.11
- Planetary systems near the galactic center experience frequent nearby supernova eruption events and gamma-ray burst events.
The frequency of such events is deleterious if not catastrophic to both the origin and survival of life.12
The bottom line is that the neighborhood of the center of our galaxy is a long ways from being a favorable location for either the origin or survival of life. As it is, Earth is barely far enough distant from the galactic center to be a site for the long-term survival of life and of human civilization in particular. Panspermia, which was ruled out in the vicinity of our solar system, is an even less likely possibility in regions near the center of our galaxy. This latest attempt to find a naturalistic answer for the existence of life in our galaxy adds to the already burgeoning weight of evidence that nothing less than the supernatural intervention of the God of the Bible can rationally explain why life exists in the universe.
Featured image credit: 2MASSG