In the two decades since paleontologist Peter Ward and astronomer Donald Brownlee published the book, Rare Earth,1 the rare earth doctrine has become firmly entrenched in the scientific community. While many astrobiologists remain hopeful that one day we will find a planet on which microbes might reside, they acknowledge that the only place in the known universe where conditions are not hostile for intelligent life and advanced civilization is Earth.
Now, there are signs that a rare planetary system doctrine might take hold. In 1995 astronomers first discovered a planet, 51 Pegasi b, outside of the solar system that was orbiting a nuclear burning star.2 At that time, most astronomers presumed that once the list of discovered planets numbered in the hundreds or thousands, many, if not most, of those planets would manifest characteristics very similar to one of the planets in our solar system. They also assumed that we would find many planetary systems that would be carbon copies of our solar system.
Today, the list of discovered extrasolar planets stands at 3,610.3 Five orbit pulsars. The remainder orbit nuclear burning stars. The masses of the planets orbiting nuclear burning stars range from 2% of Earth’s mass to more than 20 times the mass of Jupiter. For comparison, Mercury’s mass = 37% of Earth’s mass and Jupiter’s mass = 318 times that of Earth’s mass.
The only extrasolar planet that comes close to matching the characteristics of a solar system planet is Upsilon Andromedae e. Its mass compared to Jupiter’s is 1.06+ (because the inclination of Upsilon Andromedae e’s orbit is not known, only a minimum mass has been determined). Its distance from its host star compared to Jupiter’s distance from the Sun is 1.01. Like Jupiter, Upsilon Andromedae e has a nearly circular orbit about its host star.
Several features of Jupiter that are critical for life on Earth, however, are not shared by Upsilon Andromedae e. For example, Jupiter is the closest to the Sun of all the solar system gas giant planets and it possesses more than twice the mass of all the other solar system planets combined. Upsilon Andromedae e, though, is accompanied by two much larger planets, Upsilon Andromedae c and d, with masses = 14.57+ and 10.19+ times greater than Jupiter’s, respectively. These two planets orbit the system’s host star 6.1 and 2.1 times closer, respectively, than does Upsilon Andromedae e. Their orbits also are highly eccentric (elliptical). Moreover, the host star is 1.28 times more massive and 3.4 times more luminous than is the Sun. The system includes a dim star orbiting the main star at 750 times farther away than Earth orbits the Sun. The characteristics of both the main star and the two heavy planets accompanying Upsilon Andromedae e rule out the possibility that another planet in the same system could possibly support advanced life.
The extrasolar planetary systems discovered so far look nothing like the solar system. The norm for these systems is that they are populated with hot and warm gas giant planets—that is, with planets ranging from the mass of Uranus (14.5 times Earth’s mass) to many times the mass of Jupiter that orbit their host stars significantly closer than Jupiter orbits the Sun. Their proximity to their host stars rules out the possibility of any kind of advanced life on a rocky planet in the same system.
These planetary systems also frequently contain one or more super-Earths, planets ranging in size from 1.5–10 times Earth’s mass and that orbit their stars equal to or less than Earth’s distance from the Sun. Furthermore, whereas the known extrasolar planetary systems are filled with many planets possessing high-eccentricity orbits, the solar system has only one—diminutive Mercury, which resides so close to the Sun as to pose no risk of disturbing any of the other seven solar system planets.
Astronomers now have a good understanding of why the solar system seems unique. For most planetary systems, the larger planets form far from their host stars and migrate into close orbits about their host stars. For the other known extrasolar planetary systems, the larger planets either do not migrate at all or migrate only short distances toward their host stars. By contrast, the large planets in the solar system performed a “Grand Tack” migration. I discussed the amazing fine-tuning of the Grand Tack migration of Jupiter, Saturn, Uranus, and Neptune in a recent blog post.4
Thanks in large part to research on extrasolar planets, astronomers also know that every planet in the solar system fulfills a key role in making advanced life possible on Earth. Two Brazilian astronomers showed that even tiny adjustments in the orbits of Jupiter, Saturn, Uranus, and Neptune would prove catastrophic for life in our solar system.5 Regions beyond the precise orbital positions of Jupiter, Saturn, Uranus, and Neptune abound in destructive mean motion resonances. As it is, Uranus is close to a 7:1 resonance with Jupiter (where Jupiter would make exactly 7 orbits around the Sun for every single orbit of Uranus), a 2:1 resonance with Neptune, and a 3:1 resonance with Saturn. Meanwhile, Jupiter and Saturn are very close to 5:2 resonance. If any of the solar system gas giant planets’ orbital positions were to shift ever so slightly, that shift would destabilize the orbit of one or more of the eight planets in the solar system with catastrophic consequences for a long history of life on Earth.
Three Canadian astronomers further demonstrated that the orbital positions of Venus, Earth, and Mars must be fine-tuned so as to break up mean motion resonances that could be damaging for life on Earth. They showed that even the orbital features of the Earth-Moon system must be fine-tuned for this purpose.6 The Earth-Moon system suppresses a resonance in Venus’ orbit that is generated from the orbital patterns of Jupiter, Saturn, Uranus, and Neptune. Unless the Earth-Moon system is configured the way it is, both Venus’ and Mercury’s orbits would destabilize and generate destructive chaos throughout the inner solar system.
Every planet in our solar system and Earth’s Moon contribute to making advanced life possible on Earth. The solar system’s array of eight planets must be exactly the way it is. Have you thanked God today for Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune?
Featured image: The Sun’s eight planets are shown to approximate size scale relative to the Sun. The distances are not to scale.