Since the first discovery of an extrasolar planet in 1995,1 astronomers have detected more than 400 extrasolar planets, most of which are gas giants like Jupiter, Saturn, Uranus, and Neptune. The great majority of these gas giants orbit their stars closer than two astronomical units. (An astronomical unit = Earth’s distance from the Sun.) It’s impossible for such planets to form so close to their star. (In order for there to be sufficient gas for the planets to accrete, gas giants can only form beyond the “snowline”—the distance from the star where water and carbon dioxide freeze.) Thus, astronomers discerned that these planets arrived at their present locations through inward migration. Such migration occurs as a result of gas giants interacting with copious amounts of interplanetary gas and dust. Recently, however, astronomers have observed through direct imaging a few giant planets located as much as 50–120 astronomical units away from their host stars.2 For comparison, Neptune, the most distant solar system planet, is situated at 39 astronomical units from the Sun. At 50–120 astronomical units the accretion timescales are much too long for giant planets to form. Researchers realized that some mechanism must be responsible for driving these gas giant planets out to such great distances.
Calculations performed by a team of two mathematicians and an astronomer demonstrate how gas giants forming just a few astronomical units beyond the snowline could be thrust out to a hundred astronomical units and more.3 The thrusting mechanism they found to be especially effective was mean motion resonance, where the inner planet was much more massive than the outer one. Planetary mean motion resonance occurs when two planets orbiting a star exert a periodic gravitational influence on each other as a result of their orbital periods manifesting a ratio of two small integers (such as 3:2). In other words, with mean motion resonance each planet affects the orbit of the other. The strongest mean motion resonance takes place when the inner planet makes exactly two orbits for every single orbit of the outer planet—a one:two resonance.
Led by Aurélien Crida, the team showed that (under commonly expected conditions) a pair of planets formed in the 5–20 astronomical units region can be pushed out to a region up to ten times farther away from their host star. This result would explain––without the need to appeal to planet scattering––why the gas giants have been observed orbiting their star at great distances. For planet scattering to provide an adequate answer the planetary system must be comprised of closely interacting planets where the individual planets exhibit chaotic orbits.
The scenario developed by Crida’s team is testable. A gas giant planet has been discovered orbiting Fomalhaut (the brightest star in the constellation Piscis Austrinus) at a distance of 115 astronomical units. The researchers predict that a second, more massive, planet will be found orbiting Fomalhaut at a distance of about 75 astronomical units. Though difficult and time-consuming to detect, such a planet is discoverable with current instrumentation.
In their paper, Crida’s team declines to comment on the relevance of their discovery to the solar system and to life on Earth. Similarities and dissimilarities emerge.
Grade schoolers learn that Jupiter and Saturn dominate the Sun’s gas giant planetary system. Together these two planets account for 92 percent of the total mass of the Sun’s planets. As in the Crida team scenario, the Sun’s inner gas giant planet is the most massive. Jupiter is three and a third times more massive than Saturn.
Jupiter and Saturn also went through a one:two mean motion resonance.4 In fact, this resonance touched off the Late Heavy Bombardment.5 This event, however, led to only a slight outward migration of Jupiter and Saturn, which placed both planets at the ideal orbital distances relative to Earth.6 These orbital distances grant Earth maximum protection from collisions by asteroids and comets without disturbing its orbit to a degree that would threaten the well-being of advanced life.
Unlike in the scenario produced by Crida’s team, the solar system’s retinue of gas giants formed under exquisitely fine-tuned circumstances that placed them in the best possible positions to favor advanced life on Earth. This new research yields yet one more example of how the more we learn about planets, both solar and extrasolar, the more evidence we discover for the supernatural design of our Milky Way Galaxy and solar system for the specific benefit of the human race.