Among all species, humans are unique in that we, as individuals, take an exceptionally long time to develop physically and mentally. Human males do not reach full physical maturity until the age of twenty-four. Educational training can take even longer; in my case it took until age thirty to complete all my education and post-doctorate research. We are called to fulfill highly complex, meaningful, and purposeful roles—this is the main reason we take so much longer to develop than other species.
Today, astronomers and geophysicists are discovering that what is true of human beings is also proving true of the solar system’s planets, moons, asteroids, and comets. More and more research shows that it’s one thing for the solar system to develop to support unicellular life; it’s quite another matter to reach the ability to host advanced life. Supporting human civilization presents even more daunting requirements.
In more than 40 different research papers published from late 2003 to early 2011, over a hundred astronomers and physicists have pieced together all major events that took place between about 30 million to a billion years after the solar system’s birth and that were essential for advanced life. In the next several Today’s New Reasons to Believe articles, I will describe what these scientists have uncovered and how these new discoveries explain, in part, Earth’s unique capacity to sustain human civilization.
In the final article I will go past what the researchers have published and analyze how their discoveries, in the context of additional work on the statistics of the properties of extrasolar planets, produce a litany of new evidences establishing the supernatural design of the solar system’s planets, asteroids, and comets for humanity’s specific benefit. In the same article I will also briefly describe how these researchers’ work produces several new refutations of young-earth creationism. In this, the first article, I will describe some of the finely tuned features of the solar system’s birth and infancy that set the stage for the even more astounding design of its youth.
Solar System’s Birth
To understand the fine-tuned nature of the solar system’s youth, it is important to place it in the context of the solar system’s birth and infancy. That both the solar system and Earth experienced a remarkable birth and infancy is now well established and understood. Compared to other known planetary systems, our solar system is exceptionally volatile-poor and refractory-rich.1 That is, it possesses little in the way of gases and liquids but a superabundance of light and heavy metals. Goldilocks planets are like Earth in that they possess both the carbon and liquid water that life requires. However, Earth possesses about 1,200 times less carbon-based gases in its atmosphere and about 500 times less liquid water compared to other “goldilocks” planets of similar size and approximating Earth’s surface temperature.
Astronomers now understand that the solar system’s exceptional status results from its birth in a huge cluster of at least 3,000 stars located well inside the Milky Way Galaxy’s corotation distance (see figure 1). The corotation distance is the distance from the center of a spiral galaxy, where a star would revolve around the galaxy’s center at the same rate that the galaxy’s spiral structure rotates. To put it another way, at the corotation distance a star has zero relative velocity with respect to the spiral arm pattern.
For the Milky Way Galaxy (MWG) the corotation distance is about 26,000 light-years out from the galactic center. Presently, the solar system resides just inside the MWG’s corotation distance.
Figure 1: Milky Way Galaxy’s Corotation Distance
A little less than half way out from the center of the optically visible portion of our galaxy, stars revolve around the galaxy’s center at the same rate that the spiral arm pattern rotates. Stars closer to the center will revolve faster and stars more distance will revolve slower. The farther a star is from the corotation distance, the more frequently it will cross one of the galaxy’s spiral arms.
Background image courtesy of NASA/JPL-Caltech/R. Hurt (SSC)
At the corotation distance mean motion resonances disrupt star formation. This disruption limits both the stellar density and the abundance of metals, especially heavier metals. Given Earth’s extremely high abundance of such metals as phosphorus, fluorine, aluminum, titanium, uranium, and thorium, our planet and its solar system companions could not have formed anywhere near the corotation distance. They must have formed closer to the galactic center where the galactic metallicity is near peak values (see figure 2).2
Figure 2: Metal Abundance Relative to Distance from the Galactic Center
Powerful “waves” waft metal-rich gas outward from the galactic center resulting in aggressive star formation. The larger of these stars expel huge quantities of even more metal-rich gas into the galactic medium. Beyond 4 kiloparsecs (13,000 light-years) from the galactic center the star formation rate declines steadily (and with it the abundance of metals), reaching a minimum at the corotation distance (dotted line). Beyond 11 kiloparses (36,000 light-years) the star formation rate and metal abundance declines steadily again.
The drawn curve is based on data taken from Mishurov, Lépine, and Acharova, Astrophysical Journal Letters 571 (June 1, 2002): L113–L115.
Background image courtesy of NASA/JPL-Caltech/R. Hurt (SSC)
Only when closer to the galactic center would a birthing cluster of 3,000 or more stars be possible. Likewise, the supernova production rate would be high enough to enrich the solar system with its full panoply of heavy elements only when closer to the galactic center.
Within its birthing cluster, the Sun and its emerging system of planets formed adjacent to several supergiant stars that exploded as supernovae. The explosions were not close enough to destroy or seriously disrupt the planets, but they were close enough both in distance and timing to supplement the newly forming planets with a profusion of heavy elements and to bathe the entire solar system in aluminum-26, a radioactive isotope with a half-life of 717,000 years.
Different supernova eruptions produce different suites of heavy elements; so, it was crucial for Earth’s advanced life that the Sun’s disk of protoplanetary material be exposed to the different eruptions almost simultaneously. Exposure to at least four different kinds of eruptions (including a very rare faint supernova with mixing fallback) is necessary to explain Earth’s past and present mix of abundances of the 94 naturally occurring elements.3
Additionally, the Sun’s protoplanetary disk must be exposed to certain asymptotic giant branch stars and white dwarf binary stars at just the right distance and timing. It takes the precise exposure to all of these different kinds of stars at the just-right times in their evolutionary history to supply the Sun’s emerging planets with adequate amounts of all 94 natural elements (plutonium and neptunium though present on early Earth have since completely decayed away) in the periodic table.
Some of the lighter elements, especially hydrogen, carbon, and nitrogen, would be far too abundant for the existence of advanced life if it not for the primordial solar system being bathed in huge quantities of aluminum-26. In fact, while aluminum comprises less than 0.01 percent of the universe’s ordinary matter, it makes up 8.1 percent of Earth’s crust (900 times more abundant).
A just-right combination of nearby supernova eruptions showered the solar system’s developing planets with aluminum-26. The intense heat released from the isotope’s radioactive decay drove off most of the volatiles (gases and liquids), leaving the solar system planets volatile-poor and refractory-rich. The aluminum-26 drove off a greater percentage of volatiles from planets smaller and closer to the Sun.
In 2009, two British astronomers pointed out that early exposure to an unusually extreme amount of aluminum-26 explains why the solar system’s planets are so very volatile-poor compared to the hundreds of exoplanets now discovered.4 Plus, the solar system’s early exposure to aluminum-26 also helps explain why Earth’s crust is so abundant in aluminum-27 (aluminum’s non-radioactive isotope), because large quantities of aluminum-26 mean large amounts of aluminum-27.
Solar System’s Infancy
For advanced life to ever be possible, the Sun and its young planets, asteroids, and comets could not remain long in their birthing star cluster. Too much time in the birthing cluster would result in the planets suffering gravitational disturbances and radiation exposure from nearby massive stars. However, a just-right encounter with a highly specified set of giant stars at the just-right time strongly ejected the entire solar system from its birthing cluster intact.
Not just any kind of ejection would do, however. The solar system had to be ejected outward in the opposite direction from the galactic center, along a trajectory that avoided any points of danger. That is, the trajectory path steered clear of close encounters with giant stars, x-ray and gamma-ray sources, and giant molecular clouds. The ejection brought the solar system’s radial velocity (outward movement within the MWG) to a halt just before the solar system reached the corotation distance.
As noted earlier, residing right at the corotation distance would have resulted in destructive mean motion resonances. But just inside the corotation distance the solar system experiences few spiral arm encounters and few gravitational encounters with other stars.
For the possibility of advanced life on Earth, the solar system had to be born in one of the most dangerous locations in the MWG and then ejected relatively quickly into the MWG’s safest location. The solar system’s birth was not ordinary. Everything about the solar system’s genesis appears exquisitely fine-tuned to make possible the future existence of advanced life and human beings in particular.
In part 2 of this series I will describe events that took place during Earth’s toddlerhood (the first ten and a hundred million years of our planet’s history).
1. Linda T. Elkins-Tanton and Sara Seager, “Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets,” Astrophysical Journal 685 (October 1, 2008): 1237–46; Hugh Ross, “Planet Formation: Problems With Water, Carbon, and Air,” Today’s New Reason to Believe, January 12, 2008: http://www.reasons.org/rtbs-creation-model/cosmic-design/planet-formation-problems-too-much-water-too-much-carbon-and-too-much-air; J. D. Gilmour and C. A. Middleton, “Anthropic Selection of a Solar System with a High 26Al/27Al Ratio: Implications and a Possible Mechanism,” Icarus 201 (June 2009): 821–23.
2. Yu. N. Mishurov, J. R. D. Lépine, and I. A. Acharova, “Corotation: Its Influence on the Chemical Abundance Pattern of the Galaxy,” Astrophysical Journal Letters 571 (June 1, 2002): L113–L115; S. Scarano and J. R. D. Lépine, “The Effect of Corotation on the Radial Gradient of Metallicity of Spiral Galaxies,” in Chemical Abundances in the Universe: Connecting First Stars to Planets, Proceedings of the International Astronomical Union, IAU Symposium 265 (March 2010): 251–52.
3. A. Takigawa et al., “Injection of Short-Lived Radionuclides into the Early Solar System from a Faint Supernova with Mixing Fallback,” Astrophysical Journal 688 (December 1, 2008): 1382–87.
4. Gilmour and Middleton, “Anthropic Selection,” 821–23.