The Invisible Majority: Systems without Gas Giants

The Invisible Majority: Systems without Gas Giants

Coined in the nineteenth century, the phrase “silent majority” referred to the dead and the fact that the number of people who have died far outnumbers the people who are alive. In 1969, President Richard Nixon called upon “the silent majority” for support, defining this group as the great majority of Americans who, having not joined in the large public demonstrations against the Vietnam War or in the counterculture movement, were invisible to the media.

Now astronomers are invoking “the invisible majority” to describe the preponderance of planetary systems lacking gas giant planets like Jupiter, Saturn, Uranus, and Neptune. This designation seems strange given that, of the 492 discovered extrasolar planets, 454 are larger than Uranus (=14.5 times the mass of Earth). However, planet detection methods, especially the predominant Doppler radial velocity technique, are strongly biased toward higher planet mass.

In “debiasing” the current catalogue of extrasolar planets, three different astronomy research teams concluded that about two-thirds of all planetary systems in the Milky Way Galaxy lack gas giant planets as large as, or larger than, Saturn within 20 astronomical units of the respective host star.1 (An astronomical unit = 92.956 million miles, the mean Earth-Sun distance. For comparison, Jupiter’s and Saturn’s distance from the Sun = 5.2 and 9.6 astronomical units, respectively.) Recently, three American astronomers, Andrew Mann, Eric Gaidos, and Scott Gaudi, presented “230 realizations of a numerical model of planet formation in systems without gas giants” so as to determine the cause of the paucity of gas-giant planetary systems and the features of the remaining planets.2

The researchers found that in planetary systems where the host star’s gaseous disk dissipates within 2–6 million years, planets forming beyond the “ice line” or “frost line” (the distance from the host star where water remains permanently frozen) will fail to accrete massive gas envelopes. What planets and planetesimals do form become chaotic during the first million years but eventually settle into stable orbital configurations within 10–100 million years.

Mann, Gaidos, and Gaudi ran their numerical simulations out to the equivalent of five billion years of planetary system history. These simulations consistently produced:

  1. a planet 5–9 times the mass of Earth and situated at a distance of 0.25–0.6 times the position of the ice line (for our solar system the distance would be 63–150 million miles from the Sun); and
  2. an inner system of two dominant rocky planets, like Earth and Venus, lacking stability or being unable to form because of gravitational disturbances arising from the above-described planet.

Theses two results categorically rule out any possibility for a planet capable of supporting advanced life existing in any of these planetary systems (see figure 1). This conclusion is independent of an advanced-life-supporting planet’s need for a set of gas giant neighbors of the same masses, distances from the host star, and orbital eccentricities as our own. (Such gas giants protect a planet friendly to advanced life from too many asteroid and comet collisions without disturbing its orbit.)

Figure 1:  Artist’s Conception of a Distant Planetary System

The discovery of hundreds of extrasolar planets and the detailed theoretical modeling of planet formation continue to provide ever stronger evidence for the rare-Earth doctrine, namely, that other Earth-like planets with the capability of supporting advanced life are either nonexistent or extremely rare.

Image credit: NASA/G. Bacon (Space Telescope Science Institute)

The simulations performed by Mann, Gaidos, and Gaudi help refute the central analogical evidence presented by Stephen Hawking and Leonard Mlodinow in their best-selling book, The Grand Design, to prove that a personal God does not exist. They wrote:

In the same way that the environmental coincidences of our solar system were rendered unremarkable by the realization that billions of such systems exist, the fine-tunings in the laws of nature can be explained by the existence of multiple universes.3

Ironically for Hawking and Mlodinow, discoveries of extrasolar planets and theoretical analyses of planet formation like the one performed by Mann, Gaidos, and Gaudi establish the opposite premise. Rather than demonstrating that the “coincidences” of our solar system are unremarkable, they show that the more astronomers learn about exoplanets, the more evidence they uncover for the idea that our own planetary system’s characteristics and configurations are amazingly designed to permit the existence of advanced life.

To put it another way, Mann, Gaidos, and Gaudi’s research adds to the weight of evidence for the rare-Earth and rare-solar-system doctrines. These doctrines state that while planets the size and mass of Earth may prove to be abundant, planets with the just-right planetary partners, characteristics, and physical and chemical composition to enable the support of advanced life will prove either rare or nonexistent. Such concepts are consistent with the Bible’s message that God supernaturally designed Earth, its planetary partners, and its life for the specific benefit of human beings.  

  1. Charles H. Lineweaver and Daniel Grether, “What Fraction of Sun-Like Stars Have Planets?” Astrophysical Journal 598 (December 1, 2003): 1350–60; Andrew Cumming et al., “The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets,” Publications of the Astronomical Society of the Pacific 120 (May 2008): 531–54; A. Gould et al., “Frequency of Solar-like Systems and of Ice and Gas Giants beyond the Snow Line from High-magnification Microlensing Events in 2005–2008,” Astrophysical Journal 720 (September 10, 2010): 1073–89.
  2. Andrew W. Mann, Eric Gaidos, and B. Scott Gaudi, “The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants,”Astrophysical Journal 719 (August 20, 2010): 1454–69.
  3. Stephen Hawking and Leonard Mlodinow, The Grand Design (New York: Bantam Books, 2010): 165.