Is Life Possible on a Moon?

Is Life Possible on a Moon?

The case for the supernatural design of the human-friendly Milky Way Galaxy and solar system continues to build.

A team of American astronomers recently announced the discovery of the first known planet outside our solar system to spend its entire orbit within the “habitable zone.”1When astronomers talk about a habitable zone for a planet they simply mean that the planet is orbiting within that distance from its star where surface liquid water would be possible–assuming the atmosphere of the planet is fine-tuned so as to trap the just right amount of heat from the planet’s star.

The newly discovered planet, 55 Cancri f, orbits the star 55 Cancri and weighs in at 45 times the mass of Earth, making it a gas giant. All astronomers recognize that the dense atmosphere that such a planet must possess precludes any possibility of habitation. However, as Debra Fischer, leader of the research team, explained to reporters, “The gas giant planets in our solar system all have large moons. If there is a moon orbiting this new, massive planet, it might have pools of liquid water on a rocky surface.” 2

The characteristics of 55 Cancri and four known planets orbiting it rule out the possibility of life anywhere within the 55 Cancri planetary system. I discussed these hostile conditions on the November 10, 2007, Reasons To Believe Science News Flash. A more general discussion centers on whether or not moons for any planetary system make realistic candidates for life.

As Fischer says, all the gas giant planets in the solar system possess large moons. Compared to the mass of Earth, the largest moons for each of the solar system’s gas giants are: Jupiter’s at 2.48%, Saturn’s at 2.25%, Neptune’s at 0.36%, and Uranus’ at 0.06%. Earth’s moon weighs in at 1.23%

However, life requires a strong, stable magnetic field to protect the moon from the charged particles emanating from the star that would otherwise sputter away the moon’s atmosphere. Life also needs strong, stable plate tectonics to contribute to the necessary mechanisms for compensating for the star’s increasing luminosity over its burning history. Unless the moon’s mass exceeds 23 percent of Earth’s mass, plate tectonics and a strong magnetic field are impossible. For such necessities to last for a few billion years requires a mass and a density virtually equivalent to Earth’s.3 Thus, the solar system’s largest moons fall short by a factor of forty or more from being large enough for life.

The orbital zone where surface liquid water could theoretically exist on some rocky body is relatively broad if one adjusts the atmosphere to compensate. For example, Earth’s orbit could be pushed out to a little more than halfway beyond the mid-point between Earth’s and Mars’ present orbits and still sustain surface liquid water—if greenhouse gases saturated Earth’s atmosphere. Removing all of Earth’s greenhouse gases would allow liquid water to persist on Earth even if it were situated 10 percent closer to Venus’ orbit than it is now. But, a liquid water zone does not equate to a truly habitable zone. Without adequate greenhouse gases, life—at least life more advanced than bacteria—dies. Conversely, overly plentiful greenhouse gases would cripple the operations of breathing organs like lungs. Therefore, existence of plants and animals demands a much narrower zone than what astronomers refer to as “the habitable zone.”

Actually, for any given planet or moon the true habitable zone would be far more narrow yet. If Earth’s orbit moved about a half percent farther from the Sun, Earth’s surface would experience a runaway freeze. Cooler temperatures caused by the greater distance from the Sun would result in more snowfall. Since snow reflects sunlight more effectively than soil, greater snow cover causes Earth’s surface temperature to drop even more, which results in still more snow falling and an even greater drop in temperature until all of Earth’s surface freezes over.

On the other hand, if Earth’s orbit moved about a half percent closer to the Sun, Earth’s surface would experience a runaway evaporation of water. This happens because the closer distance to the Sun yields a higher surface temperature on Earth that causes more water to evaporate. Water vapor is a greenhouse gas. Thus, more water in Earth’s atmosphere causes the surface temperature to rise, which results in more evaporation. The cycle continues until all of Earth’s water evaporates.

If the moon orbits too close to its planet it becomes tidally locked, culminating in one side of the moon perpetually facing the planet. Typically, this causes long cold nights and long hot days. Such a close-in orbit would also generate destructive tides and volcanic eruptions; and if the planet possesses a strong magnetic field, its magnetosphere could wreck havoc on the moon’s atmospheric layers. Orbiting too far away from its planet causes a moon to experience enormous seasonal temperature differences as its journey about the planet puts it alternately closer and farther away from its star.

Close-in or far-out, a large moon orbiting a large gas giant planet would experience heavy bombardment from comets and asteroids. Just as the gravity of Jupiter attracts a lot of asteroids and comets to veer into its vicinity, so, too, the gravity of large gas giant planets implies that any big moon orbiting them will receive a lot of life-catastrophic collisions.

Even moons with interior ice-water environments heated through tidal friction from the planet’s gravity (such as may be the case for Jupiter’s moon Europa) also provide poor long-term life sites. Without a carbonate-silicate cycle, such a moon cannot properly compensate for the star’s increasing luminosity. However, this cycle will not operate without plenty of dry land exposed to a specific atmosphere where that dry land contains particular kinds and quantities of life.

With such a narrow true habitable zone, life on a moon becomes especially problematic. More reasons than the few described here rule out moons as potential life sites for any kind of long-term plant and animal life.4 Since the discovery of the first extra-solar planet in 1995, astronomers have cataloged 265 extra-solar planets. All these planets have taught us much about the formation and dynamics of planetary systems. The record of the past twelve years shows that the more astronomers learn about the characteristics of planets, moons, and their stars, the more evidence they uncover for the exceptional rarity of the solar system’s conditions that permit the existence of life. The case for the supernatural design of the human-friendly Milky Way Galaxy and solar system continues to build.

  1. Ian Sample, “Could This Be Earth’s New Twin? Introducing Planet 55 Cancri f,” The Guardian (Wednesday, November 7, 2007).
  2. Ian Sample, The Guardian (Wednesday, November 7, 2007);
  3. Diana Valencia, Richard J. O’Connell, and Dimitar D. Sasselov, “Inevitability of Plate Tectonics on Super Earths, “Astrophysical Journal Letters,” 670 (2007): L45-L48.
  4. Hugh Ross, Kenneth Samples, and Mark Clark, Lights in the Sky and Little Green Men (Colorado Springs, CO: NavPress, 2002): 39-41.