Moon's Stabilizing Influence Depends on Star Size

A connection between multiple factors was noted in a recent model of how a moon helps a planet remain habitable.

With a total lunar eclipse in Earth’s rear view mirror (Oct 8), it’s worth pondering what the dark orb does for us when it’s not blood red.  “Moons Can Help Planets Remain Stable Long Enough for Life to Form” is the headline of a piece by Charles Q. Choi in Astrobiology Magazine.  A new study published in the International Journal of Astrobiology examined the stabilizing influence of a moon on a planet.

One finding confirms what has long been believed: our moon helps life on the Earth.  “If the Earth did not have the Moon, the Earth’s axial tilt would have changed rapidly and the climate of the Earth would have changed often,” lead author Takashi Sasaki (U of Idaho) said. This supports what astronomer Kevin Grazier said in the documentary film The Privileged Planet in 2004: “If our moon didn’t exist, neither would we.”

Another interesting finding is that the habitability conferred by the moon and sun are related.  Because our sun is fairly large as main-sequence stars go, our moon stabilizes Earth because the Earth-moon system orbits within a habitable zone far from the sun.  The team looked at what would happen at Earth-like exoplanets around smaller stars.  For some  of the putative habitable planets orbiting exoplanets discovered by the Kepler spacecraft, having a large moon wouldn’t help:

The scientists found that stars with less than 42 percent of the Sun’s mass may not be good places to look for complex life because moons cannot survive for more than 5 billion years in these systems. This is because the habitable zones are closer to stars that have dimmer and lower masses than in brighter, higher-mass star systems. For instance, in solar systems with stars 40 to 50 percent of the Sun’s mass, the habitable distance is approximately one-quarter of the distance between the Sun and Earth. Since these planet-moon systems are so close to their host stars, their stars gravitational pull perturbs the planet-moon systems too much for the moons to remain around their planets, Sasaki said.

There’s also a connection with the planet’s composition.  Tweaking the habitable radius requires matching the planet’s mass and density:

The scientists found that Kepler-62e would have to be composed almost entirely of a high-density material, such as iron, for a moon orbiting it to exist for more than 5 billion years. They also discovered that Kepler-62f could have a moon for more than 5 billion years if it had a variety of different compositions, particularly if it had an absence of oceans or only deep oceans, either of which would cause the planet to dissipate less tidal energy.

Those kinds of tweaks, needless to say, would have corresponding impacts on the kind of life that could exist on those two candidate “Earth-like” planets.  This goes to show that it’s not just size and distance that affect habitability.  Multiple factors have to come together at the right place and time, and in the right relationships, to make a planet habitable.

Choi and the astrobiologists assume that a planet needs billions of years for life to hit the lucky combinations of mutations to get complex life to evolve:

The Moon is more than just Earth’s partner in space — it may have helped stabilize Earth’s orbit enough for it to become hospitable for the evolution of complex forms of life.

A new study suggests that large moons can form and remain stable for long times around distant planets as well, potentially helping alien life evolve….

A planet whose axial tilt fluctuates wildly like Mars may not maintain a favorable climate for a long enough time for complex forms of life to evolve.  For example, it took about 3.8  billion years for life on the 4.6-billion-year-old Earth to evolve from single-celled organisms to multicellular life such as plants, animals and fungi.

“Because the Earth has had a long-term stable climate, life on the Earth has had time to evolve from single cells to complex life forms,” Sasaki said.

Actually, though, if life, the sun, the Earth and the moon were created—as most people believe—billions of years of habitability are not required.  Intelligent causes know how to manage the constraints and tradeoffs for habitability from the start, if life is the purpose of the planet.  As for life by chance, many studies have shown that time is no help; impossible things never happen.

Add to this the unnecessary “coincidence” that our moon is just the right size to produce total solar eclipses, and we have here a number of interrelated factors that seem to have conspired to make our world not only habitable, but ideal for scientific discovery, as argued by Gonzalez and Richards in The Privileged Planet.  No other planet in our solar system has this combination of factors.  It’s the only one inhabited by sentient beings able to study and appreciate it.

If this were a law of nature, we would see every star observed by the Kepler spacecraft having the same combination.  They don’t.  These factors are all contingent; they could be otherwise.  These observable realities should make one seriously consider the design hypothesis over the chance hypothesis.  Of the design hypotheses out there, none is better than Genesis 1.  Come now, and let us reason together.