Good News, Alien Seekers: E.T. Probably Doesn’t Need a Freaky-Big Moon Like Ours

By Seth Shostak | January 24, 2012 1:01 pm

Seth Shostak is Senior Astronomer at the SETI Institute in California, and the host of the weekly radio show and podcast, “Big Picture Science.”

The Moon is a ball of left-over debris from a cosmic collision that took place more than four billion years ago. A Mars-sized asteroid—one of the countless planetesimals that were frantically churning our solar system into existence—hit the infant Earth, bequeathing it a very large, natural satellite.

OK, that’s a bit of modestly engaging astrophysics. But some scientists think there’s a biological angle here. Namely, that elaborate terrestrial life might never have appeared if that asteroid had arrived a few hours earlier, and sailed silently by. Put another way, if every night were moonless, you wouldn’t be around to notice the lack of a moon.

But is that true? Did our cratered companion really make our existence possible?

Well, there’s no doubt that the Moon is more than a handy night light and a hair restorer for werewolves. It’s responsible for the substantial amplitude of earthly ocean tides. These are of obvious influence if you’re a geoduck, a type of clam that people dig up at low tide. But even if the Moon didn’t exist—even if it had been vaporized billions of years ago by cantankerous Klingons—there would still be (somewhat lower) tides raised by the Sun. For creatures dependent on the oceans’ ebb and flow, life could go on.

However, the Moon does something else that might be of greater significance: It stabilizes the Earth’s spin. In other words, it keeps the rotation axis tilted at roughly the same angle relative to the plane of our orbit around the Sun. This tilt is about 23 degrees, a fact once known by school kids. If the Earth were in deep, deep space, the tilt—which is called “obliquity” by cognoscenti—would never vary.

But our world has neighbors; in particular, the Sun and the other planets of the solar system. Because the Earth is not a perfect sphere (it bulges a bit at the equator, much like most men over 40), these neighbors torque our world with their gravitational pull, causing our spin axis to change. The strongest effect is due to the Sun. But our path around the Sun is endlessly repetitive. So while Sol will change the direction of our planet’s spin, it doesn’t change the tilt angle.

On the other hand, the arrangement of the planets relative to Earth doesn’t repeat. Consequently, the effect of their torques can build up, and lead to a dramatic “tipping over” of our planet. The unpleasant consequence would be climate havoc.

Enter the Moon. It’s not as big as the planets, but it’s a heck of a lot closer, so its effect on the Earth really counts. Over the course of a month, it pulls first one way, and then the other. And since this cycle occurs over and over, it serves to overwhelm the long-term chaos that the planets would otherwise wreak. The Moon stabilizes Earth’s obliquity.

Well, almost. The tilt actually varies between 22 and 24.5 degrees—and the variation is enough to induce such environmental inconveniences as the occasional ice age. Without the Moon, it might be much worse. Consider Mars, whose own moons are puny. The Red Planet suffers swings in tilt that range from 11 to 45 degrees.

A moonless Earth, some scientists say, would have polar caps wandering like feral cats—greatly, perhaps fatally, inhibiting the evolution of complex life.

OK, but so what? After all, we do have the Moon. Yes, but our moon is an accident, a fairly improbable one. Of the habitable planets around other stars—planets that are similar in size, composition and temperature to Earth—few will boast a natural satellite similar to our moon. Therefore, if a large moon is really crucial for big-time biology, we shouldn’t expect a lot of sophisticated cosmic company. A dozen years ago, the popular book Rare Earth listed a big moon as one of the very special features of our planet, and one essential to the evolution of intelligent beings.

However, some recent work by NASA scientist Jack Lissauer and his colleagues paints a more propitious picture. They’ve run computer simulations of Earth’s spin to see how it would behave if there were no Moon. The simulations were done hundreds of times, because the various gravitational effects—with small torques building over time—cause the system to be chaotic. A slightly different initial arrangement of planets, or tiny perturbations, such as a small meteor hitting the Earth, could change the situation significantly millions of years down the road. This is somewhat like the proverbial butterfly whose flapping wings eventually cause thunderstorms on the other side of the globe.

The simulations were a big undertaking, but the results are in: Without our moon, we would, indeed, suffer larger swings in Earth’s tilt. But here’s the good news: Even over the course of hundreds of millions of years, these swings are not fast and traumatic. Lissauer’s team reckons that the tilt of a moonless Earth would be stable enough, for long enough, to allow complex life to gain a claw-hold, or at least to adapt to new environmental circumstances when the poles moved.

Bottom line? Those who search for life in space can take heart. For an Earth-like planet, a large moon is helpful, but not essential. Unless, of course, you’re a werewolf.

Image: Shutterstock


CATEGORIZED UNDER: Environment, Top Posts
  • Chris

    Link to paper?

  • John R

    I’m glad they did the research, but all we really know for certain is that the only planet we know of that has life, complex life, and intelligent life also has a freakishly large moon.

  • Peter John

    This does not address the real issue, as far as I can see. Life on Earth has evolved to our level because of our planets tremendous capacity for biodiversity. Our regular and rhythmic tides allow for greater biodiversity at the earlier stages of evolution. This combines with the tilt of the access allowing broader temperate zones, again more diverse biomes and greater biodiversity. The less biodiversity at early evolutionary stages, the lower the probability of sufficient variations to adapt species through catclysmic climate change– the lower the probability for life to survive long enough for intelligence to emerge. In addition the Moon could prevent environmental disasters by deflecting larger meteors from hitting Earth. These are all questions applying to the probability of intelligent extraterrestrial life, especially of human quality, which exceed merely finding planets in the habitable zone.

  • Kreegor

    According to Dr Richard Ghail, a Research Associate at the Imperial College in London, the key to life on Earth has less to do with the stabilizing effect of the Moon and more to do with the increase in the Earth’s density when the Moon was created.

    When a Mars-like body collided with the Earth 4.5 billion years ago, lighter material was flung off which formed the Moon. As a consequence, the Earth has a higher density than that of Venus. This gives Earth a core that is partly solid, which swirls around generating a magnetic field, unlike Venus’ liquid, quiescent core.

    Earth’s magnetic field protects the planet from solar bombardment and has allowed it to retain its water, whereas Venus’ water was blasted away. Water lubricates plate tectonics, which recycles the crust and scrubs carbon dioxide from the atmosphere. Without the fortuitous collision that created the Moon, the Earth would have suffered the same runaway greenhouse effect that turned Venus into a lifeless hellhole.

    (Reference: BBC,

  • Seth Shostak

    Interesting comments. A few germane (I hope) remarks:

    1. The average density of Earth and Venus differ by ~5 percent, and most of that is accounted for by the fact that Earth is a bit larger. Seems remarkable to imagine that this very slight difference would account for the magnetic field difference of the two worlds, and could be ascribed to the Moon.

    2. Actually, you could make the case that having a less stable spin axis would actually foster GREATER evolution, by stressing the environment. Such stresses have been suggested as provoking the Cambrian explosion a half-billion years ago on Earth. Heck, if the Moon didn’t exist, perhaps we’d have the cure for death by now.

    — Seth

  • Tim

    Speaking of magnetosphere, I’m curious what effect global tilt has on it, if any — does the core keeps on spinning in the same direction, deep under the surface, or does it experience a significant torque due to the overall tilt of the planet? Does this cause magnetic north to differ significantly over the course of thousands or millions of years? (Scales over 10Ma may not be as useful, given plate tectonics pushing around the geographical north pole; if the spin axis is used as reference, then this remains valid to wonder about.) Lastly, at such high angles of incidence, is there any impact of solar wind pushing on the core?


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  • G

    I thought one of the benefits of a large moon was in keeping the iron core liquid & flowing, providing a magnetic field shield against UV radiation. ie, what Kreegor says

  • B. Fox

    The two necessary conditions, from what I understand, for complex life to thrive on an Earth-like planet are plate tectonics and a magnetic field. Plate tectonics being necessary to maintain stable levels of carbon dioxide, and recycle materials life depends on. And a magnetic field being necessary to keep an atmosphere from being blown off by solar winds – also keeps surface lifeforms from being irradiated by solar and cosmic radiation (a meter of water would be enough to shield aquatic life from those forms of radiation).

    The question I have is whether the Moon and its formation is a necessary or sufficient condition to cause these phenomena, or is it altogether arbitrary to plate tectonics and having a magnetic field? Was plate tectonics initiated from having an ocean on the surface for millions of years (arguing here that Venus’s was too short lived), or was it caused as a byproduct of the Moon’s formation and subsequent tidal forces on the crust? Is the magnetic field an attribute of a more dense, iron-rich core created from a sort of smelter from when that asteroid collided with the proto-Earth and formed the moon, or is our iron-rich, spinning core a function of the Earth’s rotation and the crust-mantle-core convection that tectonics helps to drive?

    If plate tectonics and having a magnetic field for a rocky sphere around the size of our planet can occur independent of a Moon-like formation, then I would guess that Earth-like planets are more numerous. If not, then they are few and far between. I would have to agree with with Seth Shostak’s point 2 where a less stable environment, as long as it was not catastrophically unstable (i.e. constantly completely or almost completely wiping out life), would drive greater evolution, particularly if you subscribe to punctuated equilibrium – where evolution occurs at an even slower rate, if at all, during long bouts of environmental stability.

  • Hannibal

    So much is involved in when/how/if intelligent life formed on earth. All of the above had a role as well as such things as the dinosaur demise; who knows what life would be like today were the dinosaurs not wiped out giving small mammals an edge?

  • t.currie

    One of the most important effects of the Earth-moon collision was to strip away the lighter crustal elements over the heavy metal core and to expose the heavy elements that make complex life possible. Complex life uses many trace heavy elements to enable its complexity and that could not happen if they were still buried under a hundred kilometres of silicates and aluminates.

  • Wayne

    The most important prerequisite for complex life is the presence of liquid water – not just for millions of years, but for billions of years. Had the oceans ever boiled completely away or frozen solid to the bottom, it is unlikely complex life would have survived. Given the range of temperatures and pressures in the universe, this is a very narrow range. This must be maintained for billions of years given a star’s increased output over this time period.

    The amount of water is also important. The earth has only 0.02% water by mass. If this is increased to only 0.05%, the plant would be a water world, which may not preclude complex life, but would preclude technology since fire could probably never be used to manufacture anything. If this amount of water is less than a certain amount, the planet would consist of only a few salty lakes like the Great Salt Lake or the Dead Sea, and there would never be rain.

    If a freaky big moon is a requirement for complex life due to stabilizing the earth’s spin, keeping the core molten and thus maintaining the magnetosphere, affecting tides or plate tectonics, or stripping off crustal material, then I think it may be very unlikely we’ll find a planet like this one.

  • amphiox

    Complex life uses many trace heavy elements to enable its complexity and that could not happen if they were still buried under a hundred kilometres of silicates and aluminates.

    But complex life only needs trace amounts of those heavy elements. Even without a moon-type impact, it seems unlikely that ALL of these elements would be completely buried in a hypothetical earth-like planet without a moon. Even if impacts were necessary to mix up the material for this, this would not be contingent on the specific type of collision. Large collisions are very common in the early planet formation process. Large collisions with the specific angle required to produce a large moon are rare. But just mixing up elemental composition seems like it would not be so constrained by such fine details of the big crash.

  • amphiox

    I recall reading somewhere, with respect to molten metal cores with magnetic fields and plate tectonics, that models of planets larger than earth (ie, the super-earths) were rather promising with respect to these planets ability to spontaneously have both, without need for external help, like a big collision, thanks to their larger size, greater retention of internal heat, etc. (Stronger gravity would also slow down the loss of atmosphere associated with not having a magnetic field, possibly allowing such a planet to have several billion years worth of habitability before dessicating out like Mars did. And a parent star with a lower than average stellar wind would also help.)

    Whereas the smaller the planet, the less likely it would be able to spontaneously support plate tectonics and a liquid core, and the cut-off was actually very close to the size of Earth/Venus itself, suggesting that Venus is too small, and Earth just barely big enough, or perhaps also too small.

    In other words, it’s possible that an earth-sized planet needs a big moon and the moon-forming impact, but a slightly larger planet would not.

    The implication for this, of course, is that, far from being an ideal abode for complex life, earth is actually a marginal environment, just barely making it into the region of likely habitability, and requiring several fortuitous circumstances to push it over the edge, and that we, in our searth for ETs, might, in looking for exact earth analogs, be barking up the wrong tree.


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