Life on Alien Planets May Not Require a Large Moon After All

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Ever since a study conducted back in 1993, it has been proposed that in order for a planet to support more complex life, it would be most advantageous for that planet to have a large moon orbiting it, much like the Earth’s moon. Our moon helps to stabilize the Earth’s rotational axis against perturbations caused by the gravitational influence of Jupiter. Without that stabilizing force, there would be huge climate fluctuations caused by the tilt of Earth’s axis swinging between about 0 and 85 degrees.

But now that belief is being called into question thanks to newer research, which may mean that the number of planets capable of supporting complex life could be even higher than previously thought.

Since planets with relatively large moons are thought to be fairly rare, that would mean most terrestrial-type planets like Earth would have either smaller moons or no moons at all, limiting their potential to support life. But if the new research results are right, the dependence on a large moon might not be as important after all. “There could be a lot more habitable worlds out there,” according to Jack Lissauer of NASA’s Ames Research Center in Moffett Field, California, who leads the research team.

It seems that the 1993 study did not take into account how fast the changes in tilt would occur; the impression given was that the axis fluctuations would be wild and chaotic. Lissauer and his team conducted a new experiment simulating a moonless Earth over a time period of 4 billion years. The results were surprising – the axis tilt of the Earth varied only between about 10 and 50 degrees, much less than the original study suggested. There were also long periods of time, up to 500 million years, when the tilt was only between 17 and 32 degrees, a lot more stable than previously thought possible.

So what does this mean for planets in other solar systems? According to Darren Williams of Pennsylvania State University, “Large moons are not required for a stable tilt and climate. In some circumstances, large moons can even be detrimental, depending on the arrangement of planets in a given system. Every system is going to be different.”

Apparently the assumption that a planet needs a large moon in order to be capable of supporting life was a bit premature. The results so far from the Kepler mission and other telescopes have shown that there is a wide variety of planets orbiting other stars, and so probably also moons, which we are now also on the verge of being able to detect. It’s nice to think that more of the terrestrial-type rocky planets, with or without moons, might be habitable after all.

22 Replies to “Life on Alien Planets May Not Require a Large Moon After All”

  1. The key comment for me here is not just that every planet is different, but that “every system is different”, and what might appear to be the same type of planet in one system might very well be a very different type of planet in another system. That means there are more permutations and combinations than even we thought in our galaxy. Fascinating.

  2. Personally I think “life” crawling out of the “primeval” swamp needed the tides to wash it up on the shore for millions of years to make that big difference we have with plants and mammals. So just stability I don’t think is the only influence. It’s reflective properties to me are important too.

    1. It’s just as well planets without big moons still have tides then, isn’t it? The Sun contributes about 1/3rd of the total tidal energy seen here on Earth, so while the tides would not be a big, they would still be significant.

    2. Personally I think “life” crawling out of the “primeval” swamp needed the tides to wash it up on the shore for millions of years to make that big difference we have with plants and mammals.

      Plants, fungi and animals originated earlier than their populating the land. According to a fresh paper, the animal body plan toolkit were probably originating ~ 800 Ma bp [million years before present] way before the Cambrian. We have definitive animal fossils from ~ 560 Ma bp. While animals (millipedes) were populating land ~ 420 Ma bp.

      Something similar can be seen in plants. (Which interestingly in the context, originated as multicellular 4 times, as brown algae, red algae and two types of green algae. One of the later went on and become green plants.)

      There are hypotheses that makes tides important for land population, making a nutrient supply for both plants and animals by washing ashore sea life. However it doesn’t seem to make a predictive difference in evolution pathways taken.

      There are also hypotheses that makes tides important for abiogenesis, building nucleotides or protein enzymes by cycling. However it doesn’t seem to make a predictive difference in evolution pathways taken. And cycling is also provided around hydrothermal vents.

      It’s reflective properties to me are important too.

      Certainly many parameters modify the essential habitability requirement of temperature and water availability (i.e. liquid water). But nothing that goes into the Rare Earth category.

      The only finetuned parameter for life seems to be amount of water, since a factor of ten would make the difference between an arid and ocean Earth. But that is only essential for a technological civilization. (Since our kind of technology demands access to dry spaces.)

    3. Even without a moon of meaningful mass, you’ll still have solar tides and weather action to do this…

    4. Even without a moon of meaningful mass, you’ll still have solar tides and weather action to do this…

  3. I assume spin axis stability depends on the size of the planet, its angular momentum and the presence and arrangement of gas giants in the system.
    i.e. a moonless earth in a system without Jupiter and Saturn for example might be very stable.
    Ditto a 4x earth super earth with a smaller jupiter and the same ~24 hr day.

  4. If there were no moon, all the things that cratered it would most likely have impacted upon the earth, so as a cosmic body guard it appears, IMHO, that it is just as important an influence in this regard as any other,,, without it the cosmic reset button may have been pushed many more times than it has, eradicating life many times over, keeping it from getting a firm toe hold on land, and things would surely have evolved much differently. Don’t you think?

    1. Consider this: if the Earth were 1 inch in diameter, the Moon on that scale would be just over 1/4 inch in diameter and about 30 inches away. Hmm… not much of a “cosmic body guard”, is it?

      1. The big stuff can and has gotten through many times. But consider also ocean impacts, weather erosion, continental drift (including mountain building and subduction), volcanism, and assorted other forces that hide and/or destroy impact crater evidence, over long periods.

        The Moon has none of that. Lava flows wiped clean some areas early in its history. But today, the only way to eliminate a crater there, is to wear it down (subdue it) with eons of micrometeorite impacts, whose cumulative effect is like sandblasting…

        …or obliterate it with a later, larger impact, creating an even larger crater.

      2. The big stuff can and has gotten through many times. But consider also ocean impacts, weather erosion, continental drift (including mountain building and subduction), volcanism, and assorted other forces that hide and/or destroy impact crater evidence, over long periods.

        The Moon has none of that. Lava flows wiped clean some areas early in its history. But today, the only way to eliminate a crater there, is to wear it down (subdue it) with eons of micrometeorite impacts, whose cumulative effect is like sandblasting…

        …or obliterate it with a later, larger impact, creating an even larger crater.

    2. Earth has suffered as many impacts as the Moon, but the atmosphere makes all the difference:

      1. The atmosphere causes smaller objects to burn up before they hit the surface.

      2. Craters left by larger impacts are eroded away due to weather.

      Only the largest or relatively new impacts remain visible.

    3. Do not compare the Moon with Jupiter which can realy cleanup a lot of debre from the space. Actualy if a medium size meteor would hit the moon i bet a lot of debre including huge rocks would fall on Earth. It`s sad that the scientists can`t imagine life in different conditions than here on Earth .It`s like watching all those Star Trek series where all the aliens were humanoid types…(ok, i saw 1 where the alien girl had 3 titties but that was all the difference). If life emerged from the oceans, believe me that the Moon had nothing to do with it.

    4. The reason some solar system surfaces have many craters and some few is in general that the later have younger surfaces.

      The Moon haven’t been remodeled by anything else than impactors since the lava plains solidified some million years after the Moon at large did.

      While Earth surface is constantly remodeled by plate tectonics and weathering. Craters are erased and even our large craters are barely recognizable after a while.

      without it the cosmic reset button may have been pushed many more times than it has, eradicating life many times over,

      No, I don’t think so, for good reasons it seems to me.

      When you actually model it, no plausible impact rate would eradicate already existing cellular life:

      “Our analysis shows that there is no plausible situation in which the habitable zone was fully sterilized on Earth, at least since the termination of primary accretion of the planets and the postulated impact origin of the Moon.”

      The basic reason it takes a giant impactor like the one that created our Earth-Moon system is because cellular populations procreate and disperse much faster than impactors can keep up. Later models found a Goldilocks survival zone ~ 1 km down the crust, where we know prokaryotes and even eukaryotes (nematodes) live, that would make crust busters survivable.

      Likely only crust removers (Earth-Moon impactor) would sterilize a planet.

  5. Life here is adapted to here. We know nothing of other life outside earth or it’s requirements so all we can do is guess. Biology adapted to a planet with different condions that are as chaotic as a moonless world suggests would certainly have an interesting and varied evolutionary history!

  6. I wonder how much play this finding will get in the “Privileged Planet” (creationist) community. Not much I’d wager. They love to trumpet any scientific finding that makes Earth appear to be closer to being a unique specimen in the Universe (regarding the hosting of intelligent life) but anything that makes it more likely that life could be found elsewhere is typically dismissed as materialist bias.

  7. I thought it was very obvious looking at the retrograde Venus. There are only a few attempts to explain its apparently stable counter spin with couplings to the Sun, none too successful what I know of.

    So I never made much of the hypotheses. Especially since the Rare Earth creationist contingent lashed onto it, then you can be fairly certain it is a dud theory. =D

    As for large moons, unless Venus retrogression is a result of early giant migration, all our terrestrials and Pluto could have had late giant impacts. Mercury has a relatively thin crust that could result from a giant impact, Venus is retrograde that could result from a giant impact, Earth-Moon has a large impact moon, Mars has remnants of impact moons that never coalesced.

    Either 1 out of 4 or 2 out of 5 (if Pluto-Charon counts) terrestrial type bodies seems to have large moons caused by frequent giant impacts.

  8. Did ANYONE look at Mars? Maybe these guys should go back to the drawing board.

    Assuming they are anywhere near right, which I seriously doubt, given the climatic changes that would accompany a 40 to 100 degree swing in the polar alignment, “evolving” species would have a hard time making it to the point of multi-celled beings. Look at what a degree or less shift did to the North African Savannah.

  9. Did ANYONE look at Mars? Maybe these guys should go back to the drawing board.

    Assuming they are anywhere near right, which I seriously doubt, given the climatic changes that would accompany a 40 to 100 degree swing in the polar alignment, “evolving” species would have a hard time making it to the point of multi-celled beings. Look at what a degree or less shift did to the North African Savannah.

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