The Earth and Moon as seen from Mariner 10 en route to Venus. This could be a similar view of two moons as seen from Earth. Image credit: NASA/courtesy of nasaimages.org

What if the Earth had Two Moons?

Article Updated: 24 Dec , 2015
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The idea of an Earth with two moons has been a science fiction staple for decades. More recently, real possibilities of an Earth with two moons have popped up. The properties of the Moon’s far side has many scientists thinking that another moon used to orbit the Earth before smashing into the Moon and becoming part of its mass. Since 2006, astronomers have been tracking smaller secondary moons that our own Earth-Moon system captures; these metre-wide moons stay for a few months then leave.

But what if the Earth actually had a second permanent moon today? How different would life be? Astronomer and physicist Neil F. Comins delves into this thought experiment, and suggests some very interesting consequences. 

This shot of Io orbiting Jupiter shows the scale between other moons and their planet. Image credit:NASA/courtesy of nasaimages.org

Our Earth-Moon system is unique in the solar system. The Moon is 1/81 the mass of Earth while most moons are only about 3/10,000 the mass of their planet. The size of the Moon is a major contributing factor to complex life on Earth. It is responsible for the high tides that stirred up the primordial soup of the early Earth, it’s the reason our day is 24 hours long, it gives light for the variety of life forms that live and hunt during the night, and it keeps our planet’s axis tilted at the same angle to give us a constant cycle of seasons.

A second moon would change that.

For his two-mooned Earth thought experiment, Comins proposes that our Earth-Moon system formed as it did — he needs the same early conditions that allowed life to form — before capturing a third body. This moon, which I will call Luna, sits halfway between the Earth and the Moon.

Luna’s arrival would wreak havoc on Earth. Its gravity would tug on the planet causing absolutely massive tsunamis, earthquakes, and increased volcanic activity. The ash and chemicals raining down would cause a mass extinction on Earth.

But after a few weeks, things would start to settle.

Luna would adjust to its new position between the Earth and the Moon. The pull from both bodies would cause land tides and volcanic activity on the new moon; it would develop activity akin to Jupiter’s volcanic moon Io. The constant volcanic activity would make Luna smooth and uniform, as well as a beautiful fixture in the night sky.

New Horizons captured this image of volcanic activity on Io. The same sight could be seen of Luna from Earth. Image credit: NASA/courtesy of nasaimages.org

The Earth would also adjust to its two moons, giving life a chance to arise. But life on a two-mooned Earth would be different.

The combined light from the Moon and Luna would make for much brighter nights, and their different orbital periods will mean the Earth would have fewer fully dark nights. This will lead to different kinds of nocturnal beings; nighttime hunters would have an easier time seeing their prey, but the prey would develop better camouflage mechanisms. The need to survive could lead to more cunning and intelligent breeds of nocturnal animals.

Humans would have to adapt to the challenges of this two-mooned Earth. The higher tides created by Luna would make shoreline living almost impossible — the difference between high and low tides would be measured in thousands of feet. Proximity to the water is a necessity for sewage draining and transport of goods, but with higher tides and stronger erosion, humans would have to develop different ways of using the oceans for transfer and travel. The habitable area of Earth, then, would be much smaller.

The measurement of time would also be different. Our months would be irrelevant. Instead, a system of full and partials months would be necessary to account for the movement of two moons.

A scale comparison of the Earth, the Moon, and Jupiter's largest moons (the Jovian moons). Image credit:Image Credit: NASA/courtesy of nasaimages.org

Eventually, the Moon and Luna would collide; like the Moon is now, both moons would be receding from Earth. Their eventual collision would send debris raining through Earth’s atmosphere and lead to another mass extinction. The end result would be one moon orbiting the Earth, and life another era of life would be primed to start.

Source: Neil Comins’ What if the Earth had Two Moons? And Nine Other Thought Provoking Speculations on the Solar System.

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25 Responses

  1. James Lovell says:

    The pull from both bodies would cause land tides and volcanic activity on the new moon; it would develop activity akin to Jupiter’s volcanic moon Io.

    I’m sceptical. Io orbits Jupiter at roughly 400,000km and this hypothetical moon would orbit at roughly 200,000km – twice as close, but orbiting a body 300 times less massive. I doubt that the Earth (even with the Moon’s help) would exert so much tidal stress on “Luna”, even it were Moon-size. I may be wrong.

    The constant volcanic activity would make Luna smooth and uniform

    It’s worth noting that Ganymede orbits Jupiter at roughly 1,000,000km (just over 5 times as far – but again, orbiting a planet 300 times more massive) and its surface is highly cratered – suggesting that its surface is largely defined by impacts rather than volcanic activity. I would expect Luna’s surface to be similar.

    The combined light from the Moon and Luna would make for much brighter nights, and their different orbital periods will mean the Earth would have fewer fully dark nights. This will lead to different kinds of nocturnal beings

    Given how much of a wildcard cloud cover plays in the brightness of any given night, I doubt there’d be that much difference in nocturnal evolution. As this paragraph notes toward the end – the evolution is driven by relative fitness of prey and predator – regardless of how much light is going around.

    the difference between high and low tides would be measured in thousands of feet.

    If Luna is Moon-size, and it sits half as far away, wouldn’t the pull on the tide be only 4 times as great? Perhaps this would result in thousand-feet tides but again I’m sceptical.

    Eventually, the Moon and Luna would collide; like the Moon is now, both moons would be receding from Earth.

    I’m no celestial mechanician but if they’re both receding how would they collide? Would the inner moon necessarily recede faster?

    • magnus.nyborg says:

      “Eventually, the Moon and Luna would collide; like the Moon is now, both moons would be receding from Earth.

      I’m no celestial mechanician but if they’re both receding how would they collide? Would the inner moon necessarily recede faster?”

      I suspect you are correct in your implication. Two important factors:

      1. The ‘inner’ Luna would probably receede slower than the outer Moon. The tidal interaction that causes this, is also dependant on the differences between earth rotation and moon orbit. As an example, a moon in a geosynchronized would not receede at all. It is not that simple, but assuming faster recessions seems unwarranted.
      2. Orbital interaction between Moon and Luna would require investigation, and likely they would keep some form of resonance.

  2. HeadAroundU says:

    With 2 moons you would be Tamy Megashira Tautel. 😀 :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. 😀

    Amyway, I liked the article even though it is kinda fictional.

  3. HeadAroundU says:

    With 2 moons you would be Tamy Megashira Tautel. 😀 :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. 😀

    Amyway, I liked the article even though it is kinda fictional.

    • The Bobs says:

      ” what are you? German/Japanese???, that would be a scary combination.”

      What are you, some kind of racist? Oh wait, you put a smiley face on it, so it must be OK.

      • HeadAroundU says:

        Well, what’s your origin sounds better. I’m sorry guys. I don’t want to sound like a racist. Otherwise, is it taboo to ask?

    • The Bobs says:

      ” what are you? German/Japanese???, that would be a scary combination.”

      What are you, some kind of racist? Oh wait, you put a smiley face on it, so it must be OK.

  4. HeadAroundU says:

    With 2 moons you would be Tamy Megashira Tautel. 😀 :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. 😀

    Amyway, I liked the article even though it is kinda fictional.

  5. Torbjörn Larsson says:

    I’ll pitch in with squidgeny, Comins seems severely off on particulars. Especially his biology sucks:

    The size of the Moon is a major contributing factor to complex life on Earth.

    Preposterous! We don’t know that. _Some_ theories puts tides as important for abiogenesis (simple life) and land life (complex life). But besides being in the minority, they have very non-parsimonous predictions by their very nature, having to drag in tides with them.

    It is responsible for the high tides that stirred up the primordial soup of the early Earth,

    What does that even mean? The soup, if that is the theory under consideration, wouldn’t need stirring to evolve. In fact tides would be, as it is for life today, a problem to adapt to.

    Theories that rely on tides are not soup theories in general. There are specifics. Like Miller’s freeze/thaw cycles of ice promoted biochemistry that would interact with tides. And there is the “protein pump” theory, where wet/dry cycles polymerize proto-protein polymers to build a “protein first” world.

    it’s the reason our day is 24 hours long,

    Yay! Think of a 23.5 hour day. How distressing!

    The Moon was also the reason our initial day after impact was something like 7 hours. FWIW. [/sarcasm]

    it gives light for the variety of life forms that live and hunt during the night,

    But it doesn’t give any more light to the sun the immense variety of life forms that live and hunt during the eternal night of the deep sea.

    Yes, it gives variety, but presumably a lack of Moon light would have contributed even more. Now night adapted eyes does just fine, no need for fancy stuff like phosphoresence.

    it keeps our planet’s axis tilted at the same angle to give us a constant cycle of seasons.

    It doesn’t keep the same angle, obliquity changes from time to time. And this shows how Comins and other Rare Earth theorists are dated: we would do just fine without a large, close moon:

    “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. […]

    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.””

    The need to survive could lead to more cunning and intelligent breeds of nocturnal animals.

    We don’t really know why intelligence develops.

    As a side note, I think the latest hypothesis concerning hominids large brain is both robust, humorous and tragic. There is an anti-correlation between having a large brain of fatty deposits. Therefore the hypothesis becomes that animals choose their form of storage.

    If species like hominids were often subjected to starvation and bottlenecks, as it seems, we could have rather counterintuitively developed large energy-hungry brains as a buffer mechanism. When starvation hits a large fatty proteinous brain is excellent to metabolize, while keeping some minimal wits around when there is food again.

    We are intelligent because we were stupid. (O.o)

  6. Anonymous says:

    I’ve read Comins new book, (and its predecessor, “What if the Earth had No Moon”, and I don’t think it’s very good. Unlike the first book, his ideas are poorly considered and constrained (why didn’t he have the second moon be there at the start instead of getting captured after formation, which is very unlikely)? And he also neglects to consider resonances in the two-moon scenario, which would prevent the moons from colliding.

    “What if the Earth Had Two Moons” is full of incomplete ideas (the chapters just suddenly stop in the middle of things, as if he’d just ran out of ideas and stopped writing) and contains some rather poor fiction to introduce the chapters. I would recommend his first book instead, which is much better and more interesting.

  7. Anonymous says:

    I’d be more worried about being born female. Biological rhythms would be an absolute mess, and biological / reproductive sex of all the creatures of the Earth would be completely different.

  8. hdmi says:

    What if your mom was your dad?

  9. sigidunum says:

    No offense to Mr. Comins, but this is pretty junky.

    Just to point out one problem: all other things being equal, tidal forces are proportional to the inverse cube of the distance. So if Luna were the same mass as the Moon (he doesn’t say this, but let’s assume) and the same density as well, then it would produce tides 8 times as great. That would result in some huge tides, but even in the Bay of Fundy it wouldn’t mean “thousands of feet”.

    To be even more precise: solar tides account for about 1/3 of ocean tides, and lunar tides the other 2/3. (That’s why we have spring and neap tides — when the two are working together, the tides are bigger.) So in this alternate Earth, we’d have solar tides, lunar tides, and Lunar tides, with the latter generating (8 x 2/3) as much force as a normal high tide. So a full-power spring tide with all three working together would be about (1/3 + 2/3 + 8/3 =) about four times as high as a high spring tide on Earth today. We’d definitely see big differences, but it wouldn’t result in coastlines moving by thousands of feet. Honestly, that’s just silly.

    Given that Comins gets basic stuff like this wrong, it’s hardly surprising that he doesn’t even address questions like orbital resonance and orbital evolution.

    Really, though, this is just slapdash. It reads like something written by a high schooler, and it’s a bit embarrassing to see it here on Universe Today.

    Doug M.

  10. Anonymous says:

    I am not likely to read this book. It is too much of a “what if?” However, the rather extensive predictions here seem to mean this moon has a mass equal to the mass of our current moon. A rather modest body, say the mass of Enceladus or even small such as the martian moons would be far less perturbing. In fact if we could park a 10 km diameter asteroid at geosynchronous orbit we might actually stand a chance of building a space elevator. An Enceladus massed moon might be mildly perturbing. If it were in a close orbit to Earth it might actually be stable.

    LC

  11. Anonymous says:

    Reading through comments, I thought to add this as an, au contraire, to a few points:

    From Astrobiology Magazine ( 10/29/07 ) by Bernard Foing (SMART-1 principal scientist): PhD in Astrophysics & Space Techniques; Astronomer for 3 years at ESO, Chile…; Professor of Astrophysics; researcher at CNRS since ’86 — Wikipedia

    “The crust of the Earth is also affected. THE MOON’S TIDAL FORCING causes significant heating and dissipation of energy to take place. Part of this energy is heating the Earth, and part of it is dissipated by forcing the Moon to recede from the Earth over time. There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the Earth that led to the multi-plate tectonics. The other planets don’t have the same tectonic cycle. For most of them, the crust is like a lid that doesn’t move much horizontally, and the magma and heat are blocked by this lid on the surface. The Earth instead has rolling convective motion that drags the crust, and then the crust plunges back down into the mantle and gets recycled. ….

    “The Moon has been A STABILIZING FACTOR FOR THE AXIS OF ROTATION OF THE EARTH. If you look at Mars, for instance, that planet has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the Earth’s moon has helped stabilize our planet so that its axis of rotation stays in the same direction. For this reason, we had much less climatic change than if the Earth had been alone. ….

    “Finally, the Moon had a KEY ROLE IN THE EMERGENCE OF SCIENCE, and in our understanding of our place in the universe. We saw the repetition of the phenomena of lunar phases, and we observed solar and lunar eclipses. These were big challenges to our understanding of nature, and a few astronomers were put to death because they weren’t able to predict the eclipses. This challenged us to develop accurate predictions for the motion of the sun and the motion of the Moon.

    “Studying the Moon helped us determine distances in the solar system and the size of celestial objects. By studying lunar phases, for example, people were able to determine how far the Moon is from the Earth, the size of the Earth, and our distance from the sun. More recently, the Moon was the terrain where the space race took place between two political systems, allowing for great technical and scientific achievements.” (emphasis mine) —>

    http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=2507
    __________________________

    The Earth-Moon “double planet” System is extraordinary. If nothing else, these “What If” articles can help us appreciate what a beneficent role this one moon plays on our Homeworld stability, this life-platform, Earth. No violent accident, this precisely-matched, time-significant, and Sun-aligning coupling in Solar Space.

    • Torbjörn Larsson says:

      That touches many or the Rare Earth topics, including those that are raised by creationists:

      THE MOON’S TIDAL FORCING causes significant heating and dissipation of energy to take place.

      We don’t really know all the characteristics of plate tectonics, including the lower limit. It is believed that Earth is marginal, so the same argument can be made for a little less water. (The presence of water is that makes subducted oceanic plates sufficiently malleable and melting.)

      However, it is believed from modeling that larger terrestrials have plate tectonics with or without “rare” amounts of water, “rare” amounts of early large and close moons, et cetera.

      The Moon has been A STABILIZING FACTOR FOR THE AXIS OF ROTATION OF THE EARTH.

      As per the comments, irrelevant by new results.

      the Moon had a KEY ROLE IN THE EMERGENCE OF SCIENCE,

      Ah yes, the “Privileged Planet” of creationists.

      The fact is we don’t know how much the Moon meant for science and its emergence in terms of help/problem and timing. Certainly a twin sun, the more normal system, would have been much more useful to elucidate gravity and orbital mechanics if the Moon was.

      Also, as the recent GRAIL post reminds us, the presence of lunar eclipses caused by such presumed ‘finetuning’ makes Moon science much more difficult.

      As opposed to Rare Earth bayesian reasoning. we have every testable reason to believe the Moon have had severely adverse effects on Earth habitability.

      – The Earth-Moon impactor may have been as late as 4.36 Ga bp (billion years before present) while Earth aggregated 4.54 Ga bp. If the crust solidifies in a few tens of Ma, the 180 Ma was sufficient time for abiogenesis.

      The Nuvvuagittuq tentative dating of 4.28 Ga bp rocks means a crust and liquid water aggregates in at most 80 Ma. (For reasons of gene family dating putatively reaching back to ~ 4.31 Ga bp, I believe ~ 40 Ma is the more reasonable upper limit.)

      So perhaps the Moon follows from the only globally sterilizing event the Earth ever saw. Hopefully the GRAIL mission will go some distance in elucidating the Earth-Moon systems early days.

      – The early Moon with its larger liquid core had a magnetic field that affected Earth’s own. It is likely that the then strength was not enough to affect atmosphere erosion.

      But it is not certain, so maybe we would have a more dense atmosphere without it.

      We would certainly had more volatiles for extended biosphere lifetime without the Earth-Moon impact. It has been claimed that the upper limit will be increased mineral uptake of carbon from the ever hotter Sun making the carbon dioxide level too small for plant life in some ~ 0.2 – 0.5 Ga. We would have needed the carbon the Moon stole from us!

      – Tides may help abiogenesis and complex life in some less likely scenarios, but as I noted before it is probably more of an environmental problem to adapt to. And early pre-cell and proto-cell systems were not that suited for adaptation, so this was a risk and presumably an evolutionary delay successfully conquered.

      The Moon is a romantic piece of property, but it must be recognized that every property has its positives and negatives.

      • Anonymous says:

        We would certainly had more volatiles for extended biosphere lifetime without the Earth-Moon impact […] We would have needed the carbon the Moon stole from us!

        But wouldn’t that carbon have been in the impactor that formed the Moon anyway? If the Moon hadn’t formed, then presumably that impactor would never have existed.

      • Torbjörn Larsson says:

        Good and quick catch! I hadn’t thought that one out.

        I guess this could be argued both ways:

        – If we are not given the impactor, we are worse off on volatiles.

        – If we are given the impactor, we are better off without the Moon. The ideal result would have been deorbited satellites, as for Mars. Most moons from a presumed impact deorbited/will deorbit (Phobos), it will keep only Deimos indefinitely.

      • Torbjörn Larsson says:

        Suddenly I lost edit capability. To add to r

      • Anonymous says:

        Well numerical simulations are one thing, real-world physics are another. Mars does seem to have a tale to tell in that regard. There may be a story in the retrograde motion of Venus to hear, as well, another lifeless wreck of a world, in the local neighborhood of stark contrasts to one Life-World turning in time within an array of fine-balances through space: Moon of just the right weight, size, distance (in month cycling orbit, part of our celestial time-clock).

        Maybe a big moon is not so critical in a Billion-year emergence-of-life Evolution frame-scenario – maybe; I, for one, would not want to bet the future of our Geo-physical stability (oceans-continents-atmosphere: tectonics motions and ocean movements (levels too); atmosphere circulation and climate variation, along with axis orientation, and myriad moonlight rhapsodies of life-rhythms, …, and who knows what else!?) on cold theoretical models of computer simulations – and live on a Moonless Earth. Would You?

        [ Composing this, it occurred to me, Does the Moon have some vital role to play regarding Earth’s dynamic – shielding – geomagnetic field: does it serve some “gear-work”-like purpose regarding the Core of our Planet? ]

        Of course, if a school of thinking does not like the “Rare Earth” view of things, this is one way to try and demote Earth’s life-crowning place, chip-away at the concept of a “Miracle Planet”.

        [ By the way, Mr. Bernard Foing’s essay is squarely framed in the Evolutionary World View. ]

        Because water is important to Plate Tectonics, does automatically play down that play down any possible importance of the Moon’s role? And yes, the terrestrial Planets Venus and Mars may once have had crustal plate mechanisms – and moons and oceans! Obviously, no longer. They are dead and in ruin.

        “The fact is we don’t know how much the Moon meant for science and its emergence in terms of help/problem and timing.” An amazing statement!

        Well, you will have to excuse me, Torbjörn Larsson, but the word of a astrophysicist- astronomer, and professor, will have a bit more weight with me ( and what I remember from past readings and programs ).

        “…the presence of lunar eclipses caused by such presumed ‘finetuning’ makes Moon science much more difficult.” – I do not read the author’s thoughts to mean study of the Moon itself (although, lunar mountains and valleys might have been visible to Hawk-eyed observer’s during Total Solar Eclipses, before the telescope, provoking some deep thought…)

        Well, you will have to excuse me, Mr./Miss Torbjörn Larsson, but the word of an astrophysicist- astronomer, and professor, will have a bit more weight with me.

        What about Solar science? Solar Eclipses?

        “Rare Earth eager pattern searching bayesian reasoning [ as opposed to “’frequentish’ school” of reasoning? – http://www.economist.com/node/5354696?story_id=5354696 ], we have every testable reason to believe the Moon may have had severely adverse effects on Earth habitability.” You will understand, if I take that with a large grain of salt.

        I expressed my view on the Earth-Impactor Model, one that strikes me as incredulous.

        Regarding biosphere life-times(?), carbon levels, what was lost and what might have been gained (global warmest line of thinking?) – the global fact is, life has flourished and thrived on this protected planet (another minor role the Moon plays?). Though its ailing badly, now, needless to say.

        Maybe I have been blinded in moonlight, Torbjorn, but I see no negatives in one Earth-embracing “estate” of “Magnificient Desolation”.

  12. some fiction this. and it sure started a debate.. a good read, fictionally, but as the commentators have pointed out skilfully, lots of junk. or should we say, debris? 😉

  13. Robert Geller says:

    I think some perspective might be helpful regarding the criticisms made below, because too many interesting and plausible scenarios are being disregarded by the commenters. The book by Neil Comins, “What if the Earth Had Two Moons” is a serious work by an accomplished physics and astronomy professor, and author of one of the country’s leading astronomy textbooks (Discovering the Universe). This Universe Today article summarizes some results, but can’t be as detailed with the book’s assumptions and methodology. For example, the tidal forces and orbital characteristics discussed in the book are based on high-quality simulations of the sort used elsewhere in astrophysical research, and the book explains why various masses, radii, etc, where used in simulations.

    By its very nature, discussing an earth with two moons as well as nine different lunar-planetary-solar scenarios requires various assumptions and – just as he states in the book’s subtitle – “though-provoking speculations”. The author is very careful to let the reader know when established phenomena ends and science-based speculation begins. Scientific understanding is constantly being refined, but this book reflects current scientific thinking about the origins and potential for life in the universe.

    As we discover new exoplanetary systems with Kepler, this book becomes more and more relevant, and also great reading for professional astrophysicists (myself and others) thinking about the bigger picture of these new worlds. However, the book is written for the general public, and succeeds greatly in explaining the rich potential for a variety of different planetary systems. The book is filled with many solid scientific gems spanning geology, planetary science, solar science, and the origins of life, and is a great read for all those trying to put together this larger scientific picture.

    As an aside, I think a few of the scenarios discussed in the book would make an excellent episode for Astronomy Cast by Fraser Cain and Dr. Pamela Gay (such as what might have happened if the Moon orbited backwards, and if the Earth orbited two suns instead of one).

  14. Robert Geller says:

    I think some perspective might be helpful regarding the criticisms made below, because too many interesting and plausible scenarios are being disregarded by the commenters. The book by Neil Comins, “What if the Earth Had Two Moons” is a serious work by an accomplished physics and astronomy professor, and author of one of the country’s leading astronomy textbooks (Discovering the Universe). This Universe Today article summarizes some results, but can’t be as detailed with the book’s assumptions and methodology. For example, the tidal forces and orbital characteristics discussed in the book are based on high-quality simulations of the sort used elsewhere in astrophysical research, and the book explains why various masses, radii, etc, where used in simulations.

    By its very nature, discussing an earth with two moons as well as nine different lunar-planetary-solar scenarios requires various assumptions and – just as he states in the book’s subtitle – “though-provoking speculations”. The author is very careful to let the reader know when established phenomena ends and science-based speculation begins. Scientific understanding is constantly being refined, but this book reflects current scientific thinking about the origins and potential for life in the universe (as well as the other topics involved).

    As we discover new exoplanetary systems with Kepler, this book becomes more and more relevant, and also great reading for professional astrophysicists (myself and others) thinking about the bigger picture of these new worlds. However, the book is written for the general public, and succeeds greatly in explaining the rich potential for a variety of different planetary systems. The book is filled with many solid scientific gems spanning geology, planetary science, solar science, and the origins of life, and is a great read for all those trying to put together this larger scientific picture.

    As an aside, I think a few of the scenarios discussed in the book would make an excellent episode for Astronomy Cast by Fraser Cain and Dr. Pamela Gay (such as what might have happened if the Moon orbited backwards, and if the Earth orbited two suns instead of one).

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