Extraterrestrial life may be more likely than we thought

The only planet on which we know for sure that life exists is Earth. Even though this gives us just a sample of one, it is tempting to think that conditions here must be close to the ideal for life to emerge and that if there are other planets on which life exists, they must be Earth-like in terms of size and occupying the so-called ‘stellar habitable zone’ (SHZ) from its star so that the temperature range is similar to ours.

But a new paper in the journal Astrobiology titled Superhabitable Worlds by Rene Heller and John Armstrong suggests that it may not be the case that our planet’s characteristics make it the most hospitable, and in fact maybe merely marginally habitable, and that there may be so-called superhabitable planets that are two to three times bigger than Earth and orbiting stars that are somewhat less massive than our sun.

The authors look at the many factors that affect habitability: habitable surface area, total surface area, land-to-ocean fraction and distribution, plate tectonics, magnetic shielding, climatic thermostat, surface temperature, biological diversification, multihabitability and panspermia, localization in the stellar habitable zone, age, stellar mass, stellar UV irradiation, stellar lifetime, early planetary bombardment, planetary spin, orbital dynamics, and atmosphere.

After looking at the interplay of all these features, they conclude:

Eventually, just as the Solar System turned out to be everything but typical for planetary systems, Earth could turn out to be everything but typical for a habitable or, ultimately, an inhabited world. Our argumentation can be understood as a refutation of the Rare Earth hypothesis. Ward and Brownlee (2000) claimed that the emergence of life required an extremely unlikely interplay of conditions on Earth, and they concluded that complex life would be a very unlikely phenomenon in the Universe. While we agree that the occurrence of another truly Earth-like planet is trivially impossible, we hold that this argument does not constrain the emergence of other inhabited planets. We argue here in the opposite direction and claim that Earth could turn out to be a marginally habitable world. In our view, a variety of processes exist that can make environmental conditions on a planet or moon more benign to life than is the case on Earth.

As someone who hopes that there is life out there somewhere, I find this study encouraging.


  1. Rob Grigjanis says

    suggests that it may [not?] be the case that our planet’s characteristics may make it the most hospitable

  2. David Marjanović says

    Very interesting. But… how common are such large planets around such small stars? Aren’t they similarly rare?

    And what about the requirement (Ward & Brownlee 2000) of a large moon to stop a planet from changing the axis it rotates about every once in a while?

  3. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says

    Extraterrestrial life may be more likely than we thought

    As someone comfortable with saying, “I don’t know,” I must not be included in your “we”.

    Why is it that knowledgeable people have trouble saying “I don’t know”.

    Isn’t your “I don’t know” how you identify new research opportunities?


    Well, I will continue to find the question curious and continue to not know until

    1) we quantify how worlds are dispersed within the milky way
    2) we undertake a systematic surveys of worlds randomly selected from each different stellar environment in the milky way that might affect capacity for life (globular clusters, arm centers, galactic bulge, etc. – whatever other research determines is relevant).
    3) we quantify how these environments vary between the milky way and other galaxies
    4) we quantify how worlds are dispersed between those environments in other galaxies (for instance, do galactic collisions affect this?)

    I’m willing to accept the research of aliens we might meet if we can examine and validate their data and analysis.

    Up until that point, I’m more interested in the details of individual worlds than someone’s guess at Drake Equation values.*

    *I’m not saying that this research was just about quantifying Drake Equation values, but it certainly seems like that was part of the point…and the title of the post emphasized that part, it seems to me.

  4. Crip Dyke, Right Reverend Feminist FuckToy of Death & Her Handmaiden says


    Y’know, I’ve considered the moon “requirement”.

    It really depends on how fast and how often the axis changes. Extinctions on earth were drivers of new variations. It seems like the moon’s absence could **only** affect life in a macro sense – the micro-mixing of abiogenetic chemistry doesn’t appear likely to be affected by axial tilt as it appears to require local sources of energy, not macroscopic energy-exchange systems like sun->plant->animal

    The Snowball Earths of the late proterozoic show how hard single celled organisms are to kill with cooling/warming trends. Though we can’t have data on the topic, it seems such wanderings would have less impact than a conversion from ice free to totally frozen.

    On Earth that means the effects would be limited to the evolution of metazoans, and even here it’s more likely to affect nektons than planktons, and land life than oceanic.

    Even ward and brownlee concede this is the case.

    So we’re talking more about the potential for trees and people being cut short than for life itself to be cut short.

    And, I have to wonder, wouldn’t such environments favor the evolution of organisms that can respond to such changes if they were sufficiently frequent? And if insufficiently frequent to create ongoing evolutionary pressure, what would it do to have “sunset clocks” on current species? Would that really constrain the ability to evolve complex characters?

    I think Ward & Brownlee are right that earth is just right for us, and that it would be very rare to see life like us elsewhere…

    …but that’s a pretty small constraint on life.

  5. brucegee1962 says

    Was the study looking for the odds of life developing, or intelligent life? I ask because one of the constraints they mentioned was land-to-ocean fraction. Obviously intelligent (by which I think we mean tool-using) life is more likely on land, but since life doubtless began in the oceans, it seems like a large ocean percentage would make life overall more likely even while it made intelligent life less likely.

    As for me, when I look at the history of life on earth in light of the Fermi Paradox, it seems to me that there is in all possibility a bottleneck at some point between single-celled replicators and us, but it probably isn’t at the formation of life itself. Heck, our whole planet would probably still be ruled by dinosaurs if it hadn’t been for one lucky meteor. And our own ancestors were practically wiped out and got down to just a few thousand at one point. Or maybe it was the development of sexual exchange of DNA that we got lucky. I’m guessing that the future Captain Kirks (or future telescopes that can detect planetary atmospheres) are going to find many, many worlds with various flora and fauna, but nobody with our biologically unnecessary brains — after all, we still haven’t even figured out why they were useful enough for our ancestors to expend all that energy on.

  6. Mano Singham says


    The paper was looking at just life. Of course, the longer that life exists, the greater the odds that intelligent life will emerge.

  7. editor-b says

    Then, perhaps, is there fossils or life elsewhere? But, isn’t the emergence and maintenance of life a process of radical contingency? That is, is a unique and unrepeatable past totally necessary? Or does life emerge through space like mushrooms when some conditions are present? So, how many conditions are necessary: three, four, trillions, infinite? Only one, water or any sort of God? Is God the word that means infinite conditions, absolute necessity, habitability? Anyway, how did the life that emerge in a given conditions resist when switching to a different moment? How does life resist time itself, the effects of entropy? But, is it possible for human beings to recognise a simpler life than their own brain only? On the other hand, beyond likeness, is it possible to recognise a complex life than their brain, is this the extra-terrestrial life that some people are searching unsuccessfully? However, is there an origin of life or would it be as finding a cut in the material history of the universe, an infinite void that human language patches now? Along these lines, there is a peculiar book, a short preview in goo.gl/rfVqw6 Just another suggestion, far away from dogmas or axioms.

  8. Robert B. says

    after all, we still haven’t even figured out why they were useful enough for our ancestors to expend all that energy on.

    @6: Though I do like the Machiavellian Hypothesis, that the proto-human society itself imposed a survival pressure and so we evolved to outsmart our neighbors. Still, it’s a hypothesis not a theory.

  9. bobmunck says

    I’m curious why discussions like this limit their scope to the surfaces of planets and to beings that are mostly the solid and liquid states of matter. Wouldn’t the environments of stars be much more likely to produce and nurture life?. There’s a lot more energy, a lot more movement, a huge amount more total volume in the universe. There are strong and complex magnetic and (I think) electrical fields to hold things together.

    Has this possibility been discussed elsewhere, and I just missed it? If so, where and by whom?

  10. Mano Singham says


    The temperatures of stars, even in the cooler parts like the surface, are in the tens of thousands of degrees which is why there is unlikely to be life there.

  11. bobmunck says

    tens of thousands of degrees which is why there is unlikely to be life there.

    How does a particular temperature rule out the possibility of life? Are you assuming that it has to be solid matter? That it couldn’t, for example, be formed from a complex interaction of various kinds of plasma and magnetic fields? (Just to calibrate the discussion, my advisor at Brown was Leon Cooper (BCS). I know a little physics.)

  12. Lassi Hippeläinen says

    If our planetary system is in any way representative, there should be more rocky satellites around gas planets that there are rocky planets. If life is ever found, it’s more likely to be on a satellite. They are more variable that the planets, and due to the tidal forces of their mother planets, they can be warn enough even at the distance of Jupiter. And their axes won’t wobble.

  13. Mano Singham says


    I was assuming that life (it could be liquid or gas) would still consist of entities made up of atoms and molecules and at those temperatures they could not exist because the atomic and molecular bonds would be broken.

    For a plasma of nuclei and free electrons to form something like life, there would have to be some force that made subset of them behave in a collective manner.

    The paper Rob linked to was interesting but the plasma they were referring to contained dust grains that could exist in the interstellar medium. In stars, the dust would quickly decompose into nuclei.

  14. nobonobo says

    It is only natural to think of earth, as it is today, as the ideal for life. I just started reading a book that points out the trouble with this idea:

    Yet what exactly is an “Earth-like” planet? As anyone who has watched or read any science fiction knows, such a planet has water and a breathable-oxygenated atmosphere. But we are coming to realize that our view of an Earth-like planet usually means one with an atmosphere similar to that found on Earth at the present-day. Yet our current atmosphere is but a slice of a forever-changing entity and is greatly different from the atmosphere at most times in Earth’s history. It is currently suited to us mammals—hence the high diversity of mammals alive today. But again, this has not always been the case. Two not-so-ancient versions, in astrobiological timescales, of our “Earth-like atmosphere” very nearly wiped out our furry ancestors some 250 million years ago and then tried again some 200 million years ago. If a small premammal named Thrinaxodon, whose delicate skulls have been collected in lowest Triassic strata, had not survived, what would life on Earth be like now?

    Ward, Peter (2006-09-26). Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere (Kindle Locations 158-165). National Academies Press. Kindle Edition.

  15. birger johansson says

    Brucegee, I agree. In fact, future star travellers (AIs and robots?) will probably find lots of stromatolites, but that is like finding plaque on your teeth. You can not have a useful dialogue with it.
    Life on land will be lichen analogues, if we are lucky*. And sadly we will find many postzooic worlds , where the water has been lost or tectonic activity has ceased (making any biosphere unstable),

    BTW for literature I would recommend Stanislaw Lem, who adressed the “silentiuum universii” in many of his stories.

    *which explains the Fermi paradox.

  16. Rob Grigjanis says

    birger johansson: I would also recommend this essay by Lem. It’s a criticism of the state of SF in 1977, but can also apply to our more mainstream attitudes to what’s “out there”.

    The SF of today resembles a “graveyard of gravity,” in which that sub-genre of literature that promised the cosmos to mankind, dreams away its defeat in onanistic delusions and chimeras—onanistic, because they are anthropocentric.

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