What is a “Habitable” Planet, Anyway?

By Corey S. Powell | July 31, 2018 11:45 pm
Exoplanets (planets around other stars) ranked by researchers at the Planetary Habitability Lab at the University of Puerto Rico. Note the abundance of disclaimers; the truth is, we know little about these planets beyond their size and their distance from their stars. (Credit: UPR Arecibo)

Exoplanets (planets around other stars) ranked by researchers at the University of Puerto Rico’s Planetary Habitability Lab. Note the abundance of disclaimers; the truth is, we know little about these worlds besides their size and the distance they orbit from their stars. (Credit: UPR Arecibo)

We are living at the greatest moment in history for finding life beyond Earth. We have robots poking and sniffing around on Mars. We have a space probe in the works that will take a close look at Europa, to examine the likelihood of life on an ice-covered ocean world. Above all, we no longer have to wonder if our solar system is unique: We know of thousands of planets around other stars, and it’s clear that the total number of planets in our galaxy alone must number in the billions, if not the trillions.

But with all these rapid advances have come acute intellectual growing pains. It is only natural that people want to know how many of those distant planets could truly support life–to know whether living worlds like our own are common or rare (or unique). The desire for answers runs far ahead of the ability of science to provide them, unfortunately. And so we end up with whiplash conclusions based on the latest speculative study: Red dwarf stars are great places to find habitable planets…or maybe they are deadly. Earth couldn’t sustain life without the Moon…or maybe the Moon is not necessary at all.

Too often these days, astrobiology research sounds like the ping-pong headlines from the world of health studies: One day coffee gives you cancer, the next day it saves your life. It’s hard to make sense of what we really know about the likelihood of alien life. Case in point: Does a habitable planet need active, Earthlike geology, including plate tectonics? A couple months ago, a long, thoughtful article in Quanta magazine made the case that plate tectonics is crucial to terrestrial life. Then two days ago, out comes a press release from Pennsylvania State University: “Plate tectonics not needed to sustain life.”

It’s not that either story is wrong; it’s that we don’t know enough to say which is right, or rather which is more right. There is no question that Earth’s system of geology, with moving plates and the constant cycling of crustal rock, has had a huge influence on our planet’s environment. Plate tectonics returns carbon into the air and mineral-rich water into the oceans, influencing the global environment. Obviously, all of that is important, but is it essential to life? As the new Penn State study indicates, we just don’t know.

The same goes for arguments about whether biology could take hold on planets around red dwarf stars. Small, dim red stars are by far the most common kind in our galaxy. They seem to preferentially have small, Earth-size planets. They also live a very long time, about a thousand times as long as hotter, more profligate stars like our Sun. If red dwarf planets can support life, then habitable worlds are all over our galaxy. Proxima Centauri, the very nearest star after the Sun, has an Earth-size planet (Proxima b) orbiting where its surface temperatures could plausibly support liquid water. Another nearby red dwarf, Trappist-1, has seven known Earth-size worlds.

On the other hand, there are some major problems with red dwarf stars. They tend to emit powerful x-ray flares that might strip away a planet’s atmosphere. They are so dim that any habitable planet would have to be in a very close orbit to keep sufficiently warm, and at such close range the planet would probably become tidally locked, meaning that one hemisphere would bake in constant starlight while the other hemisphere would languish in eternal night. Some studies also suggest that such planets would form with too much water, smothered in deep global oceans.

What we imagine vs what we know about Proxima b: The illustration shows a possible landscape on a nearby Earth-size planet. The inset shows its gravitational tug on its star--the only concrete data we have about it.

What we imagine versus what we know about Proxima b: The illustration shows a possible landscape on a nearby Earth-size planet. The inset shows Proxima b’s gravitational tug on its star–the only concrete data we have about this tantalizing world. (Credit: ESO/M. Kornmesser/G. Anglada-Escudé)

So which is it? Do red dwarf stars provide abundant real estate for life in the cosmos, or are they dead zones that fry and drown any potential living thing? Search the news and you’ll find plenty more of the back-and-forth claims. Search the scientific literature and you’ll find the same thing. I’ve got to bring out the hand-claps again: We. Just. Don’t. Know.

Fortunately, red-dwarf planets are unusually easy to study because they tend to be nearby, and because they appear relatively large compared to their parent stars. The new Transiting Exoplanet Survey Satellite (TESS) will expand the search for planets around local red dwarfs. The James Webb Space Telescope–if it ever gets off the ground–will be able to make much more detailed studies of these planets. Soon we’ll know a lot more.

That is the critical thing to keep in mind when you take in the latest headlines. We are still learning. There is no shame in that! The pace of progress is astounding. Sixty years ago, it seemed plausible that there were steamy jungles on Venus. Thirty years ago, we didn’t know of a single planet around a star other than the Sun. Proxima b was discovered less than two yeas ago.

We should be proud of how far we’ve come, while remaining humble about how far we yet have to go. Beware of sweeping claims about which worlds can support life, but be equally skeptical about claims regarding which ones can’t. Scientists have barely begun to chip away at the mysteries of why and how life began on Earth. After a half-century of space exploration, we still can’t say if there are microbes lurking somewhere on Mars, and we’ve taken only the first baby steps to examine the case for life on Europa or Enceladus.

I roll my eyes at stories making confident claims about utter unknowns, such as the idea I mentioned that life on Earth could not exist without the Moon. In a universe of vast possibilities, such an argument seems motivated less by science than by an emotional desire to assert our uniqueness. Sure enough, a new study came out last week indicating that some moonless exoplanets might be perfectly happy and stable all the same. It’s fun to speculate, but it’s far more satisfying to gather the real data (and if you can help–by doing the research or by supporting the funding that makes it possible–so much the better).

Results will be pouring in from TESS very soon. New space telescopes, new giant observatories on Earth, and next-generation instruments will soon be finding and analyzing planets in ways never before possible. Soon they will be scrutinizing Proxima b, the Trappist-1 planets, and dozens of other intriguing worlds. What they will find will enlighten us and, no matter how much we stretch our imaginations, will almost surely surprise us as well.

For science news as it happens, follow me on Twitter: @coreyspowell

  • TheBrett

    Case in point: Does a habitable planet need active, Earthlike geology, including plate tectonics?

    . . . . Maybe? I think the problem is that without plate tectonics, you could get something like Venus, where the single crust allows the planet’s interior to heat up like a pressure cooker until it releases the pressure in a ton of violent volcanic explosions that almost completely recover the surface.

    Red dwarf stars are great places to find habitable planets…or maybe they are deadly.

    I’m still leaning towards “deadly”. That long pre-main sequence period at a much higher luminosity is going to wreck most planets in the habitable zone.

  • OWilson

    Speculation about life’s origins is fun, but speculation is too often conflated with science.

    There is , so far, no evidence that even if a planet is found identical to our Blue Marble, it will produce life.

    Until we actually find it existing elsewhere independently, there is no reason to “believe” life on earth, like the universe itself, is not unique!

    The origin of Life is still one of the great unexplained mysteries.

    And may remain so.

    • jonathanpulliam

      You are not very bright.

      • OWilson

        Be nice!

        In one of your infinite M-Theory extra-dimensional “replicant” multiverses, I may be your boss! :)

        • jonathanpulliam

          Point taken

  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    Mponeng gold mine, 4+ km deep, hosts culturable primordial life in its gold-bearing seams (originally surface swamp). “Is there life?” is hardly interesting. Three other questions are pertinent.

    … 1) Is it exotic versus protein L-amino acids and D-sugars? ( science)
    … 2) Can it be weaponized? (value)
    … 3) Can it play pinochle? (criminality)

  • Not_that_anyone_cares, but…

    Reading the environmental and climate articles published each day I often feel that Earth would be habitable were it not for humans.

    • cpmanx

      It is the nature of living things to alter the environment around them, often to the detriment of other living things, sometimes to the detriment of themselves. Humans are capable of rational, directed action, but mostly we still act according to ancient evolutionary impulses, just filtered through modern social institutions. I hope our modern, more rational side can yet prevail.

      • OWilson

        Man is an animal that needs energy and protein to survive.

        Like all animals, more energy intake than expended in the hunt for food!

        In the old days, hunting the mammoth, and following the seal.

        Today we hunt at the mall, where truly massive slaughtering institutions (butcher, meat processors) do our dirty work, and McDonalds, KFC, Publix and Safeway present it to us fresh and conveniently wrapped! :)

  • Mike Richardson

    I have to agree with the next-to-last paragraph of this article, in particular. The “Rare Earth” arguments have always struck me as reaching a little too far based on supposedly exclusionary factors unique to our world. While there is reason to be concerned about the habitability of planets around red dwarf stars for the reasons discussed above, we do not know all the possible mitigating factors that may come into play (although there are quite a few involving atmospheric composition and magnetic field strength). The next generation of space telescopes and giant ground based observatories will provide answers to these questions.

    Additionally, though they are the most abundant stars in the galaxy, and therefore of great interest in determining how abundant habitable planets might be, we should look beyond red dwarf stars to other spectral classes, such as “K” ( orange dwarf stars, like Alpha Centauri B ), “G” (Sol, Alpha Centauri A), “F” (brighter than Sol — Zeta Leonis is an example), and possibly “A” (Sirius A, Vega). The orange dwarves are the second most common type of star, and while smaller than the sun, are not so small that planets have to orbit close enough to become tidally locked like red dwarf planets. They are also less prone to major flares in their youth than their smaller red cousins. Stars like ours, Sol, are less common, but we know from our own example that they can provide the necessary environment for a habitable world. Though F class stars are bigger, brighter, and shorter lived than Sol, they have wider habitable zones and may remain on the main sequence long enough for complex life to emerge. Stars like Sirius, a good bit bigger and brighter, could remain stable long enough for life to emerge on their planets, but such life would need to deal with much higher levels of ultraviolet light.

    In years to come, we will get better idea of how likely habitable worlds are around each type of star, or if they truly are rare. We will develop the ability to directly image terrestrial planets around other planets, then their atmospheres, eventually discerning likely biomarkers, if they exist. The question of whether life has emerged on other worlds will probably be answered during the lifetimes of many of those reading this article. It’s truly an exciting time to be alive!

  • David

    What difference does it make if exoplanets are “habitable” when there is no means to get there? Heck, NASA can’t even get the James Webb telescope off the ground. It’s been nearly 50 years since man went to the moon. A Mars mission is probably decades away and that is a short hope compared to interstellar travel. Then there’s the pesky fact that the energy required for mass to reach light speed is, well, pretty much infinite. I don’t see humans getting to another “habitable” planet for a very long time if ever.

  • gorak

    Red dwarfs have 2 key problems:

    1) Planets are tidally locked, meaning the weather is insane.

    2) Massive flares drenching the planet in rays of sadness, damaging the atmosphere and our electronic equipment.

    At this stage of our civilization we should focus on nice yellow stars. When we are a few thousand years more advanced we can go for the dwarf stars.

  • Darby42164

    So we keep talking about planets that can host organisms like humans, not just life. Microbes in the soil of a planet around a flaring red dwarf will do just fine thanks. So kind of need to define life here, you mean human life. We have a fungus that grows inside nuclear facilities bathed in lethal radiation, and growing without a care in the world. No competition after all. If microbes can grow there, there are boatloads of planets that could harbor life in some pretty harsh conditions.

    But since it is intelligent life we are really talking about, I think we miss one likely point. Intelligent life may well be a millions years ahead of us. I would surmise that over time, species that advanced likely would have learned to shed a fragile biological body for a much sturdier, long living android one. Hard to see for humans today, but what about humans a million years from now? In many ways we are already moving in that direction but are at the very early stages with artificial hips etc. So if advanced life is more android in nature, well they could live on the moon theoretically. You don’t need air, you don’t need to grow food, you are not as worried about radiation (although this is still an issue), you only need energy to keep your body going, that is it. Plenty of energy on the moon, on airless planets close to stars (and its energy producing starlight) etc. So we constantly talk about life, including advanced life as being like us, but if they are that advanced, really why would they? Born to die in say an 80 year period? Lugging around a body so fragile it can only work on rare planets like earth that are “just so”. Screw that, they would live for millenia if not more discarding the biological and likely will. We are barely looking for advanced life, only primitive intelligent life like us.

  • fourpennyguy

    Life surely exists on Mars, Europa, Titan, all of which have water. Imagine swimming in 100,000 foot deep ocean in Europa, or hydrocarbon filled seas of Titan, and the pockets of waters, if not rivers under the martian surface. Can you imagine watery worlds without life? I can’t. I also happen to think that advanced life forms already visit and have visited earth for thousands, if not millions of years. Their wondrous technology likely allows them to create gateways to any location they desire. So, they don’t need ships, they simply walk through gates. Similarly, there are likely hot and cold planets whose life forms may or may not seem similar to our own. Trillions of barren planets? I hardly think so. NASA found evidence of life on Mars in 1976 and then questioned those results. Blue skies? Life? Impossible. No, Mars has both. And, so, too, do the countless planets around other stars.

  • Craig the Czech

    We’re alone. Scientists want life to exist elsewhere because they believe God will finally be dead in the wake of the discovery.

  • Alchymist

    The best we can say at present is that we are gradually pinning down some of the factors that make up the Drake equation. We have a better idea of the rate of star formation, the proportion of stars with planets and the proportion of planets in the habitable zone than we used to have. The rest is still guesswork though.

  • jonathanpulliam

    We are:
    a.) Not alone.
    b.) There are infinite numbers of near precision copies of our Earth, and of each of us.
    c.) The Multi-Verse and Many-Worlds model cannot, and will not ever be able to be, corroborated, confirmed, or even observed using Einstein time/space constrained signals. The only hope is bypass of the event horizon via folding of the string branes beynd the normal 4 dimensions we perceive, coupled with our increasing mathematical understanding of how our “replicants’ constituent sub-atomic particle “spins” are to what degree in absolute synchronicity with our own, while still permitting the differing outcomes of random chance in each of the “different” random-chance governed “Earths”.


Out There

Notes from the far edge of space, astronomy, and physics.

About Corey S. Powell

Corey S. Powell is DISCOVER's Editor at Large and former Editor in Chief. Previously he has sat on the board of editors of Scientific American, taught science journalism at NYU, and been fired from NASA. Corey is the author of "20 Ways the World Could End," one of the first doomsday manuals, and "God in the Equation," an examination of the spiritual impulse in modern cosmology. He lives in Brooklyn, under nearly starless skies.


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