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We May First Find Aliens on Eyeball Planets (nautil.us)
281 points by dnetesn on May 22, 2018 | hide | past | favorite | 156 comments

The background for the article is 'tilt erosion time' and the large number of tidally locked planets in the habitable zones of numerous low-mass stars. Many new discovered planets are tidally locked: Kepler-438b, Kepler-442b, K2-3d, K2-155d. It was previously assumed that tidally locked planets would not be habitable. New climate modeling seems to change this view.

No snowball on habitable tidally locked planets https://arxiv.org/abs/1705.08904

Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets https://arxiv.org/abs/1307.0515

Tidal obliquity evolution of potentially habitable planet s https://arxiv.org/pdf/1101.2156.pdf

"New climate modeling seems to change this view."

And this modeling was confirmed to be accurate... how, exactly?

This strikes me as a classic case of goosing the model until it says what you want it to say, and then getting in the news. We have no reason to believe these models. We have no reason to believe prior models, either. We have no reason to have any confidence in our ability to model a tidally locked planet, of unknown and arbitrary chemistry, around an unknown star, for unknown periods of time, with unknown orbital characteristics across geological time, with zero data points. We particularly have no reason to believe in any estimates of how long such a system could last; it doesn't seem to do life much good if there's a place for it to exist for a couple of million years, followed by freezing. The simulation is almost certainly unstable here; very small changes in the stability of this configuration will have massive changes to the result over even a few million years.

There's also more to life than just a comfortable temperature. Life is possible on Earth in part because we have a moon and plate tectonics keeping this stirred up, so some of our vital nutrients don't just sequester themselves somewhere chemically convenient after a couple of million years and then hang out there for the rest of the planet's existence. Do these types of planets have a solution for that? We have no idea. (One advantage of the Earthlike planets is that we do in fact have an idea... we know it's at least possible once.)

>We have no reason to believe these models ....

What is your level of expertise or knowledge of planetary astrophysics and climate modeling?

Sufficient to know our data point count is zero, and the model can not possibly have been verified against anything.

This does not require extensive study.

Furthermore, as a bona fide, credentialed expert in computer science, I observe with the full power of my credentials that other fields frequently abuse modeling to get into the news, and that as I said, this shows all the hallmarks of being one of those. There is a profound, mathematical way in which models simply spit back out at you what you put in. This profound mathematical understanding seems to be broadly lacking, and it makes people grant wildly excessive credence to unverified models. As just a single for instance, I expect you have no idea how many models they ran that produced lifeless planets until they finally found one that yielded a result conducive to life. Given the almost-certainty that these systems are deeply, deeply unstable, I expect it is almost certainly the case that they had a large number of runs where the system simply ran away in one direction or another.

Edit: I challenge the downmodders to produce a single data point about an eyeball planet in a habitable zone and demonstrate a model that correctly represents it. "We have to have data before we can verify a model" is not some sort of wild anti-scientific statement; the belief that a model can be trusted without validation is the wildly anti-scientific position!

If some of you are mistaking this for a position on the climate debate, note that we do have data for Earth's climate. Not as much as we might like (could always use more!), but it's certainly much greater than zero. You can literally get more than zero data just by walking outside and observing the temperature right where you are. This point has nothing to do with the climate debate on Earth.

>This profound mathematical understanding seems to be broadly lacking, and it makes people grant wildly excessive credence to unverified models.

3d General circulation models and basic energy balance models are verified against range of temperatures, pressures, and atmospheric conditions in the Earth, Mars, Jupiter and Venus. The same model used for exoplanets is used to model paleo-Mars, paleo-Venus, and Titan. If the model is somehow completely wrong outside known limits, so is the parent model that is used in the Earth Climate change studies. The models work just fine at this level of required accuracy. You put parameters that describe the Earth, Mars or Venus into the and you get good description of atmosphere and climate in Venus and Mars that matches observations.

We don't have to model specific planets to get interesting information. In generally the interest is to model different categories of the planets and discover how their environments vary when we vary the parameters. It's possible to say something generic about tidally locked planets and their climate. This is what these simulations do.

GISS modelE GCM https://www.giss.nasa.gov/tools/modelE/

ROCKE-3D version of GISS modelE GCM https://www.giss.nasa.gov/projects/astrobio/

Thank you for providing these links. Some people, especially on HN, seem to be surprised how far GCM modeling for exoplanets has gone.

Besides the examples you mention, research groups are now doing data assimilation for Mars (https://www2.physics.ox.ac.uk/research/geophysical-fluid-dyn... https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...). Other groups are testing and improving models for Jupiter and Saturn, as you mention, based on various remote sensing observations. (For Saturn, see: https://theoryofclimate.sciencesconf.org/conference/theoryof...).

All of these cases have allowed modelers to observationally test their models and model physics against non-Earth conditions. This work has been going on for a decade now, but it somehow has not reached the technically-aware audience.

The case of tidally-locked planets is another step beyond the above solar-system planets. Thank you for the references you supplied, in your original comment, on GCMs for this case.

I think Jerf's point is that for systems that complex, it is more sane to expect that we will be off by a lot. Remember that weather casting is frequently wrong in your own city.

I enjoy modeling, and I think it's useful, interesting as well as respectable work. But a little humility would in science is always a good things.

I don't think this is a good take. Firstly, we're discussing climate, not weather, which is easier to predict than chaotic swirling fluids. Secondly, if we were basing a practical decision off this information, then we should apply some kind of higher threshold, but this research is essentially a discussion piece and it seems pretty plausible that this might be the case on some worlds.

Regardless, it's more useful to point to particular areas of uncertainty than to point to a general miasma of uncertainty around climate modeling as though nothing can be learned. It doesn't really move any conversations forward.

Weather forecasting (short term details) may be wrong, but climate modeling (long term trends) is generally correct.

Just because we can't predict when an individual hurricane will occur doesn't mean that we can't predict "hurricane season".

Climate modelling on Earth generally refers to the scale of tens or hundreds of years, not to billions of years. Our climate model for the next few billion years is essentially "Eventually the sun will turn into a red giant". And we live here.

edit: Apparently it's not even certain whether the Sun will engulf the Earth or not!

>> Furthermore, as a bona fide, credentialed expert in computer science

That's alright. I'm a credentialed expert in computer science myself and I'm also always right about everything, including entire scientific fields I know next to nothing about.

If you understand how computers work, you understand how everything that uses computers works!

My favorite example of this phenomenon remains Soylent. Their first formula neglected to include trivial, unnecessary "nutrients" like iron.

Somebody - not me - told his mother about Soylent.

She said, "That's like SlimFast."

Since it was marketed for tech guys, it was a tech discovery.

I mean, you’re not wrong, despite the hivemind’s disapproval. Math and CS are our fields, and that’s what these other fields are using and abusing, and many of us have significant cross-disciplinary expertise and experience that gives us good intuition about bullshit. The lack of skepticism is why the replication crisis is such an issue... your first reaction to any publication should be doubt.

A bunch of programmers talking crap on astronomers because "Math and CS are our fields"... Oh geez.

Somewhere in an alternative universe, there must be an astronomers' forum talking crap on developers because Heartbleed and Spectre show they all misunderstood Goedel's theorem, and believe me, I totally know what I'm talking about, I use computer every day.

Fucking hell, astronomers do much more complex mathematics than 95% of computer scientists. The fucking hubris in this thread is astounding. Us "computer scientists" can't even design secure, working systems in our own fields, what authority do we have to lecture other fields?

"don't roll your own crypto" is a frequent reminder in programming threads that most people don't understand cryptographic mathematics and we shouldn't play with toys we aren't old enough to use yet.

But apparently the same is not valid for complex fields like astrophysics, spacephysics or astronomy.

To be fair even the title says "may" in it. The context is that eyeball planets we're not actively considered, and climate considerations are a weaker rejection criterion for habitability. There is also a secondary result which is that systemwide habitability zones may be extended in the case of tidally locked planet by generating a geographically fixed planet-scale habitability zone. These are interesting results, even if they don't take into account the full range of factors contributing to habitability, it's just a message saying don't count them out on this one factor that naively might be considered exclusionary due to unfamiliarity.

Btw tidal locking can cause tectonic activity, as in the case of the Jovian moons.

Perhaps relevant is that we have also been very bad at modelling the recent and short term future climate of our own damn planet!

Things are finally settling down to the point where models match independent data, e.g. the recent research into sea level changes as measured by satellite.

So now, finally, we can start making informed policy choices based on cost benefit analysis. (Actually, no, who am I kidding - Greens worldwide will keep hating on industry and capitalism, and everyone else will keep pretending AGW isn't real. But at least we could start forming rational policies if we wanted.)

Back on topic, to assume that we can make any kind of predictions about the climate of life bearing exoplanets seems like the height of arrogance given the state of the art in climate modelling.

Mainstream models disagree by less than 2 K (±0.3%) over the next 100 years. That's pretty darn accurate! 2 K makes a difference to us, because we built so many cities within 100 m of sea level (1% of an ocean depth)

Predicting whether life might exist on a planet can probably tolerate ±10% temperature accuracy. There's no reason to believe that smart geophysicists can't achieve that level of accuracy.

Yes but a) they weren't originally that accurate, and b) have only just been validated against independent data (satellite measurements of sea levels).

My point, perhaps snarkily made, was that we are only now getting the hang of this, after decades of work.

Unless I'm missing something, we're not going to be approaching anything like 10% error bars for planets for which we have far less data.

> I challenge the downmodders to produce a single data point about an eyeball planet in a habitable zone and demonstrate a model that correctly represents it.

At the risk of misunderstanding “eyeball planet”, you’re living on the most obvious example.

Elsewhere in Sol, Mars is in the “optimistic” habitable zone (I think that means “it would be if only it had been massive enough to retain an atmosphere”).

Earth climate models you already believe in so I won’t bother to name.

Can anyone else tell me the accuracy of The Ames Mars General Circulation Mode?

I hadn't heard it either, but the term does have a specific meaning: https://en.wikipedia.org/wiki/Eyeball_planet

> At the risk of misunderstanding “eyeball planet”, you’re living on the most obvious example.

You should read the articles on this website please.

I dont disagree with OP.

There is such a gigantic massive amount of information we dont know.

Any words spoken about how life is created should be met with the same skepticism we have toward religion. We require evidence and the scientific method, a simulation made my students who need to be correct for a PhD isnt evidence.

It's still useful though. We create models, and then when we find more data points on tidally locked planets, we can compare those points to these models. For example, we may be soon able to determine atmospheric composition of both sides of the planet thanks to different spectrograph data available from our perspective. These models can tell us something about the concentration of gasses in those atmospheres at different times. Then those models become testable models.

I don't understand this anger about having untestable models. How would science progress without having theoretical possibilities before testable possibilities?

Indeed, it's though all the false starts that one gathers subject knowledge. All the people who build "untestable models" for their PhDs have at least learned something about atmospheric modelling, what numerical approaches work and what approaches work less well.

Ah yes, the “you’re not qualified to judge made up models” attack. Got heem.

Every challenge he puts forth requires at best high school level critical thinking skills.

No every challenge he puts forth demonstrates he has no idea about how modeling is used. He also clearly has no idea nor interest in the current state of astrophysics and astrochemistry if he's declaring that we definitely know nothing about planets we observe.

By his own "logic", we can't even be sure they're tidally locked (because how did we determine that? Modeling!) Which I mean, is technically true because hey welcome to science where confidence intervals are a thing.

> current state of astrophysics and astrochemistry

Surely the specialist knowledge comes after the general knowledge? If a Nobel prize winning Astrophysicist says "there are exoplanets where you can make a perpetual motion machine" you would not let him get away with that?

The guy further up was just saying that there are general things about modelling (a very broad topic that reaches across all of science) that don't add up.

Ultimately, reasoning about observations are authoritative, not credentials.

> If a Nobel prize winning Astrophysicist says "there are exoplanets where you can make a perpetual motion machine" you would not let him get away with that?

You're demonstrating exactly why general knowledge isn't enough to analyze specialized concepts! Your rhetorical question may seem to the layman to be a witty retort but it just demonstrates ignorance of the topic at hand.

In the case of your perpetual motion machine example, anyone with even slightly specialized knowledge of (astro)physics would know that there are no fundamental phenomena like the conservation of energy that prevent life on an eyeball planet, unlike with a perpetual motion machine. This is the rhetorical equivalent of comparing a Mount Everest expedition to intergalactic teleportation.

> The guy further up was just saying that there are general things about modelling (a very broad topic that reaches across all of science) that don't add up.

No, he was just saying that there are general things about how modelling is used. He didn't say a single concrete thing about the article itself and proceeded to demonstrate vast ignorance wrt this specific subfield of scientific modelling.

No, you have totally and completely missed the point. The point is that being an authority in a particular area is irrelevant when what you're saying is badly supported.

He did say something concrete, which is that we have no observations to compare the model to.

> He did say something concrete, which is that we have no observations to compare the model to.

Which is wrong. That's my point. You wouldn't know that without some specialized knowledge in astrophysics and how the models are developed and verified.

You're completely missing the point: challenge the ideas, not the person.

This is why it's so easy to tell the planet anything.

The people with the highest credibility tells the rest how it is, and we don't question it. Now it's magic eyeball planets, next year it's spinning discs with Santa in the middle.

As long as credentials are impressive, it all checks out doesn't it...

The most interesting thing that came to my mind reading this was how different life would be without any distinction between night and day. If the amount of sunlight reaching the planet were consistent at all times, would life (assuming it followed an evolutionary path like earth's) still evolve to have frequent sleep or rest periods?

This is so intriguing indeed.

Maybe most creatures on such planets would evolve towards unihemispheric sleep, as some Earth species did (eg. dolphins).

Unihemispheric slow-wave sleep (USWS) is sleep with one half of the brain while the other half remains alert.


I was wondering if such creatures would sleep at all with 24 hour daylight. They might evolve other mechanisms to get the same benefits.

Lions sleep 22 hours/day, even though it is light during many of those hours.

Light/dark is only one factor in sleep, there are others.

Evolution on earth is heavily influenced on dark/light cycles through its entire history. No evolutionary path that has received this influence can tell us anything about what would happen, on things that are highly correlated to it, for one that has never received this influence.

of course, but the reasons lions sleep far more than the day/night cycle requires are a factor that applies elsewhere, and thus might result in something similar.

It's tough to say, there is more than one purpose for sleep, and it may have served other purposes in the evolutionary history that no longer benefit all organisms. For example, humans no longer need to be safely holed up while predators roam the Savannah at night.

It's also hard to know of the infinitely many evolutionary twists and turns lifeforms in the Universe may take, how many of those pathways involve something like sleep - especially on planets without a diurnal cycle.

Anything and everything is a wild guess based on a sample of one where extraterrestrial life is concerned.

Why would you assume there'd be animals at all?

Rogue planets [0] are also very intriguing. Planets drifting through interstellar space without any star or neighbors.

Although they may never get sunlight, I suspect the view in their sky would be spectacular indeed (like ours when away from cities and light pollution, but even better.)

Maybe all that unfiltered starlight would be enough to coat the planet in a unique twilight all around, or maybe life there would specialize in other senses besides sight.

The planet may generate sufficient heat from its core and most of the life there might live underground, or there might be some other heat-providing geochemical processes on the surface.

[0] https://en.wikipedia.org/wiki/Rogue_planet

[1] http://www.bing.com/images/search?q=Milky+Way+from+Earth

One of the Star Treks visited an inhabited rogue planet. If life could find a way in such circumstances, I’m sure it would be very interesting. I imagine it would also be uniquely likely to visit and leave star systems.

I love this imagination!

There are a ton of questions to be raised about how life on such a planet would work. Life on earth is driven by rhythms: a circadian rhythm, tidal rhythms, monthly rhythms, seasonal rhythms - it begs the question whether life is even possible without these built-in cycles.

My initial hunch is yes, and perhaps we can look for evidence of such in caves, deep ocean, and single-cellular life.

We have very similar conditions just a few dozens meters under the sea, where sun light doesn't reach. If I remember correctly, creatures that live there still sleep, although their cycles are not linked to 24 hours.

I wonder if there are still some kind of seasons down there or if the tides have an impact.

I suspect tides have an impact until you leave the continental shelf and get into deep, deep water.

The thermal vents from volcanic activity seem to be reasonably well insulated from cyclic activities at the surface. They would likely only see variation from cycles with much longer scales, such as the 26 ky axial precession cycle, the 41 ky axial tilt cycle, the 100 ky orbital eccentricity cycle, the 112 ky apsidal precession cycle, the 300-500 My tectonic supercontinent cycle, etc--the ones that can change climate and geology, rather than just weather.

The vent dwellers might get an inkling if an ice age has been going on the surface for a while, or if runaway greenhouse effect is boiling off the ocean surface.

Sleep is the natural state of life, it uses far less energy and we only wake up as a means to procure more energy. On a planet where plants have a constant source of energy to grow the animals may sleep more.

Sleep is the natural state of life on this planet. Other planets may have evolved different biology where the concept of sleep is unknown, unneeded or impossible.

Your metabolism does not slow down while sleeping so it does not really save that much energy.

Imagine if such a planet had a small residual rotation left - any civilization on it would be constantly rediscovering millennia-old sites as previously abandoned regions rotated back under the inhabitable region.

This is a beautiful idea, would love to read a novel based on this premise.

The sort of forced nomadism from "only" having ~1000 years before a location becomes uninhabitable would make for some interesting dynamics. Real estate at the leading edge would be very valuable, potentially with some kind of homesteading dynamic for claiming the land (as nobody has lived there for thousands of years). Also value of land would depreciate over time because it would have an 'expiration date'.

Eventually, the surface-dwellers start to migrate into a region where a previous civilization went underground, and there is dramatic conflict as the nomads try to claim territory that the diggers never abandoned.

Also, the polar territories would only have to worry about precession, and their land might have a longer expiration date.

Start with a tribe nestled in an Eastern valley, with just enough shade and a lake, isolated from hostile neighbors to the North and South.

But soon the Bright comes, melting the glacier which holds the lake back from the Dark. They can't cross, it's now full of chunks of ice, and they have to get through hostile neighboring lands.

Call it Sunrise.

As for fiction w.r.t celestial dynamics, here are two of my favourites that come to mind: Cheela Series and Three Body Problem.

I think its doubtful a society like that would adopt a capitalist land rights system.

First opportunity is always something to capitalize on.

Reminds me of a writing prompt I saw a while ago [0].

[0] https://www.reddit.com/r/WritingPrompts/comments/35mgnn/wp_a...

And constantly scooting back into the warm side themselves.

It'd be an interesting variation on being nomadic, where you would indeed migrate, but you'd stay in the same "logical" location on the planet. Basically, only migrating laterally, and only once every few thousand years or so.

While I agree with most of the other comments that there's little evidence for the argument of the article, it is refreshing to see a different take on the whole "earth-like planets" topic.

I am regularly shocked by how little imagination commentators in the popular science press seem to have on this subject. Even highly-respected scientists frequently refer to "requirements" for extra-terrestrial life such as carbon-based chemistry, the requirement for water, a mechanism of natural selection, etc, etc. (Actually I consider the last one, posited by Dawkins, to be probably the closest to the truth.)

I guess it's simply the anthropic principle at work, but even the nature we see on our own planet far surpasses our imagination often (especially at the microbial level), so why should we be placing any constraints on the rest of the universe?

You can have all the imagination you want, but it doesn't change the distribution of elements by mass in the universe. The most common are hydrogen, helium, oxygen, carbon, neon, iron, and so on. The likelihood that hydrogen/oxygen in the form of water are the necessary solvent for life is simply a statistical likelihood. They form a polar solvent, which can dissolve many different molecules and move them around. Water is a great medium for life to form in, do you have a different solve to propose? I've heard some try to say liquid methane, but it doesn't have some of the great properties that water does, like solid water being less dense than liquid water (really useful for bodies of water, which keep liquid water underneath a frozen surface).

Additionally, carbon itself forms stable amino acids, stable nucleic acids, and does so not only on Earth but even in celestial samples we have taken from outside Earth origin. So, would you rather a non-scientific writer leave the confines of sound science to speculate wildly? As a scientist myself, I'd rather they confine themselves to the most likely and sensible scenarios the majority of the time, since their purpose is to inform and inspire the public. Some small amount of speculation is fine, but education should be first and foremost.

I don't disagree that water is probably a unique solvent, and you make a good point about the distribution of the elements. But once we get into nucleic acids I think we're once again into the territory of applying constraints based on our own experience.

Good answer though.

My personal bet is that carbon-based life built on amino acids, proteins, fats and sugars is the low-hanging fruit in the universe, more-or-less for the reasons said above. Fats and sugars are pretty basic carbon compounds; amino acids arise spontaneously in the right inorganic conditions, and it's a hop and a skip from amino acids to the incredibly useful proteins.

From there it probably gets a lot more divergent. I'd also bet that cells and something like DNA is pretty common wherever carbon-based life is found, but my understanding is that the exact way DNA codes for proteins is pretty arbitrary, and the fact that all life on Earth more-or-less uses the same coding scheme is an artifact of the common origin of life rather than because it matters.

Body plans and the details of multi-cellular life are probably going to be wildly divergent, with some caveats. For instance, eyeballs evolved multiple times on Earth, so it would be surprising if eyeballs didn't evolve pretty often elsewhere.

(Incidentally, if aliens are made of proteins, sugars/starches and fats, that means that whatever they look like, we can probably eat 'em, barring the usual toxins and allergens.)

I want to add to saal's point-- Carbon is _fantastic_. It easily forms bonds in 4 directions, but can instead form bonds in 3- or 2- or occassionaly 1- direction if need be. If you are choosing an element to be the "lego backbone" of a bunch of molecules, carbon is a great choice because it gives that versatility. Nitrogen can easily form 3 bonds, and Oxygen 2, and Boron 3.

So, you ask, what about Silicon? Silicon also has 4 valence electrons, so it ought to be a nice substitute, right?

The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room. So you still only have 4- connectors, but it's a larger element.

There's clearly an advantage into being able to form precise and small molecules. Carbon, with it's small size and 4 valence electrons, is a clear choice for a backbone.

> The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room

I'm a complete layman, but how does that play out in planets with gravity force significantly stronger or weaker than Earth? Woul that skew which elements make the best building blocks?

No, gravity for microscopic purposes may as well not exist. Surface tension matters for very small creatures.

Silicon is basically impossible as a building block for life. It forms bonds that are too strong, meaning it requires much more energy to fuel life processes and reducing the rate of chance collisions which lowers the prospects for abiogenesis.

Carbon is the only realistic choice; it is the only element abundant enough with the right balance of stability and versatility.

I guess I was thinking more about pressure than gravity (though the two are related). At extremely high or extremely low pressure, I would imagine things to play out differently

I think that in most situations the pressure would still not change much as far as atom-to-atom distances go. And in those situations, carbon is still smaller and workable.

Carbon-based chemistry isn't the worst requirement to throw in there, since the only plausible analogue - silicon - for example doesn't form stable analogues of the usual amino acids at room-temperature.

Now it would be reasonable to say "well what about extreme environments" but we are specifically looking for planets with temperatures and "ambient energies" similar to ours - so it's reasonable to think that whatever chemistry is there probably has to follow Earth to a large degree.

If instead we were talking about high-pressure hot Jupiters or something, then it gets more interesting - but that's going to be something so different we're unlikely to recognize it at all (what consciousness does sentient life which evolved in a gas-environment have?)

Well carbon based compounds are significantly more stable when they grow in size than any other atom.

That bond stability had to play a gigantic role over the last billion years of evolution.

Exactly. We think of things recognisable to us, in scale and environment, but as the article pointed out before it got too fanciful, it seems far more likely we'll find actual life by looking outside these constraints.

Well, the problem with life that doesn't look like ours is that we'd find it very hard to recognise it as life in the first place, even if oozed to our doorstep and rang the doorbell. So it makes sense to look for habitable planets that could harbour life like ours first- because we're much more likely to recognise them if we can find them.

And knowing that life like ours has already developed on a planet like ours, it makes sense that there will be others like it out there, so we might as well look for them first.

I'm not saying that ET life can't be very different to ours, or develop in conditions radically different to ours- but it makes sense to start searching at the most likely place for the most likely thing, no?

> it makes sense to start searching at the most likely place for the most likely thing, no?

It may be the most likely to be easily recognized but not necessarily the most prevalent. You have two variables to balance: (a) likelihood that the planet you're looking at contains life and (b) likelihood that you'll know it when you see it.

If you can increase (a) significantly then it may not matter if (b) is very low or not

I taught an undergraduate class on this topic once and was pleased to find that, in the early 1990s, existing textbooks and papers and articles in things like Scientific American had a ton of imagination about non-liquid-water, non-carbon, etc.

One thing I don't get is why nobody ever points out that these are the kinds of planets we're finding because these are the kinds of planets we can find.

We'd have no way of identifying an earth-sized planet at an earth-sized distance from a sun-sized, star, as far as I know.

There could be a huge amount of exactly earth like planets out there that we have no way of finding. In fact -- all the sun-sized stars we've looked at that we can't find planets at all around, might be ones with solar systems just like our own.

My understanding of the current situation is that statistically it appears that nearly every star has planets, but smaller stars are more common and longer-lived than medium and large stars, and that planetary formation models suggest they should be more likely to have rocky planets in the habitable zone. So, the presumption is based more on statistics and models than observations, although of course observations still stand a chance to obliterate the models when we have better instruments to find exoplanets.

But I am not an astronomer, would love to see some data on this.

It’s our knowledge of chemistry at work. I think https://en.m.wikipedia.org/wiki/Hypothetical_types_of_bioche... is pretty disenchanting.

There is a third option besides tidal lock and night/day cycle.

Some moons in the outer Solar System have Chaotic Rotation[1]. These bodies don't have a fixed rotation axis or period.

There could still be such a Moon orbiting a planet in the habitable zone, or even a small planet in a belt like structure orbiting close to a red dwarf.

I wonder what lifeforms could evolve when the amount of light/energy received is unpredictable and varies constantly?

[1] https://en.wikipedia.org/wiki/Chaotic_rotation

Chaotic rotation occurs primarily in bodies that are not spherical. These tend to be smaller, because gravity forces larger bodies into spheres, and would have a thinner atmosphere if any due to the small gravity.

Not saying life is impossible on such a moon, but it would be even weirder than just strange day night suggests.

But wont millions of years of chaotic rotation cause a large body to become round?

Thousands of years this sounds realistic, but remember the scale of things is in billions of years.

I believe the chaotic rotation would stop as soon as the body became round, to conserve angular momentum.

Sun in a fixed position in the sky, impassible ice and desert barriers on all 'sides'. These planets must be full of flat 'earthers'.

Two of their most memorable technological milestones might be:

1. First crossing of the hot side, passing through the point where "sun" is directly overhead. (Having the sun directly overhead would probably seem very profound and unusual to them.)

2. First crossing of the cold side, passing through the opposite point.

Both would have unique challenges. Crossing the hot side would require an amazing ability to keep cool. You can keep warm by burning stuff and using lots of insulation, but how do you do the opposite?

Crossing the cold side would have temperature challenges and also very difficult navigation because of the continuous darkness. Maybe there would be moon(s) and stars for some light, though.

In both cases, I wouldn't be surprised if the first crossing had to wait until the invention of the airplane. You definitely aren't doing it by sea the first time like we did here on Earth.

IIRC from various articles, there might also be hefty storms around the terminator where the thermal difference between hot and cold plays out...

THIS planet is full of flat earthers. :)

It was anyway. Now they're a blessedly rare species.

Flat Earthers are an elaborate hoax ;)

On the other hand, it'd be far more straightforward to circumnavigate a planet like this since there's only one dimension in which you can travel!

Reminds me a lot of the world of One Piece manga

And cylinder 'earthers'!

Wow this brings back memories!

When I was 18, a group of friends and me were going to create a computer game based on this tidally locked concept. The story went as follows: An explosion would occur in a huge spaceship close to such a planet. Both parts of this ship would make an emergency landing on this planet. One part on the cold ice side, one on the hot desert side. Both teams would work their way towards the habitable ring, in a true real-time-strategy (Command & Conquer) fashion.

Needles to say, the game never went further than this concept phase.

Although we did visit a small local game developer here in Belgium called Larian, to get a feel of running a game development company (must be back in 1998). Larian is now known for the critically acclaimed Divinity: Original Sin.

I like it! And you could make sequels where they each develop into a different civilization with different technologies.

The article explains in length what Eyeball Planets are but gives little arguments why life should be more abundant there compared to other planets apart from a vague number argument.

Here are some reasons:


- Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighbourhood of the Sun

- Red dwarfs exist for a long time (longer than ten billion years) which gives life a long time to spring into existence (or start via panspermia) and evolve

- Planets in the habitable zone of a red dwarf are tidally locked ("eyeball planets") due to their small distance to the star

So there is very likely to be a huge number of these types of planets with stable conditions over a long period of time.


Flares by the red dwarfs (which are not so rare) could destroy life on these planets

>Flares by the red dwarfs (which are not so rare) could destroy life on these planets

As a layperson, I'm curious how "bad" those flares are. Is it in the realm of possibility that life that develops on these planets could "hide" from these flares when they happen, or evolve defenses against them for when they happen? Or is it more "turn the surface into lava" kinds of events?

~100 times the intensity required to kill even the hardiest microorganisms, bad.


In the places that are exposed to the UV light.

You could have life that's 100m deep underwater, or living in protected structures (caves, etc). Life could evolve more protections against UV (e.g. some kind of carapace that the organism resides within).

Maybe the hard part is figuring out a way to use UV-heavy light to generate energy.

There is also the detectablility aspect. Closer planets mean shorter years which means we can detect them in a short time of observation. It's not that we don't want to find earth like planets, it is that it takes longer, so why not take the easy path even if the planets are questionably habitable.

Eyeball planets are quite common, easier to detect, and potentially life bearing.

Not necessarily more abundant, just more within reach of our current tools.

It's possible they are more abundant, because (according to a post above), red dwarfs are by far the most common type of star in this part of the galaxy, and any habitable-zone planets around a red dwarf are most likely tidally-locked, meaning they'd be eyeball planets.

It isn’t that life should be more abundant, simply that Eyeball planets are easier to find. Because they have the capacity to have life (contain a Goldilocks region), that’s where we are most likely to find life first (given current technology). It is a numbers argument, but so is finding alien life in general.

I don't think it made that claim (The HN title appears somewhat incorrect) - just that we are primarily detecting what are likely to be eyeball planets, so if life is out there on any of the planets we are detecting, then it'll most likely be an eyeball planet.

It's a bit circular and not really saying anything useful, but at least it's not claiming what the HN title (nor your criticism) is saying

We are detecting planets that are close to their host stars because they are relatively easy to detect using current methods.

If a planet is closer to its star, it's more likely to be tidally locked. It can however also be in a spin-orbit resonance higher than 1:1.

Tidally locked planets have a greater range of distances they can be from the central star while having a habitable zone. Non-locked planets need to be at just the right distance.

As to which kind of planet we'll find life on, depends on whether tidally locked planets with habitable zones are more frequent than non-tidally locked planets in a habitable orbit. I don't know that we know the numbers here.

> Non-locked planets need to be at just the right distance.

We are learning that there are liquid oceans all over our solar system so I'd argue that the classic concept of a habitable zone is outdated because it doesn't take them into account.

The habitable zone doesn't mean "anything inside supports life, everything outside doesn't".

It specifically means "the light from the planets sun is in the range to support surface-level life".

If a distant moon in our solar system has a liquid ocean that A) doesn't mean it's water or anything else sane and B) that habitable zones are poorly defined.

Liquid oceans generated by orbital- or geothermal activity aren't covered in the habitable zone definition and aren't very useful as they usually require a parent body to provide energy to heat the ocean (jupiter for example) but just not enough to boil it off the moon.

That's not what it says. Or at least it's not what the article says; the title is a bit dodgy. It's saying that the early habitable zone planets we're currently finding are likely tidally locked, because they're easier to find with current observational techniques.

So, how exactly is evaporation to outer space less of a problem in these red dwarf systems? Shouldn't eyeball planets suffer of that depletion especially?

It has been said that the equators of eyeball planets are the most romantic places in the universe, as they are in a perpetual never ending sunset. Great place for a honeymoon.

Would be interesting to see how a real estate market would develop on a planet like this. My guess:

- Side of planet closest to sun would be great for solar, agriculture.

- Equator would be great for retail, recreation - constant sunset makes everything look beautiful.

- Just further than the equator might be an idea place for housing - it's always dark, but short commute to light.

- Further away from the equator would be cold, which could be useful for heavy industry (particularly exothermic processes)

I wonder how a society's 'clock' would work without the day/night cycle. Would people coalesce around a certain common "day" cycle or would everyone be on different schedules so society operates in shifts?

It’s clear that the equator would be the playgrounds of the rich, where grass is always green and the lighting always perfect. Properties facing the sunset would fetch a premium over those facing the dark empty sky.

Going away from the center, properties would get cheaper due to harsher conditions, the poorest of people living in increasingly hotter and colder conditions. But they would also live closer to the factories that lie toward the poles of the planet, making for faster commutes.

Somewhere in the very hottest and coldest extremes, would be a good place to keep prisons.

If enough capital is available, I'd expect the rich to grade the equator so it slopes towards the sun, allowing for more real estate to get sunny views.

> Further away from the equator would be cold, which could be useful for heavy industry (particularly exothermic processes)

Bitcoin mines!

I wonder how much benefit, if any, data centers get from operating in colder regions.

This kind of reminds me of Norwegian mythology where, as far as I can remember, there was an icy world in the North and a fire in the South, and as the fire melted the ice, a thin strip of life began forming.

Man, living on an eyeball planet sure would be convenient for producing solar energy.

I wonder whether you'd ever achieve the level of technology required to take advantage of it, though. Mining would be brutally difficult outside the habitable zone.

On a planet the size of Earth, or a few Earths, it seems the habitable zone should be large enough to develop a major civilization, no?

No idea. It just seems based on our own experience that resources aren't likely to be evenly distributed.

The article reminds me of another[1] HN article... this one says such a planet might be covered in oceans. I wonder what kind of life develops on an ocean planet? :)

(This is all idle speculation but it's fun to think about)

[1] "Group of scientists suggest that octopuses might actually be aliens " https://news.ycombinator.com/item?id=17110874

I find that it completely unlikely that 'eyeball' planets would have a life-supporting zone.

Over geological time period, the zone is likely to suffer unpredictable and novel variability which is not conducive to developing life.

Can you elaborate there? This seems like a very speculative statement to conclude 'completely unlikely'.

Geological-timescale variability is VERY conducive to evolution.

We also don't have timescales for abiogenesis in a sterile environment - i.e. given the right ingredients and energy input, no existing life forms...how long does life take to appear? There's no particular reason that timescale has to be very long - and we know life on earth got started pretty damn quick after the planet cooled off enough (EDIT: in the sense that the planet is ~4 billion years old, the earliest life is dated to 3.5 billion years old, so the number is somewhere between a little over 0 and 500 million years).

> Over geological time period, the zone is likely to suffer unpredictable and novel variability

Earth does that too, though. Ice ages, the odd large impact, etc.

I meant far more frequently than on Earth, and far more extreme.

"Life finds a way" - but if the temperature suddenly jumps 100 degrees because your goldilocks zone librated into the sun, then you ain't got much capacity to evolve protection from that.

A pretty good short story set on a tidally locked planet is "Weep For Day" by Indrapramit Das.

True thing. In "Another World" it looked totally different, both in fauna and flora.

If only Hal Clement were around to write hard science fiction set on an eyeball planet...

Life, at least it's origination, requires cycling - this is what allows for the original buildup and release of entropy gradients which follow spacetime trajectories different from the ones of the dead matter. For example you get cells (and especially precells) to get divided by temperature cycling.

This would make for a great blockbuster movie. Something mad max-esque

I would have added "maybe" at the end of the title :-)

Would this eyeball planet structure affect geography? I.e. would the terrain be flatter? Would there be more or less volcanoes? No blanket statements could be made because it would depend more on composition?

That's a lot of assumptions based on top of other assumptions. All asserted as the truth. I'd be more willing to listen if the author was aware of this potential for error.

The argument seems to stand on the greater risk to life on a planet with relative axial rotation. So one without is more likely to have "sweet spots."

As an earthling this is probably the first thing you think about when you read about Mars - too hot for life in the day, too cold for life at night. But that makes way too many assumptions relative to _human_ life. It assumes nothing could be rigid enough to adapt to day and night extremes greater than Earth's.

It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet.

>It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet.

I always felt the same way about astronomers linking liquid water with life but then again where do you start if you don't use the only known instances of life in the universe as a template? Maybe it happens that there's a viable evolutionary path for sentient Nitrogen clouds but how would we know that?

Furthermore it doesn't sound too absurd that the very complex chemical constructs necessary for life would have a greater chance to stabilize in less extreme environments with a smaller temperature amplitude. Especially if you're looking for complex life and not merely microbes (which tend to be a lot more resilient).

The rationale is because carbon chemistry is the richest chemistry out there. Other elements are not nearly as flexible to sustain complex molecules while maintaining the possibility of reversability in chemical reactions. Thats actually a very sound argument.

I always thought that most likely place to have life in solar system (except for Earth) is Venus. Also hardest to discover for us.

Most likely because looking at extremophiles on Earth, you just need somewhat stable energy gradient and you'll find some life that draws energy from that gradient using weird chemistry.

Hardest to find for us because most of our chemistry research is geared towards carbon compounds around 20deg C, not sulfur compounds around 500deg C that dissolve most of our equipment in minutes.

liquid water acts is a catalyst for a large number of chemical reactions. life on earth is complex in a way that only carbon base molecules with liquid water can support. Maybe life can exist where average temperatures are 90C (but this in itself changes the possible chemical reactions)

Life on this planet doesn't seem all that constrained. There is life in the deepest spots in the ocean and in the highest reaches of the atmosphere and everywhere in between.

I think the point is that when you move to an interstellar scale, the "extremes" on Earth are actually a very, very narrow band of possible environments. Even just within our solar system the differences in temperature alone are many orders of magnitude beyond those found on Earth. That's ignoring the differences in gravity, pressure, chemical makeup, etc.

Disregarding the possibility of life elsewhere that couldn't possible survive on Earth discounts most of the planets we know about.

There are some relatively clear 'bands' where we expect life to be possible. Anywhere from -273 right up to 1,000 degrees Celcius. That's a 'narrow range' by the range available within the solar system but lower you won't go and higher has some interesting problems associated with stability of the vast majority of materials that could be your building blocks. Gravity and pressure are less of a consideration though those would 'shape' life much as life on earth has adapted to the pressure gradients available. Gravity is even less of a concern. Chemical makeup is important, it determines the available building blocks which is one reason we are concentrating on second generation stars because they have enough complex building materials lying around. First generation star systems are chemically too simple to support life as we can imagine it.

So if survival of those life-forms depends on gravity, pressure or chemistry then we definitely should not rule out places where those are different than on earth, in fact that is to be expected. But temperature is a very important factor and the make-up of the star itself is also very important.

So it makes sense to check the likely places first before spending time and effort on much more unlikely places.

I was going to comment that I thought 100C was the upper bound for life that relies on water, since any higher than that and it boils (although in retrospect that only happens at standard pressure). But I googled a bit and found a BBC article on the Uzon caldera in Siberia [1], which hosts microbes that can thrive up to 122C! (1000C is still far out of reach, though).

Some of these organisms also have novel ways of acquiring energy, so it seems like research like this is probably our best near bet for understanding how life can thrive in extreme conditions.

[1] http://www.bbc.com/earth/story/20160209-this-is-how-to-survi...

I would suggest that something like a virus can probably "survive" far higher temperatures (though to my knowledge none that do are isolated atm) but they would require leaving that environment (for example via the water steam) to replicate.

I'd put the lower bound for life at closer to -100C, as you go below 0C there just isn't enough energy available for life.

It won't be fast, that's for sure!

"It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet."

I'm having a hard time finding all the people stodgily insisting that only life exactly like Earth's is possible through the hordes of people screaming about how it might not be. Everybody already knows that life might not be exactly like Earth life. Any illusions to the contrary have been shattered by Earth life itself and the concrete existence of extremophiles, which are themselves already not what most people imagined "Earth life" to be.

I'll make that claim. Even extremophiles are very earth like life forms: the have similar chemistry behind them.

Is going for the jugular like that really fair? It's clearly speculative, but backed up worth reasoned argument.

>That image may very well be completely off-base. There is good reason to think that the first potentially life-bearing worlds that are now being detected around other stars...

"may be", "Good reason to think". Not exactly ex-cathedra pronouncements from on high.

I tend to agree, however with the universe being so vast and the number of planets out there simply impossible to comprehend I always end up thinking "that sounds far-fetched but it almost certainly happens somewhere in the universe".

Now of course if we consider the much, much smaller subset of planets we may hope to actually observe then of course it might not be so likely.

I also think the initial point of the article in insightful, although rather obvious: since the planets we're currently looking for are not Earth-like (because we're currently unable to detect planets such as Earth in other solar systems) it means that if we find something it'll probably be very different than what we're used to. Now of course the author goes on to flip that around by saying "since we're looking for planets that are not like earth we're going to find this and that" which is obviously a bit presumptuous. Still, fantasizing about alien worlds is something I always greatly enjoy so I'll allow it.

Disregarding the matter of life and whatnot, it was interesting to read a piece on what the climate and behaviour of a planet like this would be. Not something I've seen in science fiction yet.

The closest I've come across is Kim Stanley Robinson's Blue Mars, which has a city on Mercury that rolls on rails along the terminator, pushed along by the thermal expansion of the rails behind it.

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