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Comment by ionwake | Hacker News Reader

Out of interest , although dimmer, are there any potential hazards simply due to the proximity of a dim star?

> are there...hazards simply due to the proximity of a dim star

Tidal locking [1]. Similar to how our moon shines at us ass first all the time, the planets around TRAPIST-1 would be expected to have one face locked to their star. Instead of an equatorial belt bookended with loops of temperate zones, as we have on Earth, we should expect, climatically, an "equatorial" face and a "polar" face separated by a tropic-to-temperate transition area.

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

Fascinating thank you

Of course, #askNasa didn't win me a question, but could any astrobiologists comment on this question: Tidal locking means the planets would always have a side facing the star, leading to very different thermodynamics in terms of climate and weather on the planet, not to mention radiation. Doesn't that very much hurt the chances for life on those planets?

I would be very interested in seeing comparative weather simulations for a fictional tidal locked planet to Earth and the other Sol planets.

I'm not an astro-biologist, but I assume that it wouldn't be conducive to life as we know it, but that doesn't rule out all life.

I don't see why. If anything, life might be even easier if you're constantly getting energy from above, rather than intermittently. In any case, "different climate and weather" doesn't seem to me to imply "hurt the chances for life."

If they're in the habitable zone, then they're getting similar radiation as an organism at the far north latitudes of Earth are getting during the summer. I don't know if it makes that much difference to a short-lived bacteria whether it's full sunlight for several weeks at a time, vs full sunlight forever.

I remember reading about 55 Cancri e (1), another tidally locked planet (2) featured in NASA's Galaxy of Horrors as "The Twilight Zone" (3), where life in the boundary seems unlikely.

1. https://exoplanets.nasa.gov/custom_planets/2

2. https://exoplanets.nasa.gov/resources/149

3. https://exoplanets.nasa.gov/alien-worlds/galaxy-of-horrors

The debate rages. The problem is that one side of the planet is essentially exposed to space and thus tends to be at cosmic background temperature, and the other side exposed to the star. It is thought by a lot of people that anything volatile enough to be a gas, ever (so, including water), would rapidly end up migrating over to the frozen side, so even if the planet started with the right mix of water and whatever other volatiles are needed it would shortly not have any in the temperate range. Some have objected that the planet could wobble and periodically unfreeze some of that stuff, and maybe life could exist in a resulting narrow temperate zone. My personal feeling is that it would still far-too-rapidly simply freeze out again, this time out of reach of the wobble, so the parched zone would simply grow to include everything ever touched by the sun rather than forming a stable temperate zone.

But we won't really know until we go look at a planet like this, because we don't have one locally to look at. (Plenty of tidally-locked bodies, plenty of bodies with volatiles and enough energy to keep them moving, I'm pretty sure nothing in the Solar system with both.)

Oceans can move a lot of heat around a planet, so the cold side could stay well above freezing. Remember it's T^4 so cold areas radiate far less heat.

That said, if there where minimal atmosphere then the effects your talking about become a larger issue. Atmosphere is largely a function of solar wind and volcanic activity which relates to the isotopes in a planets core making this hard to predict.

But the planets are close enough to interact gravitationally. Perhaps that would accomplish more than just a slight wobble.

If water freezes on the dark side, moisture in the atmosphere would probably move from warm to dark side and fall down permanently as a snow gradually removing liquid water.

If the oxygen and nitrogen freezes and snows down in the dark side (-220 C or so), the warm side can't have atmosphere either.

Unless there is some way tidally locked planet can have stable air or sea convection between the sides. I don't think they have.

If there is enough gravitational pull, the planet itself might be warmed by geologic flexing like Io.

I think one idea is large mountains on the dark side and the water slides back down as a glacier that melts, and eventually vaporizes, and back in a circle.

You have to watch out though - make your mountains too high, and you shift your center of gravity, and now your mountains are no longer elevated.

So if the water and gasses migrate to the cold side, I wonder if that would eventually make the cold side heavy enough to be rotated towards the star, causing the planet to periodically(probably over several millenia) reverse the side facing the star.

You're thinking of Earth, where the polar ice builds up perpendicular to the sun, causing tidal effects to wobble our axis a bit.

In this case, the ice is building up in such a way to make the planet even longer in the direction that tidal locking already had elongated it. The effect of which is that tidal locking would get stronger; not weaker.

I don't think it matters too much that it'd be icy on one side and rocky on the other; I think it matters more that it's oblong. But even supposing it does matter, if your intuition is that the more-dense end would want to fall towards the star, then that'd be the rocky end; not the icy end. Yes, the icy end got heavier supposing you measure from the rocky planet's center. But if you shift your frame of reference to be the planet's center of mass, then both halves of course kept exactly 50% of the mass; the densities are what shifted.

That sounds like a fun simulation to run

I know people have tried. I believe you can get both results if you put the right numbers in. Personally I consider this one of the canonical examples of where a simulation is a waste of time; with absolutely no ability to validate the model against reality, the model is in my humble-but-strong opinion utterly useless, which I mean in the strongest metaphysical sense possible. There's more ways to be wrong than to detect that you're wrong. (That's not just a flippant cute turn of phrase; I mean that as a rather fundamental issue with the entire idea.)

What about tidal and geological heating? or a greenhouse effect similar to Venus?

Any heat source exposed to space has the basic problem of its volatiles running away and not coming back. On geological time frames, this effect is probably nearly instant, so it isn't very helpful that maybe a new volcano could pop up somewhere and melt lots of stuff or something, because all that does is put a hole in the permanently frozen layer. Any heat source not exposed to space might harbor life, but that's not particularly true of these planets; we already have things like Europa locally with the same possibility.

The issues with valuable resources running away and not coming back is a serious one even for Earth, which I think might surprise some people. The geological carbon cycle [1] in particular is one that may surprise people who have not encountered it before, or not thought about it in terms of extraterrestrial life. On geological time scales it's surprisingly easy for entire important elements to go find themselves a low-energy configuration they like and disappear from any putative biosphere. The issues with a tidally-locked planet providing such solutions to "all gasses liquids" is merely an extreme example of the case, and really makes one wonder how it would be possible for such a planet to stay "stirred" enough for life to have access to what it needs to develop.

[1]: https://en.wikipedia.org/wiki/Carbon_cycle#Geological_carbon...

The link you provided suggests that one of the main ways for carbon to enter the atmosphere or ocean is through volcanic eruptions and geological hot spots. Isn't increased geological activity one of the results of orbital resonance?

If the 'central' planet has 6 close neighbours of equivalent size in resonant orbits would this not provide a very large amount of 'stirring'? These planets are also much more massive than Europa which is only 0.008 times the mass of the Earth compared to ~.6 and ~1.3 Earth masses for the ones in the habitable zone. They may have a more substantial mantle and core, possibly global magnetic fields and definitely more gravity to hold on to any fugitive gasses.

Considering geological timescales, this system is also billions of years younger than our own, potentially only a little over 500 million years old. The processes you draw concern to may simply not have had time to play out yet.

I'm also not convinced that the atmosphere would be cold enough to freeze out gasses on the night side if there were significant oceans or enough atmospheric circulation to transfer heat from the day side.

Apparently what would hurt the chance of life in a tidally locked planet the most is that due to being tidally locked the planet would have a reduced magnetic field to protect the atmosphere, and therefore there is a good chance that the planet would lose it's atmosphere over time.

https://en.wikipedia.org/wiki/Habitability_of_red_dwarf_syst...

Agreed, but there are other variables involved, including strength of stellar wind, density of atmosphere, whether a magnetic field is induced by the stellar wind, the mass of the planet (strength of its gravity), etc.

Does atmosphere include the ocean? Because life developed in the oceans first here, and some has never left.

Without an atmosphere you can't maintain a surface ocean, because liquid water can't exist in a vacuum. It's possible for the ocean to exist under a crust of ice though. See Europa for an example. So it's still theoretically possible for life to exist in a subsurface ocean, but I think it's a lot less likely compared to a planet with an atmosphere and temperatures that would support liquid water on the surface.

> It's possible for the ocean to exist under a crust of ice though. See Europa for an example.

That's exactly what I was thinking of. :)

> I think it's a lot less likely compared to a planet with an atmosphere and temperatures that would support liquid water on the surface.

Sure, but we have little information on how common life is. We're not even sure there's no life on Europa. If Europa supports life, it may be that (basic) life is somewhat common given certain criteria, and while I agree it's probably more common on planets with atmosphere and your statements were not incorrect, it may be that life is actually fairly likely on a planet like that (which is what I, possibly incorrectly, interpreted your statement as ultimately trying to convey).

Maybe but I like to think there are creatures living in the temperate rim of a tidally locked planet somewhere debating whether life could possibly evolve on a rotating planet.

"I mean, there'd be temperature fluctuations of tens of degrees every day! It would be dark half the time! How could life survive in such an unstable environment?"

And don't forget: there will be long-term cyclical trends of dozens of degrees due to geometric properties of the orbit and hypothesized tilts!

I'm not a biologist, but here's a fascinating article on the subject: http://m.nautil.us/blog/forget-earth_likewell-first-find-ali...

TL;DR is that there may exist a band around the planet where the two zones meet that is habitable. I think it's a reasonable supposition that most of the rest of the planet would be uninhabitable.

http://www.space.com/20856-alien-planets-eyeball-earths.html

This would be my first thought as well, but the original article says that they are near-orbitally resonant. It seems possible to me that the regularly passing nearby planets could apply enough of a torque to counteract the tidal locking---though I don't have a good sense of the order of magnitude of the forces, so maybe not.

Fascinating! Also – off topic, but – this description reminded me of Twinsun[1], the planet in the game Little Big Adventure. It remains one of my fondest game memories of that era, despite all its weird bugs. I've thought about finding a copy and playing it again at some point, but I fear it might spoil my good memories of it..

[1]: http://lba.wikia.com/wiki/Twinsun

Dammit, why did you have to remind me... Both LBA and LBA2 stand out as extremely memorable from my childhood. It does look like there's a revival of game-styles from that era, given Thimbleweed Park[0], Pathway[1], and apparently 2Dark[2] from (one of?) the authors of LBA. Any more?

[0] https://thimbleweedpark.com [1] http://www.pathway-game.com [2] http://www.2dark.cc

Wow, those look amazing! Thanks for the tips!

You can pick up Relentless on GOG :] https://www.gog.com/game/little_big_adventure

And apparently there's a macOS version too – SOLD! Thanks buddy! :o)

For what it's worth I did this a few years ago and was not disappointed - great to hear other people grew up with this game too!

Great to hear! I just bought LBA and LBA2 from GOG, from the sibling commenter's tip. Now I just need to find a few free hours!

https://play.google.com/store/apps/details?id=com.dotemu.lba

It's not a given, though. We used to think Mercury is tidal locked, but that turns out not to be the case. These planets, as I understand it, are in similar orbits, so they may not be locked either.

Well, Mercury is in a tidally locked orbit of sorts. Its rotation is resonant with its orbit -- it experiences two days every three years, and this isn't a coincidence, it's a relatively stable outcome of its tidal forces with the sun.

Depending on the spectrum of the star, I would be concerned about non-visible radiation doses when in close proximity to the star.

IANA astrophysicist, though.

Actually, adding onto that, I'm wondering whether orbits this tight would result in a noticeable centripetal force -- that is, that you'd feel lighter on the night side of the planet than on the day side.

Good point, this is in TFA:

"Although at least some fraction of each planet could harbor liquid water, it doesn't necessarily follow that they are habitable. TRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have."

But it's complicated. For example, apparently the energetic radiation can help by stripping away the H/He atmosphere. For more, see:

https://academic.oup.com/mnrasl/article-lookup/doi/10.1093/m...

Michael Okuda has raised the question on his Twitter feed of whether the planets would be close enough to the star that tidal force could be an issue---hard to say at this point (in the sense that they're not loose clouds of rocks, so tidal forces haven't sheared them apart, but I'd assume it's unclear whether they might have interesting geological activity, such as frequent earthquakes or unexpected extra heat due to internal friction).

Like others have pointed out, the combination of the planets being tidally locked so only one side gets sunlight and the strong X-ray radiation and flaring from the host star make it considerably un-Earth-like, and harder for life as we know it to develop.

Actually they are fewer. A dim star has less high energy radiation and a lower probability of harmful solar flares.

But the danger of both of those things surely decreases with the square of the distance to the star?

More radiation = more mutations = healthier/faster evolution!

Then the surface of stars ought to have more life than planets.

That doesn't follow, at all. If we're talking about Earth-like life, then any such life would need protection from radiation to be able to live long enough to reproduce. If such protection wasn't provided by the planet's atmosphere, then life there wouldn't move out of the water. In any case, the mutation rate would need to be extremely low, as it is with Earth life.

Life is more than capable of evolving mechanisms to increase or decrease it's mutation rate. On Earth Eurkaryotic cells with large genomes have developed nuclei to reduce their mutation rate and bacteria are more of a mixed bag with some even deliberately uping their mutation rate above what would be cause by radiation, etc. So in the long run eukaryotic life would just have to waste more resources on mechanisms to resist the effects of the radiation.

That's not quite true. These planets are very close to the star and can receive large doses of the little bit of harmful radiation that the star puts out. Additionally, these planets put out a lot of infrared, but not a lot of higher energy photons, and life-as-we-know-it needs those higher energy photons. That infrared is also absorbed by water in any atmospheres or oceans that are present, leaving even less for life. Finally, these types of stars tend to have rather variable output in their spectrums, which may make it difficult for life to adapt.

That's backwards. Red dwarfs have extremely powerful flares relative to their luminosity. So much so that it's unlikely that life would survive on the surface of an orbiting planet. (Though life would probably survive underwater.)

https://en.wikipedia.org/wiki/Habitability_of_red_dwarf_syst...

https://en.wikipedia.org/wiki/Flare_star

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