
We Might First Find Life on Tidally Locked Planets - dnetesn
http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets
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scythe
Tidal locking isn't always 1:1. Mercury, the lone tidally locked planet in our
Solar System, is locked at a 3:2 resonance. For simple thermodynamic reasons
one would expect that in most cases the planets locked at an "offset" ratio
are far more likely to be habitable than the ones locked at 1:1. This happens
when there are outlying large planets that disrupt the orbit:

[http://en.wikipedia.org/wiki/Mercury_%28planet%29#Spin.E2.80...](http://en.wikipedia.org/wiki/Mercury_%28planet%29#Spin.E2.80.93orbit_resonance)

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JoeAltmaier
I though Mercury was 3:2 solely because of its elliptical orbit? The fast
rotation at the closest orbit continues during the long eccentric part, where
it has time to make an extra half turn. Just geometry.

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jkot
Someone already calculated in 1950 that such planet is very unlikely to have
an atmosphere. Temperature difference is about 300 degrees celsius and most
gasses would either froze on cold side, or escape to space on warm side.

In best cases you get permanent hurricane (Venus)

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JoeAltmaier
Right, that idea that water would flow constantly across the terminator is
nuts. The water would blow to the cold side and freeze there.

Also, give all that was said, probably no life there at all. Our Moon may have
been instrumental in generating life. Tides and rotation meant constant motion
of water on shores, combined with day/night heat/cold wet/dry. Every grain of
sand on every beach was a different biochemical petri dish for billions of
years, resulting ultimately in the cell.

A tide-locked no-day-night planet would likely be sterile.

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lisper
> The water would blow to the cold side and freeze there.

You missed the part where the glaciers on the night side get big enough to
flow despite being frozen.

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JoeAltmaier
Depends on how much water.

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rflrob
I was at an astrobiology conference this week, and one of the ideas that we
discussed is that life on Earth is good at finding and exploiting gradients
for energy harvesting---thermal, chemical, even electrical. I'm not sure how
to weight the whole "the atmosphere will freeze out" argument, but it does
seem like having a constant, predictable thermal gradient, coupled with lower
average high energy radiation (not much UV at sunset), should be conducive to
life, all else being equal.

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JoeAltmaier
Conducive to existing life adapting. But what about life forming in the first
place?

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mapt
"Tidally locked"

There, was that so hard? Extraordinary effort here to coin a neologism for
something that's already an established phrase.

This is also a blogpost promoting a paper about cloud formation in tidally
locked planets, which it appears someone forgot to link, inside the phrase
"Gory details here".

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s_q_b
There's also a thousand variations on the theme of tidally locked planets in
science fiction: from Larry Niven's (of Ringworld fame) world Jinx which was
tidally locked to a gas giant, right back to Stapledon's _Star Maker_ from
1937, which features a habitable planet tidally locked around a red dwarf.

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pavel_lishin
The whole time I was reading this was "selection bias".

> _The easiest planets to find are those that orbit close to their stars._

> _Tides drive the planet’s obliquity to zero, meaning that the planet’s
> equator is perfectly aligned with its orbit. The planet will also be
> “tidally locked”_

Seems likely that the first planets we'll find life on are the planets that
are easy to find.

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cwp
Yeah. That's the premise of the article. Given the kind of planet that's easy
to find, what sort of conditions might support life?

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cturner
I'm surprised that tidal locking happen for a planet with liquids on the
surface, except in a jury-rigged toy scenario. I'd expect that the energy
would cause fluid/molden volumes to circulate. This would create motion
relative to the energy source.

It might be possible to use this same kind of circulation effect to have a
self-contained space-craft. It would consume solar energy, and then
strategically pump volumes of fluid from one end to the other, to deliberately
change its momentum.

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Potando
Pumping fluid wouldn't give long term motion to the planet (can't travel
further than it's own length). That would violate momentum conservation no
matter how it's powered. You need to give it momentum to make it move, and
then you just get a solar sail.

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jdonaldson
I don't know enough about tides and atmospheres to say whether or not this
sort of arrangement would be _more_ conducive towards generating life.

However, there's plenty of evidence that our rotating Earth gives a lot of
benefits. There's the jet stream, which helps to spread moisture, there's the
tides themselves, which helped create thriving transition zones between
aquatic and terrestrial life (tide pools, etc.)

I'm also thinking of Jared Diamond's Guns Germs and Steel. In our own case of
planet Earth, we had pretty big geographic obstacles that both enabled and
prevented the spread of certain forms of life (Deer can easily migrate within
the larger temperate zones, enabling their species to spread easily. Viruses
can't easily cross oceans, making it unlikely that they wipe out an entire
species.)

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ak217
Aside from the obvious problems with keeping an atmosphere, can tidally locked
planets have a magnetosphere?

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Eupolemos
Sure, that comes from a molten core IIRC.

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s_q_b
A spinning liquid metallic outer core, with a certain composition, but yes (we
think.) Even Earth's magnetosphere isn't fully understood.

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analog31
I wonder if the earth would be tidally locked without having bumped into the
moon at some point.

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waingake
I imagine Game of Thrones is set on such a planet :)

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lordnacho
Surely there are day and night scenes in the same place?

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waingake
True. So a little tilt then, enough so the sun sets just below the horizon - a
day and night in the habitable zone. But when "winter comes" the tilt gets out
of whack for a while.

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comrade1
A couple problems - when there's even a tiny wobble the goldilocks zone will
alternate from extreme freezing to extreme heat. The other problem is that the
atmosphere will eventually fully condense on the cold side of the planet.

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JoeAltmaier
Further, early cells depended on an ocean environment (algae) to protect their
fragile forms. No ocean on these weird planets.

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pavel_lishin
What about the icy eyeballs, where the substellar point is the center of a
large ocean?

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JoeAltmaier
There, no land, no beach. Just deep water.

