# Liquid water on both sides of a tidally locked planet. Feasible?

I'm brainstorming for a rocky planet with similar mass to that of Earth's, orbiting a red dwarf star. It is tidally locked with no natural satellites, yet I'm bent on having liquid water on both sun-facing and "nighttime" sides of the planet.

1. For that to be possible, I explored the possibility of a denser atmosphere, perhaps a higher content of CO₂, ocean and winds that diminish the temperature differences between night and day sides, tidal heating, volcanic activity, and strong mantle convection.

2. Since it orbits a red dwarf star, a strong magnetic field must exist to prevent the planet's atmosphere from being stripped away by solar winds. Though it wouldn't be a flare star, as a red dwarf, I think it would still be less stable in terms of luminosity than our sun. Despite having slow rotation, could the planet's powerful magnetic field be justified by strong mantle convection and plate tectonics?

3. It has a year of roughly 15–30 days, and though it doesn't have any natural satellites, I'm considering another rocky planet with an orbit close enough to exert a gravitational pull that would cause strong tides (hopefully making a liquid ocean on both sides of the planet more feasible).

4. I'm also considering an axial tilt that would allow inhabitants to measure time through seasons in the year-long day.

I'm having a go at world building for a high school project, and I'm starting with the planet. A reality check from this community seemed necessary as soon as I found you.

I'm still very much an amateur at this. I'd appreciate any information.

• The science-based and reality-check tags are alternatives. If you really want science-based, then you don't need reality-check. – Brythan Feb 24 '17 at 6:29
• I'm new to the site. That's good to know, thanks! – kirani100 Feb 24 '17 at 6:30
• Question in the title is OK, but it seems that in the body you actually ask many questions? Please, one question per question, and make sure it's clearly visible what you are asking about, and what you already decided and provide only as reference. – Mołot Feb 24 '17 at 6:45
• You are just combining arbitrary properties, wilfully, and now want those "explained". That's not the way nature works. – Karl Feb 24 '17 at 7:32
• @Karl: This site has questions about the evolution of life in hard vacuum and how to create a biologically accurate dragon. Combining arbitrary properties and then trying to explain them is a part of the way this site works. – Joe Bloggs Feb 24 '17 at 9:36

## In a nutshell: Yes, you can keep your water with a thick atmosphere but you've set yourself a difficult scenario with a lot of hoops to jump through to survive.

There is a lot in your question, I'm going to take some of the bits separately to discuss them:

Rocky planet with similar mass to that of Earth's, orbiting a red dwarf star. It is tidally locked with no natural satellites.

Given the tidal locking, similar earth mass and orbiting a red dwarf things are already looking risky, you need a strong magnetic field which in turn requires rotation for a magnetic dynamo. The faster the rotation, the stronger your field. We're rotating rather slowly in our several days of orbit.

However if you increased the size of the liquid iron core (and probably the size of the planet a little) then we could get a largish magnetic field due to the convective flow of material within the planet.

But now we come to the red dwarf, you've got very little chance of your atmosphere surviving the coronal mass ejections. The red dwarf calms down later in its life though, so perhaps - if we assume your atmosphere survived - we can discuss other methods.

For that to be possible, I explored the possibility of a denser atmosphere, perhaps a higher content of CO2, ocean and winds that diminish the temperature differences between night and day sides, tidal heating, volcanic activity, and strong mantle convection.

Yup, a stronger global warming effect would definitely help (though you need to have kept a thick atmosphere for this). Your strong volcanic activity could help with the production of CO$_{2}$ and other gasses.

Since it orbits a red dwarf star, a strong magnetic field must exist to prevent the planet's atmosphere from being stripped away by solar winds. Though it wouldn't be a flare star, as a red dwarf, I think it would still be less stable in terms of luminosity than our sun. Despite having slow rotation, could the planet's powerful magnetic field be justified by strong mantle convection and plate tectonics?

As I mentioned above, we can (potentially) hold onto the atmosphere if we have a large liquid iron core and strong convection. You should aim to hold onto the atmosphere until your star calms down and then use volcanic activity to repopulate your atmosphere.

It has a year of roughly 15-30 days, and though it doesn't have any natural satellites, I'm considering another rocky planet with an orbit close enough to exert a gravitational pull that would cause strong tides (hopefully making a liquid ocean on both sides of the planet more feasible).

This wouldn't be a stable system, the two planets would eventually fall in towards one another. In this case, too, your planet would in fact be a dwarf planet since it hasn't sufficiently cleared the surrounding area.

I'm also considering an axial tilt that would allow inhabitants to measure time through seasons in the year-long day.

You would still get seasons since the rotational axis of the planet is stationary relative to the orbital axis.

I'm still very much an amateur at this.

None of us are "professional", searching the internet for relevant information is a skill but everyone has it to some degree. It just takes practice.

Summary

You need:

• Large liquid iron core to provide the convection required to support a magnetic field and warm your planet a little.
• Your atmosphere to survive the early years until the star has calmed a little, you can also use your strong volcanic activity to repopulate the atmosphere.
• A thick atmosphere this provides the global warming effect and the gasses will mix, providing high winds which keep the temperature comparable on both sides.
• To be slightly further out in the habitable zone since your atmosphere will keep you warm, you don't want to be too warm and evaporate off your water or atmosphere (though a slightly higher mass will help here).
• If the world began as a gas giant, might it have lost most of its atmosphere as the star aged, leaving you with a metallic core and some atmosphere? Red dwarfs age slowly, so you have lots of time for this to occur. – papidave Feb 25 '17 at 15:47

though it doesn't have any natural satellites, I'm considering another rocky planet with an orbit close enough to exert a gravitational pull that would cause strong tides

if this rocky planet has enough of a gravitational influence to generate tides and orbits close, it can also influence the orbit of the planet itself, eventually ending up in either an expulsion from the system or a satellite relation.

For you reference Jupiter has gravitational influence on the inner solar system, but doesn't generate significant tides in our seas.

I explored the possibility of a denser atmosphere, perhaps a higher content of CO2, ocean and winds that diminish the temperature differences between night and day sides, tidal heating, volcanic activity, and strong mantle convection

for tidal heating to occur, you need to remove tidal locking. Tidal heating happens when the material is stressed by the tidal wave. In a tidally locked configuration the minor body simply assumes a "pear" shape, deformed toward the major body, and this deformation is static.

You can rely on strong vulcanism or radioactive decay to keep the dark side warmed up above the temperature of equilibrium with space. This can very well keep water liquid, depending on the atmospheric pressure.

I'm also considering an axial tilt that would allow inhabitants to measure time through seasons in the year-long day.

I think that even if the axis is tilted it will follow the tidal lock and change orientation along the orbit, same as the moon does (reference here, as posted in an answer to this other question). Therefore no seasons.

# Yes

Using Venus as a guide, a planet can be much much warmer than it should be due to a heavy atmosphere of greenhouse gasses. To make the Venus situation work for your planet, simply dial down the output of the sun (you already said it is a red dwarf, so great) until the planet should be chilly and frozen...like Mars here in our solar system. Then dial up the greenhouse effect and include 'super-rotating' 100 m/s winds.

That should do it. The greenhouse effect warms the planet from its black-body temperature of 200 K to a balmy 300 K; and high atmospheric winds distribute temperatures relatively evenly around the planet.

Now, whether your people want to live under a crushing carbon dioxide atmosphere the thickness of water is another matter.

Also tidally locked planets have around circular atmospheric movements, see the clouds of the Venus:

Note, although the Venus rotates, it does it very slowly.

The circular motion has a different, more complex cause as the Coriolis-force in the Earth.

Furthermore, it can have an enough thick atmosphere to equalize the temperature (like in the Venus).

This is a reasonably model of a tidally locked planet that could have liquid water present on its surface. The OP has identified the majority of the features the planet needs to satisfy the situation.

There already is an earthlike planet that has many of these features already, with the exception of being tidally locked. That is, the planet Earth itself. The atmosphere, the oceans, and clouds are excellent mechanisms for redistributing heat on Earth. Earth's relatively faster rotation means temperature variations won't be too extreme.

The presence of greenhouse gases such water vapour, CO2, and methane in the Earth's atmosphere also help smooth out temperature variations.

It is not unreasonable to assume similar mechanisms will smooth temperature variations to allow for the presence of liquid water on the proposed tidally locked planet. However, the more prolonged heating on the dayside of the planet, especially compared to the nightside, will mean there will be a steeper thermal gradient. Effectively strong wind systems will be necessary to carry heat from the hot dayside to what could have been a very frigid nightside. This will keep the nightside from being locked in permanent frozen night. This a planet where extreme weather will be very extreme, but these extremes, by our standards, will be their normal weather. Expect to find a planet of storms.

You might consider the possibility of an orbital resonance to allow the planet to be less tidally locked. This will permit both sides of the planet to be exposed to its primary star. The conditions will be less extreme, but the periods of day and night will still be quite long and the weather stormy. This is only a suggestion for the OP to consider to see if it makes a better world for his purposes.

Let's address this in order:

1. We don't have any examples of a tidally locked world with an atmosphere to work from, but the theory is that a deep thick atmosphere will even out thermal inputs across the world, that is somewhat supported by Venus which has a longer day than it's year so the sub-solar point, the place where the noon sun is directly above is always moving but very slowly. It's unknown whether that pattern of atmospheric convection can be supported with no rotation at all. Similarly a world with deep salty oceans that supports a large web of currents redistributing heat from equator to poles and in this case light to dark would likely work for keeping water at liquid temperatures across the globe but the problem again is recycling the cold water back from the darkside where all the energy is going out of the system without it all freezing up on the journey around the night side. Assuming you have a liquid water distribution you should see corresponding atmospheric mixing and heating so in some ways the water system is more important than the overlying atmosphere.

2. Red dwarves can be as active, or even more active, than their yellow counterparts like Sol so you can have it flare if you want, it's variability is entirely up to you. You will still need a strong magnetic field due to stellar proximity just to keep the solar winds from tearing the atmosphere off the planet at an Earth similar thermal input rate. That means that there is strong rotation at depth in the core, Earth's liquid metal outer core is thought to be our primary field driver, that may or may not translate into an active crustal system, that depends on the relative thickness of the crust, the mantle and crustal chemistry and a raft of other issues. Most known red dwarves are very old and metal poor but if you use a fifth generation red dwarf with a similar chemistry to Sol and a planetary system with similar chemistry then you can easily justify an entirely Earthlike world with similar geophysical systems. If you use a sixth or seventh generation star like Sirius (in metal abundance not temperature) an orbiting world should be richer in heavy metals like Iron and also in transuranics meaning the world will be more geologically active and have a smaller hotter core and a larger liquid outer core that would create a massively strong magnetic field.

3. Strong tidal motion would certainly help with thermal distribution, but it would only be really influential if it was sufficient to actually move a reasonable amount of water right around the planet as it goes. If it's moving in a separate orbit at a vastly different rate of motion (which it would have to be to generate that kind of tide if it's not an orbiting satellite) that's not likely to be a stable orbital arrangement in the longer term, in fact it's likely to result in the tide inducing world being ejected from the system after a couple of passes. You could have a reasonably large world that causes some tidal melting of surface ice and small Roche Tides at superior conjunction without it being big enough or close enough to cause orbital instability, or in fact planetary scale tidal mixing. Earth experiences about 5 metres of Roche Tide from the transit of the moon after all. Such a passage is going to cause internal tidal heating from those Roche forces which helps the overall thermal equation for the world as well as mobilising large surface ice deposits due to the same effect. It's also going to cause any amount of geological upheaval.

4. Axial tilt makes no difference whatsoever on a completely tidally locked world because the axis is going to remain in the same relative orientation along with the star facing hemisphere. You need to look at orbital eccentricity to create seasons on a tidally locked world, which may also help with thermal distribution, but I don't think it does.

While you're looking at strange worlds you could also look at using a Mercury analogous three-two spin-orbit resonance instead of a tidal lock too. The planet completes three days every two years. This removes a lot of the problems with a single continuous input point in the thermal cycle. It also creates strange phenomena like the fact that the sun appears to travel backwards across the sky for part of the year.