# Could a planet, tidally locked with the sun, have molten rock on the side that faces the sun?

I am designing a solar system, with the first planet located very close to the parent star. This planet has become tidally locked to the star. Is it possible to have a molten lava ocean on the side that is facing the sun?

What would the conditions be like for on the other (solid) side of the planet?

Could a settlement be built on the side facing away from the sun?

• When you say settlement do you mean something like an earth-based town or a moon-base/large dome? Aug 10, 2018 at 8:51
• @ChrisH A moon base with a small dome. This base essentially exists to supply and repair solar satellites and conduct science on the sun. Aug 22, 2018 at 2:20

I am designing a solar system with the first planet located very close to the parent star.

Probably start with the basic "ballpark" expression for a planetary surface temperature ( the Effective Temperature ) :

$$T_{eff}=\left[ \frac {A_{abs}}{A_{rad}} \frac {L(1-\alpha)}{4\pi\sigma\epsilon D^2} \right]^{\frac 1 4}$$

For a tidally locked body $$\frac {A_{abs}}{A_{rad}} = \frac 1 2$$

It's a little easier to work with quantities related to Earth and the Sun :

$$\frac {T_{planet}}{T_{earth}} = \left[ 2 \frac {L_{star}}{L_{sun}} \frac {1-\alpha_{planet}}{1-\alpha_{earth}} \frac 1 {R^2_{planet}}\right] ^ {\frac 1 4}$$

where the distance from the star ($$R_{planet}$$) is in AU and the $$\alpha$$ values are albedo values and the luminosity values ($$L$$) can be in any unit as long as it's the same for both (it's common for these values to be quoted relative to the Sun in astronomy).

The factor of two comes from Earth having not being tidally locked compared with your planet which is.

This planet has become tidally locked to the star.

Is it possible for the sun facing side to have a molten lava ocean ?

In short yes, the central area will certainly be able to do this, although the boundaries for the end of this ocean would depend on a lot of details (it's practically a science paper in itself to calculate for any given body).

What would the conditions be like for on the other side of the planet, where the surface is still solid ?

Solid but what temperature ? This is tricky, but there's a more significant problem.

Remember you have to have an atmosphere (and would have a ghastly one) for such a body to exist with any possibility of having life.

The closest thing in real life to such a body would be an exoplanet called Trappist-1f although it's not quite what you're looking for.

The "hotspot" would be ballpark 1400 K and the opposite side's "cold spot" would be of the order of 219 K (i.e. extremely cold). Somewhere along a narrow band there might be a region where it's livable.

However the atmosphere is a problem. It's going to generate extremely strong winds (on that exoplanet and on yours). It has to - you've searing heat on one side and freezing cold on another. The resulting atmospheric circulation (which has to happen) would be violent.

It would also transport gases from the lava ocean (none of them friendly to human-like life) to the otherwise habitable regions. I can't see a way to avoid this. Given you're looking for an ocean sized hotspot of lava, this would be really hellish even in the "temperate" regions.

Could a settlement be built on the side facing away from the sun ?

Maybe, but probably a lot, lot easier (and safer) to build an larger orbiting settlement. Any settlement (for human like species) would be essentially sealed and would need excellent climate control internally to cope with fluctuations in climate.

There's also a potential issue with seismic activity. You've a planet heated to molten temperatures for the crust at one side and cold as the worst of Earth's polar conditions on the opposite side. This is bound to generate seismic (tectonic) activity on a significant scale. I'd imagine the "temperate" region to be worst affected as this is the region where the cold plates meet the warm plates.

• +1 and the settlement should also move parralel to the livable region.
– Mr.J
Aug 10, 2018 at 7:26
• Sorry I forgot to specify this, but the planet doesn't have much of an atmosphere and I'm not expecting life to exist on it naturally. It was meant to be a planet human colonists would settle. This settlement would basically be a mining station, since the planet is almost entirely made out of metals, and be used to aid facilities around the sun since the Delta V requirements of sending minerals that close to the sun would be ridiculous. Aug 11, 2018 at 3:59

Yes it's very possible but not likely to be a life bearing planet. Life can really only survive at the boundary between zones.

One side has extremes of heat and the other side frozen. Any settlement would at the edge so the heat and cold balance out and there is light. Any further into the cold and it is darkness 24/7.

I'm not sure about completely molten. I suspect you need a large moon to keep the core molten

• Reminds me of Nomad City: starwars.wikia.com/wiki/Nomad_City Of course in that case the city had to move to stay in the habitable zone and on a tidally locked planet it wouldn't but still... Aug 10, 2018 at 4:33
• The planet is molten because it's extremely close to the sun. It orbits closer to the sun than even Mercury. Aug 11, 2018 at 4:00