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Its a very wip idea: I have a star that's not very hot, and the planet is very close to it. Can the planet be so close to the star to be livable on the surface, but flying at cruise height(10 km) would mean melting or burning. And if yes? How close? And how hot should it be?

The sizes of the sun is roughly half of the sun, the same applies to the planet (half the size of earth). If any information is needed just ask.

Since this is probably impossible in real sience, let's not be stopped from that. Its a universe with huge wormholes and object faster than light, so bending the physics a little more won't hurt. The size of both object can be changed too if needed

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    $\begingroup$ wouldn't hat make the planet unlivable? $\endgroup$ – shieldedtulip Oct 3 '17 at 17:10
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    $\begingroup$ I think for this planet, hat would be great idea. $\endgroup$ – Willk Oct 3 '17 at 17:18
  • $\begingroup$ Could there be a dense atmosphere that helps shield the planet at lower altitudes but going above that causes melting? $\endgroup$ – Miles Engel Oct 3 '17 at 17:19
  • $\begingroup$ When you say flying do you mean through an atmosphere? $\endgroup$ – Joe Bloggs Oct 3 '17 at 17:34
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    $\begingroup$ @JackIta: it very much is important. If the atmosphere traps any appreciable amount of heat then eventually the whole planet will be hot enough to set things on fire. $\endgroup$ – Joe Bloggs Oct 3 '17 at 18:04
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Can the planet be so close to the star to be livable on the surface, but flying at cruise height(10 km) would mean melting or burning

Not through nearness to the sun, for a number of reasons:

  • that close, it would be within the star's corona
  • solar flares and magnetic reconnections would blow the atmosphere off
  • attrition with the solar atmosphere would send the planet spiraling to its doom in a few years
  • thermal equilibrium would mean the planet itself gets melted

But @Will has the right of it: you can have most of the radiation being reflected back by a cloud layer. Playing with physics, you can imagine a sort of "liquid" atmosphere or a layer of gases with so low a resistivity as to behave like a metallic mirror, reflecting 99% of the incoming radiation.

In our Universe, there are no life-sustaining dense liquids; in your Universe, there might be, and they could form the planet's atmosphere (around 5,000 times denser than Earth's, and much less thick). So you could have a whole world completely covered in "liquid" (except that its denser inhabitants call it "air"); to make things worse, above the liquid, a thin layer of molten metal such as gallium ensures a reflectivity of 99.5%. Only a half of one percent of the incoming radiation ever makes it to the surface. On the night side, the metal oxidizes and becomes black, re-radiating heat into space.

Flying over this "shield" would expose you to a radiation level four hundred times surface normal, and possibly also to a very different chemical composition of the "atmosphere", causing immediate ignition. A hell of a myth of Icarus.

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I understand you are not talking about earth but about your world. You could set your planet up this way by covering it with low thick clouds with a very high cloud albedo.

From https://en.wikipedia.org/wiki/Cloud_albedo

Studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds and their albedo.[6] The variations of the albedo of typical clouds in the atmosphere are dominated by the column amount of liquid water and ice in the cloud.[2] Cloud albedo varies from less than 10% to more than 90% and depends on drop sizes, liquid water or ice content, thickness of the cloud, and the sun's zenith angle. The smaller the drops and the greater the liquid water content, the greater the cloud albedo, if all other factors are the same.

Addition of cloud nuclei by pollution can lead to an increase in solar radiation reflected by clouds.[7] Increasing aerosol concentration and aerosol density increases cloud droplet concentration, decreases cloud droplet size, and increases cloud albedo.[2][6] In macrophysically identical clouds, a cloud with few larger drops will have a lower albedo than a cloud with more smaller drops.

So thick, low clouds with lots of particulates. Beijing world? Wet, low, bright (when seen from above) clouds. Seattle world?

In any case, if you have clouds with albedo of 90% covering the ground, you can have it be substantially hotter above at cruising altitude - both from unintercepted solar radiation coming from the sun as well as 90% of that amount of radiation being reflected up from below.

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    $\begingroup$ I was wondering how to get around insolation causing intolerable atmospheric heating. Completely forgot albedo!!! +1 $\endgroup$ – Joe Bloggs Oct 3 '17 at 17:50
  • $\begingroup$ So basically similar to Venus? $\endgroup$ – Vylix Oct 4 '17 at 1:40
  • $\begingroup$ @Vylix: as regards the shinyness from a distance, yes. Hopefully not as regards the ground temperatures. $\endgroup$ – Willk Oct 4 '17 at 13:01
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A different solution:

Have your planet be tidally locked to the star (so that one side always faces the sun) and much closer to the star than Earth equivalent. The population lives in the twilight habitable lands between the burning hot side and the freezing shadow side.

Any aircraft flying in the twilight region rising above a certain altitude would pass outside of the shadow of the planet and be exposed to the full force of the nearby star. Kind of like watching a sunrise on Mercury.

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A distance of 10km is too small relative to the astronomical distances of planetary orbits to make any significant difference on habitable planets (and probably on any planets).

A fanciful but easy to understand example. The temperature at the centre of the sun is 15 million degrees C and the distance from the centre of the sun to the earth is 150 million km. 150million/15million = 10 giving a 1 degree drop in temperature every 10km. So even assuming a linear relationship it wouldn’t work. In reality the relationship of energy to distance is an inverse square so the difference would be miniscule.

Although complicated by very different types of atmosphere and different albedo values a very crude idea of the temperature differences involved can be seen from this link – a few hundred degrees over tens of millions of kilometres.

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"Very close" is a relative term in astronomy. The closest planet to our sun is Mercury, which is 57,910,000km from the sun. I can say with great confidence that flying at a cruising altitude of 10km, bringing you to 57,909,990km is not going to make all that much difference.

What makes it warmer to get closer to the star anyway? The temperature of the radiation reaching you is exactly the same. The only difference is that the star is making up a larger portion of what you can see, so it can heat you faster. You would be looking for the star to be so ungodly close that moving up towards it actually noticeably increases its size in the sky.

To get the effect you describe, I think you would want to have a brown dwarf instead of a star (brown dwarfs are nicknamed "failed stars" because they lack the mass to fuse hydrogen, but can fuse other elements). Brown dwarfs can range from anywhere from 3000 degrees down to the temperature of hot coffee. Then you want it close enough to touch. If you can have a transition from purely radiative heating to conductive heating as you go up. However, you're going to run into issues with drag. Such a planet is on a death-spiral into the sun.

Perhaps the easiest way is to have a substantial cloud layer. If your planet is permanently covered in highly reflective clouds which are transparent to low temperature radiation (like that from the ground), the ground would be quite cool. However, ascend above the clouds, and you suffer the full gaze of the star.

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You can't have the situation you describe with the reasons you describe because the distance to the sun is in from earth is 150 million kilometers and going up 10km is not even noticeable at that scale. This same kind of math apply to your situation.

Going 0.0000001% closer to the sun isn't going to suddenly turn normal Earth temperatures to burning and melting. This is 6 order of magnitudes of difference and scaling down does not really solve the problem. For 10km to make a difference, your planet would need to be extremely close to the star and the star would need to be extremely small. Even the smallest star would not make this possible.

Generally, because of the atmosphere, a planet is also warmest lower down and gets cooler as you move up. On Earth, at 10km up, its freezing cold.

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  • $\begingroup$ Actually, the question is about "scaling down" the system, but the answer is still no. It's not possible to scale down so much that 10 km would make a significant difference. $\endgroup$ – Alexander Oct 3 '17 at 17:19
  • $\begingroup$ @Alexander I have edited the answer to make that more apparent. $\endgroup$ – A. C. A. C. Oct 3 '17 at 17:26

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