How would our skies look if our sun was a white dwarf instead?

Question

What I'm using your information for:

My planet is small, has a thicker atmosphere than earth and it's orbiting a white dwarf. The main reason I chose it to be a white dwarf is that I read somewhere that these would take much longer to cool than a regular star, because they have a much smaller surface. The reason I need the star for such a long time is, that I want to communicate how unlikely it is that an intelligent alien species similar to ours would evolve. The time between humanity and my alien species should feel like an eternity. At the same time my species is one of many thousand similar ones that lived on that planet one after another. The whole ecosystem stayed roughly the same and just this one species gets replaced every few 100'000 years. -This process should go on for as long as possible.

What I want to know:

The main reason I won't do my research without you is, that I'm really really bad at it: The white dwarf I'm "looking for" should burn its energy as slowly as possible. That's why my planet will orbit quite closely to it, still allowing my ecosystem to exist. It should also have the biggest mass possible and look the same over a long time which (I think) means, that its spectral light will rather be white than blue? -Here's where it ends for me.

• Does the light of the white dwarf I need look differently from each angle than that of our sun? How?

Edit regarding the comments:

To L.Dutch. Yes, it's tidally locked.

To AlexP. I could imagine that dead ends in evolutionary lines are possible, if the conditions are right. Our way of evolution isn't the only possible anyways. I even wanted to explain parts of it: There's a bacteria-like species that lives in almost every organism. Rumors tell that an intelligent species that lived long ago had developed them to make certain species go extinct for ever. They support the organs of all rather complex species, also their breeding chambers... To your second comment: I never thought about it this way... Species evolving that have comparable sight to us will see their suns color as white anyways. Everything else would depend on the atmosphere and the weather... Thanks.

To Alexander: Exactly.

To Stephen G: I understand what you mean. Why would someone care about the sun, when the main problems are asteroids, the rotation of a planet, the planet cooling down, etc? - There's another bigger planet orbiting the white dwarf blocking/pulling asteroids away. The planet will be heated by the same other planet whenever it comes by, like Titan, just less often. The species aren't like the ones we have on earth. Most of them are able to live for thousands of earth-years and they don't reproduce how "we" do. (would take long to explain) Everything else can be explained by luck or "not talking about it", I guess. The longer the sun stays the same, the more likely it is that there's a very long window where also everything else is in place for the stagnating environment I want my stories to take place in.

to cmm: thanks. Now there's only the "main question" remaining.

to TheDyingOfLight: "universe sandbox" looks really interesting. Thanks for the link.

to Mike Scott: Thanks! Such info was exactly what I was hoping for. Hopefully I'll find a way/an example to explain this...

Answer 1

How the sky will look depends on the light of the star, the consistency of the atmosphere, and the weather. You have given clues to each of these in your multi-part question.

The starlight will be "white" to any visual system that has evolved in that illumination. We see white light because our experience of "white" is that "white" is the color of natural light from which nothing has been removed. If the sun were cooler, or hotter, the spectrum would change, but eyes which evolved in that spectrum would see it as white.

Your planet's atmosphere is denser than earth. The atmosphere may need a generous quantity of greenhouse gasses. If the star is to last longer than the earth's sun, it would use fuel more slowly, and the emitted energy would be lower. Up to a point the lower stellar output can be compensated for by being closer to the star, but an atmosphere of greenhouse gases helps a lot. You can't be too close to the star or your planet will be ripped apart, so greenhouse gasses may be a friendlier alternative.

The weather is trickier. You say that nothing changes in the ecology, which must mean that mountains aren't growing, the seas aren't changing, and the climate is consistent. I don't know how to do that. Wind and rain cause erosion, which cause change. I think you might need to give up on you "constant ecology", and make something more like earth, with a dynamic balance between growth and destruction with species evolving in competition with others, and species disappearing as they fail to adapt to changes. It would be more likely that your succession of intelligent species could evolve in such a dynamic relative equilibrium that in a world that is completely static.

Answer 2

There is a scientific paper titled "TRANSIT SURVEYS FOR EARTHS IN THE HABITABLE ZONES OF WHITE DWARFS" by Eric Agol that focusses on the possibility of habitable planets orbiting white dwarfs, and it has quite a bit of information that you may find useful.

In it he claims, although he does not show his calculations, that the star would appear to be of a very similar apparent color and apparent size in the sky to that of the sun, if the planet was orbiting at the optimal position for the planet to be habitable for the longest possible time, which he concluded to be at an orbital radius of approximately 0.01 AU, which, yes, is tiny, but white dwarfs are very dim.

It is also worth noting that the more massive a white dwarf is, the smaller it's radius, up to an optimal mass of approximately 1.44 solar masses, at which point a white dwarf will collapse into a neutron star. However, whatever your atmospheric composition is will likely affect the apparent color of the star, so you may need to take that into account.

One side effect of having a planet orbit a white dwarf star that you may not realize is that, because white dwarfs produce either very little (if the white dwarf is extremely young) or no solar wind, which results in there being no Auroras.