I've settled on using a blue star for my setting. It'll probably either be a small B or large A-type main sequence star, somewhere between 2 and 4 solar masses. There is one relevant planet. I've toyed a bit with various calculations, including this neat resource https://www.astro.indiana.edu/ala/PlanetTemp/index.html. For now:

-largely icy planet (so probably a high albedo - I went with 0.75 - 0.8)

-average global temperature vaguely around -20 degrees Celsius

-potentially a captured vagabond planet (so it needn't have formed around this particular star)

Right now, I'm getting the feeling that I should place the planet from 2.something to 3 AU away from a smaller (2 solar masses) star, or 6-8.5 AU away from the larger (4 solar masses) star. [It's not a binary system, it's a single star, I'm just undecided how large to make it on the scale from 2 to 4 solar masses]. The range (very) roughly accounting for some variation in greenhouse gases. For the reason mentioned in the title, I've been wondering whether I should increase greenhouse gases even further, in order to be able to move the planet further from its sun while keeping the temperature.

Basically, from my understanding blue suns emit more UV. Would simple distance be enough to protect the planet to the point that a) it's not stripping away its atmosphere (to a significant level), but also b) it's not deadly for its squishy humans? I'm fine with a less bright sun, but would ideally still like to keep enough light that a standard human eye could see colors during the day. Is there a balance where I can keep both colors, and UV low enough that simply walking outside is not much more dangerous than it is on Earth?

I would appreciate your thoughts!

Edited for clarification.

  • 3
    $\begingroup$ Detailed atmospheric composition is the important variable here and it appears to be missing from the details you have provided. Earth's ozone layer is measured in parts per million and makes a huge difference to how we cope with UV hereabouts. $\endgroup$
    – Ash
    Commented Sep 20, 2021 at 0:48
  • 2
    $\begingroup$ I would discourage going any further out. You already have to crank up the greenhouse gases to keep out of a deep freeze. More distance isn't going to save you. The two big levers for dealing with UV are ozone (as Ash mentioned already) and a strong magnetosphere (to keep you atmosphere safe). $\endgroup$
    – legio1
    Commented Sep 20, 2021 at 1:53
  • $\begingroup$ There are some problems with your set up which may have to be modified. I will write an answer later. $\endgroup$ Commented Sep 20, 2021 at 2:06
  • $\begingroup$ your planet would need to orbit both stars at once with these distances. spending time closer and further away from your blue star. $\endgroup$ Commented Sep 20, 2021 at 5:11
  • $\begingroup$ Walking outside will be immediately lethal, because there’s no oxygen, because there’s no life, because the star is far too young for it to have evolved. $\endgroup$
    – Mike Scott
    Commented Sep 20, 2021 at 5:58

3 Answers 3


Perhaps you should figure out where you want your story to be on the Mohs Scale of Science Fiction Hardness.


The "harder" you want it to be, the more you need to read what follows.

Problem One:

In the early 20th century geologists discovered that the Earth was billions of years old. They also discovered fossils of Earth lifeforms hundreds of millions and eventually billions of years old. Those fossils showed that the surface temperatures on Earth remained fairly constant for billions of years.

Astronomers and physicists calculated how long the Sun could shine with energy from gravitational contraction, and they found that the Sun could shine for only tens of millions of years, a tiny fraction of the age of the Earth, and thus of the Sun, according to geologiests.

So there were strong disagreenments about the age of the Earth between geologists and phycists and astronomers. I have read that actually led to punches at one scientific conference.

Then in the 1920s, 30s, and 40s, astronomers nand uclear physicists worked out the nuclear fusion processes which actually powered stars. They began to calculate how long a star of a specific spectral class would shine as a fairly steady main sequence star before becoming a red giant and then a white dwarf, killing all life on its planets.

At the same time geologists established that Earth was about 4.6 billion years old, and found evidence that there had been life on Earth for billions of years, and that some lifeforms on Earth had eventually produced the oxygen rich atmosphere which humans and all large multicelled lifeforms on eArth need to life. They found that Earth only had an atmopshere breathable for humans during the last few hundred million years, a small part of the history of Earth.

Combining those facts, it became obvious to the more scienficially literate science fiction writers that only main sequence stars belonging to some spectral classes could have planets that could possibly - other factors being right - support human life or be otherwise interesting for most types of science fiction stories set on other planets of other stars.

So in Robert A. Heinlein's juvenile science fiction novels Starman Jones (1953) and Time for the Stars (1956) it was mentioned that main sequence spectral class G stars would be most suitable for having planets habitable for humans and for lifeforms with similar environmental requirements.

Stephen H. Dole published Habitable Planets for Man (1964) discussing the scientific factors necessary for a planet to be habitable for humans or for lifeforms which have similar requirements.


On pages 67 to 72, he discussed the properties necessary for a star to have a habitable planet, calculating lower and upper limits of mass and luminosity.

On page 68 Dole calculated the upper limit of stellar mass for for a star to possibly have a habitable planet is aobut 1.4 stellar masses, a spectral class F2V star.

And if you ignore such calculaitons you may run the risk of being considered 60 or 70 years behind the times when it comes to selecting the spectral classes of stars in your fictional star system.

The only way for a writer to get around the spectral class limitations for habitable planets is to claim in the story that a highly advanced civilization terraformed a young and uninhabitable planet in the system of a young star to make it just as habitable as planet billions of years older would become. Or maybe those aliens took an older planet that was already habitable from another and older star system and mmoved it to the younger star system of the story.

Your idea that your planet could have been naurally torn from orbit in one solar ssytem, traveled through interstellar space for countless millions of years, and then been recapured into orbit in your solar system is very dubious. That would be a very rare sequence of events.

If it is possible in your story for a super advanced civilization to deliberately move a planet form one star system to another, that should happen countess millions of times more often than a planet accidentially moving from one star system to another.

And if a planet accidentially and naturally moves from one solar system to another, it should freeze up for the countless millions of years it will take to do so, killing all life on the planet. But if a superadvanced civilization can deliberately move a planet from one star ystem to another, they should have no problem keeping the planet warm and lighted while it crosses interstellar space, thus keeping life alive on the planet.

Or you can simply put your planet in orbit around a star of the proper spectral type.

Or if you don't care how hard or soft your story is, you can use any type of star you want.

Problem Two:

A planet habitable for humans can not be at just any distance from its star. Each star has a specific luminosity, and each specific luminosity has a different sized circumstellar habitable zone, where an otherwise suitable planet would have the proper temperatures for having liquid water on its surface.

Calculating the size of the circumsteller habitable zone around a star is easy in Theory. Just take the luminosoity of the star compared to that of the Sun and scale the habitable zone of the Sun up or down.

But the size of the circumstellar habitable zone of the Sun is not known with certainty.

Here is a link to a list of recent estimates of the inner or outer edges or both of the Sun's circumstellar habitable zone. Note how much they differ.


One way to make certain a fictional planet will be within the habitable zone of its star is to calculate the exact distance from that star that a plent would receive exactly as much radiation from the star as Earth receives from the Sun, which I call the Earth Equivalent Distance or EED.

The answer by user177107 to this question:


Has a table with a list of data about differnt spectral classes of stars. The date includes the disances of planetary orbits at the EED for each listed type of star.

Note that according to Dole, the most massive star capableof having a habitable planet would be an F2V class star, with a mass about 1.4 times that of the Sun. Such a star would have an EED of 2.236 AU according to the table.

You say:

Right now, I'm getting the feeling that I should place the planet from 2.something to 3 AU away from a smaller (2 solar masses) star, and 6-8.5 AU away from the larger (4 solar masses) star.

I assume you want the smaller star with two solar masses to more or less orbit around the larger star with 4 solar masses, and for the planet to orbit around the smaller star with 2 solar massess.

According to the table of stars I mentioned earlier, a star with 2.05 solar masses would be a class A2V star, and a luminosity 21.243 times that of the Sun, and its EED would be at 4.611 AU. Thus at a distance of 2 to 3 AU the planet would receive consideriably more radiation from the smaller star than Earth gets from the Sun, and so should be considerably hotter than Earth despite having a higher albedo. Your temperature of about minus 20 degrees C seems improbably cold even without considering the radiation the planet would get from the larger and more distant star.

Vega is a class A0Va star with a mass of about 2.135 solar masses, and a luminosity 40.12 times that of the Sun, and thus its EED should be at about 6.334 AU.

A spectral class B8V star would have a mass of 3.8 solar masses, and a B7V star would have a mass of 4.45 solar masses. So your larger star with "(4 solar masses)" would be between a B7V and a B8v star, and closer to the B8V star.

18 Tauri is listed as a B8V class star, and has a mass of 3.34 solar masses and a luminosity of about 160 times the luminosity of the Sun. Thus its EED should be at about 12.649 AU.


Thus a planet that is 6 to 8.5 AU from a star with 4 solar masses should be hotter than Earth, no matter how high its albedo, even if it wasn't also even closer to the smaller star in the system and also heated up by that smaller star.

Problem Three.

Your planet needs to have had a stable orbit for a long time.

A planet in a binary star system can have one of two types of orbit. Exoplanets with both types of orbits have been found.

A circumbinary or P-Type orbit is when the planet orbits around both of the stars, which are much closer to each other than the planet is to them.

An S-type obit is when the planet orbits one of the stars and the other star is much farther from the planet. Your description with one star several AU farther away from the planet the plane tthan the other star indicates it is an S-Type orbit.

In non-circumbinary planets, if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.5


In your example, with the planet orbiting one star at a distance of 2 to 3 AU, and the other star being 6 to 8.5 AU distant, the ratio of distances is 2 to 4.25, less than the 5.0 minimum ratio for orbital stabiity.

According to this list, the closest known orbital distance between two stars with a planet orbiting one of them in an S-Type orbit is about 12 to 17 AU. Since the planet orbits one of the stars at a distance of about 0.7 AU, the distance ratio is about 17 to 24.


Problem Four

You desire that the average surface temperature of the planet is about minus 20 degrees C. That is below the freezing point of water, so there should be no liquid water on the planet and no liquid water using lifeforms. Thus photsynthesis whould never have produced an oxygen atmosphere on the planet, and it should be uninhabitable for humans.

So the colonists you mention in your question should never have colonized the planet.

Maybe they colonized the planet to mine it, and they live in pressurized buildings like in a moon base, and work in mines deep underground or in massive excavating equipment in open pit mines. Thus they should be protected from ultra violent ultraviolet rays by the roofs and walls of their buildings and by their vehicles, and by rock and/or ice when they work in the mines.

Since the air is unbreathable they will have to wear breathing gear outside, and since the climate is so cold they will have to wear very warm clothing outside. So I guess that they might as well wear protective gear when outside which covers their entire bodies to keep them warm and supply oxygen, and which also prevents all ultraviolet radiation from reaching their skin.

Problem Five.

The Earth's atmosphere keeps a lot of ultraviolet radiation from reaching the lower atmospher and the ground. Since Earth has a breathable atmosphere wiht a lot of molecular oxygen (02), some of that is converted by various processes into ozone (03), and some of that ozene forms the ozone lawyer in the upper atmosphere which blocks a lot of ultraviolet radiation from reaching the surface.

So if your planet is not so cold that it has no native life and no oxygen in the atmosphere, but instead has native life and a breathable oxygen rich atmospehere, like most planets which are colonized by humans in science fiction stories, it should have an ozone layer which block a lot of ultraviolet rays from reaching he surface.

And possibly you could find a way to change the composition of the atmosphere to block out much more ultraviolet radiation and make the planetary surface safe for your colonists.

  • $\begingroup$ Thank you very much! I'd like to comment, not because I want to be combative but because I love getting into the nitty-gritty of this stuff. [Note: there's only one star, whose size may range from 2 to 4 solar masses, not two. edited to clarify] 1) Larger stars aren't typically listed as habitable due to their shorter lifespans. However, for the smaller estimate (2 solar masses), the total lifespans I got ranged from 1.7-2.5 bill year. An atmosphere could be conceivable if we assume that certain improbable events (such as the transition to an eukaryote-like-complex-cell, and multicelularity) $\endgroup$
    – Laura
    Commented Sep 20, 2021 at 8:51
  • $\begingroup$ would happen faster than they did on Earth. Essentially cutting out the 'boring billion' of Earth history' plus some on top. Nevertheless, as improbable as it is, I'd like to keep the rogue planet in the story. Would it be impossible that some life could be maintained in the oceans, around hydrothermal vents? This inspired me youtube.com/watch?v=M7CkdB5z9PY 2) Welp, I need to look into more detail to see how that source differs from the one I list in my question. Thank you for spotting that. 3) My mistake. It's only one star. $\endgroup$
    – Laura
    Commented Sep 20, 2021 at 8:52
  • $\begingroup$ 4) Even if the rogue planet idea doesn't work out, would it not be feasible that the planet wasn't always a snowball? I mean, Earth was a snowball for large parts of its history, and photosynthesis still happened. 5) Thank you! $\endgroup$
    – Laura
    Commented Sep 20, 2021 at 8:52
  • $\begingroup$ A nice read, However -20°C is the average temperature and doesn't rule out local temperature above 0°C: one idea is a planet tidally locked to its star, so that one particular place is warmed up above 0°C and allows for life to appear, whereas the rest of the planet can still be populated once organisms gradually adapt to lower and lower temperatures. Of course when you think about that, much higher temperatures than 0°C are possible for a planet with -20°C average, which allows for some rotation of the planet. Also volcanoes. $\endgroup$ Commented Sep 20, 2021 at 15:43

Less of a problem for dark skinned colonists.

Constitutive high melanin levels are protective against UV damage. It is not a myth!

The Protective Role of Melanin Against UV Damage in Human Skin

Black and Australoid persons often have high constitutive melanin levels and might be expected to have less trouble as regards exposed skin / UV damage. Maybe your colonists are such persons? Of course if your story needs some fair skinned persons they can wear stylish broad-brimmed hats when out and about.

  • $\begingroup$ Thank you! I was intending for them to be dark-skinned. I was just unsure of the scales I'm working with here (e.g. if kep my planet close enough to the sun for my desired temperature and shove some ozone on top, would that be enough or am I still left with deadly, potentially atmosphere-stripping levels of radiation?). *but stylish hats for tourists is a neat idea ^_^ $\endgroup$
    – Laura
    Commented Sep 20, 2021 at 9:26

M. A. Golding wrote a great answer but like me probably thought that it was supposed to be a binary system.

Regardless of your choice between the 2 to 4 solar mass stars. Your brain will just automatically adjust. From the planet looking up you would see a white star just like on earth. If you looked out over the landscape it would just look like earth. You would only notice a difference when you were comparing photo's from earth and your planet but only if you kept the camera settings the same. So for a story on planet all you would change is the amount of UV-radiation, the length of a year, and the possible lifeforms already able to have evolved on the planet.

The human eye is a remarkable thing able to perceive a lot in low light conditions compared to what our sun provides. If you have ever tried to run a high speed camera you can attest to this. Having to need lots of high powered lighting indoors while you just could move outside into the daylight to get even more light. So there shouldn't be a worry about that.


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