# Double planet climate

How far would 2 tidally-locked (to eachother) planets need to be from their blue star host to be habitable, but have a cold-weather climate similar to our Ice Age?

• To which is the planet tidally locked, its sister planet, or the star? Because it can't be both. Sep 20 at 17:18
• You're asking a lot of different questions in this post. Worldbuilding has a strict 1 question per post requirement. Can you edit it so you're asking 1 specific answerable question. Sep 20 at 17:22
• Please give a good read at our help center, in particular to the section about how to ask a good question. You are now asking many different questions, without showing any of the research you have made.
– L.Dutch
Sep 20 at 17:23
• Thank you for the edit, I've retracted my close vote. Sep 20 at 19:18
• DItto, the questions clear and answerable now. Sep 20 at 19:41

TL;DR: binary planets probably handwaveable to be OK, iceage climate can also be arranged. Making a habitable world around a blue star is basically a non-starter, though. Too hot, too bright, far, far too much UV. You should probably change that.

It is difficult to say exactly how the climate of a binary planet will differ from the situation on Earth... the day length will be longer, and so the days will likely be warmer and the nights colder. It is possible that some atmospheric super-rotation will help even things out... strong prevailing winds will help warm the night side and cool the day side to some extent.

Having an ice-age like cold weather climate is easy though... just slap em in the habitable zone and call it a day. After all, Earth has been both ice free and entirely frozen over in its time, without having any kind of dramatic orbital change. Just handwave in your desired conditions, and you'll be basically good to go.

The habitable zone of a blue star is a bit more awkward, though. There are multiple problems with having such a hot star as the primary, but I won't cover them all here. Lets consider a a star like 18 Tauri. It is a main sequence B8V star, so it is quite small and cold by the standards of the some hot blue stars. As you can see from the Wikipedia page, it is larger than the sun... nearly triple its radius. Its luminosity though is more like 160 times that of the Sun. Its surface temperature is over 13000K. That's pretty hot and bright.

In order to receive about the same amount of solar irradiance, your worlds might need to be a factor of $$\sqrt{160}$$ or more than 12 times further away from the star than Earth is from the Sun (this is a very lazy approximation for a habitable zone, but it'll do as a starting point). That gives your star, as seen from your worlds, and angular diameter of about 0.12 degrees. Compare this with the Sun's apparent angular diameter of more like 0.52 degrees... your star has a quarter of the diameter and so a 16th of the apparent area. It'll look like a tiny, blindingly bright light in the sky, producing hard-edged shadows. It won't look like a cool blue version of the sun. The very blue-white light will make everything look quite washed out and sterile.

That's just aesthetic though. More concerning is the nature of its radiation. . The sun, with a surface temperature of ~5778K, has the peak of its radiation spectrum at 510nm, a faintly bluish green, and only about 3% of the total output is of UV wavelength or shorter. 18 Tauri has a peak at 210, which is hazardous and destructive UV-C. More than 20% of the total radiated output is vacuum UV, and nearly 50% of the output is any kind of UV (or worse). (Worked out using another handy calculator on hyperphysics)

This will be exceedingly destructive to pretty much anything made of chemical bonds. The oxygen in your atmosphere is particularly good at absorbing UV (that's what the ozone layer does on Earth). The increased rate of oxygen-bond splitting increases the amount of high-altitude monatomic oxygen and ultimately atmospheric escape. Your atmosphere will be rendered unbreathable much more quickly than it will on Earth.

Even if you handwave that away things will still look very weird and alien... plants won't be green. I'm not sure quite what colour they will be, but I could imagine a white or black might work. It will look very alien indeed.

• Upvoted.. completely overlooked the aspect.. a blue star.. can it have an habitable zone at all.. maybe only a goldilock zone (water + ice) Sep 20 at 21:03
• On last paragraph: White or black to an Earthling, but whatever lives there (if anything lives there) will likely have their visible spectrum sitting around the UV region, so will perceive colour completely differently to Earthlings. Sep 21 at 7:00
• Can you perhaps tune wavelengths somewhat by using a dust cloud (maybe ex-moon) between the planets and the sun? Even if expected particles are too big for Rayleigh scattering you could have reflectance wavelength-dependent. Say silver, gold and copper all reflect visible much better than UV so you could solve problem somewhat. Besides, we have N=1 sample of habitable planets and even here some bacteria are annoyingly UV resistant, so I believe life would find a way. Sep 21 at 8:02
• Green plants will be Okay-ish. The plant usable spectrum is generally limited by the chemical bond stability and not the available light spectrum. So is human vision. On the other hand, blue stars tend to evolve quickly and may not allow for a complex life evolution. Sep 21 at 8:58
• @ZizyArcher life could be fine... chasmoendolithic and deep ocean biomes under an ice cap could both theoretically work here, assuming they had time to evolve before the parent star went foom (the light that burns twice as bright, etc etc). I get the impression that the OP isn't quite so interested in that particular kind of setting, though. Sep 21 at 11:05

One very well studied planet, the planet Earth, has a distance from its star, the Sun, which is exactly 1 Astronomical Unit (AU) by definition.

You ask for climate like Earth had during the Ice Age, which was really a bunch of separate ice ages, separate glaciations. In those glaciations Earth's average temperature was a little lower than now, and glaciers covered a much larger percentage of the surface than now, and the lands at the edges of the ice sheets were (obiously) not warm enough to melt the ice very vast when the edge of the ice was advancing or when the edge was standing still.

Those events happened when Earth was at a distance of one AU from the Sun. And as I rememeber, during the last one or two million years the glaciations have been much longer than the interglacial periods like the one we are now in, though the maximum extent and peak of the ice lasted for a much shorter period than the total period of each glaciation.

So Earth's ice ages happened when Earth received about as much energy from the Sun as Earth receives now.

And before the last million or two years ago there was a period of tens or hundreds of millions of years with no glaciations on Earth, even though Earth received about the same amount of radiation from the Sun then as it did during glaciations and as it does now.

You should study the various theories about the causes of Earth's glaciations and see how well you can give your planet similar contributing factors.

https://en.wikipedia.org/wiki/Ice_age

Since Earth is now in a state of recurring glaciations, you should calculate what I call the Earth Equivalent Distance (EED) of the star, the distance at which the planet receives as much energy from its star as Earth receives from the Sun, and put your planet at that distance from its star. That should be enough to explain the glaciation the planet is currently experiencing, if you can also give the planet the factors which contribute to the onset of glaciations on Earth.

I note that during the glaciations on Earth, the lands adjacent to the ice fields and at high altitudes were quite cold, but there were vast regions near the equator which were quite warm and lush.

I don't know if that is acceptable to you. Maybe you want most of the planet to be covered with ice caps, and the small regions of bare land and sea to be cold and occpupied by cold wether plants and animals.

In that case you might need something more like the periods of snowball Earth than like ice ages.

Periods of snowball Earth supposedly lasted for millions or tens of millions of years and happened several times in Earth's history.

I note that astrophysical calculations suggest that the Sun slowly and steadily gets more luminous, and so it seems an obvious guess that the end of a period of snowball Earth might result from come the amount of radiation Earth gets from the Sun increasing past a critical tripping point. But that could not explain several different periods of snowball Earth separated by warmer periods.

Thee sems to be considerable uncertainty about the causes of those snowball Earth episodes.

https://en.wikipedia.org/wiki/Snowball_Earth

Both the current period of alternating glaciations and interglacial periods, and the periods of snowball Earth, are examples of Earth during icehouse periods. The climate of Earth seems to have alternated between greenhouse and icehouse periods over geologic time.

So possibly some of the icehouse periods in Earth's past may have been better for your purposes than either the more recent glaciations or the more distant snowball Earths.

https://en.wikipedia.org/wiki/Greenhouse_and_icehouse_Earth

Goldilock- or habitable zone

For each type of star, there exists a certain minimum and maximum distance for planets, that could support fluid water. When your Ice Age would be anywhere earth's, you'd still have fluid water.. so when these planets are both Earth-like, you could put it at a position comparable to Earth, but not closer.. Earth is actually quite hot.. on the inner edge of Sun's habitable zone, which is 0.95 AU[*] wide. You could put your planet e.g. where Mars is, in the middle.. 1.524 AU from the Sun.

https://physicsworld.com/a/earth-is-closer-to-the-edge-of-suns-habitable-zone/

Atmosphere

If Mars would have an Earth-like atmosphere, you'd get a lot of snow and ice. With a denser atmosphere, planet Mars could become Earth-like, or an ocean planet.

Double planets

This planetary system of yours better be stable ! For climate, I see only one effect between the planets affecting temperature. Size matters.. if your 2 planets are close and differ considerably in size, the smaller planet gets periods of solar eclipse, which can cool things down. But apart from that, atmosphere and sun-planets orbit distance is the key to get an icy, inhabitable planet.. they could have 2 different atmospheres, one -70 degrees Celcius and Mars like, the other Earth like.

[*] AU = Sun-Earth distance

NOTE: this explanation ignores the fact that we have a blue star. As Starfish Prime has pointed out, these are quite hot and emit deadly radiation. There could be a goldilock zone (supporting water) however it may be doubtful there is a habitable place on these planets, unless its inhabitants are very resistent to radiation.