Is ~-40C to 40C the only temperature range for any lifeforms to exist? Or could there be lifeforms that we cannot imagine at -150C or 4000C?

there really is a concept of temperature in space beyond the 3K background

I disagree. 3 K is the only physically meaningful temperature for space (and it really only applies to deep space).

For instance, knowing the temperature of a room is only meaningful because we can expect objects within the room to approach the room's temperature. Note that this expectation holds regardless of an object's position, orientation, surface area, emissivity, etc. Can we define an analogous temperature for space?

First of all, this 'temperature' will depend on the distance from the sun. This isn't an insurmountable problem: we can just say that the temperature of the solar system depends on position. Maybe at 1 AU (Earth's orbit) the temperature is ~300 K, while at 5 AU (~Jupiter's orbit) the temperature is ~130 K, an so forth. But the equilibrium temperature depends on much more than position:

  • For instance, the equilibrium temperature on Earth has been slowly increasing (i.e. global warming). This is caused by a change to the Earth itself, not just the distance between the Earth and the Sun.

  • The Earth and the Moon are the same distance from the Sun, yet they have different temperature.

  • A spacecraft with radiators (e.g. ISS) is much cooler while the radiators are running. Also, spacecraft can stay cooler by constantly rotating.

It's clear that the equilibrium temperature of an object in the solar system depends on many things, most of them specific to the object itself. Therefore it is impossible to define a meaningful temperature anywhere in the solar system.

Is this making sense? It might sound like I'm nit-picking, but I'm really not. Would you say that the space 1 AU from the sun is 300 K for an Earth like planet with 400 ppm CO2, 310 K for an Earth like planet with 500 ppm CO2, 400 K for a non-rotating spacecraft in LEO with high beta-angle, 350 K for a rotating spacecraft in LEO with a high beta-angle, 310 K for a rotating spacecraft in LEO with a low beta-angle, 309.8 K for a spacecraft rotating slightly quicker in LEO with a low beta-angle... I think you get the picture. You can't put a number on the 'temperature of space' in the solar system.

But then why can we say the the temperature of deep space is 3 K? Most of the complications we saw earlier were caused by asymmetry in the system. Basically, the sun is only heating you from one direction. You can change the equilibrium temperature by changing the rate of radiation from the 'dark' side of the object (i.e. using radiators, rotating the spacecraft, etc.) But when you are in deep space, you are getting equal radiative flux from all directions, so none of this matters anymore. So we can reasonably expect an object in deep space to come to an equilibrium temperature of 3 K, therefore 3 K is a physically meaningful temperature for deep space.

/r/askscience Thread Parent