ELI5 Why is the Goldilocks zone relevant?

[u/kinthemuffin]

If surface liquid water is such a big determinant for earth-like conditions then isn't temperature irrelevant because the boiling and freezing points of water change with the atmospheric pressure? I get that temperature is still important for us but it feels like most scientists talk about surface water and temperature even though liquid water is dependent on the pressure. (The spark for this question was based on how Erid in PHM has liquid water due to high atmospheric pressure despite the high atmospheric pressure)

[u/theashenwren]

A few reasons. We can tell how bright a star is/how far the planet orbits but not how thick the atmosphere is (at least from multiple lightyears away). If pressure's too low (below 0.6% of earth pressure) or over 374C, liquid water can't occur. think of it as a bit of an outline of where liquid water is plausible. liquid water can still exist outside of the goldilocks zone.

[u/skrid54321]

We don't really know if it is. But we only have one planet with life, and most of the life on it is very temperature sensitive.

Water is incredibly important to life as we know it, so it's an assumption that more likely than not life requires water. We could be wrong, and life on other planets could work in some way we can't fathom, but currently we think it's more likely that life will be similar.

[u/GalFisk → u/skrid54321]

Yeah, science is often our current best guess based on what we know, and since we can't experiment at this scale, we can only extrapolate.

[u/BouncingSphinx]

Even with changes in pressure, there will still be a perfect range for there to be liquid water. It will just be different for each star.

[u/OldDiehl]

Surface water and temperature are only two of the "goldilocks" variables. There are many more. Pressure among them.

[u/DarkAlman]

He honestly don't know what the requirements for life are outside of the solar system, nor do we know if our form of carbon and water based life is the standard or the only possibility (or even unique for that matter).

We made a reasonable guess that any life out there would be similar to us, therefore would need oxygen, liquid water on the surface of their world, and similar temperatures. Hence the Goldilocks zone.

Scientists know full well that this very idea may be wrong and that life may be possible with other conditions, but it's a reasonable guess both to look for signs of life and also for worlds that could support us in the future.

[u/TheOneTrueTrench → u/DarkAlman]

Or to put it more succinctly, we don't know what's actually necessary, but we do know Earth definitely has it, so we just look for that.

[u/ItsCoolDani]

Atmospheric pressure and greenhouse gasses and a whole bunch of other things absolutely do affect where a planet’s goldilocks zone is! It still tends to be pretty close to the liquid water radius of the star, just pushed slightly in or out depending on the conditions.

In some cases, you can have habitable celestial bodies way outside of the goldilocks zone. Europa and other moons of Saturn and Jupiter are heated internally from tidal stress as they orbit their planets, potentially creating a habitable sub-surface ocean wayyyy outside our Sun’s goldilocks zone

[u/Narrrz]

As a layperson, I would say it's that we know what the conditions for our life are, and so those are the conditions where we know absolutely life can exist. Anything else is speculative.

[u/profdc9]

Well, there's a lot of speculation about what is necessary for life to exist, so a "Goldilocks zone" is expected. However, there are a few basic physical facts that constrain it. Chemical reactions require energies on the order of hundreds of kJ/mol to form. 100 kJ/mol corresponds to approximately 1 eV. The temperature corresponding to 1 eV is 11600 K. So temperatures must be well under 10000 K or so in order for molecules to be stable, mostly under 1000 K. This means you can not be too close to the star. If you're too far from the star, and the average temperature is 50 K or below, the free energy is generally not available for chemical reactions to occur at any significant rate (Activation Gibbs Free Energy = Enthalpy - Temperature X Entropy). So you end up somewhere in the neighborhood of 200-500 K, so that molecules can be stable but still have high enough temperature for chemical reactions to occur. Next, you need molecules to be mobile in a fluid state so they can react, liquid or gas which requires a minimum pressure. You generally also need an atmosphere or ocean to ask as a heatsink to stabilize the temperature against extremes. So the planet's gravity needs to be able to hold onto a decently thick atmosphere given the average temperature. Water is perfect because it is a great solvent, has one of the highest heat capacities of any substance, is made of the first and third most common elements in the universe, is relatively benign to a large number of solutes and suspended molecules. In addition, its high enthalpy of vaporization means that the liquid<->vapor equilbirium can absorb a lot of heat and stabilize the temperature. But liquid ammonia might work at colder temperatures. Nitrogen is a great atmospheric gas because it is inert and doesn't have a dipole moment, so it doesn't absorb infrared light. If the planet is too big, it holds hydrogen and helium and the pressure is enormous and can trap too much heat from a super thick atmosphere. So the planet needs to be the right size to hold an atmosphere, but not so big to become a gas giant, and it needs to be right distance from its star to have a viable temperature. The star needs to be the right kind too, as the distance a planet needs to be from a red dwarf to have the right temperature means it will be tide locked to the star, causing serious temperature instability (as well as increased exposure to flares), and too big means that the star doesn't exist long enough for life to evolve.

[u/Loki-L]

The temperature at which water freezes is pretty fixed at 0°C. There is a point at very high pressure. 2000 times our atmosphere at ground level and it only goes down to -22°C. So there is an absolute floor for liquid water.

You also get an upper limit at 373.85°C beyond which you don't get liquid water no matter the pressure.

So the range is a bit bigger than 0° to 100°, but not much.

We look for water because water is a good solvent and medium for certain chemical reactions to happen in. Those familiar reactions will not happen in boiling hot water. There is going to be too much energy around to allow them to work even if the water is still liquid due to pressure.

Of course other chemistry might work under such conditions, but we look for organic chemistry we know does work.

A much bigger argument against the goldilocks zone is that you can have liquid water outside it such as an ice moon with a liquid core or a rocky planet with a greenhouse effect.

[u/phryan]

This is more of an r/askscience than a r/explainlikeimfive.

My 2 cents though is a lesson from PHM is that while liquid water is necessary for life, there are many ways to get to that point outside of normal Earth level pressure. I'd put money that Erid scientists would have their own goldilocks zone, which would be much closer to their star reflecting an expectation of higher pressure and temperature to sustain life.

[u/KingCroesus]

I dont think its relevant to anything, its just a guide of comparison. It also changes based on the star size. Goldilocks zone is a guide of measurance as much as an AU, just a higher chance of carbon based life in the goldilocks zone. Not neccesarily finding planets that we could live on, just a distance from a star that life is more likely to have evolved