Question: So I'm guessing you guys know about the speculation that life exists on Titan. To me, this is very interesting but many point out that it would probably only be microbes and unicellular life because the freezing point and melting point of liquid methane is so close together. 

But! More complex life could exist in that environment if its temperature never went to either extreme. So let's say there is an Earth-sized planet with all that nice stuff on the surface of Titan that makes life suspected to be able to live there, and it's at the right distance to have all conditions like Titan has. So what conditions would make it so that the planet never had a temperature that reached the freezing or vaporization point of methane? Maybe two moons?

Answer: 
**A Planet With Stable, Methane Seas** 

Distance from a star, atmosphere, and planet rotation are the primary aspects that determine surface temperature of a planet. Even then, in order for a planet to remain at a [constant average temperature](https://www.acs.org/content/acs/en/climatescience/energybalance.html), the total energy radiated from its surface (and atmosphere, if any) must equal the total energy absorbed from the sun. 

Even with a maintained temperature, considering that it would generally need to remain between −161 °C (−257.8 °F), methane boiling point, and -182°C (-295.6°F), methane freezing point -- it would be far too cold to sustain any life that we currently know of, [including microbial](https://courses.lumenlearning.com/microbiology/chapter/temperature-and-microbial-growth/).

[![Growth rate of bacteria as a function of temperature.](https://i.stack.imgur.com/Uu2oo.jpg)](https://i.stack.imgur.com/Uu2oo.jpg)

Since water is essential to all known life (which is all carbon based). Without the presence of water, any speculated existence of life is [purely hypothetical](https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry).

Even if a planet were to begin with an oxygen rich atmosphere, the existence of methane seas would completely destroy it. Evaporation and rainfall, of liquid methane, result in a toxic enviroment.  

In fact, methane was one of several gases that canaries were used for in coal mines, since they would die before levels reached 5% (the lower explosive limit).

The levels of methane would [destroy any existing ozone layer](https://www.nasa.gov/vision/earth/lookingatearth/methane.html), causing the planet to be completely unprotected from radiation. Over time, the sunlight would break apart methane into carbon and hydrogen and create a thick haze across the planet, which would become [somewhat similar to an ozone layer](https://www.universetoday.com/40242/titans-haze-acts-as-ozone-layer/).

Aside from that, the amount of methane alone would cause asphyxiation/suffocation for any known form of life.

Therefore, even with sustained temperatures, non-volatile, methane seas would still result in extreme cold, unsustainable to life, as well as no water and no oxygen.
Titan is already pretty close to such a world. There is nowhere on its surface where methane actively boils. So I don't know where this speculation that the limited liquid range would restrict organism sizes is coming from. Especially since there are plenty of places on Earth where water either freezes or boils, and complex life clearly gets along just fine in *this* environment....

Additionally, oceans and lakes on Titan, or a larger Titan-like world, are very unlikely to be pure methane. While rain is mostly methane, because of it's much lower boiling point and correspondingly higher vapor pressure, the surface liquid is a mixture of methane and ethane. And ethane has a *much* larger liquid range. That mixture is what any lifeforms are likely to use as their biolsolvent, not pure methane (and a good bit of their metabolism may consist of transforming one into the other, and either releasing or consuming hydrogen), so the liquid range of methane itself is not quite so important.

However, if you want the temperature to remain relatively stable, there are a few general principles to observe:

1. Minimize orbital eccentricity, so the insolation doesn't change much over the course of the year. If the planet always receives the same amount of input, it'll always maintain the same average temperature, modulo major internal changes.
2. Give the planet a large thermal mass. A thick atmosphere and a large ocean, with high heat capacity, are helpful there. This is why coastal regions on Earth have generally milder climates than inland areas--oceans soak up heat during the day and during the summer, and release is during the night and during the winter, smoothing out the variations in sunlight.
3. Make it easy to transport heat around the planet. Again, a thick atmosphere helps here, as does a freely-flowing ocean. If the ocean is broken up by lots of small landmasses, making it difficult for fluid to flow from place to place, you'll have larger temperature variations than if there are large, open avenues for fluid to move all around the planet. A suitably thick atmosphere, such as Venus and Titan already have, may make this moot, but generally you will want some landmasses distributed to force mixing of oceanic currents, or else you might end up with a situation like Antarctica, which is unusually cold because the ocean can circulate continuously around it, and there is little mixing with warmer waters from lower latitudes. Compare the Arctic region, which is warmed by water that's forced to flow north-and-south around the American and Eurasian continents.

So, replace Saturn and Titan with an Earth-sized version of Titan, you've pretty much already got the sort of low-variance environment you want: a thick atmosphere, low eccentricity, and plenty of surface liquid.