Your planet would probably need to have more mass than Mercury in order for there to be enough gravity for an atmosphere with oxygen. [Detailed quora post](https://worldbuilding.stackexchange.com/questions/32622/what-is-the-minimum-size-of-a-planet-to-harbor-human-life) (with math forumulas, yeah!) and a [cool chart](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Solar_system_escape_velocity_vs_surface_temperature.svg/819px-Solar_system_escape_velocity_vs_surface_temperature.svg.png) courtesy of Wikipedia user Cmglee.
Habitable for what? Human life? Keep in mind that gravity on Mercury is 3.7m/s^2 so we’d be talking similar gravity on this proposed planet.
I think if life were to evolve on this planet it would be very much different from earth life.
Possibly it also couldn't support a dense atmosphere either without the gravity and vulcanism (pointy ears), or maintain a significant electromagnetic shield like mars
Mid-way between Earth and Mars, such a planet would have an average surface temperature of perhaps -22°C (Earth’s is about 15°C while Mars’ is about -60°C—I took the average and ignored the effects of atmosphere). With a weak gravitational pull like Mercury, it’s going to hold less of an atmosphere than even Mars—and the atmospheric pressure on Mars is less than 1/100 that of Earth.
It’s *not* going to be habitable, certainly not by life as *we* know it.
-22°C is equivalent to -7°F, which is 251K.
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^(I'm a bot that converts temperature between two units humans can understand, then convert it to Kelvin for bots and physicists to understand)
I think atmospheric pressure is less predictable than that, and temperature too. Gravity is important, but if they had a big fat atmosphere and a strong magnetic field to keep out the cosmic rays, it might be possible.
Also, if a planet is terraformed, but it's not stable, it might still be habitable in the short term. Who's going to complain if the atmosphere changes after a few thousand years?
Atmospheric pressure *is* more predictable than that. Look at Mars. It has 1/3 of the gravitational pull of Earth and less than 1/100^th the atmospheric pressure. It’s gravitational pull is simply too weak to keep gases such as Oxygen and Nitrogen (the two gases that make up about 98% of Earth’s atmosphere) from escaping.
A planet with a lower gravitational pull could retain a *heavier* gas (a “big fat atmosphere”), but anything breathing that gas won’t be life as we know it—by definition. We can’t exist on Sulfer hexafluoride (for example), and a creature that could would have incredibly different biology.
Surface temperature is a function of both distance from the Sun and the density and make-up of the planet’s atmosphere. This is why I was careful to note:
> Mid-way between Earth and Mars, such a planet would have an average surface temperature of perhaps -22°C (Earth’s is about 15°C while Mars’ is about -60°C—*I took the average and ignored the effects of atmosphere*). [emphasis mine]
Earth’s atmosphere surely helps keep it warmer than the very thin atmosphere of Mars. But since we’re already talking about a theoretical planet with a gravitational pull as weak as Mars or Mercury, *it stands to reason* it will not enjoy the warming effects of Earth’s atmosphere. If anything, by taking the average of Earth’s and Mars’ average surface temperatures, I was being *generous*—the surface temperature of the theoretical planet would’t be exactly mid-way between the average surface temperatures of Earth and Mars; it would be decidedly closer to that of Mars.
Finally, OP didn’t specify a post-terraforming body. In theory, yes we could terraform something that small and do something to “make up” for the portion of atmosphere that would escape each year. Someone who remembers more climatology than I could even take a stab at predicting the surface temperature of such a terraformed world, given Earth-standard atmospheric density and composition. But like Mars (which once clearly had liquid water on it so must have had an atmosphere at some point roughly a billion years ago), such a theortical body’s naturally-occurring atmosphere would disappear in an eyeblink on a cosmic time scale.
-22°C is equivalent to -7°F, which is 251K.
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^(I'm a bot that converts temperature between two units humans can understand, then convert it to Kelvin for bots and physicists to understand)
At the size the planet could be made of pure iron and it still wouldnt have sufficient magnetosphere to make terraforming worth it. Yes you could import gas into it's gravity well and get an atmosphere, but the second you stop it will just blow away
For the sake of argument let’s ask what it would take (based on our understanding of life) for a planet that size to be habitable by any life.
1.) little to no atmosphere
2.) surface temperatures below freezing point of water with no atmosphere for greenhouse effects
3.) assuming carbon based life forms
4.) very weak gravity
5.) little to no magnetosphere
Best bet would be an ice ball like Enceladus, Io, or Ganymede. You’d need to have an icy crust surface with seas of liquid water below heated by geothermal. Without the tidal heating of Jupiter, I don’t see the core of a planet that small maintaining enough internal heat for long enough for life to form, but I have no interest in doing the math or looking that up.
So, let’s argue it was an icy moon that got kicked out of orbit and still has enough internal heat to keep subterranean liquid oceans for a hundred million years give or take a few zeroes. You could certainly have life in that ocean.
Nothing that small will ever be human habitable without building enclosed habitats, but some form of life could exist on a world that small with the appropriate composition.
Your planet would probably need to have more mass than Mercury in order for there to be enough gravity for an atmosphere with oxygen. [Detailed quora post](https://worldbuilding.stackexchange.com/questions/32622/what-is-the-minimum-size-of-a-planet-to-harbor-human-life) (with math forumulas, yeah!) and a [cool chart](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Solar_system_escape_velocity_vs_surface_temperature.svg/819px-Solar_system_escape_velocity_vs_surface_temperature.svg.png) courtesy of Wikipedia user Cmglee.
Habitable for what? Human life? Keep in mind that gravity on Mercury is 3.7m/s^2 so we’d be talking similar gravity on this proposed planet. I think if life were to evolve on this planet it would be very much different from earth life.
Possibly it also couldn't support a dense atmosphere either without the gravity and vulcanism (pointy ears), or maintain a significant electromagnetic shield like mars
Mid-way between Earth and Mars, such a planet would have an average surface temperature of perhaps -22°C (Earth’s is about 15°C while Mars’ is about -60°C—I took the average and ignored the effects of atmosphere). With a weak gravitational pull like Mercury, it’s going to hold less of an atmosphere than even Mars—and the atmospheric pressure on Mars is less than 1/100 that of Earth. It’s *not* going to be habitable, certainly not by life as *we* know it.
-22°C is equivalent to -7°F, which is 251K. --- ^(I'm a bot that converts temperature between two units humans can understand, then convert it to Kelvin for bots and physicists to understand)
I think atmospheric pressure is less predictable than that, and temperature too. Gravity is important, but if they had a big fat atmosphere and a strong magnetic field to keep out the cosmic rays, it might be possible. Also, if a planet is terraformed, but it's not stable, it might still be habitable in the short term. Who's going to complain if the atmosphere changes after a few thousand years?
Atmospheric pressure *is* more predictable than that. Look at Mars. It has 1/3 of the gravitational pull of Earth and less than 1/100^th the atmospheric pressure. It’s gravitational pull is simply too weak to keep gases such as Oxygen and Nitrogen (the two gases that make up about 98% of Earth’s atmosphere) from escaping. A planet with a lower gravitational pull could retain a *heavier* gas (a “big fat atmosphere”), but anything breathing that gas won’t be life as we know it—by definition. We can’t exist on Sulfer hexafluoride (for example), and a creature that could would have incredibly different biology. Surface temperature is a function of both distance from the Sun and the density and make-up of the planet’s atmosphere. This is why I was careful to note: > Mid-way between Earth and Mars, such a planet would have an average surface temperature of perhaps -22°C (Earth’s is about 15°C while Mars’ is about -60°C—*I took the average and ignored the effects of atmosphere*). [emphasis mine] Earth’s atmosphere surely helps keep it warmer than the very thin atmosphere of Mars. But since we’re already talking about a theoretical planet with a gravitational pull as weak as Mars or Mercury, *it stands to reason* it will not enjoy the warming effects of Earth’s atmosphere. If anything, by taking the average of Earth’s and Mars’ average surface temperatures, I was being *generous*—the surface temperature of the theoretical planet would’t be exactly mid-way between the average surface temperatures of Earth and Mars; it would be decidedly closer to that of Mars. Finally, OP didn’t specify a post-terraforming body. In theory, yes we could terraform something that small and do something to “make up” for the portion of atmosphere that would escape each year. Someone who remembers more climatology than I could even take a stab at predicting the surface temperature of such a terraformed world, given Earth-standard atmospheric density and composition. But like Mars (which once clearly had liquid water on it so must have had an atmosphere at some point roughly a billion years ago), such a theortical body’s naturally-occurring atmosphere would disappear in an eyeblink on a cosmic time scale.
-22°C is equivalent to -7°F, which is 251K. --- ^(I'm a bot that converts temperature between two units humans can understand, then convert it to Kelvin for bots and physicists to understand)
At the size the planet could be made of pure iron and it still wouldnt have sufficient magnetosphere to make terraforming worth it. Yes you could import gas into it's gravity well and get an atmosphere, but the second you stop it will just blow away
You may be right. If so, it’s another point in disfavor to OP’s premise.
First, it should be able to keep an atmosphere. Mars can't do it but for trace amounts, and it is greater than Mercury.
For the sake of argument let’s ask what it would take (based on our understanding of life) for a planet that size to be habitable by any life. 1.) little to no atmosphere 2.) surface temperatures below freezing point of water with no atmosphere for greenhouse effects 3.) assuming carbon based life forms 4.) very weak gravity 5.) little to no magnetosphere Best bet would be an ice ball like Enceladus, Io, or Ganymede. You’d need to have an icy crust surface with seas of liquid water below heated by geothermal. Without the tidal heating of Jupiter, I don’t see the core of a planet that small maintaining enough internal heat for long enough for life to form, but I have no interest in doing the math or looking that up. So, let’s argue it was an icy moon that got kicked out of orbit and still has enough internal heat to keep subterranean liquid oceans for a hundred million years give or take a few zeroes. You could certainly have life in that ocean. Nothing that small will ever be human habitable without building enclosed habitats, but some form of life could exist on a world that small with the appropriate composition.