# How small of a planet can sustain life as a desert planet?

I'm writing about a small desert planet (about the size of pluto) that humans can live on. Is this possible?

• "planet (about the size of pluto) that humans can live on." That's your first -- and biggest, and immediately fatal -- problem: too small to retain anywhere near a viable magnetosphere or atmosphere. – RonJohn May 16 '17 at 0:11
• What do you mean by planet size? Do you mean mass or diameter? What about the composition of said world? And welcome to world building! – Joe Kissling May 16 '17 at 0:17
• @RonJohn a planetary body with the same diameter as pluto could easily have the gravity required to sustain a thick atmosphere. I'm waiting on the author to give some feedback before I submit my answer. – Joe Kissling May 16 '17 at 1:15
• other than the size, what about its mass? – L.Dutch - Reinstate Monica May 16 '17 at 4:26
• There are existing Questions here about adding an atmosphere to the Moon etc. It will not last for geologic timescales, but will be just fine for people to move in and live there. – JDługosz May 16 '17 at 13:59

So as a simple answer no.

The diameter of Pluto is only 2,390 km across. Just for comparison, that's about 70% the diameter of the Moon. And it's a fraction of the size of the Earth; about 18% of the Earth's diameter. In terms of volume, Pluto only has 6.39 × 109 km3. As far as planetary objects go, this is tiny. Pluto is considered a dwarf planet.

In order to sustain life, you need an Atmosphere. In order to have one of those, you need gravity. For us humans we need something along the lines of earth or mars size. We can go smaller, but there are issues with retaining an atmosphere and that is the main issue here.

to keep this shortish. You'll probably wanna make it roughly Earth sized, maybe a bit smaller lets say Mars sized. Now your next question should be how to sustain human life on a desert planet. Presumably if there are no questions already asking that exact thing.

Hope this helps!

• You need to proofread especially when pasting stuff «6.39 x 109 km3» is not the same meaning as you intended! (I didn’t fix anything else — you still need to.) – JDługosz May 16 '17 at 1:45
• Thank you! This helps a lot, I will probably make the planet Mars-sized. – pin-up enthusiast May 16 '17 at 3:47
• Couldn't you increase the planet's density, to increase its gravitational pull? – Masked Man May 16 '17 at 8:17
• @MaskedMan Yes technically. – Warm Shadow May 16 '17 at 8:28
• "In order to sustain life, you need an [a]tmosphere" - counterexample: Europa. Carbon-based life can do just fine without being enveloped by gas. Though living in vacuum would be a challenge. – John Dvorak May 16 '17 at 10:43

## Sure is

But this world will have to have some very specific properties. Also, I shall assume that when you say the size of Pluto, you mean its radius. Pluto has a radius of 1190 km, so that will be the figure I use for this world.

Firstly, this world is going to have to be rather dense, denser than any other planet in the solar system. It needs to be to have reasonable surface gravity to hold an atmosphere. Let's say it's almost entirely iron giving it an average density of 7.0 g/cm³. For comparison, Pluto has an average density of 1.8 g/cm³, Earth's is about 5.5 g/cm³. It has a giant iron core with a thin crust of silicates.

Plugging in those values to this calculator give a planet with a surface gravity of 2.33 m/s² which is about a third of Earth's. For some perspective, Titan has less surface gravity than the moon (1.3 m/s²) and yet has an atmosphere thicker than Earth (1.45 atm).

This planet would be very dry, because most of the water, if any was ever there to begin with, would be consumed in rust. A desert may be hot or cold, so place it wherever you like around its star.

This world may, however, have difficulty maintaining its atmosphere. Low gravity does mean it's easy for the host star to strip atmosphere. Small planets' interiors cool quickly, so the Geodynamno responsible for making a protective magnetic field dies quickly too. However, a planet rich in iron may have a residual field that provides protection. This stripping is also dependent on the distance from the star and how thick the atmosphere is in the beginning.

Summary: you could have a small desert world that humans could live on.

• Is Titan's atmosphere so dense because it's 98.4% nitrogen. – RonJohn May 16 '17 at 1:55
• @RonJohn In the sense that the atmosphere is almost all nitrogen yes. But specific to it being nitrogen, no. Oxygen is actually denser. – Joe Kissling May 16 '17 at 1:59
• OK. Changing subject: "a planet rich in iron may have a residual field that provides protection." Mars' core is iron/nickel, but lost it's magnetosphere long ago, and there's no residue. arstechnica.com/science/2015/11/… – RonJohn May 16 '17 at 2:07
• @RonJohn Admittedly, I used the term residual loosely. There is more than one way to get a magnetic field. Mars may have had a residual field, but it has faded. Planets can also gain a field by interacting with a larger one. In any case, it's really a non-issue. Orbiting around a less than active star, or far from it, a very thick atmosphere, being a moon of a gas giant. There are plenty of options. In the end, if the star is stripping the atmosphere it only needs to be breathable for the duration of the story. Bonus pretty auroras. – Joe Kissling May 16 '17 at 2:17
• @JoeKissling - my point was that rust doesn't consume water. It consumes oxygen. If you don't have water because there's not enough hydrogen, human life can still persist as long as you're careful about water - like a normal desert. If you don't have water because there's no oxygen, then you have bigger problems. – Ben Barden May 16 '17 at 19:48

No is the right answer, but one of the great authors of the past got around this simple impossibility by feeding an oxygen rich gas giant to a small star...

In Larry Niven's The Integral Trees, the atmosphere which the humans breath takes the form of a torus in tight orbit around the star, and the ground which the characters stand on is a tree growing along the center of that gas cloud. Like your miniature planet, the tree does not have enough mass to hold an atmosphere, but some of its surface has breathable air simply because it is located at the right place and time.

I seriously doubt that Niven's creation could stand up to astronomical time, but as long as your characters didn't have to evolve on that little world, you should be okay for the length of most stories. Just give them a wrecked spaceship and a castaway origin story, and move on with writing your story.

Downsizing planet to size of Pluto would not work.

Nevertheless, if you want a small desert planet that still is habitable:

• Make it quite dense, with big iron core (like Mercury has)

• It's hot - actually it faced serious atmospheric escape, thus it avoided fate of Venus

• only polar regions are habitable, the rest is hell, and during day the temperature can go on equator to some insane temperatures (like over 100C)

• better low axial tilt

For inspiration, a slimer version of: https://en.wikipedia.org/wiki/Future_of_Earth#Loss_of_oceans

Possible? Yes, but with conditions: - Decide what combination of planetary diameter and gravity you want, and then calculate the density of the planet - upper limit is the density of osmium, but you might be able to get away with a combination of lead and iron. Note that surface gravity on a small planet will be higher than a larger planet with the same mass (because the surface is closer to the center of mass). - Decide what kind of atmosphere your aliens require for life. Easiest is to assume they require 21% oxygen like we do, but they might be able to breathe sulfur dioxide or something else. - Decide what atmospheric pressure you need. - Decide how much water you want the planet to have. If the core is lead, or the crust is mostly similar to Earth's, then you won't have the water all bound up in rust, as JDługosz suggested. - How far from the parent star? The hotter and bluer the star, the farther it can be.

• Note that what matters isn't the percentage amount of oxygen, but rather the oxygen partial pressure. On Earth the oxygen partial pressure is about 200 millibar, so if your chosen atmospheric composition maintains that, then you should be able to get away with doing so in a reasonable range of pressures. – user May 16 '17 at 14:37
• Michael Kjörling - good point. Also, if humans are going to breathe that air, the partial pressure of nitrogen needs to be low enough not to cause nitrogen narcosis. – Charles B Dorsett Jr Jun 13 '17 at 7:59
• The planet needs to have some life form that makes oxygen at a rate that the other life forms consume it. – KalleMP Jun 15 '18 at 17:17