Pluto before terraforming
Main article: Rocky Planets (Theoretical Models)
A Kuiper (or Tritonian) type planet is a theoretical model of an Outer Planet orbiting far from its parent star. The name is not official and comes from the adjective tritonian meaning related to Triton, the largest moon of Neptune and from the Kuiper Belt.
For the Solar System, Neptune fits around the center of the region. A few centaurs and Kuiper Belt objects also can be found where a Tritonian planet would orbit.
Relation with parent star[]
Around majority of stars, the planet will receive small amounts of heat and light, with aSolar Constant of 0.004 to 0.0004. Temperatures on the surface, without an atmosphere, should be at best -200 C.
Around M - type stars (modeled for Barnard's Star):
- Distance: 190 to 600 million km
- Visual constant: 0.00002 to 0.0002 (for yellow wavelength)
- Revolution period: 3.2 to 18 years
- Stellar gravity: 0.012 to 0.12
- Hill sphere (assumed Earth's mass): 3.2 to 10.3 million km
The planet will not have enough light for plant life. Compared to the Solar System, it will be located in the Asteroid Belt.
Around K - type stars (modeled after Epsilon Eridani):
- Distance: 2040 to 6400 million km
- Visual constant: 0.0002 to 0.002 (for yellow wavelength)
- Revolution period: 56 to 309 years
- Stellar gravity: 0.0004 to 0.004
- Hill sphere (assumed Earth's mass): 22 to 68 million km
Only at the inner edge of the region, the limited amount of light can hardly support plant life.
Around G - type stars (modeled after Sol):
- Distance: 3300 to 10500 million km
- Visual constant: 0.004 to 0.04 (equal with solar constant)
- Revolution period: 104 to 589 years
- Stellar gravity: 0.0002 to 0.002
- Hill sphere (assumed Earth's mass): 33 to 105 million km
This distance includes Neptune and the Kuiper Belt. Plant life can survive around Neptune. Some unicellular algae might survive further away, but it is not known how far.
Around F - type stars (modeled after Procyon):
- Distance: 9900 to 31500 million km
- Visual constant: 0.0002 to 0.002 (for yellow wavelength)
- Revolution period: 440 to 2500 years
- Stellar gravity: 0.00003 to 0.0003
- Hill sphere (assumed Earth's mass): 86 to 275 million km
Only close to the inner limit, some plants could hardly survive the low luminosity conditions.
Around A - type stars (modeled after Sirius):
- Distance: 16 700 to 53 000 million km
- Visual constant: 0.00005 to 0.0005 (for yellow wavelength)
- Revolution period: 830 to 4700 years
- Stellar gravity: 0.000016 to 0.00016
- Hill sphere (assumed Earth's mass): 132 to 419 million km
It is too dim for plants to survive.
Around B - type stars (modeled after Rigel):
- Distance: 132 000 to 415 000 million km
- Visual constant: 0.00003 to 0.0003 (for yellow wavelength)
- Revolution period: 5500 to 30 100 years
- Stellar gravity: below 0.00003
- Hill sphere (assumed Earth's mass): 460 to 1400 million km
It is too dim for plants to survive.
Around O - type stars (modeled after R136a1):
- Distance: 8 000 000 and 25 500 000 million km
- Visual constant: 0.0000036 to 0.000036 (for yellow wavelength)
- Revolution period: 700 000 to 3 000 000 years
- Stellar gravity: below 0.00001
Hill sphere (assuming Earth's mass): 12 000 to 37 000 million km
It is too dim for plants to survive.
Around L - class brown dwarfs:
- Distance: 7.9 and 25 million km
- Visual constant: 2.9E-10 to 2.9E-11 (for yellow wavelength)
- Revolution period: 20 to 111 days
- Stellar gravity: 1.7 to 18
Hill sphere (assuming Earth's mass): 0.21 to 0.67 million km
The planet will have a stable orbit and will not be tidal locked.
Around T - class brown dwarfs:
- Distance: 4.45 and 14 million km
- Visual constant: 8E-15 to 7E-16 (for yellow wavelength)
- Revolution period: 8.4 to 47 days
- Stellar gravity: 5.7 to 56
Hill sphere (assuming Earth's mass): 0.12 to 0.38 million km
The planet would be tidal locked or will be spinning slowly.
Around Y - class brown dwarfs:
- Distance: 0.7 and 2.23 million km
- Visual constant: below 1E-30 (for yellow wavelength)
- Revolution period: 0.5 to 3 days
- Stellar gravity: 2300 to 22 000
Hill sphere (assuming Earth's mass): 19 000 to 60 000 km
The planet will be close enough to experience strong tidal stress.
Physical and chemical composition[]
These planets experience lower temperatures, so, in theory, they should contain higher amounts of volatiles, including methane and other gasses. From a certain mass, they can keep helium in their atmospheres. Many of them are exposed to cosmic rays, since the magnetic field of their parent star is weaker at that distance. This contributes to the formation of tholins.
Internal composition of outer planets might not differ too much. The vast majority of moons orbiting the gas giants of our solar system consist of a central rocky core surrounded by a mantle of ice, sometimes with a subsurface ocean. The only difference between Galilean and Tritonian planets should be a higher concentration of volatiles on their surface and the fact that many of them should be frozen (including methane and nitrogen).
Atmosphere[]
Given the average low temperature, in many cases most of the atmosphere should be frozen on the surface. Gasses sublimate from the surface to form a tenuous atmosphere. The fact that majority of these planets have long seasons means that gasses travel from one hemisphere to another, according to seasons. Planets closer to the inner edge of the region should have larger atmospheres, while planets close to the outer limit, should have most of the gasses frozen.
Terraforming[]
Terraforming these planets is a very hard task. Given the fact that majority of plants will not survive in the low luminosity conditions, terraforming is questionable.
The tiny amount of energy these planets receive from their stars is too little. A strong greenhouse effect can be created with the use of greenhouse gasses, but the energy needed to melt the ices is huge, meaning that it would take millennia for this to happen. So, settlers will have to heat-up the planets with another technology (nuclear fusion, maybe). Once temperatures are suitable for life and the ice has melted, settlers will have another task: to transform the atmosphere. Again, since the amount of light is too small, plants will barely survive and will take a very long time to transform carbon dioxide into oxygen. So, the only feasible solution is to use huge amounts of energy, to artificially produce oxygen and organic compounds.
Once terraforming is complete, still there will not be enough light to support agriculture. Settlers will have to adapt to this and use artificial light.
Climate simulation[]
The type of climate one would get on such a planet is name monoclime. All over the planet, there will be almost the same temperatures. Here is a model for a planet similar to Earth, located at the orbit of Neptune:
Latitude T(C) 90 13 75 16 60 17 45 17 30 18 15 18 0 18
The following is a temperature model for a terraformed planet, the size of Earth, orbiting at the outer edge of the region:
Latitude T(C) 90 15 75 17 60 18 45 18 30 18 15 18 0 18
The values indicated for poles are in fact the temperatures expected during a polar night. One can see that differences are very low, about only 5 degrees for the orbit of Neptune.
Such small temperature variations also means that winds will be very slow. Without major air currents, gasses in the atmosphere will separate. Greenhouse gasses, which are heavier, will move closer to the ground. The temperature difference between different area swill be huge. It might be requested for small artificial air currents to be made. As gasses move to the ground, higher layers of the atmosphere will freeze.
Another problem is that stratification will create huge temperature variations between gas layers. The high mass of greenhouse gasses contribute to this separation. It is possible that at a certain point very cold gas from a higher layer to go down. This will automatically cool nearby gas, reducing its volume and pressure and creating winds. There will be both horizontal and vertical winds. The process will result in a fast decrease of temperature, raining (and probably also snowing) and mixing of gas layers. Then, the planet will slowly heat again, for a few years, until this process starts again.
This kind of planets is the outermost that can support life without the use of artificial light, even if that light is not enough to sustain agriculture.