Main article: Rocky Planets (Theoretical Models)
A Sednoid 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 Sednian meaning related to Sedna, the largest known object that transits the area.
For the Solar System, the area for such a planet is between Kuiper Belt's extension (the scattered disk) and the inner Oort Cloud.
Relation with parent star Edit
For a planet orbiting a star similar to the Sun, the planet will be exposed to extremely low temperatures, but still, with the help of greenhouse gasses, it could, at least in theory, be possible to rise temperatures to a value above water freezing point. The luminosity will be very low, so plant life will not survive without artificial light. The Solar Constant will be very low, usually between 0.0004 and 0.00004. Basically, this is the outer limit where a planet can be warmed enough with greenhouse gasses.
There are stars and brown dwarfs who emit most of their light in infrared. Since infrared is reflected by greenhouse gasses, it cannot be used for heating. So, there will be in certain solar systems Sednoid type planets that cannot be heated and will be considered above the heating limit.
Around M - type stars (modeled for Barnard's Star):
- Distance: 600 to 1900 million km
- Visual constant: 0.000002 to 0.00002 (for yellow wavelength)
- Revolution period: 18 to 103 years
- Stellar gravity: 0.0012 to 0.012
- Hill sphere (assumed Earth's mass): 10 to 33 million km
In practice, these stars emit most of their light in infrared. So, the outer limit should in fact be around 600 million km.
Around K - type stars (modeled after Epsilon Eridani):
- Distance: 6400 to 20 400 million km
- Visual constant: 0.00002 to 0.00022 (for yellow wavelength)
- Revolution period: 300 to 1800 years
- Stellar gravity: 0.00004 to 0.0004
- Hill sphere (assumed Earth's mass): 68 to 218 million km
These stars are somehow similar to our sun.
Around G - type stars (modeled after Sol):
- Distance: 10 500 to 33 000 million km
- Visual constant: 0.0004 to 0.004 (equal with solar constant)
- Revolution period: 580 to 3300 years
- Stellar gravity: 0.00002 to 0.0002
- Hill sphere (assumed Earth's mass): 105 to 330 million km
Since these stars emit most of their light in visible, the outer limit might be further away, maybe up to 45 000 million km.
Around F - type stars (modeled after Procyon):
- Distance: 31 500 to 99 000 million km
- Visual constant: 0.00002 to 0.0002 (for yellow wavelength)
- Revolution period: 2500 to 14 000 years
- Stellar gravity: below 0.00005
- Hill sphere (assumed Earth's mass): 275 to 865 million km
The outer limit is close to the one listed here.
Around A - type stars (modeled after Sirius):
- Distance: 53 000 to 167 000 million km
- Visual constant: 0.000004 to 0.00004 (for yellow wavelength)
- Revolution period: 4 700 to 26 000 years
- Stellar gravity: below 0.00001
- Hill sphere (assumed Earth's mass): 132 to 419 million km
The outer limit should be very close to the one listed here.
Around B - type stars (modeled after Rigel):
- Distance: 415 000 to 1 320 000 million km
- Visual constant: 0.000003 to 0.00003 (for yellow wavelength)
- Revolution period: 30 000 to 173 000 years
- Stellar gravity: below 0.000003
- Hill sphere (assumed Earth's mass): 1500 to 4600 million km
These stars give most of their light in UV. The outer limit for the region is not well known.
Around O - type stars (modeled after R136a1):
- Distance: 25 500 000 and 80 000 000 million km
- Visual constant: 0.00000036 to 0.0000036 (for yellow wavelength)
- Revolution period: 4 000 000 to 22 000 000 years
- Stellar gravity: below 0.00000001
Hill sphere (assuming Earth's mass): 37 000 to 112 000 million km
Since most of the light is in infrared and X, we don't know exactly where the outer limit might be.
Around L - class brown dwarfs:
- Distance: 25 and 79 million km
- Visual constant: 2.9E-11 to 2.9E-12 (for yellow wavelength)
- Revolution period: 0.3 to 1.7 years
- Stellar gravity: 0.18 to 1.8
Hill sphere (assuming Earth's mass): 0.7 to 2.1 million km
Brown dwarfs emit most of their light in infrared and only limited in visible spectra. Therefore, using their radiation for heating in a greenhouse system is useless. Infrared will be reflected by greenhouse gasses into outer space. The outer limit where greenhouse heating is possible up to water melting point, is at a visual constant of 0.00002, corresponding to an average distance below 100 000 km. At that distance, the planet will be ripped apart into a ring.
Around T - class brown dwarfs:
- Distance: 14 and 44.5 million km
- Visual constant: 7E-16 to 7E-17 (for yellow wavelength)
- Revolution period: 47 to 265 days
- Stellar gravity: 0.5 to 5
Hill sphere (assuming Earth's mass): 0.38 to 1.2 million km
The planet will experience tidal forces similar to Venus or Earth. It could host satellites.
Around Y - class brown dwarfs:
- Distance: 2.23 and 7 million km
- Visual constant: below 1E-31 (for yellow wavelength)
- Revolution period: 2.9 to 17 days
- Stellar gravity: 23 to 225
Hill sphere (assuming Earth's mass): 60 000 to 190 000 km
The planet will be tidal locked and will suffer from moderate tidal stress.
Physical and chemical composition Edit
The main difference is that nitrogen and methane will be on the surface and will not sublimate. The only gasses that might form an atmosphere are hydrogen and helium, with much lower boiling point. However, they are both very light, so with a too low gravity, they will escape into cosmos.
If the planet has significant amounts of nitrogen, methane, carbon monoxide and other gasses with low freezing point, they can be deposited in a thick solid layer. Below, we might find a crust of water ice, with a possible subsurface ocean. Deeper, we can expect a solid nucleus made of rock.
If the planet is active, an interesting cycle can start: Volcanoes can heat the subsurface ocean. Then, water can escape through the ice crust, heating the upper layers of frozen gasses. Maybe the water will not reach the surface, but it will heat the gasses. We might see seasonal seas, seasonal atmospheres and methane geysers. Stellar radiation will slowly transform methane in tholins. However, since tholins have a much higher melting point, they will sink into the occasional seas. Given the low temperatures, atmosphere will not survive, it will freeze on the surface and will cover the whole planet with a white layer. Since white reflects light, this will further keep the planet cold.
If the planet is inactive, over time, interstellar radiation will alter the methane and make deposits of tholins. This will slowly change color of the planet from white to dark red, also gently increasing temperature.
The only gasses that might be around a Sednoid are hydrogen and helium. Helium, given the low temperatures, could survive an atmosphere of a planet the size of Ganymede. Hydrogen, on the other hand, is far lighter and will need gravity of a larger planet. However, helium has the lowest greenhouse effect of all gasses.
If the planet is active, cryovolcanism can melt and sublimate some other gasses, like methane, nitrogen or carbon monoxide. However, they will snow back to the surface.
Sednoid type planets can be terraformed (at least in theory), but with huge costs. If we want to form an atmosphere with the same density of Earth's, it will consist at least 25% of sulfur hexafluoride or at least 33% of nitrogen trifluoride.
The energy these planets receive from their sun is little. That energy will slowly be transformed into heat and that heat is supposed to melt all frozen gasses and the ice. This would be possible only in millennia, if not in millions of years. On the other hand, greenhouse gasses will freeze before they can receive enough heat to form an atmosphere. So, before everything, we need to artificially heat the planet (maybe, with nuclear fusion or with a gigantic impact).
Once the planet is heated and we have an extremely strong greenhouse effect, we will need artificial technology to make breathable air. Once the former gasses are melt, we will have huge amounts of nitrogen, methane, carbon monoxide and dioxide, ammonia and water vapor. Luminosity is not enough for plant life, so we will have to do everything with the use of technology. From the organics we can make floating materials for artificial continents. Creating oxygen and nitrogen is also possible, but with a huge energy consumption.
In the end, we will get a breathable atmosphere around a dead planet. We will need to use artificial sources of light to grow crops.
The major problem is that, without an energy income, air currents will be extremely slow, below the speed of 1 km/h. Gasses will separate. Greenhouse gasses, which are heavier, will move to the surface. What remains above (nitrogen and oxygen) will freeze and will fall down as snow or haze. As they move down, they will cool the greenhouse layer. In the end, the entire planet will freeze back. If we use artificial lights (that also create some heat), we can make small temperature differences and we can make some winds to exist.
Since air currents will be extremely slow, it is expected that air ionization will be almost absent. This will be a major thread for life.
Climate simulation Edit
The type of climate one would expect for a terraformed Sednoid is called monoclime. Here is a theoretical model:
Latitude T(C) 90 16 75 16 60 16 45 16 30 16 15 16 0 16
Even if the planet is tilted to 90 degrees and we are at solstice, we will not see temperature differences between the poles above 0.5 degrees Celsius. Day-night fluctuations will be negligible, as will be differences between various latitudes.
Since the amount of light is too little for plant life to survive and the costs will be too high, it is questionable if any civilization will terraform a Sednoid class planet. However, at least in theory, it is possible to increase the temperature and to create a breathable atmosphere. If at some point the artificial lights will be closed, all plant life will die, but until the atmosphere freezes, the air will still be breathable for many years, maybe centuries.