Main article: Rocky Planets (Theoretical Models)
A Hermian type planet is a theoretical model of a planet orbiting close to its parent star. The name is not official and comes from the adjective hermian meaning related to Mercury. A hotter planet will be a Vulcanoid Type Planet.
The Solar System has one Hermian planet, helping us to understand how they will behave around other stars. Also, the Kepler Space Telescope has detected many similar planets.
Relation with parent star Edit
A Hermian planet will be exposed to higher amounts of heat then Earth. Its Solar Constant (the amount of heat radiated from the parent star) is somewhere between 5 and 50, while the surface Temperature is estimated (without atmospheric interference) to be at 180 to 500 degrees C.
Around M - type stars (modeled for Barnard's Star):
- Distance: 1.75 5.4 million km
- Visual constant: 0.25 to 2 (for yellow wavelength)
- Revolution period: 9 to 30 days
- Stellar gravity: 150 to 1500
- Hill sphere (assumed Earth's mass): 30 to 90 thousand km
The planet has a stable orbit, but is exposed to strong tidal forces, so it must be tidal locked.
Around K - type stars (modeled after Epsilon Eridani):
- Distance: 15.5 5.7 million km
- Visual constant: 2.8 to 37 (for yellow wavelength)
- Revolution period: 42 to 155 days
- Stellar gravity: 5.6 to 76
- Hill sphere (assumed Earth's mass): 0.16 to 0.6 million km
The planet will be similar to Solar System's Mercury, but exposed to stronger tidal forces.
Around G - type stars (modeled after Sol):
- Distance: 30 to 94 million km
- Visual constant: 5 to 50 (equal with solar constant)
- Revolution period: 70 to 230 days
- Stellar gravity: 2.5 to 25
- Hill sphere (assumed Earth's mass): 0.3 to 0.9 million km
Mercury and Venus are inside these boundaries.
Around F - type stars (modeled after Procyon):
- Distance: 90 to 280 million km
- Visual constant: 2.7 to 25 (for yellow wavelength)
- Revolution period: 180 to 560 days
- Stellar gravity: 0.43 to 4.1
- Hill sphere (assumed Earth's mass): 0.8 to 2.4 million km
These planets are heated like Mercury and Venus, but experience lower tidal forces, similar to Earth. So, they would rotate probably as fast as Earth and would have a higher chance to host moons.
Around A - type stars (modeled after Sirius):
- Distance: 150 to 470 million km
- Visual constant: 0.61 to 6.3 (for yellow wavelength)
- Revolution period: 0.7 to 2.2 years
- Stellar gravity: 0.2 to 2
- Hill sphere (assumed Earth's mass): 1.2 to 3.7 million km
The visual constant is low enough, matching the values needed for Earth plants (1.98). Even if the average temperature is high, the luminosity is perfect. Also, given the low tidal stress, there is a high chance to find moons.
Around B - type stars (modeled after Rigel):
- Distance: 1200 to 3700 million km
- Visual constant: 0.4 to 3.9 (for yellow wavelength)
- Revolution period: 1.7 to 5.2 years
- Stellar gravity: 0.04 to 0.36
- Hill sphere (assumed Earth's mass): 4 to 13 million km
Around these hot stars, there is less visual light and more ultraviolet. Red light is especially found in low amounts, while blue light is more then enough. If somehow we manage to reflect the ultraviolet, these planets would become the best places for human life.
Around O - type stars (modeled after R136a1):
- Distance: 70000 and 230000 million km
- Visual constant: 0.04 to 0.4 (for yellow wavelength)
- Revolution period: 28 to 87 years
- Stellar gravity: 0.0001 to 0.01
Hill sphere (assuming Earth's mass): 110 to 340 million km
The planet does not feel the tidal force from the star and can host a wide system of moons. The major problem is that the amount of visible light is small, especially red light. The amount of light needed for plants is similar to what we find at the orbit of Uranus. As a direct result, a Hermian planet located here will not be able to sustain food production for a large number of settlers.
Around L - class brown dwarfs:
- Distance: 75000 and 250000 km
- Visual constant: 0.000003 to 0.0000003 (for yellow wavelength)
- Revolution period: 0.8 to 2.7 days
- Stellar gravity: 18000 to 200000
Hill sphere (assuming Earth's mass): 2000 to 7000 km
A planet will be below the Roche limit and will break into a ring. Anyway, the amount of visible light received is too small to sustain any sort of life. It will be like staying at one meter to a campfire. You can heat yourself, but the light is hardly enough to read.
Physical and chemical composition Edit
Hermian planets are heated, but not up to extreme limits. If the planet has enough mass, it can sustain a large enough atmosphere (as does Venus). However, atmospheres will have a great risk of runaway greenhouse effects. If there is water, it would be as a gas inside the atmosphere, together with carbon dioxide, sulfuric acid and many other toxic gasses.
If the planet is not large enough to support an atmosphere, it will be something similar to Mercury. They will have a tenuous atmosphere, created by solids that sublimate from surface and by particles brought by the solar wind. Water and other volatiles (like carbon dioxide, ammonia, methane, nitrogen) will be located inside the craters close to poles (if the planet does not have a tilted axis). If the planet is tidal locked, there is a high chance we will find ice and volatiles on the dark side.
The planet is expected to contain a similar or slightly higher amounts of heavy elements then Earth.
Because the surface is heated, the core will not cool fast enough. If the planet has a runaway greenhouse effect, then the cooling is slowed down further. In response, internal dynamos might be stronger. This also means that the planets can be geologically active.
The first atmosphere could be tenuous or very large. However, terraformers will transform these planets into something more similar to what we have on Earth.
A planet with a diameter of 3000 km and an internal density of 5.0 would be risky for creating an artificial atmosphere if placed at the orbit of Mercury, but would be acceptable for a few generations, at the orbit of Venus. In this case, the atmosphere would reach around 80 km height.
If the planet has a diameter above 8000 km and a density above 5, then terraforming can occur without risks, the planet will not lose its gas for millennia.
There are two different ways to terraform a Hermian planet, depending on atmosphere.
If the planet lacks atmosphere, then we have to bring one. The first step is to divert comets and smash them to the planet. This will create oceans and the first atmosphere. While doing this, we also have to create something to reflect solar radiation. Space mirrors can be used (with the risk of colliding with spaceships), as well as atmospheric mirrors (floating balloons) and anti-greenhouse gasses (not much studied today). Unlike a Vulcanoid planet, Hermian planets don't need too much shielding, so it is possible with current technology to cool them.
But if the planet has an atmosphere (and most probably it has undergone a runaway greenhouse effect), then the first step is cooling the planet. Everything must be done to decrease temperature. During day, the light must be reflected, but during night, the planet must be allowed to radiate heat. One way to do so is by creating a huge fleet of tiny balloon robots, that will open mirrors during day and close them during night. As temperature falls, first will the sulfuric acid condensate on the ground, together with water. Then, we will have to use genetically modified bacteria and algae, to transform all carbon dioxide into oxygen and carbon. Also, water and oxygen can be created from sulfuric acid, leaving sulfur behind. Slowly, the oxygen will react with other toxic substances. Giant terraforming plants will conduct chemical reactions, working together with bacteria and algae. Then, the excess carbon and sulfur needs to be stored underground or under the sea surface, preventing ignition.
The terraforming of a planet similar to Venus is probably the most ambitious of all terraforming processes.
Climate simulation Edit
We don't know exactly how a terraformed planet would look like.
Latitude T(C) 90 -255 75 -47 60 -10 45 15 30 34 15 50 0 63
This temperature model for a planet between Mercury and Venus, assuming the atmosphere will be able to cool fast and the planet will only use small anti-greenhouse gasses.
Latitude T(C) 90 -73 75 4 60 17 45 27 30 34 15 40 0 45
This model is for a planet with some anti-greenhouse gasses, but with a lower ability to cool down.
Assuming a day length of 24 hours, day-night temperature changes should be around 14 degrees.
The shield will not protect perfectly the planet. Gaps in the shield will appear. Light beams will reach the ground, but they will not kill anyone. They will increase local temperature. The shield must reflect excess light during day and must allow heat to escape during night. There are two scenarios: one in which the atmosphere is able to radiate the heat fast during night (and temperatures will drop dramatically, mainly if the planet is tidal locked) and one in which the shield will reflect much heat (and the planet will not heat too much during day). If the planet is tidal locked, in the first model, temperatures on the dark side might get below -150 C, so that the air will freeze. A small planet will be more safe.
The shield will interact with seasons. So, summers will not be too hot, because the shield will reflect more light, but the winters will be cold, since the shield's role is to cool the planet.