An inner planet is a planet located between its host star and the classical Habitable Zone. Within our Solar System, Mercury and Venus are good examples, though the status of the latter is debatable, since it exists on what is generally considered the inner edge of the Habitable Zone.

Environment Edit

In addition to the two inner planets found within our solar system, there are countless extrasolar worlds that also meet the basic criteria. There is no restriction in terms of planetary classification. Inner planets can be gas giants, super - Earths, or otherwise unremarkable terrestrial planets (see Gas giants (theoretical models) or Super-Earth, Asteroids (theoretical models), Earth - like planet and Average planets and moons (theoretical models). Though the precise composition of extrasolar planets is often difficult to determine, it is believed that in rare cases Oceanic Planets may also fall into this category.

Despite this potential diversity, this article will focus primarily on the average planets and moons that tend to be the best candidates for terraforming. In theory, a few models of planets could be found around the galaxy, each one with its own challenges.

Runaway Greenhouse Planet Edit

Venus is a classic example of this phenomenon. Planets suffering from a runaway greehous effect typically demonstrate immense atmospheric pressures, temperatures soaring to extreme values, and a complete absence of liquid water on their surfaces. If any water is found to exist, it will occur almost exclusively within the atmosphere. As heat helps chemical reactions to occur, highly corrosive mixtures may also be commonplace in the air or on the ground.

In order to trigger a runaway greenhouse effect, a few conditions must be met. First, solar radiation output must be higher than on Earth, but not too high, to avoid blasting the atmosphere apart. Second, the planet must have enough gravity (and therefore mass) to keep the atmosphere from escaping into the vacuum of space. Finally, solar winds must be not too violent (or the planet must have a magnetic field). This threshold is relatively high, however. It would require a roughly tenfold increase in the solar wind to begin stripping the atmosphere of Venus.

as the greenohuse process accelerates, water is expected to vanish slowly. Water has a lower mass than carbon dioxide or molecular nitrogen. Since the high temperature will effectively prevent surface or near-surface condensation of water vapour, it will remain trapped in the atmosphere and eventually cycle through to the stratosphere where it will be exposed to cosmic radiation, breaking it into its molecular components of hydrogen and oxygen. The lighter hydrogen will eventually escape into space, as the gravity of most earth-sized or near earth-sized terrestrial worlds is not capable of holding onto it for a long period of time. Though a magnetic field can slow this process, it cannot be entirely stopped. If the planet retains any water within its atmosphere, this might be found in chemical compounds. Venus, for example, has clouds of sulfuric acid, a highly corrosive compound that nonetheless can be split into sulfur dioxide and water.

Tenuous Atmosphere Desert Edit

The best-known example is Mercury. Due to its high average temperature, molecules in its atmosphere accelerate above the gravity escape limit. There are not many volatiles found on the surface and within the regolith. Extremely small amounts of water can still be found in craters around the poles or locked inside of rocks. Other gasses, like nitrogen, are found in even lower amounts.

Overheated Inner Planet Edit

Many extrasolar planets discovered so far appear to be members of this group. They orbit their host star at too close a distance. Temperatures may rise to 1000 C or more. Moisture and molecular compunds trapped in rocks sublimate and create a tenuous, transient atmosphere. Solar winds are often overwhelming at that distance, as these planets can even pass through the corona. In many cases, they are so close that they may be Tidally Locked Planets or  Low - spinning planets. Mercury falls dangerously close to meeting the latter criteria. The result is that, on one side temperatures often rise to the point where rocks will exist in a semiliquid state, while on the other side, they plummet below -100 C. In these cases, the atmosphere condenses on the dark side or is blown there by solar winds. These worlds are generally believed to be volcanically active, thanks to strong tidal forces. However, there is one notable exception. Planets orbiting B - type stars or O - type stars will face extreme temperatures even if they are at the same distance that Earth is from Sol, because these stars are very bright. So, an overheated inner planet will be further away and will not be exposed to strong tidal stress.

At temperatures above 1000 C, many substances sublimate. Planets that experience temperatures in this range are all-but certain to contain no traces of water. They are also thought to be unable to support metals with a low boiling point, such as lead. However, miners might might find themselves interested in some of the materials found on these inhospitable worlds. Tungsten, titanium and uranium have high melting points and they could be refined there by nature itself.

A special category (at least in theory) is that of a planet orbiting within the corona of its host star, with surface temperatures reaching 3000 C or above. If such a planet were to exist, its rocks would be liquid. Its lifetime would be quite short as solar winds would slowly blow its mass away, assuming that it is not first torn apart by extreme tidal forces.

Terraforming Edit

See also Anti Greenhouse Gasses

The terraforming process of an inner planet is much more complicated than what is required for an Outer Planet.

Runaway Greenhouse Planet Edit

This wiki already contains a plausible and fairly comprehensive model for Venus. However, it conditions will not be consistent from planet to planet. One of the most important factors is the existence of usable amounts of water. If it exists in the atmosphere (alone or as part of other molecules), it can be put to work later on in the terraforming process.

Cooling the planet would be the first and most difficult task. There are a number of (often prohibitively expensive) ways of achieveing this:

  • Strip part of the atmosphere away, to decrease greenhouse effects
  • Lock part of atmospheric gasses into chemical compounds (from the planet or brought from elsewhere, like fixing carbon dioxide with calcium or magnesium)
  • Use space mirrors or a space lens to deflect a percentage of the incoming solar energy
  • Use a special kind of gasses, able to create greenhouse effect at low temperature and be permissive to infrared at high temperature (this has not been studied yet)
  • Create an atmospheric shield, made of robotic balloons
  • Use large ground mirrors or paint huge amounts of rocks in white
  • Create clouds during day and clear sky during night (technology already exists on Earth, but is expensive).

Once the greenhouse effect has lowered enough, pioneers will also need to reduce their struggle to lower temperatures. Then, as they will continue to monitor and transform Water, Minerals and Atmosphere and mainly when they will insert the first living organisms, they will have to control temperature.

In some cases, it is possible to keep some inner planets habitable, without ways to keep temperature down. As shown for the Oceanic Planet, as long as water doesn't boil at the equator, the planet is safe. Settlers will look for polar regions. In case of a Desert planet, without oceans, even a planet at the orbit of Mercury can be inhabited without means to keep temperature down.

Keeping temperature down will be a costly job. Space mirrors or lens are expected to interact one with each other, slowly forming a ring. They will also collide one with each other. Atmospheric balloons will also be affected by weather, radiation and age.

Tenuous atmosphere desert Edit

In the case of Mercury, pioneers will have to create an atmosphere and an ocean, most probably by diverting comets and other Kuiper belt objects. Then, depending on the amount of water, the planet will become a desert or a new Earth. Maintenance will also be needed, in order to keep a planetary shield in position and to replenish air and water token away by solar winds.

To cool down the planet, settlers will need a continuous work, using the methods described above. Unlike an Outer Planet or an Earth - like planet, maintenance costs will be much higher.

Overheated inner planet Edit

Only a very advanced technology will be able to create a planetary shield on a world exposed to temperatures above 1000 C. More likely, these tortured planets are suitable for paraterraforming, mainly if they are tidal locked. A city can grow on the dark side, heated by a solar plant on the illuminated side. Mining and energy producing would be the main activities found there.

Climate after terraforming Edit

After the terraforming process, climate on an inner planet might look similar to what we see on Earth today. Average temperatures vary, depending on many factors: day length, year length, orbit shape, axial tilt, geographic features, amount of water and very important, the ability to cool down the planet. Depending on cooling methods used, we have the following possibilities:

Without any shield, average temperature will be higher. We can imagine a planet where at the equator temperatures rise to near 100 C and where at the poles, temperature oscillates around 0 C. In case of a Desert planet, at the equator temperatures can rise to even 300 C, as long as there is not enough water to generate a massive greenhouse effect and as long as at the poles temperatures don't reach average values above 40 C.

By using space mirrors and lens, temperature will be kept lower during day, but some radiation will come during night. As so, in case of a Low - spinning planet, temperatures can be kept within acceptable boundaries. Some strange light can be seen during night. However, in time, all mirrors and lens will tend to group into an equatorial ring. Poles will be less protected. It is possible to see a model in which the equator is cooler then the temperate areas.

If settlers will depend on an atmospheric shield (like robotic balloons) or some special gasses that change their greenhouse effect depending on average temperature, they will face some major problems. Because of wind currents, density of balloons or gas will not be the same. Accidental heat or freeze might occur. As seen globally, the climate will be somehow similar to Earth, even if nights and winters are expected to be more temperate. Low scale analyses will reveal accidental fluctuations in temperature. Temperature decreases will not be a problem since air is moving away fast enough. A snow in summer is unlikely to be seen. Most often, problems will be caused by gaps in the shield. If the planet is too close to the star, temperatures might rise above 100 C for a limited time frame. Solar radiations can bring death and destruction to the surface, followed by violent storms and hurricanes.

If the settlers will try to reflect some light from the ground, by using mirrors and white rocks, massive cleanup operations will be needed, to take all dust and dirt away. Temperature will be lower near mirrors, but it will be far hotter at distance from them. This will create a regional climate, with wind currents strongly influenced by reflective surfaces. It is somehow similar to what you see in Central Asia, where high mountains, covered with white glaciers, keep cold and trap moisture, feeding rivers that flow to the desert.

Settlers have some tools to create artificial clouds and artificial clean sky. This implies the use of chemicals. It is known that by pulverizing tiny crystals of salt, atmospheric water is drained and clouds can be created. Other more expensive chemicals can do the same. By creating clouds in day and clear sky during night, the planet can be cooled. However, this technology is very expensive and the chemicals can have unknown long-term effects. The main advantage is that climate can be almost entirely controlled.

As seen, inner planets are harder to terraform then outer planets. However, future generations might find the opposite. For now, one thing is key. There are more cold outer then hot inner planets, so we have studied more the outer planets. Maybe future settlers will not have the same lack of knowledge as we have.

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