Dry cold planets are rocky planets that orbit away from their parent star or are free-floating in interstellar space.

Formation Edit

A planet can be formed closer to its star and can move forward under certain conditions:

  • Gravity perturbations from a nearby planet or from a passing star can make a planet change orbit from a nearly circular one close to the star to an elliptical one reaching far away. Then, other forces, like gravity forces from passing stars, can re-shift the orbit so that at perihelion the planet will never come close to its parent star.
  • The parent star can become a red giant. In this phase, the star increases its luminosity and can melt water on the surface. If the planet has a low mass, it can lose its water and atmosphere. Also, solar winds become stronger and can erode the atmosphere. Later, as the red giant becomes a white dwarf, its mass is smaller and the planet will orbit at a higher distance.
  • The parent star can undergo a nova explosion. If this is the case, the dramatic increase in brightness will also increase temperature on the planet, making it lose water and gasses.
  • The parent star can explode as a supernova. A planet orbiting far enough might survive the explosion, but it will lose all or almost all volatiles. Also, planets that might form from the debris of a supernova, will have sonly small amounts of water on their surface.
  • Around O - type stars and Neutron stars, the strong ionizing radiation can erode the atmosphere of an outer planet and can split water molecules.
  • There are also other ways Outer Planet can lose its water and gasses. This can include repeated cataclysmic collisions.

Based on these theories, dry cold planets should be frequent around stellar remnants, like White dwarfs, Neutron stars and Black holes, but might also exist in other places.

The planet Edit

The main feature of a dry cold planet is the absence of water and gasses. If the planet is close enough to the star, it will have a dim atmosphere and a dry surface. However, if the planet is far away, it will have only trace amounts of ices on the surface. Most of the planet will be covered with dry rocks.

Kicked from its orbit Edit

Gravity perturbations from nearby planets can remove a planet from its previous orbit. First, the planet will enter a long elliptical orbit. Then, other interactions will reshape the trajectory to a more circular orbit. Also, the planet might be ejected into interstellar space.

The planet will look almost as it did before. The interaction might cause some tectonic activity, but it will not last long. However, it will not look like the majority of outer planets. Imagine how would look Mercury kicked into the Oort Cloud. It will look just as it looks where it is now, only that it will be a dried world.

Survivals of red giants Edit

Let's think about the future of Solar System. When Sun will become a red giant, it will swallow Mercury and Venus and as many scientists argue, also Earth. Mars will survive, but still will be very close. What will happen to Mars? Scourged by extreme heat, it will become a ball of molten lava. It will lose all its water and volatiles. Given the extreme temperatures, also ground rocks will sublimate and as a gas or vapors, they will be sent to outer space. The strong solar wind will impact Martian surface and will be strong enough to move the dust, if there still is some solid ground. However, the red giant phase will not last forever. As the Sun will shrink and become a white dwarf, Mars will be left alone, to cool down. Rocks will solidify and the planet will take a new shape. There will be nothing left from the former Mars, nothing that would tell anyone how that planet looked before. As the rocks solidify, tectonic activity will occur and Mars will have canyons, rifts and faults. As the Sun cools down, surface temperature drops below -200 C. Volcanism might bring some gasses to the surface, forming an atmosphere that will almost completely freeze. Since all bodies in the Solar System have moved away from the Sun, the risk of meteoric impact is far smaller then before. Mars will need much more time to get filled with craters.

Now, let's think about Jupiter and Saturn when the Sun will undergo a red giant phase. What will happen to them and their moons? Jupiter has a strong gravity and will keep its atmosphere tight. Also, its moons will remain in orbit. However, ice will melt on the Jovian moons. Given their small mass, they will not be able to keep their atmosphere and oceans when the solar wind will reach them. Depending on how long this situation will last, they might be left with some ice or only with their rocky cores. In any case, they will no longer be recognizable for us. It is possible that even a gas giant will not survive the extreme solar wind and temperatures around a red giant and will lose much, if not all of its atmosphere. What is common for dry cold planets created further away from the parent star is that the solid surface does not reach melting temperatures. However, since outer planets are usually covered with crusts of ice and some of them have a subsurface ocean, what a red giant phase will do to them is that it will make their rocky cores become exposed. The planets will lack of violent volcanic activity, but will also be free of craters.

Survivals of supernovae Edit

What will happen to a planet that is unlucky enough to witness a supernova[1]? Planets closer then 10 AU from the star will not stand a chance. Planets located at hundreds AU from the star will survive. However, the vast majority will become rough planets. A small number of planets will remain in orbit around the neutron star or black hole that will be created, orbiting at hundreds of AU away. During the explosion, all water and gasses will be removed from the planet. The blast will erode the surface or at least will melt it. The surviving planet will be a dead world that will never look as it did. As the rocks will solidify, they will create completely new Geographic features. A supernova lasts less then a red giant phase, but is far more powerful. Even if it will dramatically alter the surface and will remove much of what it has melted, it will not alter the crust too deep, below what it has melted. So, when the rocks will solidify, since the remaining crust is solid, there will not be massive geological activity. The planet might look like an endless plain. And since the vast majority of survivals will be ejected into interstellar space, there will be a far lower risk of meteoric impacts. In any case, the planet will be left as a dead world.

For planets located at 10 to 100 AU, it is questionable if they will survive. Probably a Super-Jupiter will have a chance. The explosion will blast away its atmosphere and maybe a little part of the core will survive. Just imagine how would look Jupiter (assuming it were further away), after a supernova. All its atmosphere, including the layer of metal helium, will be blasted away. The rocky core will also be partially blasted away. The remaining body, with a diameter of only 5000 km (similar to Mercury), will be exposed. Made only of iron and heavy metals, the core will be very hot, at 50 000 degrees. It will be in a state of plasma. But given the fact that its heat can radiate into space fast, it will cool before completely evaporating into space. The remaining structure will be a small rocky planet, very rich in metals and with very strong volcanic activity. Settlers will find such an object as a very good destination for mining.

There is, however, a faint chance that a planet orbiting very far away, like in an Oort Cloud, can survive. The blast there will be less powerful. Suppose the planet has an ice crust and a subsurface ocean, the explosion might not melt and erode the ice entirely. What will melt, will also be removed by the blast. However, the ocean can survive. Still, the planet will be transformed and the surface will never look like it did before.

After a nova Edit

Now, there is also another possibility. Around a white dwarf, a planet is kicked from its previous orbit and moves much closer. As this happens, it might come close to the habitable zone. However, from time to time, varying from decades to nearly a million years, many white dwarfs undergo nova explosions. When this happens, the planet is exposed to extreme heat. Its atmosphere and oceans are eroded and removed from surface. Temperature does not reach high enough values to melt the rocks and even if they melt, they will soon solidify without altering too much the Geography. In the end, the planet will have the same Geographic features (craters, valleys or whatever it had), but will be dried and almost airless.

Eroded by solar winds and radiation Edit

Planets orbiting O - type stars and neutron stars are exposed to high amounts of ionizing radiations. They will lose their volatiles fast.

Such planets will lack of water and atmosphere, but will still have some features of the world they were before. For example, if they had an atmosphere, they could still have dunes. And if they had water, they could still have valleys.

Terraforming and colonization Edit

Terraforming such worlds is a hard task. First of all, you need to bring water to them, to bring air and to transform them. In many cases, you also need to bring a source of light and heat.

There is also another problem. Many planets that survived catastrophic events had their surface molten and re-crystalized. Most of them will have their surface compact, solid as stone, with no or almost no soft rock. Dust will slowly be formed by meteoric impacts and by rock fragmentation caused by exposure to temperature variations. However, since these planets will be exposed to low temperatures and impacts will be rare, dust will form very slow. Even terraformed, plants will not grow on compact rocks and water will not form aquifers.

However, these planets would not be quite a good target for terraforming. Many of them will be targets for mining and industrial corporations.

The landscapes of these planets will be very interesting. If there is enough light, they might become touristic attractions. But, more then tourists, they will attract scientists, eager to learn how these planets formed, what were they before and what enormous forces modeled them.

Ejected planets will, for a limited time, stay close to their parent stars. They can be used as outposts for interstellar travel, as trade stations serving a solar system or as military forts, protecting nearby solar systems.

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