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The term Hyperbolic Planet describes a planet that follows a hyperbolic trajectory (an ellipse with eccentricity of 1). Such a planet will come close to a star, then it will move away, without ever returning.
The planet might have formed in the solar system it is currently escaping, it might have formed in another solar system (and it is just passing by) or it might have formed in the interstellar environment.
Moved away from orbit Edit
There are a few ways a planet can be ejected from its orbit:
A Gravity Assist from another planet (most probably a gas giant) can send a rocky or a dwarf planet away from its solar system. Pluto could be ejected by Neptune (or could become a satellite) at some point, if it were not in a 2:3 resonance. Such events are predictable and since terraforming requires many years and huge investments, probably nobody will try to terraform a planet that has an unstable orbit.
A gravity assist from a giant planet will have significant effects on the ejected planet. For example, it will re-activate volcanism and will create huge tides.
A Heavy Object Flyby is a different scenario. In the documentary Evacuate Earth - Death By A Neutron Star, neutron star is detected and in 70 years it impacts the Earth. Neutron stars and Black holes are hard to detect and might come close enough to a solar system without being noticed. So, they can appear inside a recently colonized solar system without being noticed. The chance for a neutron star to directly strike a planet is very small, but perturbations to the solar system will be huge.
Suppose a neutron star passes through the Solar System. What would it actually do? Well, if it gets as close to the Sun as Earth's orbit, it will kick out all planets. Maybe Mercury will still orbit the Sun, on a highly elliptical orbit. For the gas giants, the scenario is different. If they encounter the neutron star first, their orbits will be changed so that they will soon eject the Solar System. The chance for an object to become a satellite of the neutron star is very small. The Kuiper Belt and the Oort Cloud will be the least affected. However, as the neutron star passes close to the Sun, its gravity disturbs Sun's orbit through the galaxy. So, what actually happens is that the Kuiper Belt remains almost where it were, with majority of dwarf planets moving with 4 km/s, but the Sun changes its trajectory, with roughly 9 km/s, moving away from its system.
If the neutron star crosses the Solar System further away (at the orbit of Saturn), the inner planets will still orbit the Sun (but on more elliptical orbits), however the gas giants will be ejected from the Solar System. In case of the Kuiper Belt, some objects will be removed from orbit by the passing neutron star. The Sun will be disturbed by the gravity pull of the passing neutron star and will move towards one border of the Kuiper Belt, leaving the opposite border behind. As the Sun will move, it will let behind a large part of its Kuiper Belt and Oort Cloud.
Human Interventions are currently not powerful enough to divert a planet from its orbit. In future, we might be able to see this. A highly advanced civilization might be able to change gravity fields and alter the fabric of space, diverting planets as needed. There is also a chance that a future civilization will try to create an Artificial sun from a giant planet. The process might fail and the planet might explode, sending its moons into runaway orbits.
Long Time Effects might refer to evolution of a star. Red giants lose mass, so that their planets will slowly move outwards.
Passing A New Star Edit
A planet might not be formed around a star. It can form on its own, from a small cloud of gas and dust in the interstellar space (see Rough planet) or it might have been ejected from a solar system. In either case, the planet travels for billions of years through the eternal cold and darkness of interstellar environment. If at some point the planet reaches a new solar system, it might start orbiting it or it (usually on an elliptical orbit) or it might just come and go on a hyperbolic trajectory. There is a chance that the planet will become a quasi-satellite.
There are satellites of all known gas giants in the Solar System that have retrograde orbits and are thought to be captured asteroids, but far more asteroids are reported to be kicked away by gravity of Earth and Jupiter. So, we can suppose that most often a runaway planet will be sent on a hyperbolic trajectory then captured into orbit.
Why would someone try to terraform a hyperbolic planet? There are 3 scenarios. Maybe the planet was already terraformed and at some point it lost its orbit. The second scenario is that the planet was colonized and as it moved closer to a star, it became suitable to sustain life for a limited time. In the third scenario, the planet might pass through a solar system long enough so that terraforming is worth the money.
There are many types of stars. The O - type stars, the B - type stars, possibly the A - type stars and also the Red giants have strong energy outputs and support large habitable zones, sometimes extending beyond the orbit of Pluto and even Sedna. With the help of greenhouse gasses, a runaway planet can be made habitable for a long time. A hyperbolic planet passing the solar System can be made habitable for a century, if it is moving slow enough. Around a B - type star, this might mean over 10 000 years. The longest lived human civilization was the ancient Egyptian, which last a few thousand years. If a planet can be made habitable for a millennia, it is worth terraforming.
The planet will require permanent maintenance. as it comes closer to the star, greenhouse gasses must be removed, then, as the planet moves further away, its atmosphere must receive extra amounts of greenhouse gasses.
Setting into orbit Edit
Main article: How to change orbit and rotation.
A highly advanced civilization might be able to change gravity or to modify the fabric of space. However, a least advanced civilization still has a few ways to slowly shift the orbit of a runaway planet. You can put a moon (ore use one if the planet has any) in a stationary position. With the help of a powerful engine, the moon will not actually orbit, but it will stay still. Its gravity will slowly attract the planet and will change its speed and direction. This way, you can, in hundreds of years, make the planet enter an elliptical orbit. most probably, the planet will be in orbit at a high distance from the star. Then, using the same principle, you can further attract the planet and spiral it towards the star.
Runaway Earth Edit
This is the subject of a few sci-fi novels. What would be the fate of a runaway planet, possibly the Earth?
At first, as solar gravity vanishes and solar tide disappears, there will be huge earthquakes and many volcanoes will erupt. Also, Earth's crust will crack in some places. The Moon will remain where it is in the sky as Earth (and probably all planets) will move away. In one year, we will reach the orbit of Jupiter and in 9 years the orbit of Pluto (only that both Jupiter and Pluto will also move away from the Sun).
Day length will slowly increase, since now we will use the sidereal day. Seasons will remain as they were. So, if it were winter in the Northern hemisphere, the Sun will never be seen again at the North pole, because the Sun will remain forever in the same part of the galactic sky.
There are two different scenarios: one with current technology and one requiring a more advanced future.
Current Technology Edit
With our current technology, we will not have much time left. In 6 months, as the Earth moves away and cools down, it will snow at the equator. Temperate regions will have even less time available. Most probably, all governments will allow unlimited use of fossil fuel, to heat houses at least for a while. Construction of large greenhouses will start close to thermal and nuclear plants. Also, production of other greenhouse gasses will start on an industrial scale. All fields will soon be covered with snow. In the arctic circle, people are used to extreme cold, but not to eternal freezing. They will survive for a while with what nature gives them.
Based on current orbital velocity, after an year Earth will be where Jupiter is now. In equatorial and temperate areas, a large percent of the population would have died of extreme cold. Now, since nothing is growing on former fields, the remaining population will starve. Those who have built greenhouses, are now facing another problem. Solar light is not strong enough to support large-scale agriculture. Thermal plants will have to work much harder, to produce also the missing luminosity.
The situation does not last like this for long. In 4 years, the fugitive Earth crosses the orbit of Uranus. That far, the air starts to condense. First it starts raining oxygen and then also nitrogen. Atmospheric pressure decreases fast. All greenhouses are in a risk of explosion. Threatened humans have only two choices: to move underground or to strengthen their buildings. Underground, there is warmer.
After 9 years, as Earth crosses Pluto's orbit, temperature drops below -200 C. At that point, the only survivors are underground. For a while, humans can still use fossil fuel. We can burn coal with oxygen snow. Also, we will always have water ice and solid oxygen on the surface.
Probably the population will shrink from 6 billions to less then a million. There is no way to know. After some time, human civilization will start to grow again and we will see large underground cities.
Future Technology Edit
Let's re-think this scenario, but with a more advanced civilization. Once Earth is ejected from orbit, scientists will deploy into the atmosphere huge amounts of greenhouse gasses. Temperature will rise as much as it can without destroying equatorial regions. Greenhouse gasses are also supplied later. The idea is to trap as much heat as is possible for when the planet will be away in space.
As Earth moves away, climate will change, but not so violent. Greenhouse gasses will bring heat to the poles. All ice caps will melt, but this is a needed sacrifice. At the equator, extreme heat can cause hurricanes and other problems. As Earth will pass the orbit of Mars, global average temperature should be around 30 C.
As the planet moves further away, farmers will have to use artificial light, to illuminate their crops. It might never be possible to produce as much food as before, but still this might save us from starvation. Also, artificial light will bring extra heat.
Greenhouse gasses will trap heat for up to 30 years above freezing, but in the end the planet will freeze. If the planet is moving much slower, plant life cannot survive beyond the orbit of Neptune (that is the point where we will be unable to grow crops without artificial light). Greenhouse gasses can keep our planet heated beyond the orbit of Neptune. Even at 100 AU, average temperature can be kept at 15 degrees C. However, if the Earth is moving with its orbital speed, it will reach orbit of Pluto in 9 years and 100 AU in around 20 years. The Sun will heat Earth for about 20 years, then the trapped heat will give us an extra 30 years, if we do nothing.
At that point, human civilization will have two choices: go underground or build an artificial sun. An advanced civilization will prefer creating an Artificial sun. We can create a huge nuclear power plant on the Moon to send heat and light towards the Earth and create a day-night cycle of roughly 25 hours.