Elliptical orbits of some outer Trans Neptunian Objects, with the hypothetical planet nine
Many extrasolar planets move on elliptical orbits. If terraformed, their climate will be different from what we see on planets using a circular orbit. If they cross the Habitable Zone, they might become new Earths. Some settlers might try to change their orbits, but this might not be possible in the near future.
Year length[]
Not all planets have the same year length.
Fainter stars[]
Around fainter stars, like K - type stars, the comfort zone is closer to the star then it is to our sun, Sol. As a result, the year for a planet with its semimajor axis at the comfort zone, will be shorter. The planet will experience heat during summer and cold in winter. However, since planet speed increases as it closes the star, summers will be shorter, while winters will be very long.
Let's consider a planet that has an year of 100 Earth days. Summer will be short, probably 10 days. During that time, temperatures will not have enough time to rise to deadly values. Water will evaporate at higher rates. Excess heat will generate seasonal storms. But as the planet moves away, the autumn will be longer, because the moisture in the atmosphere will create a greenhouse effect. As so, temperatures will gradually decrease and it will rain. After a 30 Earth days of autumn, the winter will come. As the planet cools, water from the air continues to condensate as rain and snow. It will start snowing on large surfaces, but not at the equator. Finally, when the planet reaches its apastron, it enters an equilibrium phase. Weather will be more calm, but very cold. As the planet starts approaching, the snow will increase its albedo, so that temperatures will not grow for a while. Spring will come faster to the equator, which is not covered with ice. Spring will come far after the planet crosses semimajor axis, when stellar radiation is enough to melt the ice at closer to the poles. After a 50 days winter, there will come a 10 days spring, with massive floods.
In theory, such a planet will never be completely covered with ice, nor its poles will be free of glaciers.
Settlers will have problems harvesting their crops. At the equator, they will have the longest time for agriculture. They could plant in autumn and harvest in late spring. In temperate regions, there will be too much time with extreme cold. Plants will find hard to survive the harsh summer and to produce food. Still, many plants will survive. On Earth, vegetation from high mountains is adapted to extreme weather changes.
In case of a Low - spinning planet, things are getting very interesting. The day on such a planet is long. In summer, as the planet moves faster, we might see for a limited time, the sun going backwards on the sky. Some places might be safe from extreme summer heat, receiving more heat in winter, while others might be illuminated in summer and during winter will face long periods of darkness. Settlers will find themselves face to face with extreme weather patterns. Temperatures might vary between +120 and -130 C, between summer days and winter nights. At such extreme conditions, only genetically modified plants can survive. It is possible that in some places (protected by natural barriers like mountains and waters) grass and other short lived plants will survive. Settlers will have to be modern nomads. Armed with the best meteorological predictions, they will have to move in time to good settlements.
Very faint stars[]
There are very faint stars like M - type stars or White dwarfs. Around them, planets should be tidal locked. However, this is not possible for a planet on an elliptical orbit. The year will be very short, between 5 and 20 days. The planet will spin a bit faster or slower then the star. This will generate internal heat and will also power planetary dynamo. A hypothetical planet orbiting Barnard's star will have a short year, of 10 days, but will have a rotation period of 7 days. As a direct result, the day will last around 33 Earth days. When planet is at apastron, settlers will see the sun moving in normal direction, from East to West. However, at periastron, the sun will move backwards. This will generate a unique climate. A day cycle will consist of planetary years. There is also the possibility for the planet to rotate a bit faster, resulting in a day of 40 Earth days, so it will consist of 4 planetary years.
If the summer strikes in the middle of a day, temperatures will rise to high values. On the most exposed place, at the equator, water will start boiling. So, settlers will look for temperate regions. Suppose there is a city, facing the extreme heat. There, settlers will face temperatures up to 50 C, followed by violent storms. The summer will last for only two Earth days, because the planet will turn further away from its sun. Autumn will come, also very short. And the winter will start. But, because the short time, heat will still exist. The winter will bring temperatures of around 20 degrees. Then, the planet will turn again towards its sun. During the winter, the rotation will slowly. The city reaches the twilight zone and night comes. Temperature slowly decreases. then, during the short summer, since the planet is moving faster, settlers will see the sun going backwards. The sun will rise again on the sky, bringing just the amount of heat that's needed. It will be a mild summer. But when the planet is mowing away from its sun, night will come. It will be a long cold night, lasting for 10 days, with temperatures falling to -30 C. This time, the planet is completing a full year. Finally, in winter, the sun will shine again from East. This time, its heat is reflected by ice, so that it is not of much use for the town. Temperatures go back to -10 C. As the winter ends and the summer comes, heat is increased and the town can see its snow melting. Then, for two Earth days, in summer, the sun is moving back beyond the horizon, for a small night. It starts shining again in autumn and will illuminate the city for a whole winter, just to melt all the ice and keep temperatures above freezing. Then, the harsh summer will return.
Another theoretical model is a liberating planet. Some tidal-locked bodies on slightly elliptical orbits (like Luna) are known to liberate. The spinning speed is constant, but the orbiting speed is not. So, for a limited time, some areas from the dark side will see light and some areas from the light side will see dark. This will result in a climate that will differ not much from what is described at Tidal Locked Planet.
Stars like our sun[]
The stars similar to our sun, Sol, are included into G - type stars. Also, F - type stars might be considered similar. Let's suppose that Earth is spinning on an elliptical orbit. This will generate a small hot summer and a long colder winter. In case of an Earth with an apastron at 1.5 AU and a periastron at 0.5 AU, we will see a summer lasting for only a month and a winter that keeps going for 5 months. Spring will come later because the snow will reflect solar radiation. Because of the absorbed heat in oceans and atmosphere and also because of water vapors in atmosphere, autumn will extend a month into the winter.
These long seasons will force the planet into an ice age. Longer winters mean that more snow will feed the glaciers, while shorter summers will mean less water flowing from them. One way to deal with this problem is to add some greenhouse gasses, but this will also make summers harder to survive.
Brighter stars[]
Let's take a look at A - type stars. A planet that will cross the comfort zone at its semimajor axis, will be further away from its sun and will have a longer rotation period. The year will last for 4 to 8 Earth years, depending of star. The planet will experience summers as long as the Earth summer, longer springs and autumns and very long winters. Summers can be long enough to melt all the ice, except for the poles, but the extremely long winters will dramatically affect life. For plants, surviving the 4-year long winter is a hard task. Trees are unable to do that, but short-lived plants can survive with help of their seeds. At the equator, extreme summer heat can reach 70 C or even beyond, sterilizing vast surfaces of land. How can plants survive this? Their seeds will be cooked! Maybe some genetically modified plants are able to survive thanks to their roots. Below ground, temperatures will remain lower. Animals will have no other choice then to migrate.
Settlers will also have to deal with long winters and short summers. A good idea is to stay for almost all of the year close to the equator and to move in summer close to the poles, in order to survive.
Very bright stars[]
An elliptical planet orbiting B - type stars or Red giants will have a very long year, of over 100 Earth years. Left alone, that planet will go from ice ages to warm ages. Most likely settlers will not agree with this. They will permanently adjust the planet, feeding the atmosphere with greenhouse gasses in winter and reflecting the extra light in summer. Without these improvements, the planet might end-up completely frozen in winter or with a runaway greenhouse effect in summer.
Elongation of the ellipse[]
No planet moves on a perfectly circular orbit. However, in case of Earth, the elongation is too small to be noted. In case of Mars, this still is not too much to result in massive climate changes. It is visible in case of Pluto. The longer the elongation, the higher the changes. However, there are some limits to the process.
Extreme elongations[]
If the planet has a too elongated orbit, it might be very hard to live on it. Let's do an imagination test and replace Halley's comet with an Earth-like planet. At apastron it will witness extreme cold weather, with temperatures falling to -200 C, while at periastron oceans will boil and will generate a huge greenhouse effect.
In this case, mainly extremophiles and genetically modified plants are able to survive. The planet will spend almost all its lifetime in extreme cold winter, where all life should be frozen. Perhaps only some life deep inside oceans will still be. After many years of hibernation, as the planet moves closer then Earth's orbit, temperature increases and ice starts to melt. Most probably, at the orbit of Venus, life will conquer large surfaces of land and all oceanic water. Then, as the planet reaches Mercury's orbit, heat gets so intense that oceans start to boil. Life vanishes from all continents. A gigantic greenhouse effect is created by evaporating water. It is becoming a new Venus. But for how long? Deeper inside oceans, life will still exist. There, temperature is always 4 C. It will be impossible to evaporate all water, so some oceans will survive. As the planet moves further away from Sol, beyond the orbit of Mars, temperatures are decreasing. Water starts to return. A massive rain, something like Noah's flood, washes all the continents, with huge erosion force. Rain keeps on going and gradually transforms into snow. Without water, greenhouse effect is not ensured and the planet keeps cooling as it passes beyond Jupiter. Atmosphere freezes. Only deep inside the oceans, life still survives. As ice accumulates, not much light enters the oceans and life becomes harder.
As seen in this model, life can survive in the oceans. Extremophiles could resist also on the ground. Other organisms might be able to migrate, to stay on ground when conditions allow, to return in water when temperatures reach the maximum and to hibernate in the long winters.
Such a planet will not be chosen by many settlers, even if for limited time it can be inhabited. Or will it? Living on an environment changing up to such high levels is a big challenge.
Habitable only at one end[]
Another, more friendly model, is to have a planet habitable only at apastron or only at periastron. Settlers might be able to stay there for a limited time, then they will move to an outer space colony or to another planet.
A hot ellipsoidal planet will be inhabitable only during the winter. In summer, temperatures will be so high that oceans will evaporate. The problem is that the extreme heat will kill all living organisms, except for extremophiles and genetically modified organisms. Still, for a limited time, people can live there.
A cold ellipsoidal planet will be the opposite. It will be habitable only during summer. In winter, all will be covered with ice. Plants could survive thanks to their seeds and marine life will still exist below ice.
Other factors[]
There are a few other factors that can influence climate on a terraformed elliptical planet.
Axial tilt[]
Depending on the spin axis orientation, this will affect the climate. It is shown in case of comet 67P/GC [1], because of its axial tilt, on one side, the comet will lose 10 times more matter then on the other side. A tilted axis can result in a planet receiving on one side more light then on the other, creating seasons. If we take Earth and change its orbit, making it closer to the sun in winter and farther away in summer, on Northern hemisphere we will have cooler summers and warmer winters, but on Southern hemisphere we will see very hot summers and extreme cold winters. settlers will most likely want to go where the climate is better.
Greenhouse gasses[]
Suppose we have a planet moving on an ellipse between the orbits of Neptune and Jupiter. That planet was terraformed with the help of greenhouse gasses. This implies a special equilibrium in the thermal balance of that planet. There will not be much heat reaching the planet during the day, but not much infrared radiation during the night, to cool the planet. The problem is discussed on Outer Planet and Low - spinning planet. The planet will experience a unique climate (monoclime), with nearly the same temperatures for entire surface and with air humidity at 100%. However, the planet is not safe when facing long term increase or decrease of solar radiation.
At apastron, that planet will face a global winter. Water will freeze, air temperature will probably fall to -20 C. Since luminosity is so small at the orbit of Neptune, plants will find so hard to survive in water, when ice will deflect part of the light. Again, at periastron, temperature will rise to 50 C. Since humidity will be at 100%, it will be impossible for people to survive the summer in opened air.
Settlers will have two options to keep their planet at a constant temperature:
- Adjust the amount of greenhouse gasses as needed.
- Set the amount of greenhouse gasses as needed at apastron, then, as these gasses tend to accumulate at lower altitudes, force atmospheric currents to blow from high altitude, to create gaps in the greenhouse gas layer, to cool the planet when needed.
Planets with elliptical orbit are found often in the Universe. Knowing how to handle them is very important for future generations of settlers.