Epsilon Eridani

Epsilon Eridani is a star located at 10 light years from us. It is a young, main-sequence K star (see Young Stars, K-Type Stars and Main Sequence Stars for details). It has a highly active corona, strong Stellar Wind and is known to host at least a planet. The system hosts two asteroid belts and a large dust disk. Settlers that one day will want to make Epsilon Eri as their home system will have to face many challenges. The environment around this star is specific to a typical young star and requires Young Planet terraforming methods.

The Star[]

Epsilon Eridani is a standard K2V star. That means its mass is 82% that of our sun, Sol, its radius is 73.5% of Sol's and its luminosity is 34%. Surface temperature is 5084 K. Being a young star, its solar wind is roughly 30 times stronger then Sol's. It rotates fast (11.2 days) and unequal (equator rotates with a different speed than the poles). The fast rotating speed causes a strong magnetic field. Compared to Sol, the heliosphere of Epsilon Eri extends far further away, up to 1600 AU, with a long magnetotail stretching to 8000 AU. Also, the star shows brightness variations which are related to coronal and magnetic activities, which are cyclic. Epsilon Eri is emitting more UV and X radiation than the Sun.

Epsilon Eridani is thought to be far younger then Sol, 0.3 to 0.6 billion years old. However, the star shows less metalicity (amount of elements heavier then helium), which is in contradiction with age. If this is true for the whole system, it means a different chemical composition compared to the Solar System for the planets found here.

The Habitable Zone around Epsilon Eridani can be calculated by using a computer simulation. The star has the following parameters:

Main wavelength: 625 (yellow-orange light)
Solar Constant (Sol = 1): 0.368
Red light (Sol = 1): 0.370
Yellow light (Sol = 1): 0.337
Blue light (Sol = 1): 0.289.

The star produces slightly more red then blue light, but not as much as to lack plants of light. Plants need both red and blue light for photosynthesis, the smallest amount of light they seem to survive is 1/1000 of what they receive on Earth (similar to the orbit of Neptune). We can calculate what is the maximum distance where plants can survive:

Outer limit for red light (1/1000 Earth intensity): 19.24 AU
Outer limit for blue light (1/1000 Earth intensity): 17.01 AU.

So, terraforming a planet located beyond 17 AU will not work without an Artificial Sun.

Terraforming a planet is theoretically possible within large boundaries. At a Solar Constant Ks = 1, a planet will behave like the Earth. Using Anti-Greenhouse Technology, it might be possible to terraform a planet experiencing 20 times more light then the Earth (Ks = 20). The outer limit, using Greenhouse Gases, is at the orbit of Neptune (Ks = 1/1000). There, the Atmosphere Cooling Effect, caused by freezing gasses outside of the greenhouse gas envelope, can send a terraformed planet into a runaway ice age. The limits where terraforming is possible can be calculated too:

Inner terraforming limit (Ks = 20): 0.1356 AU
Orbit of an Earth-Like Planet (Ks = 1): 0.607 AU
Outer terraforming limit (Ks = 1/1000): 19.19 AU.

There is also an inner limiting factor for Epsilon Eri. The star has a solar wind 30 times stronger then Sol's. The following chart will show what planets in this system have to put up with. To make things easier, the strength of the solar wind for Earth has a value of 1.

Epsilon Eri's inner terraforming limit: 54.4 (Solar System), 1632 (Epsilon Eri)
Mercury's orbit: 6.67 (Solar System), 200 (Epsilon Eri)
Earth-like planet orbit: 2.714 (Solar System), 81.4 (Epsilon Eri)
Venus's orbit: 1.913 (Solar System), 57.4 (Epsilon Eri)
Jupiter's orbit: 0.0369 (Solar System), 1.108 (Epsilon Eri)
Saturn's orbit: 0.01099 (Solar System), 0.330 (Epsilon Eri)
Epsilon Eri's outer terraforming limit: 0.00271 (Solar System), 0.0815 (Epsilon Eri)
Uranus's orbit: 0.00272 (Solar System), 0.0815 (Epsilon Eri)

As one can see, an Inner Planet is exposed to strong solar winds. It is questionable if a planet experiencing a solar wind 100 times stronger than Earth. Given the fact that Epsilon Eri has a strong magnetosphere, it is questionable if the magnetic field of an inner planet will be strong enough to deflect it. A planet in the middle of the habitable zone will experience solar winds 100 times stronger than the Earth. Similar solar winds with what we see around the Earth will be found at Jupiter's distance.

Planetary System[]

Epsilon Eridani is known to have one planet, with one or two extra planets theorized to exist. The system has two asteroid belts and a large dust disk. Here is what we know about them:

Inner Planets[]

There is no evidence of dust between the star and 1.5 AU. While asteroids cannot be ruled out, dust released by asteroid collisions has not been detected. The absence of dust can be caused by inner planets which clear their neighbourhoods of asteroids and dust. If Epsilon Eridani has rocky planets, they must be very young and volcanic, with a narrow crust and possibly with oceans of lava.

It is very hard to detect planets around a young and active star, this is why no telescope is able to detect them, even if rocky planets have been detected around other planets, even if located much further from us.

Given the large dust disks around Epsilon Eri, which is about 1000 times denser than in the Solar System, the number of comets and asteroid impacts is estimated to be 1000 times more than in the Solar System. So, whatever rocky planets exist, they are subject to heavy bombardment. In such conditions, for any terraformed planets, a planetary defence system is vital.

A planet located at the inner limit where terraforming is possible based on temperature would behave like that:

Distance: 0.1356 AU
Solar constant (Earth = 1): 20
Tidal forces (Earth = 1): 44.6
Rotation around the star: 20.1 days
Temperature: 802 K (529 C)
Temperature assuming Earth's atmosphere: 336 C
Solar wind strength (Earth = 1): 1631

For a planet orbiting at the heart of the habitable zone, we get the following parameters:

Distance: 0.607 AU (near Venus's orbit)
Solar constant (Earth = 1): 2.23
Tidal forces (Earth = 1): 44.6
Rotation around the star: 190.6 days (0.522 years)
Temperature: 379 K (106 C)
Temperature assuming Earth's atmosphere: 15 C
Solar wind strength (Earth = 1): 81.4

From these parameters, we can see that an Earth-Like Planet will experience an year equal to half an Earth's year. Seasons will be short and milder. The planet will not have enough time to warm in summer or cool in winter as the Earth does. Tidal forces are comparable to what we see for Venus. If Epsilon Eridani were the age of the Solar System, we would assume that such a planet will be a Low-Spinning Planet. However, since Epsilon Eri is a young star, planets are supposed to rotate fast. This also implies that planets have stronger magnetic fields, which implies that they are better protected against stellar winds. A hypothetical Earth-like planet would have amazing auroras.

Inner Asteroid Belt[]

There is an asteroid belt at 3 AU. Some scientists put it at 1.5 to 3, while others at 3 to 4 AU. What we know about this asteroid belt is that it contains enough dust to fill one asteroid with a diameter of 100 km. This is far more than what the Main Asteroid Belt in the Solar System has. Probably the asteroids have more unstable orbits and collisions are far more often. The high amount of dust implies that there are far more asteroids then in our system. We can also presume that the Inner Asteroid Belt hosts larger bodies than the Solar System, based on the chance that, having enough matter to feed on, larger asteroids could grow up. Probably the largest of all are dwarf planets close to the size of the Moon. Given their young age, some of these dwarf planets could have active cores and volcanisms and could maintain a tenuous atmosphere.

Planet b, known also as AEgir, acts like a shepherd, keeping the asteroids in place, in a similar way Jupiter acts on our asteroid belt.

The dust appears made of silicon or silicate rocks.

We can calculate the conditions found at the inner edge, middle and outer edge of the asteroid belt.

Inner edge asteroid:

Distance: 1.5 AU (near Mars's orbit)
Solar constant (Earth = 1): 0.164
Tidal forces (Earth = 1): 0.364
Rotation around the star: 740 days (2.03 years)
Temperature: 241 K (-32 C)
Temperature assuming Earth's atmosphere: -90 C
Solar wind strength (Earth = 1): 13.3

Middle asteroid:

Distance: 3 AU (near Ceres's orbit)
Solar constant (Earth = 1): 0.0408
Tidal forces (Earth = 1): 0.0911
Rotation around the star: 2094 days (5.74 years)
Temperature: 171 K (-103 C)
Temperature assuming Earth's atmosphere: -144 C
Solar wind strength (Earth = 1): 3.33

Outer edge asteroid:

Distance: 4 AU
Solar constant (Earth = 1): 0.023
Tidal forces (Earth = 1): 0.0513
Rotation around the star: 3225 days (8.83 years)
Temperature: 148 K (-125 C)
Temperature assuming Earth's atmosphere: -161 C
Solar wind strength (Earth = 1): 1.875

The asteroids will experience slightly lower temperatures compared to Solar System's main asteroid belt. Being in a more crowded area, they will be exposed to far more collisions and might accrete a layer of dust on their surfaces.

There is a high chance that we will see these asteroids locked in an orbital resonance with AEgir. This can be seen in the Solar System, where many asteroids are locked in 2/5 and other resonances with Jupiter.

AEgir[]

AEgir is the unofficial name of planet b. It was never imaged, but it creates significant wobbles in the star's motion to make it easy to detect. The planet orbits at 3.48 AU and has a mass of 78% of Jupiter's, orbiting the star in 2617 days. Please notice that there is a disagreement between scientists, these values are average from various sources.

AEgir behaves like Jupiter in the Solar System. It has a similar mass (compared to its host star), it maintains an asteroid belt in position, it is the largest planet of the system and it lies in similar temperature zones. Here are some parameters which we can calculate for AEgir based on the data we know:

Distance: 3.48 AU
Solar constant (Earth = 1): 0.0304
Mass (Earth = 1): 248
Diameter (assumed for its mass): 130000 km
Surface gravity (assumed): 2.39
Density (assumed): 1.29
Tidal forces (Earth = 1): 0.0677
Rotation around the star: 2617 days (7.17 years)
Hill sphere: 0.105 AU (15.7 million km)
Temperature: 158 K (-115 C)
Atmospheric temperature (assuming Earth's atmosphere): -153 C
Solar wind (Earth = 1): 2.477

As one can see, AEgir has a shorter year compared to Jupiter, but experiences similar temperatures. The solar constant is similar to what we find around Jupiter. The only major difference is that AEgir is bombarded by a solar wind that is stronger than what we see at Earth's orbit. Because of this, AEgir, even if it has a magnetic field as strong as Jupiter, is unable to hold a magnetosphere as large as Jupiter's. This might leave its outer moons exposed to solar wind.

The moons that AEgir might have receive enough light to be suitable for terraforming. However, these moons are for sure exposed to massive bombardment, more intense compared to the inner planets.

AEgir Gap[]

There is a large gap between the two asteroid belts (between 4 and 8 AU). AEgir orbits at the inner end of the gap, leaving enough space for another planet further out. If such a planet exists, it would have a stable orbit at 7 AU, where it can also work as a shepherd for the outer asteroid belt. If it exists, it would experience similar conditions to what we find at Saturn's orbit. We don't know if such a planet exists, but, based on its orbit, we can calculate a few things:

Distance: 7 AU
Solar constant (Earth = 1): 0.00751
Tidal forces (Earth = 1): 0.0165
Rotation around the star: 20.45 years
Temperature: 112 K (-161 C)
Temperature assuming Earth's atmosphere: -188 C
Solar wind strength (Earth = 1): 0.612

This planet will have a shorter year than Saturn but will experience almost similar temperatures to the moons of Saturn (a bit colder, that is). Terraforming is possible if the planet is not a gas giant or if it has moons of planetary size.

Outer Asteroid Belt[]

A second asteroid belt exists at 8 to 20 AU. It contains more dust (and hence more bodies) compared to the inner asteroid belt. Its chemical composition is different, containing more water ice. Despite having more mass, it has a lower density. Given its size and the fact that stellar gravity is far weaker, there is a higher chance for larger dwarf planets to exist here, feeding on dust and asteroids. Some of them might be up to the size of Mercury or Mars, making them suitable for terraforming. Of course, these planet-sized objects are exposed to massive bombardment and they will require protection (see Protecting Future Worlds From Impacts for details about this).

We can calculate a few features for a planetoid located in the middle of this asteroid belt:

Distance: 14 AU
Solar constant (Earth = 1): 0.00188
Tidal force (Earth = 1): 0.00418
Rotation around the star: 57.8 years
Temperature: 79 K (-194 C)
Temperature (assuming Earth's atmosphere): -213 C
Solar wind (Earth = 1): 0.153

Such a planetoid will experience low temperatures and will be able to hold carbon dioxide frozen on its surface, as well as water ice. With a weaker solar wind, large planetoids will be able to hold atmosphere. Temperature values are closer to Neptune than to Uranus, despite the distance to the star being smaller than Uranus from the Sun. As the solar constant is still above 0.001, plants on a terraformed body can survive with the little light they receive.

Plants have enough light to survive as far as 17.01 AU, which is close to the outer edge of the second asteroid belt.

Epsilon Eri d[]

A third planet is sometimes proposed at the outer edge of the second asteroid belt, to work as a shepherd, maintaining the asteroids in place. It would be at 20 to 25 AU from the star. This is beyond the limit where plants have enough light to survive (17.01 AU) and beyond the limit where terraforming is possible with greenhouse gasses (19.19 AU). No details regarding its mass have been proposed. It could be a Super-Earth or or something like Mini Neptunes. For this planet and its moons, terraforming is not feasible without an Artificial Sun. We can still see a few details about its behaviour:

Distance: 22.5 AU
Solar constant (Earth = 1): 0.000727
Tidal force (Earth = 1): 0.00162
Rotation around the star: 118 years
Temperature: 62 K (-211 C)
Temperature (assuming Earth's atmosphere): -226 C
Solar wind (Earth = 1): 0.0593

Conditions on this planet and its moons are similar to Pluto. It is a cold world. It is a place where methane, carbon monoxide and nitrogen ices can be found on the surface, maintaining tenuous atmospheres.

Epsilon Eri c[]

This planet is theorized to exist at the inner edge of the dust disk, at 40 AU. The planet works as a shepherd to the star's Kuiper Belt and has a mass of 10% that of Jupiter, with an orbital period of 102000 days and a slightly eccentric orbit. It has double the mass of Neptune and just like Neptune works as a shepherd. However, it is a far colder planet and surrounded by a much larger Kuiper Belt. We can calculate some of its parameters:

Distance: 40 AU
Solar constant: 0.00023
Mass (Earth = 1): 32
Diameter (assumed): 60000 km
Gravity (assumed): 1.5
Density (assumed): 1.7
Tidal force (Earth = 1): 0.000513
Time needed to orbit around the star: 279 years
Temperature: 47 K (-226 C)
Temperature (assuming Earth's atmosphere): -238 C
Solar wind (Earth = 1): 0.0188

This outer Neptune is too far to host any moon suitable for terraforming. It lies just inside the inner edge of the dust disk (Epsilon Eridani's Kuiper Belt) and so it must sustain heavy bombardment. There is a high chance that there are many prograde and retrograde moons, the first being natural moons and the second being captured. As so, collisions between moons are likely to occur. This is similar to how Triton's capture from the Kuiper Belt is supposed to have destroyed most of Neptune's initial moons.

Kuiper Belt[]

There is a huge dust disk located mainly between 35 and 100 AU from the star. The highest density of dust appears to be at 60 AU, while the outer limit seems to be at 120 AU.

This disk is in a state of equilibrium between dust production (as a result of collision between unseen bodies) and cleansing mechanisms (the dust is slowly cleared by the stellar wind). To be in a state of equilibrium, the dust requires a dense Kuiper Belt, with a mass of 5 to 9 Earth masses, so that impacts will be often enough to generate it.

Epsilon Eridani's Kuiper Belt contains almost 1000 times more matter than Earth's Kuiper Belt. Because of this, we can speculate that planetoids much larger than Pluto could exist, having enough matter to feed on. They could be larger than Mars, but smaller than Earth. Given the fact that Kuiper Belts contain large amounts of water ice and frozen gasses, these objects will have densities below 2, similar to Pluto. They will also have less tholins on their surface, as tholins need time to form. We can calculate some of the properties of an object located in the middle and at the outer edge of Epsilon Eridani's Kuiper Belt:

Middle Kuiper Belt Object:

Distance: 60 AU
Solar constant (Earth = 1): 0.000102
Tidal force (Earth = 1): 0.000228
Rotation around the star: 513 years
Temperature: 31 K (-235 C)
Temperature (assuming Earth's atmosphere): -244 C
Solar wind (Earth = 1): 0.00833

Compared to the Solar System, such temperatures are found beyond the Kuiper Belt, in the Scattered Disk. Because of this, the icy bodies located here will lose their volatiles much slower compared to the Solar System.

Outer Kuiper Belt Object:

Distance: 100 AU
Solar constant (Earth = 1): 0.00000368
Tidal force (Earth = 1): 0.000082
Rotation around the star: 1104 years
Temperature: 30 K (-244 C)
Temperature (assuming Earth's atmosphere): -251 C
Solar wind (Earth = 1): 0.003

At this distance, it is like beyond the furthest known objects in the Solar System. Like Sedna, atmospheres are highly unlikely to exist.

Conclusion[]

Epsilon Eridani is a fascinating system, with large asteroid belts and with many possibilities for planets and moons to exist, many of them suitable for terraforming. It is also a system with a lot of challenges, mainly from the high risk of impacts and the strong solar wind. There are still many unknowns about this system, which seems likely to host many more planets than the ones detected or theoretically predicted. Surviving A Heavy Asteroid Bombardment is tricky and difficult.

Future settlers may one day terraform the planets and moons of this system or they might just live without having the ambitious goal of terraforming. They will live a dynamic life, full with challenges. They will have to protect their worlds from impacts, which are about 1000 times more frequent compared to the Solar System. They will also have to live on young planets and moons, which are more active, with fierce volcanism, quakes and active tectonics.

From the inner planets scorched by solar winds to the gas giants with their moons and to the many asteroids offering fresh mineral resources, settlers will have many worlds to chose from and many challenges ahead.

A Young Planet requires different approaches in order to terraform.