A terraformed Callisto

Main article: Rocky Planets (Theoretical Models)

A Galilean type planet is an Outer Planet that orbits its parent star outside the Habitable Zone, but not too far. In the Solar System, such a planet could be located from closer then Ceres to further away then Jupiter. The name comes from the Galilean moons of Jupiter and is not official.

The Solar Constant is between 0.04 and 0.4 At these values, the Void Temperature (temperature of a body painted 25% gray, directly exposed to the radiation source) would be between -126 and -21 degrees Celsius. This corresponds to an average planetary temperature between -168 and -94 degrees, while for Earth's solar constant, the planetary temperature is +15 (please note that these temperatures are theoretical, there are many factors that influence them, like atmosphere composition).

The Solar System has one dwarf planet (Ceres) and one giant planet (Jupiter) within this area, but no rocky planet.

Relation with parent star Edit

A Galilean type planet would be exposed to less light and heat and would require the use of greenhouse gasses. Still, unlike other, outer planet models, it will have some common features with planets in the habitable zone.

Around M - type stars (modeled for Barnard's Star):

  • Distance: 19 to 60 million km
  • Visual constant: 0.00207 to 0.0206 (for yellow wavelength)
  • Revolution period: 37 to 209 days
  • Stellar gravity: 1.22 to 12.2
  • Hill sphere (assumed Earth's mass): 0.3 to 1 million km

The planet will have a stable orbit and will face tidal forces similar to Venus. So, it might not be tidal locked and it could be a Low - spinning planet. There is a small chance for satellites to exist. The light might barely be enough for Earth plants to survive.

Around K - type stars (modeled after Epsilon Eridani):

  • Distance: 203 to 640 million km
  • Visual constant: 0.022 to 0.22 (for yellow wavelength)
  • Revolution period: 1.7 to 9.8 years
  • Stellar gravity: 0.045 to 0.45
  • Hill sphere (assumed Earth's mass): 2.2 to 6.8 million km

The planet will be habitable and on safe orbits. Tidal forces will be similar to those on Mars. So, the planet will be spinning. There will be enough light for plants to survive. Seasons will be longer then on Earth, similar to what we would expect on Mars.

Around G - type stars (modeled after Sol):

  • Distance: 330 to 1050 million km
  • Visual constant: 0.04 to 0.4 (equal with solar constant)
  • Revolution period: 3.3 to 18.6 years
  • Stellar gravity: 0.02 to 0.2
  • Hill sphere (assumed Earth's mass): 3.3 to 11 million km

Ceres is at the inner edge of this region, while Jupiter reigns well within. The planet will have long seasons and will have enough light for plants to live. Tidal forces from the sun will be negligible.

Around F - type stars (modeled after Procyon):

  • Distance: 990 to 3150 million km
  • Visual constant: 0.022 to 0.22 (for yellow wavelength)
  • Revolution period: 5.5 to 17.2 years
  • Stellar gravity: 0.003 to 0.03
  • Hill sphere (assumed Earth's mass): 14 to 79 million km

Such a planet would have limited light for plants to survive. Seasons will be long, lasting for years. Tidal forces from the star will be too small to count. There is a high chance for satellites to exist.

Around A - type stars (modeled after Sirius):

  • Distance: 1690 to 5300 million km
  • Visual constant: 0.005 to 0.05 (for yellow wavelength)
  • Revolution period: 27 to 149 years
  • Stellar gravity: 0.0016 to 0.015
  • Hill sphere (assumed Earth's mass): 13 to 42 million km

The visual constant is low. Plants will hardly survive, if at all, especially because the lack of red light. The year will be very long, so many species will die in the long winter. The large Hill sphere suggests that many moons can orbit one planet.

Around B - type stars (modeled after Rigel):

  • Distance: 13200 to 41500 million km
  • Visual constant: 0.003 to 0.03 (for yellow wavelength)
  • Revolution period: 173 to 694 years
  • Stellar gravity: 0.0003 to 0.003
  • Hill sphere (assumed Earth's mass): 46 to 146 million km

The visual constant is low, even lower in red wavelength. Plants will not survive. Given the very long year, all life will parish during polar nights (so the poles will not be a good place to live).

Around O - type stars (modeled after R136a1):

  • Distance: 800 000 and 2 550 000 billion km
  • Visual constant: 0.00035 to 0.0036 (for yellow wavelength)
  • Revolution period: 22 000 to 125 000 years
  • Stellar gravity: below 0.0001

Hill sphere (assuming Earth's mass): 1200 to 3700 million km

On such a world, plants will not have enough red light to survive. Even that far, the planet will be exposed to strong UV and X radiation and strong solar winds.

Around L - class brown dwarfs:

  • Distance: 0.79 to 2.5 million km
  • Visual constant: 0.00000003 to 0.000000003 (for yellow wavelength)
  • Revolution period: 0.63 to 3.5 days
  • Stellar gravity: 180 to 1800

Hill sphere (assuming Earth's mass): 20 000 to 80 000 km

The planet will be outside from the Roche limit, on a safe orbit, but exposed to extremely strong tidal forces. The amount of visual light received is so small, then a person will find hard to see on the surface.

Around T - class brown dwarfs:

  • Distance: 0.445 to 1.4 million km
  • Visual constant: 7E-13 and 7.5E-14 (for yellow wavelength)
  • Revolution period: 0.26 to 1.5 days
  • Stellar gravity: 570 to 5600

Hill sphere (assuming Earth's mass): 12 000 to 38 000 km

The planet will be close to the Roche limit, with a significant chance to break apart for inner orbits. If the planet is close to the outer limit, it will suffer from tremendous tidal forces. The light will be too dim for a person to see.

Around Y - class brown dwarfs:

  • Distance: 0.07 to 0.225 million km
  • Visual constant: below 3E-25 (for yellow wavelength)
  • Revolution period: 0.016 to 0.095 days
  • Stellar gravity: 22 000 to 230 000

Hill sphere (assuming Earth's mass): 2 000 to 6 000 km

The planet will be below the Roche limit, so it will break into a ring.

Physical and chemical composition Edit

Galilean planets will have high amounts of water, usually in a frozen state. It is possible for some planets to host a subsurface ocean. There might be outer rocky planets lacking of water (Io is known to be a dry place). Unlike their outer cousins, Galilean planets will have a lower concentration of methane, ammonia and organics.

Except for cryovolcanoes, all water will be solid. Also, many gasses could be trapped beneath the ice or dissolved. As water melts, it has a high chance to become an Oceanic Planet. Minerals can also be found dissolved in water, in large amounts.

Since these planets will orbit their stars further away, the vast majority should not be tidal locked. So, they might have an active internal dynamo. Solar winds are less powerful too.

Atmosphere Edit

Given the higher possibility of an active dynamo, lower solar winds and more stable atmosphere (thanks to the lower temperatures), there is a higher chance for outer planets to host an atmosphere around them.

Around these planets, differences of temperature between equator and poles, between seasons and between day and night, will still exist, but much softer then around other outer worlds. Seasons will power winds that will help mixing the gasses. Since greenhouse gasses are heavier, winds are vital for mixing them.

Terraforming Edit

The terraforming of a Galilean planet is possible with current technology, with the use of greenhouse gasses. The temperature increases and ices start to melt. Then, with the use of genetically modified algae and bacteria, carbon dioxide is transformed into oxygen and atomic carbon. Also, many naturally occurring chemical compounds are transformed. The vast majority will become oceanic planets unless artificial continents or ground insulation techniques are to be used.

Climate simulation Edit

At the inner orbit of a Galilean planet, even without the use of greenhouse gasses, an atmosphere as dense as Earth's might be enough to create a greenhouse effect enough to keep temperatures above zero. Temperatures listed in plain text are expected with a classic atmosphere, while the values in italics are expected to happen with the use of greenhouse gasses:

Latitude  Temperature (C)
90       -273   2
75       -80    14
60       -45    16
45       -23    17
30       -15    18
15        10    19
0         22    20

The following simulation is for a planet close to the outer end of the Galilean zone. Again, temperatures listed in plain text are expected with a classic atmosphere, while the values in italics are expected to happen with the use of greenhouse gasses:

Latitude  Temperature (C)
90       -273   8
75       -161   15
60       -141   16
45       -128   17
30       -117   17
15       -109   18
0        -101   18

On this example, it is clearly visible the effect of greenhouse gasses in ameliorating temperature fluctuations over the planet. Summers will be colder and winters hotter. Also, day-night fluctuations will be lower then 2 degrees.

As one can see, Galilean type planets will have seasons. Temperatures might occasionally reach values very close to freezing point, but not for long. Ice caps will not exist. It will be like an eternal spring. However, since air currents will be slower and temperature fluctuations more rare, water vapors will accumulate in the atmosphere in high amounts. There will be fog and clouds, but with little rain.

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