Main article: Rocky Planets (Theoretical Models)

A planet inside the Habitable Zone, sometimes called an Earth - like planet, is a planet where temperatures are expected, at least in some areas, to be suitable for Earth life forms. A special type is a planet inside the goldie lock zone, where it will receive the same amount of heat like the Earth (it will be exposed to a Solar Constant of 1.98).

The habitable zone is the place where the solar constant is between 0.5 and 4. At these values, the Void Temperature (temperature of a body painted 25% gray, directly exposed to the radiation source) would be between -8 and 160 degrees Celsius. This corresponds to an average planetary temperature between -64 and +69 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 planet in the habitable zone (possibly two, if we also consider Mars at the outer boundary). Other stars are known to also have planets inside the habitable zone.

Relation with parent star Edit

A Earth - like planet will be exposed to similar amounts of radiation like Earth.

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

  • Distance: 6 to 17 million km
  • Visual constant: 0.026 to 0.2 (for yellow wavelength)
  • Revolution period: 6.6 to 32 days
  • Stellar gravity: 15 to 122
  • Hill sphere (assumed Earth's mass): 0.1 to 0.3 million km

The planet will have a stable orbit, most probably will be tidal locked. Plants will have a enough red light to survive, but blue light will be close to the lowest limit.

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

  • Distance: 64 to 183 million km
  • Visual constant: 0.27 to 2.2 (for yellow wavelength)
  • Revolution period: 0.31 to 1.53 years
  • Stellar gravity: 0.5 to 4.5
  • Hill sphere (assumed Earth's mass): 1.1 to 3.3 million km

The planet will be habitable, but exposed to higher tidal forces then Earth (something similar to Venus). In such conditions, the planet might still not be tidal locked and might actually rotate slow.

Around G - type stars (modeled after Sol):

  • Distance: 105 to 299 million km
  • Visual constant: 0.5 to 4 (equal with solar constant)
  • Revolution period: 0.59 to 2.82 years
  • Stellar gravity: 0.25 to 2.82
  • Hill sphere (assumed Earth's mass): 1 to 3 million km

Earth is closer to the inner border then to the outer border of the region.

Around F - type stars (modeled after Procyon):

  • Distance: 315 to 890 million km
  • Visual constant: 0.2 to 2.2 (for yellow wavelength)
  • Revolution period: 2.5 to 11.2 years
  • Stellar gravity: 0.04 to 0.3
  • Hill sphere (assumed Earth's mass): 3 to 8 million km

The tidal forces will be like in the Main Asteroid Belt. Seasons will last long, because of the year length.

Around A - type stars (modeled after Sirius):

  • Distance: 530 to 1500 million km
  • Visual constant: 0.06 to 0.5 (for yellow wavelength)
  • Revolution period: 4.7 to 22 years
  • Stellar gravity: 0.02 to 0.16
  • Hill sphere (assumed Earth's mass): 4.2 to 12 million km

The visual constant is low, matching values similar to Jupiter in the Solar System. Plants will not be happy about this, especially about 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: 4850 to 11700 million km
  • Visual constant: 0.04 to 0.33 (for yellow wavelength)
  • Revolution period: 39 to 144 years
  • Stellar gravity: 0.003 to 0.03
  • Hill sphere (assumed Earth's mass): 15 to 42 million km

Since the planet will pass around its sun in about 10 Earth years, life will nearly be extinct in summer droughts or long winters. The visual constant has low values, so that plants can hardly survive.

Around O - type stars (modeled after R136a1):

  • Distance: 255 and 700 billion km
  • Visual constant: 0.005 to 0.036 (for yellow wavelength)
  • Revolution period: 4000 to 18 000 years
  • Stellar gravity: below 0.0001

Hill sphere (assuming Earth's mass): 380 to 1000 million km

Despite having enough heat, the planet will lack of visible light. Plants will find very hard to live in this environment. Since most of the light is in UV and X rays, life on such a planet will be very hard.

Around L - class brown dwarfs:

  • Distance: 0.25 to 0.7 million km
  • Visual constant: 0.0000003 to 0.00000003 (for yellow wavelength)
  • Revolution period: 0.11 to 0.52 days
  • Stellar gravity: 2200 to 17000

Hill sphere (assuming Earth's mass): 7000 to 20000 km

The planet will be very close to the Roche limit, so there will be a risk it will break apart. It will be exposed to extreme tidal forces. The luminosity is in dark red and it is something like the Moon seen from Earth at crescent. Plant life is impossible.

Around T - class brown dwarfs:

  • Distance: 0.15 to 0.4 million km
  • Visual constant: 0.00000000001 and 0.000000000001 (for yellow wavelength)
  • Revolution period: 0.05 to 0.22 days
  • Stellar gravity: 7000 to 60000

Hill sphere (assuming Earth's mass): 3000 to 11000 km

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

Physical and chemical composition Edit

Earth - like planets are moderately heated by their planet. they have conditions for liquid water, at least in limited area. Closer to the inner border of the habitable zone, they will be hotter and will tend to lose water. Close to the outer border, they will have most of the water frozen, on the surface or beneath.

These planets have a risk to undergo a runaway greenhouse effect, but much smaller then inner planets. If they lack of atmosphere, water has a higher chance to survive trapped below surface.

Planets in the habitable zone are expected to have similar or relatively similar chemical compositions with Earth. They will have an inner metallic core (that might or might not sustain an internal dynamo), liquid mantle and a rocky crust. Depending on conditions, the amount of existing water might render them into oceanic or desert planets.

Atmosphere Edit

Planets in the habitable zone might sustain a large type of atmospheres. We can see planets with tenuous atmosphere like Mars or planets with a much denser gaseous layer. It is a chance that they will enter a runaway greenhouse effect, but the chance is smaller then in case of inner planets.

Around these planets, atmospheres will be dynamic. Differences between day and night will force winds to create and will mix atmospheric layers. Terraforming processes will require limited use of greenhouse or anti-greenhouse gasses.

Simulations show that at Earth's orbit, atmospheres are stable for a celestial body with a diameter of 5300 km (assuming Earth's density and no effect from solar winds). Much smaller bodies, like Earth's Moon (and even smaller, up to 3000 km diameter), could support an atmosphere for a millennia.

Terraforming Edit

The terraforming of a planet inside the habitable zone should be more easy then other planets. The first step would be to set the correct temperature (with the add of small amounts of greenhouse or anti-greenhouse gasses). The second step is correcting the amount of water and volatiles (by diverting comets to the planet if this is needed). The third step is to make oxygen (with the use of genetically modified algae, from carbon dioxide). In the fourth step, settlers will try to neutralize naturally occurring toxins and to ameliorate soil composition and oceanic salts. The fifth step will be to develop a local flora and fauna, with plants and animals brought from Earth.

These planets will have the most similar climate patterns with Earth, so they will be able to support the most diversified ecosystems.

Climate simulation Edit

Very interesting is that a planet in the habitable zone will have habitable areas, even without the help of greenhouse gasses. The following scheme is a simulation of a planet similar to Earth, close to the inner edge of the habitable zone. Temperatures expected without the use of anti-greenhouse gasses are showed in plain text, while the temperatures expected with the use of these gasses are in italic:

Latitude  Temperature (C)
90         30  -135
75        130  -19
60        148   1
45        160   15
30        169   26
15        176   34
0         183   41

The following simulation is for a planet close to the outer end of the habitable 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       -118  -26
75       -58    2
60       -48    7
45       -41    10
30       -36    13
15       -32    15
0        -28    17

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 not be over 4 degrees.

Planets in the habitable zone will have a large variety of climatic models, depending on many factors. Their position within the zone dictates many of these parameters.

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