A brown dwarf is a failed star. It is a celestial body too small to fuse hydrogen, but still able to produce some energy by fusing deuterium into helium. Even if their light is too dim for plants, still they generate enough heat to warm a planet on a close orbit.
Characteristics of a brown dwarf Edit
Formation and lifetime Edit
The life of a brown dwarf can be separated into 4 different stages:
1 - formation (M class) Brown dwarfs might form in a similar way stars are created: from a cloud of gas and dust. Only that the cloud is too small. Some of them orbit around a host star, so they were created like planets. In either ways, when matter collided, it should have generated enough heat to start a nuclear reaction. If the initial temperature was high enough, it could be sufficient to start fusing hydrogen into helium. If that is the case, the brown dwarf will increase size, so that pressure and temperature will decrease below hydrogen fusing, then it will slowly contract. In this phase, the dwarf might look and behave like very dim M - type stars, they might even have flares.
2 - deuterium fuse (L class) In this phase, a brown dwarf creates a limited amount of heat by fusing deuterium. The process does not generate too much energy. Deuterium is an isotope of hydrogen, with one proton and one neutron. Since deuterium is found more rare, it will be exhausted fast. Depending on mass, some brown dwarfs might not be able to use all their deuterium. If they are too small, internal pressures will drop fast enough, so that up to 70% of deuterium will remain unused. Larger brown dwarfs will use all (or almost all) their fuel.
3 - methane (T class) When nuclear reactions are no longer taking place, the brown dwarf is cooling gradually. As the internal heat is diminishing, surface temperature is also decreasing. At some point, temperature drops below 1000 K and methane starts to get formed in the upper atmosphere. The brown dwarf still emits a dim red light, but its spectra contains some blue, from heated methane.
4 - black object (Y class) At some point, a brown dwarf is so cold that it no longer emits visible light. It becomes like a planet, like a giant Jupiter, with a similar diameter, but with much higher mass. They can actually be smaller than Jupiter.
A brown dwarf can be directly created into M - class. However, if accretion process is slow enough and matter has time to cool itself, it might simply result a lower-class dwarf.
Sub - brown dwarf is an object that has not enough mass to fuse deuterium, but looks similar. In theory, when it was created, internal temperature could be enough to sustain deuterium fusion, but pressure was not enough to maintain the process.
Physical characteristics Edit
To get an overall look at brown dwarfs, please go here.
A brown dwarf has a diameter similar to Jupiter (slightly larger in its upper stages). Its mass is between 15 and 65 times the mass of Jupiter. Temperature varies, as shown below:
- M - class: ~2500 K
- L - class: ~1300 K
- T - class: ~700 K
- Y - class: ~200 K
M - class dwarfs have a larger diameter (probably 1.5 times the diameter of Jupiter). L - class have their name from lithium. This is because lithium decays easy in the conditions found in their core. Spectral analysis found metals in their absorption lines, this is because at their temperatures many metals evaporate and can be found as volatiles in the upper atmosphere. On the other hand, T - class dwarfs are cold enough to allow creation of methane, found in high amounts in their upper atmosphere. The Y - class is different. When they are cold enough, water clouds can be formed.
M - class brown dwarfs are flare stars, eruptive, with strong and dangerous flares, like the dim M - type stars. For example, DEN 1048-3956 was detected because a massive flare. When flares occur, light output might increase 100 times in less then a minute. A flare also comes with a massive solar wind that will destroy existing ecosystems. Since their average energy output, a habitable zone should be at maximum 5 million km away. At that distance, the effect of a flare would be catastrophic.
L and T - class brown dwarfs have a dim, red light. To have a better idea how their light looks like, their light output is similar to a burning cigarette. An L - class one will look like a cigarette when someone is inhaling, while a T - class dwarf is like a cigarette resting on an ashtray. Their light is so dim that it's hard to find your way around, on a nearby planet. Through a telescope, the Epsilon Indi brown dwarfs (T class) are hardly visible in red, clearly visible in infrared and almost invisible in blue.
Y - class brown dwarfs emit no visible light, only infrared.
Hosted planet Edit
Please see this material for a map of habitable zones.
A Habitable Zone exists around the majority of brown dwarfs. It is not a conventional zone. Light is too dim for plants. In order to survive, a plant needs a light at the intensity of minimum 0.1% of what is on Earth, both red and blue. We are talking here mainly about a zone where infrared light generates enough heat for a planet to have liquid water. Greenhouse gasses are unlikely to be suitable for terraforming a further away planet, because they will reflect the infrared light coming from the brown dwarf, in the same way they trap infrared emissions from the planet. However, if we take a brown dwarf into Solar System and place it between Uranus and Neptune, if it has a planet heated enough and with liquid water, solar light should be enough for plants to survive.
Light and habitability Edit
Since M - class brown dwarfs create powerful flares, increasing their light output 100 times (and blasting huge solar winds), their planets are unlikely to be chosen for terraforming. Only a very expensive shield could protect such a planet.
Around L - class dwarfs, there might be enough red light, but blue light is almost non-existing. Still, some genetically - modified plants could use only red light in order to survive.
For a T - class dwarf, habitable zone still exists, a place where heat is enough to keep water liquid on a nearby planet. the light should be similar to what you see at 2 cm from a burning cigarette. Settlers would find hard to walk without a lantern. Could any plants survive there? Only some genetically - modified plants could hardly use some near-infrared light. The longer the wavelength, the lower the energy.
If we think about the Y - class brown dwarfs, their energy output is so low, that planets cannot be warmed enough. A heated planet should be below the Roche limit, or the distance where dwarf's powerful gravity will make the planet to disintegrate. Still, some heat is received by the planet, not enough to keep water liquid, but to keep other fluids, like methane. Still, an orbital station can be placed close enough to an Y - class dwarf, to provide the needed heat. Note that brown dwarfs may produce strong tidal heating, so even without radiant heat the planets may warm themselves enough for liquid water to exist.
Orbit and tidal forces Edit
A planet orbiting a brown dwarf should be very close, as shown below:
- M - class dwarf: 0.1 AU (15 million km)
- L - class dwarf: 0.04 AU (4 million km)
- T - class dwarf: 0.01 AU (1.5 million km)
- Y - class dwarf: 0.003 AU (0.5 million km)
At such close orbits, planets must be tidal locked, showing the same face to the dwarf. On one face, temperatures will be high, but on the other face, it could be an eternal winter. Orbits will tend to be perfectly circular. Tidal forces should be much stronger then what is discussed for M - type stars. A tortured planet could be more violent then Io, but this is not a rule. If the planet has a circular orbit and there is not nearby object to create perturbations, tidal effect should not be so violent.
A planet will rotate fast, once a few days (for M - class), daily (for Y - class) or at a few hours (for T - class).
A satellite cannot orbit a planet because huge influences from the brown dwarf.
The brown dwarf appears very big, really huge. It is at the same size (or larger) then Jupiter seen from Io. Because of the dim light, stars are also visible. Everything appears to be dark-red around: clouds or objects.