A black dwarf is what is theorized to remain after White dwarfs exhaust all their heat.
A white dwarf does not sustain thermonuclear reactions and so it slowly cools down. Given its small radius and large mass, the time needed for a white dwarf to cool down until it no longer emits visible light is longer then the estimated age of the Universe. If we found one today, it would mean that our knowledge about the age of the Universe is wrong, the formation processes are unknown, or the object was somehow created artificially faster.
If they exist, black dwarfs are very hard to detect. The only way to find them should be by their gravity interactions with another visible companion. However, they are theorized to exist in large numbers as the Universe gets older.
Basic properties Edit
A black dwarf has the same mass and diameter of a white dwarf. That means, it has a diameter of 5000 to 10000 km (similar to a rocky planet) and a mass similar to a star (from 0.1 to 1.4 the mass of Sol, our sun). So, its gravitational effect on an orbiting planet is similar to that of a white dwarf.
Surface temperature is low, below 600K, even as low as 5 degrees kelvin. It emits no visible light, only a very faint infrared radiation, So, it cannot support light or heat an orbiting planet.
Because of its small size and high mass, its density is very high, over 1 million times that of the Sun. Also, its surface gravity is very high, millions of times higher then Earth's. As so, no spacecraft can land on its surface and no known material can resist under its own weight. This huge gravity is what keeps matter so compacted.
A white or black dwarf is made of what remained when its previous star remained without fuel. Small dwarfs might be made of helium (remnants of M - type stars), while others, heavier, contain carbon or other elements.
All white dwarfs are known to have a compacted atmosphere, made usually of lighter elements. The atmosphere is replenished by matter captured by their strong gravity (for example solar wind from a stellar companion). When the amount of hydrogen in the atmosphere reaches a certain limit, white dwarfs go nova (and the atmosphere explodes, while hydrogen undergoes nuclear fusion). In case of a black dwarf, since the required temperature for nuclear reactions is not achieved, there should be no nova and all matter should deposit on their surface, forming a larger atmosphere.
White dwarfs rotate fast (even faster then a second), generating strong magnetic fields. Since rotation speed is decreased by tidal forces, we can speculate that black dwarfs rotate much slower and don't have a strong magnetic field.
Possible planets Edit
Planets have been detected orbiting white dwarfs. Given the estimated age of a black dwarf, we could speculate that gravitational perturbations are enough to pull any planet out of its orbit into interstellar space or make it come too close to the dwarf, below Roche limit, where they will break apart in a ring.
Still, black dwarfs can capture a Rough planet. So, planets can exist around them.
If a planet exists around a black dwarf and there is no star nearby, it should have all its atmosphere frozen. With an endless night and temperatures close to absolute zero, that planet will not be too friendly for settlers.
Still, because of their strong gravity, black dwarfs can provide enough tidal heat for a planet that is close enough. Tidal heat can sustain strong volcanism or a subsurface ocean on any planet.
If a black dwarf has planets and is orbiting a star, then heat and light from the star could sustain Earth-like life on a terraformed planet. Magnetic fields of a black dwarf can protect a planet from solar wind or can maintain a deadly radiation belt
Possible Uses Edit
Given the strong gravity and high density, it is impossible to extract matter from a black dwarf with current technology. A matchbox, on the surface, will weight over 10000 kg. So, even if they have a solid surface and temperatures are not high, no object can support its own weight down there. Also, no living organism can survive in these conditions. A human settlement is out of question.
When an object falls into a black dwarf, its accelerates and smashes the surface with great speed. The impact produces much heat. If a planet collides with a black dwarf, the impact heat can be enough to start a nova explosion, threatening all human colonies within a solar system.
Planets and asteroids surrounding a black dwarf can be used by settlers. Industrial colonization might occur if there are resources. In addition, since black dwarfs are hard to detect and their planets are even harder, they can become good destinations for outlawed people, for space pirates, contrabandists, secret organizations and some cults.
However, colonization or planets and asteroids around black dwarfs will be possible only if the dwarfs themselves exist.