Brown dwarf

Brown dwarfs are a class of "sub-stellar" objects that have mass that is above the range of mass of gas giant planets, but not enough for their cores to sustain nuclear fusion of hydrogen-1 and turn them into proper stars. That's why they are often described as "failed stars".

The science
In astronomy, the term "dwarf" refers to any star which is not a giant star - even including our own Sun! The upper limit of a brown dwarf mass is about 75 and 80 times the mass of Jupiter. For comparison, the Sun is considered to be a "yellow dwarf" and it's about 1000 times the mass of Jupiter. The lower limit that distinguishes them from gas giants is less clear, but it's considered to be around 13 times the mass of Jupiter. Anything above the lower limit fuses deuterium (hydrogen-2), and anything above about 65 times the mass of Jupiter fuses lithium. In terms of radius, brown dwarfs don't get much bigger than Jupiter. Their cores consist of degenerate matter, which is readily compressed by the increased gravity, but doesn't expand much on heating: heavier brown dwarves become hotter instead of larger. Originally, they were assumed to be brown-colored like Jupiter and Saturn, but later it was discovered that they glow in purple, magenta or reddish colors depending on temperature.

Gas giant-like objects too small to fuse deuterium are called "sub-brown dwarves". The difference to the related rogue planets is that sub-brown dwarves are created from the gravitational collapse of gaseous nebulae, as brown dwarves and stars are. Meanwhile, rogue planets formed around a star, but were kicked out of the planetary system they were born in.

As with stars, brown dwarves can form multiple star systems and have planets and potentially comets, asteroids, meteoroids, planetoids, and dwarf planets. In 2013, a planet about twice the mass of Jupiter was confirmed to be orbiting a brown dwarf.

The habitability of planets orbiting brown dwarfs, i.e. their suitability to support life, has been studied using computer models. They would not be particularly habitable. The habitable zone of a brown dwarf would be narrow and would decrease with time as it cooled. With such a close orbit, the tidal forces of the brown dwarf would squeeze and pull at the planet, causing it to heat up. In a similar manner, Jupiter turns its moon Io into a volcanic hellworld with constant eruptions. Planets would need to have orbits of very low eccentricity (10-6) to avoid triggering a greenhouse effect that could render them as uninhabitable as Venus. Satellites with subsurface oceans under a crust of ice similar to Jupiter's moon Europa, which were orbiting further away, would have things easier.

Planet X/Nibiru
In various versions of the Planet X/Nibiru story, it is claimed to be either a brown dwarf or to orbit around one. Although the term "Planet X" has been legitimately used for a proposed trans-Neptunian planet, nowadays astronomers usually use the term "Planet Nine".

Visibility
Planet X/Nibiru proponents like to claim that brown dwarfs are "invisible" and can be detected only in infrared light, and this is why no amateur astronomers have been able to spot the "incoming" Planet X/Nibiru. Infrared telescopes operated by professional astronomers have no problem detecting brown dwarfs nearby. In 2010-2011, WISE (Wide-field Infrared Survey Explorer), an infrared space telescope, scanned the entire sky and found six nearby brown dwarfs, with the closest (Luhman 16) at a distance of 6.6 light-years. This is next door by galactic standards, but still very far from the solar system: in contrast, Neptune is only 0.000476 light-years from the Sun. The discoveries were confirmed with data from four other surveys. Even a large planet (Saturn-sized or larger) would have been detected up to 10,000 AU (1.5 trillion kilometers or 0.16 light-years). Earlier surveys had been completed by IRAS, ISO, Spitzer and Akari space telescopes. At this point, it's safe to say that Planet Nine, if it exists, is not a nearby brown dwarf.

In response, conspiracy theorists tend to dismiss "official" astronomy as controlled by the powers that be, a typical unfalsifiable argument.

As with many conspiracist beliefs, this claim is based on a distortion of actual scientific facts about brown dwarfs' low visibility:


 * Brown dwarfs fuse deuterium, which heats them up and everything heated emits infra-red light (and if it's hot enough, visible light too: think of red-hot iron, or see at Wikipedia);
 * small brown dwarfs may be cold enough to emit mostly infrared light;
 * thus, if we consider only light emission, a small, cool brown dwarf will be fainter in the visible spectrum and brighter in the infra-red spectrum.
 * light sources become fainter with distance, so if a brown dwarf is far away enough, its visible light may be too faint to be detected by the human eye or a telescope, while its infrared light may be still above the threshold of detection.

That doesn't mean that brown dwarfs are "invisible" to non-infrared observation, though. A brown dwarf is at least the size of Jupiter or larger and will reflect light in the same way. If one was close to our Solar System (or in it, as some claim), it would look like Jupiter at the same distance due to reflected sunlight. A brown dwarf inside the orbit of Jupiter would be as bright as or brighter than Jupiter in the night sky. (And Jupiter is one of the brightest objects in the night sky after the Moon, Venus and the International Space Station.) And even if the brown dwarf was located in the outer fringes of the Solar System and invisible to the largest telescopes, as it approached the Sun it would be brighter and brighter to the point that sooner or later an astronomer would catch it (and there would be absolutely no way to hide it once it became bright enough to be visible with the naked eye).

Effects on planetary orbits
The orbits of the four outer planets, especially Neptune, have very low eccentricities and in the case of the latter it's together with Venus' one the most circular orbit of all Solar System planets. Had a brown dwarf, not to mention a star, paid a visit to the innermost part of the Solar System ("innermost" means here where the planets orbit the Sun) they would be circling the Sun in far more eccentric and/or inclined orbits, because a massive body doesn't pass through a solar system without its gravity tugging on everything orbiting the star. Not to mention the huge mess left if said body had entered even deeper, where Earth and its siblings are located.

Saturn
In the somewhat bizarre post-Velikovsky Saturn scenario, Saturn was supposed to have been a brown dwarf, and Earth was in orbit around it within human memory, in a perpetual 'purple twilight'.