The mass of a brown dwarf is too low to support hydrogen burning. Their luminosity is probably about one hundred thousandth of the Sunís. They are therefore very difficult to detect by telescope. Their relative low temperature means that most of their energy is emitted in the infra-red range. In a sense, they are failed stars. Despite emitting a lot of heat and a little light, brown dwarfs can have upper atmospheres of a similar nature to Jupiter, even containing water vapour and methane.
Their properties are very much dependent upon their age. From up close, a young brown dwarf would look like a low-mass star, but an old brown dwarf would look more like Jupiter. Irrespective of mass, they are likely to remain about the same actual size as Jupiter. So if a brown dwarf were to be where Jupiter is now, we might see it as an extremely bright, reddish planet of the same size. Astronomers guess that there is about a 50-50 chance that a brown dwarf lies undetected within 4 light years distance of the Sun, the distance to Alpha Centauri.
These facts are at least consistent with a theory describing the 12th planet as a brown dwarf. To imagine gazing upon its bright countenance 2000 years agoí as it passed between the orbits of Mars and Jupiterí is a stirring thought. For life to be found on Nibiru, it would have to be on a planet in close orbit around the failed star, rather like Europa around Jupiter. But that planet would also be bathed in heat and weak light, making the formation of evolving life possible. Only time will tell if this proposition proves correct, as astronomers seek the entity that appears to be sending comets plunging into the inner Solar System.
How can we detect Brown Dwarfs?
As the mass of a Brown Dwarf is too low to support Hydrogen burning, these objects are going to be pretty hard to detect. The luminosity is expected to be around a hundred thousandth of the sun's (10-5 L), so a very large telescope is going to be required in order to be able to see one directly. The only one so far imaged, Gl229B, is just about visible in the Palomar 60 inch telescope.
As Brown Dwarfs are so "cold" they are going to radiate most of their energy in the Infra red, thus finding a "cold" object with a suitable spectrum and being able to determine its mass (it would need to be part of a binary system for this) would enable identification of an object as a Brown Dwarf.
stars where sustained nuclear reactions take place, lithium will quickly
disappear as it breaks down into helium at a temperature of around 2 million
degrees. Because of mixing in such stars the whole star will soon lose
all of its lithium. Thus if lithium can be detected this is a sure sign
that the object is not a hydrogen-burning star, such as a red dwarf. Luckily
for astronomers the strongest spectral line for lithium lies in the visible
region (670.9 nanometres), making it relatively easy to detect.
In the infrared part of the spectrum water (steam) should be easily seen, and Methane (CH4) should also be prominent. For a "hot" Brown Dwarf carbon Monoxide may be seen, giving way to Carbon Dioxide at lower temperatures. Using infrared filters (as in the discovery of PIZ 1) should therefore be an effective method of finding them.
Otherwise detection must depend on indirect methods, where the candidate Brown dwarf is assumed to be the lesser component in a binary system:
By measuring the displacement of the primary star against the expected track (it will appear to "wobble" because of the gravitational attraction) the relative mass of the candidate Brown dwarf can be calculated.
Radial Velocity Measurements
Again the Brown Dwarf is assumed to be part of a binary system where the other star is more massive. Here periodic changes in the wavelength of lines in the spectrum of the larger star can be neasured to give changes in the line-of-sight velocity. This can then be used to calculate the relative mass of the candidate Brown dwarf.
Where Should We Look for Brown Dwarfs?
A Brown Dwarf will be hotter, and hence brighter, when it has been newly formed because the gas coming together to create the Brown Dwarf releases its energy in the form of heat. It may also manage to initiate hydrogen burning for a while. As mentioned before, Brown Dwarfs are not very luminous, and so the best places to look are going to be nearby star-forming regions, or places where there are many young stars, for example Open Clusters such as the Pleiades and the Hyades. As all the stars in an Open Cluster are formed at roughly the same time and from the same gas cloud, then the ages and original chemical composition of all the members should be the same. This makes life a lot easier for astronomers, as the mass of the object will be the only independent variable.