Image Credit: P. Delorme and the Canada-France-Hawaii Telescope
Brown dwarfs are "failed stars," objects that form like stars, perform a few nuclear reactions early in their lives, but never ignite the basic hydrogen fusion reactions that allow stars to burn for million, billions, and even trillions of years. So, while a brown dwarf starts off as a reasonably hot star, it can't keep itself warm, and gradually cools off.
As a brown dwarf cools, its atmosphere will change. When it is very young, its atmosphere looks a lot like that of red dwarf stars -- glowing a feeble red and filled with some simple molecules, like titanium oxide and carbon monoxide. As the brown dwarf cools below the coolest true stars (about 3100 degrees F), the types of molecules change, and substances like iron hydride (FeH) appear. So, since these objects look different and are too cool to be real stars, astronomers made a new classification for these stars: Spectral Type "L". (Why "L"? Because it was one of three letters in the alphabet left for spectral classes. The others were "T" and "Y", and we'll be using those below.)
Brown dwarfs of spectral class "L" continue to cool, and complex molecules like methane can form in the atmosphere at temperatures of about 2000 degrees Fahrenheit. This changes the appearance of the star quite a bit, so we created yet another spectral class for these stars: "T." Until recently, all brown dwarfs we know were either spectral type "L" or spectral type "T" (with a few having other spectral types typical of the coolest stars). This is only because cooler stars are very hard to detect -- they are faint, and they don't put out any visible, optical light. All of their light comes out as infrared ("heat") light.
Yesterday, astronomers using the Canada-France-Hawaii Telescope on the Big Island of Hawaii announced they had found the coolest known brown dwarf, with a temperature of only about 650 degrees F! At this temperature, a new molecule has formed in the atmosphere, ammonia, and the star's atmosphere looks quite a bit different than the know brown dwarfs of spectral type "T."
Several years ago, Davy Kirkpatrick (who invented the "L" and "T" spectral classes) anticipated that the oldest, coolest brown dwarfs should have atmospheres with lots of ammonia, and he invented the spectral class "Y" for these brown dwarfs cooler than about 900 degrees Fahrenheit. Why "Y"? It was the last useful letter in the alphabet, and these stars will have ammonia in their atmospheres for eons to come. So, the CFHT brown dwarf may be the first member of a spectral class that has existed for 9 years. And we astronomers can all feel pretty clever, because now we can give any star a spectral classification.
In terms of science, though, the new brown dwarf probably represents the most common type of brown dwarf in the Milky Way galaxy, because brown dwarfs will cool to these temperatures in a few billion years, and the Milky Way is three or four times older than this. Now that one has been found, I bet many dozens of these class Y brown dwarfs will be found, and we'll be able to study some of the oldest brown dwarfs in the galaxy.
While this may be the first class Y brown dwarf to be found, it is not the first object of this kind to be found. The planet Jupiter shines partly under its own light (in infrared light; all of the visible light we see comes from sunlight reflecting off the planet's clouds), and its atmosphere is full of ammonia and similar molecules. For that reason, the "Y" class is sometimes thought of as a link between planets and brown dwarfs.
But let's be a little careful here -- planets are different from brown dwarfs. Brown dwarfs are big enough (bigger than about 15 times the mass of Jupiter) that they were able to do some nuclear fusion in the past. Also, brown dwarfs probably formed differently from planets. Planets form in disks around bigger stars, while brown dwarfs probably form more like stars do, from collapsing clouds of gas. Planets are almost always (if not always) found close to their planet stars, while very few brown dwarfs are close to bigger stars. In short, while their atmospheres and the chemistry in those atmospheres may look similar, brown dwarfs and planets are different. One cannot change into another.
And that is the only thing that really bothers me about this announcement. The new brown dwarf is billed by some news outlets as a "missing link" between brown dwarfs and planets. In some senses, this is true. But the term "missing link" carries some baggage; namely, its connection to evolution of one species into another. And, as I said, this brown dwarf is not evolving into a planet. There are links in the atmospheres, and many differences are small. But "missing link" is an unfortunate phrase here.