In the last few years, astronomers have had problems with a seemingly obvious definition: what, exactly, is a planet, as opposed to a star? Most people, thinking in an Earth-chauvinistic way, might point out that planets are hard, solid, rocky things, while stars are made of hydrogen and helium. But in fact, half of the planets in our solar system have no solid surface at all, and are composed of pretty much the same kinds of gas that makes up our Sun.
Well, then, what about the blazingly obvious observation that stars shine? Stars give off heat, light, and many other kinds of radiation. Planets have no energy sources of their own, do they? We all know that the Earth is totally dependent on the Sun's energy for heat and light.
Things are actually quite a bit different in the outer parts of our solar system. While it's true that none of the planets shine in visible light, all of the four gas giants in our solar system (Jupiter, Saturn, Uranus, and Neptune) actually give off more radiation than they get from the Sun. The extra energy, in the form of heat, comes from gravity compressing the planet's gases more and more over time. You could say that these planets really haven't finished forming yet. Gravity is still crushing together the gases that form these planets, and they will someday be denser and cooler. If you had eyes sensitive to infrared radiation, these planets would actually "shine" on their own.
Admittedly, this kind of heating through compression is much different from the hydrogen fusion that fuels the stars, and that, in the end, is what differentiates a star from a planet. Stars are able to sustain thermonuclear fusion reaction in their cores, planets are not. But, interestingly, even nuclear fusion is a little bit of a gray area. For planets about ten times the mass of Jupiter, research suggests that early in their formation, there may be a stage where deuterium (heavy hydrogen) fusion occurs deep inside their cores. For slightly more massive planets, even normal hydrogen fusion may happen for a limited time. So, in a real way, these massive "planets" may have been "stars" in the past, depending on your definition.
See the problem?
The confusion over this gray area has led to the classification of a new kind of celestial object, the brown dwarf. A brown dwarf is something that is a bit too massive to be a planet, but not quite massive enough to be a star (and the plural is indeed "dwarfs" not "dwarves"). Most scientists are getting comfortable defining brown dwarfs as objects with masses ranging from a few times the mass of Jupiter, to about a tenth of the mass of the Sun.
The word "dwarf" in astronomy usually denotes a small star. A red dwarf, for example, is a small, dim star that burns its hydrogen so slowly that all it can manage is a faint red glow in the sky. So, the "dwarf" part of a brown dwarf seems to suggest that it is some kind of star. But the "brown" part calls to mind the brownish clouds of giant planets like Jupiter and Saturn.
In fact, some of the smaller brown dwarfs may look very much like Jupiter indeed. One of Jupiter's most famous attributes is its great red spot, which is actually a giant hurricane about 15,000 miles across. Recently, astronomers may have detected weather on a brown dwarf that would make Jupiter's storm look more like a tempest in a teapot. Like the giant planets in our own solar system, brown dwarfs are steadily being compressed by gravity, and cool off over time. Most objects in space become fainter as their temperature drops, but for some reason, brown dwarfs bucked this trend. There seemed to be a transition temperature where, although a brown dwarf was indeed getting cooler, it also got brighter. Astronomers couldn't explain this until they tried to model how giant storms would influence the appearance of brown dwarfs.
To begin with, brown dwarf atmospheres are very hot, about 3,000 degrees Fahrenheit, and the farther down into the brown dwarf you go, the hotter it gets. At extreme temperatures like that, lots of chemicals would be vaporized and swirling, even stuff like iron and silicon. Over time, as the brown dwarf cools off, there has to be a transition period when the vapors in the atmosphere condense, much like steam condensing back into water droplets as it cools.
This condensation phase would fill vast cloud layers with liquid metals, which would whip up lightening storms the likes of which our solar system has never seen. As the cloud layers break up due to the turbulence, lower and hotter parts of the atmosphere would be exposed to space, and able to radiate away their heat. The extra radiation expected from the lower cloud layers turns out to match very well the extra brightness that brown dwarfs exhibit as they cool off.
It's not the only possible explanation, but for the time being, it seems to be a good one. It also gives us one a dramatic new image: somewhere, right now, titanic thunderstorms are rocking the atmosphere of a brown dwarf, creating a downpour of molten iron rain. And while it may not be linked to storm activity, some brown dwarfs give off periodic flares of X-ray radiation. Something, perhaps deep down in the tangled magnetic field of the brown dwarf, is creating surges of electrical current though the atmosphere, releasing violent X-ray jolts like huge strokes of lightening.
Observing brown dwarfs will surely become a hot topic, pardon the pun, in the coming decade. One important question to be addressed is whether brown dwarfs form more like stars or planets. At the moment, brown dwarfs are thought to form in much the same way as stars, inside the core of a collapsing interstellar dust cloud. The only thing that stopped the brown dwarf from becoming a star was that is was not able to accumulate enough mass to drive its temperature enough to create sustainable nuclear reactions.
In a way, it may be correct to think of brown dwarfs as stillborn stars. On the other hand, brown dwarfs may have begun life more like a planet, forming in a spinning disk of material around a young star. Recent models suggest that in systems where two planets at least the size of Jupiter form, one will eventually migrate in to a closer orbit around its star, while the other will get thrown off into space. Is it possible that some brown dwarfs formed this way, and have kicked their twin out of the nest?
Could life exist around a brown dwarf? It's not as silly as it sounds; just think of the moons of Jupiter, like Europa, that are heated by tidal interaction with their parent planet, instead of sunlight. Europa may have warm saltwater oceans underneath its think layer of ice, and there's no reason to assume that there couldn't be a similar moon around a brown dwarf. Somewhere between stars and planets, there's a whole new kind of world waiting to be explored.
Michelle Thaller is an astronomer at the California Institute of Technology. She dedicates most of her time to education and public outreach for the Space Infrared Telescope Facility.