A Good Definition of the Word "Planet": Mission Impossible?

3) Planets and Brown Dwarfs

Other fundamental problems have arisen as a result of the discoveries outside our Solar System. All the extrasolar planets to date have been found by the "wobble method", which observes the line-of-sight velocity (Doppler shift) of the central star caused by a planet orbiting it. By its nature, this method is most sensitive to massive planets orbiting close to their stars. One difficulty is that it depends on the orientation of the orbit. If the orbit is face-on to us the method fails completely (no velocity towards or away from us). This has the effect that each inferred mass is actually a minimum possible mass. These minimal masses range from about Saturn's mass up to the brown dwarf limit. That means that some of the more massive extrasolar planets could actually be brown dwarfs (although the statistics of the finds suggest that most of them are not).

A brown dwarf is a "failed star" (rather than a planet), but what does that mean? How does one draw the line between the two? Here, astronomers diverge in their opinions as well. "Stars" are objects that shine by nuclear fusion. That is the source that powers hydrogen bombs, in which hydrogen nuclei are fused together to make helium nuclei, releasing energy. To shine, the star must have enough mass that its gravity crushes the star's center into an extremely hot dense state. The problem with brown dwarfs is that they collected enough mass to start fusion, but not enough to sustain it for long. Most of the time a brown dwarf's energy comes from the fact it begins slowly shrinking (the release of gravitational potential energy). This works in much the same way that dropping a weight on your foot works to hurt it (mass releases energy as it drops). This same energy source works for massive planets (like Jupiter) as well. These objects cannot generate nearly the power of the Sun, and as they become more compact the shrink rate is reduced, making them slowly fade out. There is a dividing line between objects with enough mass to ever have fusion, and those so small that they never do – at about 13 times the mass of Jupiter. Most astronomers are willing to draw the line between brown dwarfs and planets there. (For more information on brown dwarfs, try this link: http://oposite.stsci.edu/pubinfo/PR/2000/29/index.html)

The brown dwarfs are too dim to be seen in a visible-light image taken by the Hubble telescope's Wide Field and Planetary Camera 2 [picture at left]. This view also doesn't show the assemblage of infant stars seen in the near-infrared image. That's because the young stars are embedded in dense clouds of dust and gas. The Hubble telescope's near-infrared camera, the Near Infrared Camera and Multi-Object Spectrometer, penetrated those clouds to capture a view of those objects. The brown dwarfs are the faintest objects in the image.

4) Born Into the Right Class?

Another possible difference between brown dwarfs and massive planets is how they are born. Our standard picture for the formation of a massive gas giant planet is that first a smaller planet forms, by the merging of even smaller bodies of rock or ice, called "planetesimals" (these are the size of the smaller asteroids, or comets). The growing planet must achieve above 10 times the mass of the Earth while the protoplanetary disk (out of which both star and planets form) still contains its primordial gas (hydrogen and helium). If so, the planet can attract this gas and quickly grow into a gas giant. It is unclear if there is a limit to how large it can grow thereafter. Of course, if it grows past 13 Jupiter masses it can begin fusion, and become a brown dwarf companion to the star. But some astronomers would say it is still a planet because of its mode of birth. They would base the fundamental definition on the mode of formation: a planet is built from planetesimals.

Brown dwarf companions, though, have usually been thought of as forming in the same way stellar companions do (and these are quite common). They do not go to the trouble of building up from small objects. A large denser region of gas collapses under its own weight, and directly makes the object. We see that stellar companions (which we think form this way) often do not have circular orbits, while circular orbits prevail in our Solar System. That had been interpreted as a natural outcome of the planetesimal merger scenario above. It was therefore a surprise to find that most of the extrasolar planets have non-circular orbits. A few iconoclasts have even suggested that therefore the extrasolar planets are misnamed. In fact, our formation theories are nowhere close to being sure about what the mass limits for either mode of formation (direct or merger) are, nor whether circular orbits should result. Furthermore, the orbits we see today may well not be the original ones. Both the remaining disk and any other reasonably massive planets in the system can alter them during the early history.

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