Today I would like to discuss the process of star and planet formation while raising certain questions concerning the nomenclature and definitions of certain categories of objects.

The dividing line between a red dwarf (the dimmest, least massive main-sequence stars) and a brown dwarf or “failed star” is definite and easy to define. It is simply a function of how much mass a star must have before its center has the requisite temperature and pressure to produce the fusion reaction of the common isotope of hydrogen. There is a small variation in the required mass, according to the exact ratio of hydrogen to helium, or hydrogen to oxygen, etc, but that is a minor detail. The planet, Jupiter, by the way, would have to have 75 times more mass to qualify as a feeble red dwarf.

In a brown dwarf star, the temperature and pressure in the center will be high enough for the fusion reaction of deuterium (heavy hydrogen) to occur for a brief span of time (brief, in astronomical terms), but not high enough for the most common isotope of hydrogen to undergo fusion. Jupiter would need to increase its mass by a factor of 13 before deuterium fusion could take place in its core, thus it is far from being a brown dwarf star.

According to stellar formation theory, stars form in the center of large disk-like structures and planets form in the disks surrounding these nascent suns. This theory is supported by numerous observations. Planets, at least small rocky ones like Earth, Mercury, Venus, and Mars, the so-called terrestrial planets, are slowly formed by accretion. That is to say very small particles were stuck together by cohesive forces, and gradually more and more material was stuck together until the proto-planet was massive enough for its own gravity to pull in material from the disk.

In a binary star system, two stars form from the same dense cloud, each with its own disk. But can a small star form from an instability or vortex in the accretion disk of a larger star or can it only form from a separate accretion disk?

Did Jupiter form from an instability or vortex in the circumstellar disk of the young sun, or by accretion of disk material, the way the smaller planets in our solar system most certainly did? Or perhaps it began its life as an instability in the circumstellar disk, and began accreting material after it attained a large enough mass to pull in significant amounts of matter.

Is there a continuum of objects ranging from large brown dwarfs to small brown dwarfs, proceeding down to large planets and then small ones? Is there a maximum size that a planet can attain, limited by accretion rates and circumstellar disk density, etc? Is there a minimum size for an object, ie. a star, that forms at the center of such a disk?

Jupiter may well have begun life as an instability in the circumstellar disk from which our solar system formed. It may have had its own disk surrounding it during its early years. Having its own disk or dense rings is possible whether it formed as a disk instability or by accretion. For that matter, the Earth may have been surrounded by a disk or rings in its early formative years. How could we ever know?

Perhaps the best evidence that Jupiter began its formation as an instability in the sun’s circumstellar disk is the discovery that the disks surrounding young stars do not last long. While studying star clusters that are demonstrably not more than 5-million years old, astronomers have discovered that stars with a mass equal or greater to our sun have lost their disks. That is highly significant. That means that planets only have a window of time of approximately 5-million years in which to form. This means that Jupiter had to attain a lot of mass in a hurry. It would not have been able to do so by accretion alone. A planet can form much faster if it begins its formation as an instability in a circumstellar disk. Accretion is a very slow process. It makes sense that Jupiter, at least initially, began its formation from an instability in the sun’s circumstellar disk. This would explain why Jupiter is so big. It got a head start. It may have begun as a disk instability and once it reached a certain mass, began to accrete any material that came within a certain range.

Let me back up a minute. There is a generally accepted definition of brown dwarf. It is an object that is not massive enough for ordinary hydrogen fusion to have ever occurred in its core, but massive enough for deuterium fusion to have occurred in its core at some point in its life. Deuterium, an unstable isotope of hydrogen, will undergo nuclear fusion at a much lower temperature and pressure than regular hydrogen. At some point in its life, a brown dwarf will experience deuterium fusion. If it is not massive for this to ever occur then it is not a brown dwarf, but is a planet. Jupiter would have to have 13 times more mass to be able to ignite deuterium fusion in its core. Does that mean that 13 Jupiter masses is the minimum that an object can have, and qualify as a brown dwarf? The other factor is the composition of the object. If there are a lot of heavier elements in the object, then it would have to be more massive before deuterium fusion can take place.

The point I am making here is that there are two competing theories on how gas giant planets form. Do they form from accretion as terrestrial planets do (There is no other possible explanation for terrestrial (earth-like) planets) or do they form from instabilities in the circumstellar disk? Or is it some combination of both processes? Of course it could be some combination of both models.

An important factor is that giant planet formation by the disk instability model is totally independent of relative elemental abundance. However, planet formation from accretion is obviously very dependent on having lots of elements heavier than hydrogen and helium. You must have dust first, which accretes into little rocks, then bigger ones, then planetesimals, and so on. If giant planets start out from accretion, then they should be found preferentially around stars with an abundance of heavier elements, rather than around older stars which lack those elements. A spectroscopic survey of the stars that host huge planets has revealed something important. They are almost exclusively stars with a heavy element content higher than the average star. This is strong evidence for the accretion model. The few exceptions to this strong trend do throw up a red flag of caution however. How could there be any exceptions? Maybe sometimes giant planets can sometimes form from disk instabilities, but usually they need some hard material to start out with. Maybe. Nonetheless, the statistics are very significant.

One thing the disk instability theory has going for it is time. It takes millions of years to form a gas giant planet from accretion alone. An unstable region in a circumstellar disk can theoretically form into a gas giant in a few hundred years. As noted above, circumstellar disks typically don’t last very long. Young fledgling stars in the throes of birth emit a fierce wind, blowing away the disk. Even worse, stars form in huge clouds, typically in groups of hundreds, and there are usually one or more supergiants in these star-forming regions. The ultraviolet light from these stars rapidly evaporates the circumstellar disks of nearby nascent solar systems. Not only the UV light, but also the incredible winds from these stars, will destroy the disks of nearby baby solar systems in a few million years. The wind from these stars is absolutely astounding. Some of the really big ones, from stellar wind alone, can lose enough mass to make many stars like our sun, before they go supernova. Now we come to another question. What would a nearby supernova do to a protoplanetary disk, i.e. circumstellar disk, surrounding a nearby star? One can imagine.

You might wonder why our solar system is not surrounded by its sibling stars if stars are created by the hundreds in huge clouds. Galactic tidal forces gradually disperse the stars and spread them apart. The leftover gas in the clouds, gradually dissipates.

There are many unanswered questions concerning the details of star and planet formation, though the broad outlines of the process are well understood. New discoveries are coming in at a breathless pace.