Currently proposed ways to distinguish between a really big planet and a really small star

Question

The Next Big Future article Rogue Exoplanet 12.7 times bigger than Jupiter is 20 light years away

Astronomers using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have made the first radio-telescope detection of a planetary-mass object beyond our Solar System. They found a rogue planet 12.7 times the mass of Jupiter twenty light years from earth.

It is at the boundary of Jovian objects and Brown dwarf stars.

[...]

Last year, an independent team of scientists discovered that SIMP J01365663+0933473 was part of a very young group of stars. Its young age meant that it was in fact so much less massive that it could be a free-floating planet — only 12.7 times more massive than Jupiter, with a radius 1.22 times that of Jupiter. At 200 million years old and 20 light-years from Earth, the object has a surface temperature of about 825 degrees Celsius, or more than 1500 degrees Fahrenheit. By comparison, the Sun’s surface temperature is about 5,500 degrees Celsius.

The difference between a gas giant planet and a brown dwarf remains hotly debated among astronomers, but one rule of thumb that astronomers use is the mass below which deuterium fusion ceases, known as the “deuterium-burning limit”, around 13 Jupiter masses.

In the spirit of

• Do astronomers generally agree that the distinction between comets and astroids is not so clear?
• Is this object an asteroid or comet, and how can it produce so many tails?

and the relatively recent reclassification of Pluto as a dwarf planet and not a planet proper, I'd like to ask what are the ways that have are currently proposed and given consideration to distinguish between a really big planet and a really small star?

Is the "rule of thumb" proposed in the block quote the leading contender or are there other definitions that receive a similar amount of support

Answer 1

The dividing line between star and brown dwarf is the mass at which hydrogen fusion via the pp-chain occurs at its core. A brown dwarf below this mass limit will never reach a sustainable equilibrium due to this reaction and will continue to cool as it gets older. A star will reach the long-lived main sequence.

The dividing line between planet and brown dwarf is much less well defined. Some want the definition to be mass-based, and there is a mass-based dividing line at about 13 Jupiter masses between brown dwarfs that fuse deuterium in their interiors (which provides only a brief hiatus in their cooling) and "planets" that do not.

Others argue that any definition of a planets should be based on whether they formed around a star, but this is problematic, because such objects may then be disrupted or escape and also it is not clear where a binary brown dwarf becomes a star plus planet.

The reason that some campaign for a different definition is that the composition and structure of a planet formed around a star could be different to that of a planet that collapses from a small gas cloud. According to the "core accretion model" for gas giant formation, the core of such a planet would consist of solid ice or rocky material, whereas it would be gas all the way down in a less massive version of a brown dwarf. However, the waters are further muddied by ideas that large planets can also form by direct collapse in the disc of a forming planetary system.

Answer 2

Like in each of the other cases you mention, when dividing really big planets from really small stars you're drawing a bright line across a continuous range of objects. Whenever you do that, you're going to get legitimate and reasonable disagreement, because different people with different purposes will see a different place to draw the (ultimately arbitrary) line as most useful.

(Emphasis: This is not to say that drawing such lines is a useless business. Human beings need categories to think clearly and to communicate productively. We need those subdivisions. We just don't always agree on exactly what they ought to be.)

Looking at stars vs. planets, there are basically two dimensions to consider: Mass and metallicity. If you look at bodies which are nothing but primordial H and He, as you go down in mass at some point you read a mass where the H didn't ignite and at an even lower mass, you reach a point where the D didn't ignite. In bodies less massive than that, no fusion ever took place. Since fusion (and the resultant high temperature and emission of radiation) is the outstanding characteristic of stars, it's very natural to say "stars are massive bodies which sustained fusion at some point in their life; planets are those that never did."

If you study the theories of star and planet formation, you see that ignition depends pretty much entirely on the object's mass. Unless the mass is large enough, conditions for fusion don't happen. (We're talking natural objects condensing out of nebulae here -- smaller fusing masses could be constructed with sufficient effort.)

When you add the second dimension of metallicity (basically how much of elements heavier than He are present) you discover that it doesn't make a huge difference. There doesn't seem to be a natural way for planets to get to the ignition mass without being mostly H and He, and these planets behave almost as if they had no heavier elements.

So while have found a continuum of objects out there ranging from pebbles to blue-white super-giants, the deuterium ignition temperature is for most people a handy place to use as the border between categories.

Note that this is not always going to be right. We can, for instance, imagine objects that are very old and have fused deuterium, but whose surfaces have cooled to the point of being solid and are made of ices. This would be a star by the above definition, but would be much more like a planet for anyone interested in studying its surface. (Kinda high gravity to visit, though!) Here the D-fusion dividing line would be ill-chosen, and a dividing line based on the nature of the surface would be better -- perhaps dividing large bodies in space into categories with surfaces that are solid or liquid from those with surfaces that are gaseous.

Also note that being hot and radiating isn't a good dividing line, since the heat of formation -- gravitational energy released during formation -- makes every massive object start hot.