“When is a planet a star?”

After 400 years of gazing at the sky through telescopes, you would come to assume that astronomers would have it more or less sorted when it comes to what planets are and what defines them. However, there is actually a slight mystery around this question and it is not as silly as it may sound.


Astronomy, being a science, is in a constant state of flux. It is an ever changing and evolving being, altering its perceptions and definitions due to new discoveries that challenge and change established ideals. (This is why science is so awesome; you never know what could be around the next corner!).

Growing up, we all knew that there were nine planets that orbited a Sun, which was a giant burning star that radiated heat from creating nuclear reactions.


It is pretty fair to state that there was no startling or defining discovery that served to change this orderly picture of our celestial world. Instead, it seemed to be more a gradual evolution of events that arose from subtle findings about the Solar System and the planetary families of other stars in the galactic neighborhood. In the early 70’s, the Pioneer 10 and 11 showed that Jupiter, what was once thought to be a planet that exerted no internal energy, actually gives off about 70% more energy than it receives from the sun when the measurements are extended to beyond the red end of the rainbow spectrum that measures visible light and into the heat radiation section.


This then raised the idea that perhaps planets did not only shine by reflect light (from the Sun) and in turn raised the further concept of how a giant gas ball like Jupiter could maybe be viewed as a failed star. That, maybe, it did not have enough nuclear reactions required to turn into something like our Sun. (more gray area, whooo!).

Let’s look at our Sun. Our Sun is an example of a genuine star, the central temperature of which is sustained by continuous H-Bomb like reactions in which Hydrogen (the raw material of all stars) is turned into Helium:

main sequence star

This is a process called nuclear fusion where the small atomic nuclei of hydrogen bundle and form together tight to make bigger forms of Helium, energy is the byproduct that is released. This is how the Sun creates heat and light. These reactions begin at the stars birth when the hydrogen cloud that it is born from is compressed (getting smaller, which creates increased heat). You can think of it kind of like a bicycle pump, the pump will get hotter as it is compressed more and more.

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Astronomers don’t really like measuring things in a conventional way, instead they think of planets, stars and celestial objects in relation to the mass of Jupiter. The Sun, for example, is about 1000 times the mass of Jupiter (yes, 1000).



Let’s say that out there, there is a much smaller baby star. Instead of 1000 times the mass of Jupiter, this guy is only 75 Jupiter masses.

What do you think will happen?

The smaller size will result in reduced gravitation compression, yes.

The temperature definitely won’t be high enough for the hydrogen to start burning…

That results in no star… right?

Well, not quite.

If the baby star was between 13 and 75 Jupiter masses, a very special thing occurs. A less energetic type of nuclear reaction kicks in. the resulting star will shine, but only very dimly and in infrared rather than visible light.


This; is the birth of a Brown Dwarf.

Brown Dwarfs are like the Axolotls** of the Gas giants and true star world; they are at a curious half stage. This can be further shown, in studies recently showing that Brown Dwarves actually experience weather, something that is usually associated with planets.

Maybe, you are thinking “damn it is hot on that thing, way too hot to rain.” Well, it isn’t. Scientists have proven that it does indeed rain; though not a conventional boring H2O type situation, it rains drops of liquid iron.


There are other types of celestial wanders that further make us question what is a planet and what is a star. These are known as “rogue planets”, “orphan planets” or FFLOP’s (Free Floating Planetary mass objects). These guys are objects that contain less material than Brown Dwarves and are thought to have originated out of the collapse of the super cute little gas clouds along side the bigger ones that created their Brown siblings.

However, (there is always a however)

There is a more current theory, where FFLOP’s are formed within planetary systems like our Solar system but were ejected from their birthplace like some King’s bastard son. This happens by some kind of disturbance (in the force, haha), for example the gravity of a passing star or an aggressive encounter with a fellow planet (yes, they fight, it happens). Perhaps then, “banished star” is a more appropriate name.


(not quite, Dumbledore.)

So how then do these little wanderers shine?

They don’t have parent stars to reflect light from… they aren’t big enough for star like nuclear reactions to stir them to luminosity… So, what happens?

Radioactive decay happens.

The energy they possess comes from the decay of radioisotopes like Uranium. The glowing they emit is nothing more than geothermal energy; this might sound unlikely, but calculations show it is possible.

I am sure you are getting the point that things are far from straight forward when it comes to defining a star or a planet, and together with exoplanets (extra solar planets) they have raised the issue of what actually constitutes a planet as a matter of serious debate.

There have been many valiant attempts at highlighting the differences between Brown Dwarfs, planets and FFLOP’s but I think the best is this:

“Free floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium (13 Jupiter masses) are not ‘planets’, but are ‘sub brown dwarfs’ (or whatever name is most appropriate)”

In other words, you might as well call them bananas.


Your friendly Science lass,

Sami x

** Axolotls are a type of Salamander that has halted at an evolutionary stage of development (and are also cute as hell).