Why Is a Planet Not a Star?

When you look up at the night sky, amongst all of the twinkling stars, airplane lights, and the satellites marching across the sky, the steady light of a planet shines unblinkingly at you—seemingly staring you in the face. It’s possible to distinguish the planets from the stars using only your eyes, mostly because planets don’t “twinkle,” and stars do (that’s a topic for a different post). But why are these little points of light different? And why should we even care?

The answer is all a matter of size. Mass loves mass, thanks to gravity.

Stars form in regions named molecular clouds, which are often called “stellar nurseries” and are essentially patches of mostly gas and dust floating in space. When this gas and dust gets close together, gravity starts to attract particles to each other, which causes part of the cloud to collapse into a dense ball. This collapse releases energy, which makes the temperature rise rapidly in what is now considered a protostar (a star-to-be). At this point, when this mass (a lot of mass—roughly equivalent to 80 Jupiters, at a minimum!) is getting squished together by its own gravity, the temperature and pressure get so high that hydrogen atoms come together to form a new element, helium, through a process called nuclear fusion. This fusion reaction ignites a cascading chain of reactions, which continue to emit even more heat and light, which is why stars are bright. Nuclear fusion continues to power the star throughout its lifetime.

But what happens if there isn’t enough mass for nuclear fusion to start?

Well, that’s when we get planets. Planetary bodies come in many different sizes even within our own Solar System, from tiny Mercury (just 5% the mass of Earth) all the way to Jupiter (318 Earth masses). Like stars, planets also formed from clumps of mass joining up with other clumps of mass. Unlike stars though, they never got massive enough for nuclear fusion to start. Planets are formed in the remnant disk of gas and dust that swirls around a protostar, called a protoplanetary disk. As this dust orbits the new star, dust grains bump into each other and form clumps. Clumps bump into and merge with other clumps, and continue to collect material as they grow, much like a snowball collecting snow as it rolls down a hill. Through this process, a planet clears out the material in its path, establishing its own orbit around its host star. This is also why small objects like Pluto are not considered planets, because they were not able to clear out their own orbits! Pluto resides in the Kuiper belt, where it shares its orbit with other similarly-sized objects.

However, as often happens when we humans try to divide the natural world up into neat little categories, it resists.

Not everything can fit nicely into our two boxes of “planet” or “star”. Astronomers have observed classes of objects that are not shining like stars but are also not orbiting a star (or anything else for that matter). What are they? Are these stars or planets?  Turns out, some of these dim objects fall into a mass range (13 – 80 times the mass of Jupiter), where they once were able to support a little bit of fusion but couldn’t sustain it and quickly died out. Astronomers call these ‘failed stars’ brown dwarves for their small size and dim appearance. The even smaller objects (less than 13 Jupiter-masses) floating out on their own have never been big enough to ignite fusion and get their taste of star-dom. These fascinating and hard-to-see objects are dramatically called rogue planets, and as their name suggests, seem to be hanging out in space all alone. Some may have formed in the same manner as a star (in a molecular cloud, but without igniting fusion), while others may have formed around a star and then later got ejected from their home system.

But really, why do we care what these balls of gas and dust are called? If stars and planets are just different sizes of the same interstellar stuff, and if not everything fits neatly into those two categories, why do we even try?  Well, even if there is a squishy grey area in between being a planet and being a star, there is still a big difference in what the end result is. In our search for life in the universe, we can be fairly certain that biological activity is better suited to planets, rather than stars. And if visiting a star would be like hanging out inside of a giant thermonuclear bomb, I’d personally take a planet any day.