Is Jupiter a failed star?
I study how stars and planets form, which is a big part of the ways that Jupiter both is and isn’t like a star.
We can consider Jupiter to look somewhat like a “failed star” because it actually has a chemical composition that is very similar to the Sun. Jupiter is mostly hydrogen and helium, with only a small fraction of any heavier elements. This means that if Jupiter was heavier, then it could undergo fusion (the source of energy in the Sun) and radiate its own light. However, Jupiter is too light, and its central temperature never got high enough for hydrogen fusion to start.
(I’ll note that Jupiter actually looks quite a bit different from stars, but that’s because it’s so cool that a little bit of the hydrogen goes into making molecules like ammonia (NH3), methane (CH4), and even heavier hydrocarbons. The Sun is so hot that almost all molecules get broken up, so you only have hydrogen and other atoms.)
The fact that Jupiter has the same composition as the Sun actually tells us something interesting about how it formed – that is, it must have formed at the same time as the Sun, out of the same cloud of gas. However, that brings us to the key difference between stars (like the Sun) and planets (like Jupiter). When a star (and its planets) form, the whole cloud collapses. Due to the conservation of “angular momentum” (i.e., the rate the material is spinning), the collapsing cloud spins up, and some of the material can’t fall all the way down to the star. This material instead becomes a disk orbiting around the star, and planets condense out of that disk. I think the best way of distinguishing between stars and planets is to ask if they were the central object in one of the big condensing blobs, or formed in the disk around some (much more massive) central object.
It’s worth noting that this can be a subtle distinction! We see many objects floating around out in the Milky Way that are actually quite small, such that they also never ignited fusion. We call these “brown dwarfs”. An emerging result in astronomy is to realize that you can make those objects in the same way as a star, that have a mass that is maybe even as low as Jupiter’s. So these objects would look totally identical to Jupiter, but perhaps they really should be regarded as “star-like objects” and not “planet-like objects”.
This causes some arguments among astronomers, because we don’t know which term to use for them! We’re still working out whether to define a “planet” as any object less than some critical mass (which we can at least measure), or to try defining it based on how they form (which is more scientific, but often hard to determine). I lean toward the second option, but stay tuned, because the answer to this question could continue to change in the future!
Prof. Adam Kraus
University of Texas at Austin