Why isn't Jupiter a star?
In our Solar System, there are two objects larger than this young star. One is the Sun, of course. The other is Jupiter, which has an average radius of 69,911 kilometers. So why is Jupiter a planet and not a star? The short answer is simple: Jupiter does not have enough mass to fuse hydrogen into helium. EBLM J0555-57Ab is approximately 85 times the mass of Jupiter. But if our Solar System had been different, could Jupiter have ignited in a star?
The gas giant may not be a star, but Jupiter is still a big deal. Its mass is 2.5 times that of all other planets combined. Except that, being a gas giant, it has a very low density: about 1.33 grams per cubic centimeter; The Earth's density, at 5.51 grams per cubic centimeter, is just over four times that of Jupiter. But it is interesting to note the similarities between Jupiter and the Sun. The density of the Sun is 1.41 grams per cubic centimeter. And the two objects are very similar from a compositional point of view. By mass, the Sun is about 71% hydrogen and 27% helium, while the rest is made up of traces of other elements. Jupiter by mass is approximately 73% hydrogen and 24% helium.
It is for this reason that Jupiter is sometimes called a failed star. Stars and planets are born through two very different mechanisms. Stars are born when a dense knot of material in an interstellar molecular cloud collapses under its own gravity by spinning as it starts in a process called cloud collapse. As it spins, it wraps more material from the cloud around it in a stellar accretion disk.
Astronomers think that, for gas giants like Jupiter, this process begins with small pieces of icy rock and dust in the disk. As they orbit around the newborn star, these fragments of material begin to collide, sticking together thanks to static electricity. Eventually, these growing clumps reach a size large enough, about 10 Earth's masses, that they can gravitationally attract more and more gas from the surrounding disk.
Starting at over 13 times the mass of Jupiter, these objects are massive enough to support core fusion, not normal hydrogen, but of deuterium. This is also known as "heavy" hydrogen; it is an isotope of hydrogen with a proton and a neutron in the nucleus instead of just one proton. Its melting temperature and pressure are lower than the melting temperature and pressure of hydrogen. Because it occurs at a lower mass, temperature and pressure, deuterium fusion is an intermediate step on the road to hydrogen fusion for stars, as they continue to increase mass. But some objects never reach that mass; these are known as brown dwarfs.
For a while after their existence was confirmed in 1995, it was not known whether brown dwarfs were undersized stars or overly ambitious planets; but several studies have shown that they are formed just like stars, from the collapse of clouds rather than from the accretion of the core. And some brown dwarfs are even below the mass for deuterium burning, indistinguishable from planets.
Jupiter is right on the lower mass limit for cloud collapse; the smallest mass of a collapsing cloud object has been estimated to be about one mass of Jupiter. So, if Jupiter had formed from the collapse of the cloud, it could be considered a failed star. But data from NASA's Juno spacecraft suggests that, at least for a time, Jupiter had a solid core and this is more consistent with the method of formation of the nucleus accretion. The modeling suggests that the upper limit for the mass of a planet, which is formed through the accretion of the nucleus, is less than 10 times the mass of Jupiter. Hence, Jupiter is not a failed star. But thinking about why it isn't can help us better understand how the cosmos works. Without it, we humans may not even have been able to exist.
