The least known main series star in the Milky Way galaxy is a true pixie of a thing.
It is called EBLM J0555-57Ab, a red dwarf that is 600 light-years away. With an average radius of about 59,000 kilometers, it is only larger than Saturn. This makes it the smallest known star that supports hydrogen fusion in its core, the process that causes stars to burn until the fuel runs out.
In our solar system there are two objects larger than this teenage star. The one, of course, is the sun. The other is Jupiter, like a giant ice-spoon, which enters with an average radius of 69,911 kilometers.
Why then is Jupiter a planet and not a star?
The short answer is simple: Jupiter does not have enough mass to melt hydrogen in helium. EBLM J0555-57Ab is about 85 times the mass of Jupiter, about as light as a star can be – if it were lower, it would not be able to melt hydrogen either. But if our solar system were different, would Jupiter be able to ignite in a star?
Jupiter and the Sun are more the same as you know
The host giant may not be a star, but Jupiter is still a big deal. The mass is 2.5 times the mass of all the other planets combined. Because it is a gas giant, it has a really low density: about 1.33 grams per cubic centimeter; The Earth’s density, 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 to each other. By mass, the sun is about 71 percent hydrogen and 27 percent helium, while the rest consists of trace amounts of other elements. By mass, Jupiter is about 73 percent hydrogen and 24 percent helium.
Illustration of Jupiter and its moon Io. (NASA’s Goddard Space Flight Center / CI Lab)
That is why Jupiter is sometimes called a failed star.
But it is still unlikely that Jupiter, just on the solar system’s own equipment, would even be close to a star.
You see stars and planets being born by two different mechanisms. Stars are born when a dense material knot in an interstellar molecular cloud collapses under its own gravity – poof! flomph! – rotate as it goes in a process that collapses cloud. As it rotates, it washes more material out of the cloud around it in a stellar growth disk.
As the mass – and therefore gravity – grows, the core of the baby star becomes tighter and tighter, causing it to become hotter and hotter. Eventually it becomes so compressed and hot that the nucleus ignites and thermonuclear fusion begins.
According to our understanding of star formation, there is a whole lot of accretion disk left once the star has finished making material. This is what the planets are made of.
Astronomers think that this process (which accumulates pebbles) for gas giants like Jupiter begins with small pieces of icy rock and dust in the disk. As they orbit the baby star, these pieces of material collide with static electricity. Eventually, these growing clusters reach a size large enough – about 10 Earth masses – that their gravity can draw more and more gas from the surrounding disk.
From that point, Jupiter gradually grew to its current mass – about 318 times the mass of the earth and 0.001 times the mass of the sun. After swallowing up all the material available – completely removing the mass needed to fuse hydrogen – it stopped growing.
So Jupiter was never even close to becoming massive enough to become a star. Jupiter has a similar composition to the Sun, not because it was a ‘failed star’, but because it was born from the same cloud of molecular gas that the Sun gave birth to.
(NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran / Flickr / CC-BY-2.0)
The true failed stars
There is another class of objects that can be considered ‘failed stars’. These are the brown dwarfs, and they fill the gap between gas giants and stars.
From about 13 times the mass of Jupiter, these objects are massive enough to support nuclear fusion – not of normal hydrogen, but of deuterium. It is also known as ‘heavy’ hydrogen; it is an isotope of hydrogen with a proton and a neutron in the nucleus instead of just a single proton. The melting temperature and its pressure are lower than the melting temperature and the 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 are still accreting mass. But some objects never reach the mass; these are known as brown dwarfs.
After their existence was confirmed in 1995, it was unknown whether brown dwarfs were stars or overambitious planets; but several studies have shown that they form like stars, from cloud collapse rather than nuclear wax. And some brown dwarfs are even below the mass for burning deuterium, and it is indistinguishable from planets.
Jupiter is right on the lower mass limit for cloud fall; the smallest mass of a cloud-collapse object is estimated at about one Jupiter mass. Thus, if Jupiter formed from the collapse of the cloud, it can be considered a failed star.
But data from NASA’s Juno probe suggests that Jupiter, at least once, had a solid core – and this is more in line with the method of forming nuclear growth.
Modeling suggests that the upper limit for a planetary mass, formed by nuclear growth, is less than ten times the mass of Jupiter – only a few Jupiter masses are shy of deuterium fusion.
So, Jupiter is not a failed star. But when we think about why it is not one, it can help us better understand how the cosmos works. In addition, Jupiter is a striped, stormy, winding miracle herb in its own right. And without it, our people could not even exist.
However, this is a different story that needs to be told another time.