(ORDO NEWS) — The smallest known main sequence star in the Milky Way galaxy is called EBLM J0555-57Ab.
The red dwarf is located 600 light-years away. With an average radius of about 59,000 kilometers, it is only slightly larger than Saturn. This makes it the smallest star known to support hydrogen fusion in its core, a process that keeps stars burning until they run out of fuel.
There are two objects in our solar system larger than this tiny star. Obviously one of them is the Sun. The other is Jupiter, with an average radius of 69,911 kilometers.
So why is Jupiter a planet and not a star?
The short answer is simple: Jupiter doesn’t have enough mass to convert hydrogen into helium. EBLM J0555-57Ab is about 85 times the mass of Jupiter. But if our solar system were different, could Jupiter turn into a star?
Jupiter and the Sun are more similar than you think.
The gas giant may not be a star, but Jupiter still matters a lot. Its mass is 2.5 times the mass of all other planets combined. It’s just that, being a gas giant, it has a really low density: about 1.33 grams per cubic centimeter; The density of the Earth is 5.51 grams per cubic centimeter, which 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 these two objects are very similar in composition. By mass, the Sun is about 71% hydrogen and 27% helium, with the rest made up of trace amounts of other elements. Jupiter is roughly 73 percent hydrogen and 24 percent helium by mass.
For this reason, Jupiter is sometimes referred to as a failed star.
But it is still unlikely that, left behind by the solar system’s own arrangements, Jupiter would even come close to becoming a star.
You see, stars and planets are born through two very different mechanisms. Stars are born when a dense knot of matter in an interstellar molecular cloud collapses under its own gravity – spinning in a process called cloud collapse. As it rotates, the star rolls more material from the cloud around it into the stellar accretion disk.
As mass – and therefore gravity – grows, the core of the young star shrinks harder and harder, causing it to get hotter and hotter. Eventually it becomes so compressed and hot that it ignites and fusion begins.
According to our understanding of star formation, when a star finishes accreting material, an accretion disk remains. This is what the planets are from.
Astronomers think gas giants like Jupiter begin this process with tiny pieces of icy rock and dust in the disk. As they orbit a young star, these pieces of material begin to collide, sticking together with static electricity. Eventually, these growing clumps grow large enough – about 10 Earth masses – so that they can gravitationally pull more and more gas from the surrounding disk.
From that point on, Jupiter gradually grew to its current mass – about 318 times the mass of the Earth and 0.001 times the mass of the Sun. Once it absorbed all the material available to it – largely of the mass needed to synthesize hydrogen – it stopped growing.
So Jupiter never came close to becoming massive enough to become a star. Jupiter is similar in composition to the Sun, not because it was a “failed star,” but because it was born from the same cloud of molecular gas that gave birth to the Sun.
Real frustrated stars.
There is another class of objects that can be considered “failed stars.” They are brown dwarfs and fill the gap between gas giants and stars.
Starting at about 13 times the mass of Jupiter, these objects are massive enough to support nuclear fusion – not ordinary hydrogen, but deuterium. It is also known as “heavy” hydrogen; it is an isotope of hydrogen with a proton and neutron in the nucleus instead of one proton. Its melting temperature and pressure is lower than the melting temperature and pressure of hydrogen.
Because it happens at lower mass, temperature, and pressure, deuterium fusion is an intermediate step on the path to hydrogen fusion for stars as they continue to build up mass. But some objects never reach this mass; they are known as brown dwarfs.
For some time after confirming their existence in 1995, it was not known whether the brown dwarfs were stars or overly ambitious planets; but several studies have shown that they form in exactly the same way as stars, as a result of cloud collapse rather than core accretion. And some brown dwarfs are even indistinguishable from planets.
Jupiter is right at the lower mass limit for cloud collapse; The smallest mass of a cloud collapse object is estimated to be approximately one mass of Jupiter. So if Jupiter was formed as a result of the collapse of a cloud, it could be considered a failed star.
But data from NASA’s Juno probe suggests that at least Jupiter once had a solid core – and this is more in line with the method of nucleus accretion formation.
Modeling suggests that the upper limit for the mass of a planet resulting from core accretion is less than 10 times that of Jupiter – just a few Jupiter masses, which is not enough for deuterium fusion.
So Jupiter is not a failed star. But thinking about why this is not the case can help us better understand how the cosmos works. Plus, Jupiter is a striped, turbulent, swirling miracle in and of itself. And without him, we humans might not even be able to exist.
However, this is another story that should be told another time.
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