The smallest main sequence star known in the Milky Way galaxy is a true pixie of something.
It’s called EBLM J0555-57Ab, a red dwarf 600 light-years away. With an average radius of about 59,000 kilometers, it is only a larger surface than Saturn. This makes it the smallest star known to support hydrogen fusion in its core, the process that keeps stars burned until they run out of fuel.
In our solar system, there are two objects larger than this small star. One is the Sun, obviously. The other is Jupiter, like a giant spoon of ice cream, which enters with 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 with helium. EBLM J0555-57Ab is about 85 times the mass of Jupiter, almost as light as a star can be; if it were lower, it would also not be able to fuse hydrogen. But if our solar system had been different, could Jupiter have lit up a star?
Jupiter and the Sun are more similar than you know
The gas giant may not be a star, but Jupiter is still big business. Its mass is 2.5 times that of the other combined planets. Is that, being a gas giant, has a really low density: about 1.33 grams per cubic centimeter; The density of the Earth, 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 in composition. In mass, the Sun has about 71% hydrogen and 27% helium, and the rest is made up of traces of other elements. Jupiter in mass has approximately 73% hydrogen and 24% helium.
Illustration of Jupiter and its moon Io. (Goddard Space Flight Center / CI Lab) from NASA
It is for this reason that Jupiter is sometimes called a failed star.
But it is still unlikely that, left by the very devices of the solar system, Jupiter would even come close to being a star.
You see, 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 – pouf! flomph! – rotating as it happens in a process called cloud collapse. As it rotates, it surrounds more cloud material that surrounds it in a stellar accretion disk.
As the mass, and therefore gravity, grows, the baby’s star core narrows more and more, which makes it hotter and hotter. Eventually, it becomes so compressed and hot, that the nucleus ignites and starts thermonuclear fusion.
According to our understanding of star formation, once the star has finished accumulating material, a whole accretion disk is left over. This is what the planets are made of.
Astronomers think that for gas giants like Jupiter, this process (called pebble accretion) begins with small pieces of icy rock and dust on the disk. As they orbit around the baby’s star, these pieces of material begin to collide and stick with static electricity. Finally, these growing groups reach a size large enough (about 10 Earth masses) that they can gravitationally attract more and more gas from the surrounding disk.
From that moment on, Jupiter grew to its current mass, about 318 times the mass of the Earth and 0.001 times the mass of the Sun. Once he had razed all the material he had at his disposal (with a high removal of the mass needed for hydrogen fusion), he stopped growing.
Therefore, Jupiter was never about to grow massively enough to become a star. Jupiter has a composition similar 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.
(NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran / Flickr / CC-BY-2.0)
The real failed stars
There is a different class of objects that can be considered “failed stars.” These are brown dwarfs and fill that gap between gas giants and stars.
Starting at about 13 times the mass of Jupiter, these objects are massive enough to support the fusion of the nucleus, not of normal hydrogen, but of deuterium. This is also known as “heavy” hydrogen; is an isotope of hydrogen with one proton and one neutron in the nucleus instead of a single 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 path to hydrogen fusion for stars as they continue to accumulate mass. But some objects never reach this mass; they are known as brown dwarfs.
For a time after its existence was confirmed in 1995, it was unknown whether brown dwarfs were low-achieving stars or too ambitious planets; but several studies have shown that they form just like stars, from the collapse of clouds and not the accretion of the nucleus. And some brown dwarfs are even below the mass for deuterium burning, which is indistinguishable from the planets.
Jupiter is just at the lower mass limit for the collapse of the cloud; the smallest mass of a cloud collapse object has been estimated at about one mass of Jupiter. Therefore, if Jupiter had formed from the collapse of the clouds, it could be considered a failed star.
But data from NASA’s Juno spacecraft suggest that, at least once, Jupiter had a solid core, and this is more consistent with the method of nucleus accretion formation.
The modeling suggests that the upper limit for a planetary mass, which is formed by the accretion of the nucleus, is less than ten times the mass of Jupiter, only a few masses of Jupiter shy of deuterium fusion.
Therefore, Jupiter is not a failed star. But thinking why it is not can help us better understand how the cosmos works. In addition, Jupiter is a marvel of buttery stripes, stormy and smooth. And without it, humans may not even have existed.
This, however, is another story, to tell again.