The smallest known star in the main sequence in the Milky Way galaxy is a true pixie of a thing.
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 smidge larger than Saturn. This makes it the smallest known star that supports the fusion of hydrogen in its nucleus, the process that keeps the stars burning until they run out of fuel.
In our solar system, there is Two objects larger than this small star. One is the Sun, obviously. The other is Jupiter, like a huge spoonful of ice cream, which comes 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 into helium. EBLM J0555-57Ab is about 85 times larger than Jupiter’s mass, about as light as a star – if it were smaller, it would not be able to fuse hydrogen. But if our solar system were different, could Jupiter have ignited in a star?
Jupiter and the Sun are more alike than you know
The gas giant may not be a star, but Jupiter is still a Big Deal. Its mass is 2.5 times greater than that of the other combined planets. It’s just that, being a gas giant, it has a very low density: around 1.33 grams per cubic centimeter; The density of the Earth, at 5.51 grams per cubic centimeter, is just over four times higher than 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 compositionally. In mass, the Sun is about 71% hydrogen and 27% helium, the rest being made up of traces of other elements. Jupiter’s mass is about 73% hydrogen and 24% helium.
Illustration of Jupiter and its moon Io. (NASA’s Goddard Space Flight Center / CI Laboratory)
For this reason, Jupiter is sometimes called a failed star.
But it is still unlikely that, left at the disposal of the solar system, Jupiter will even become close to being a star.
You see, stars and planets are born through two very different mechanisms. Stars are born when a dense node of material in an interstellar molecular cloud collapses under its own gravity – fluff! flomph! – rotation as it takes place in a process called cloud collapse. As it rotates, it rotates in more material from the cloud around it in a stellar disk.
As the mass – and therefore gravity – increases, the core of the star is tightening more and more tightly, which makes it grow stronger. Eventually it becomes so compressed and hot that the core ignites and thermonuclear fusion begins.
According to our understanding of star formation, once the star has finished accumulating material, a lot of accumulation disk remains. This is what the planets are made of.
Astronomers believe that for gas giants like Jupiter, this process (called gravel accumulation) begins with small pieces of frozen rock and dust in the disk. As the baby’s star orbits, these pieces of material begin to collide, sticking together with static electricity. Eventually, these growing agglomerations reach a sufficiently large size – about 10 Earth masses – that they can gravitationally attract more and more gas from the surrounding disk.
Since then, Jupiter has risen gradually to its current mass – about 318 times the mass of the Earth and 0.001 times the mass of the Sun. Once all the available material escaped – at a fairly large distance from the mass needed for hydrogen fusion – it stopped growing.
So Jupiter was never close to growing massive 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 gases that gave rise 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 they fill that gap between the gas giants and the stars.
Starting at about 13 times the mass of Jupiter, these objects are massive enough to support the fusion of the nucleus – not normal hydrogen, but deuterium. This is also known as “heavy” hydrogen; is an isotope of hydrogen with one proton and one neutron in the nucleus instead of one proton. The 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 that mass; these are known as brown dwarfs.
Some time after their existence was confirmed in 1995, it was not known whether the brown dwarfs were underwater stars or too ambitious planets; but several studies have shown that they form like stars, from the collapse of clouds rather than the accumulation of the nucleus. And some brown dwarfs are right under the table for burning deuterium, indistinguishable from planets.
Jupiter is right on the lower mass limit for the collapse of the clouds; the smallest mass of a collapsing object was estimated at about one mass by Jupiter. So, if Jupiter had formed from the collapse of the clouds, it could be considered a failed star.
But data from NASA’s Juno spacecraft suggests that, at least once, Jupiter had a solid core – and this is more consistent with the method of forming the base accumulation.
Modeling suggests that the upper limit for a mass of the planet, which is formed by basic accumulation, is less than 10 times the mass of Jupiter – only a few masses of Jupiter shy of deuterium fusion.
So Jupiter is not a failed star. But thinking about why it is not one can help us better understand how the cosmos works. In addition, Jupiter is a miracle with stripes, storm and belly. And without him, we humans might not even have existed.
However, this is another story, to be told another time.