Jupiter is larger than some stars, so why haven’t we had a second sun?

The smallest star in the known main sequence of the Milky Way galaxy is a true sprite of a thing.

His name is EBLM J0555-57Ab, a red dwarf 600 light years away. With an average radius of about 59,000 kilometers, it’s just a little bigger than Saturn. This makes it the smallest star known to endure the fusion of hydrogen in its nucleus, the process that keeps stars on fire until they run out of fuel.

In our solar system there are of them objects larger than this little star. One is the sun, of course. The other is Jupiter, like a giant ball of ice, entering 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 the mass of Jupiter, about as light as a star can be – if it were lower, it also couldn’t fuse hydrogen. But if our solar system had been different, could Jupiter have ignited into a star?

Jupiter and the Sun are more alike than you think

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. It’s just that, being a gas giant, its density is really low: 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 in composition. By mass, the Sun is made up of about 71% hydrogen and 27% helium, the rest being made up of traces of other elements. Jupiter by mass is about 73% hydrogen and 24% helium.

Illustration of Jupiter and its moon Io. (NASA Goddard Space Flight Center / CI Lab)

It is for this reason that Jupiter is sometimes called a failed star.

But it is still unlikely that, left to the Solar System’s own devices, Jupiter will even come close to being a star.

The stars and the planets, you see, are born from two very different mechanisms. Stars are born when a dense node of matter in an interstellar molecular cloud collapses under its own gravity – poof! flomph! – run over time in a process called cloud collapse. As it spins, it coils in more material from the cloud surrounding it in a stellar accretion disk.

As the mass – and therefore gravity – increases, the core of the little star gets tighter and tighter, causing it to get 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, once the star has finished accreting material, there is a lot of accretion disc left. This is what the planets are made of.

Astronomers believe that for gas giants like Jupiter, this process (called pebble accretion) begins with tiny chunks of icy rock and dust in the disk. Orbiting the little star, these pieces of matter begin to collide, sticking together with static electricity. Eventually, these growing clusters reach a size large enough – about 10 land masses – to be able to gravitational pull more and more gas from the surrounding disk.

From that point on, Jupiter gradually reached 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 it had – a considerable distance away from the mass required for hydrogen fusion – it stopped growing.

So, Jupiter was never even close to becoming massive enough to become a star. Jupiter is similar in composition to the Sun not because it was a “ failing star, ” but because it arose 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 the brown dwarfs, and they fill that void between gas giants and stars.

Starting at about 13 times the mass of Jupiter, these objects are massive enough to withstand the melting of the nucleus – not normal hydrogen, but 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 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 melting is an intermediate step on the path to hydrogen melting for stars, as they continue to accumulate mass. But some objects never reach this 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 underperforming stars or overly ambitious planets; but several studies have shown that they form like stars, from collapsing clouds rather than accreting the nucleus. And some brown dwarfs are even below mass to burn deuterium, indistinguishable from planets.

Jupiter is right on the lower mass limit for collapsing clouds; the smallest mass of a collapsing cloud object has been estimated to be around one mass of Jupiter. So if Jupiter had formed as a result of the collapsing clouds, it could be considered a failing star.

But data from NASA’s Juno probe suggests that at least once Jupiter had a solid nucleus – which is more consistent with the nucleus accretion formation method.

Modeling suggests that the upper limit of a planet’s mass, formed by nucleus accretion, is less than 10 times the mass of Jupiter – just a few masses of Jupiter shy of deuterium fusion.

So Jupiter is not a failed star. But thinking about why it isn’t can help us better understand how the cosmos works. Plus, Jupiter is a striped, stormy, swirling butterscotch wonder in its own right. And without it, we humans might not even have been able to exist.

This, however, is another story, to be told another time.