The smallest known main sequence star 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 slightly larger than Saturn. That makes it the smallest known star to support hydrogen fusion at its core, the process that keeps stars burning until they run out of fuel.
In our solar system there are two objects bigger than this little star. One of them is of course the sun. The other is Jupiter, like a giant scoop of ice, entering 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 melt 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 wouldn’t be able to fuse hydrogen either. 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, as a gas giant, it has a very low density: about 1.33 grams per cubic centimeter; Earth’s density, 5.51 grams per cubic centimeter, is slightly more than 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 about 71 percent hydrogen and 27 percent helium, with the rest made up of traces of other elements. Jupiter by mass is about 73 percent hydrogen and 24 percent helium.
Illustration of Jupiter and its moon Io. (NASA’s Goddard Space Flight Center / CI Lab)
For this reason, Jupiter is sometimes referred to as a failed star.
But it is still unlikely that Jupiter, left to the solar system itself, would even nearly become a star.
You see, stars and planets are born through two very different mechanisms. Stars are born when a tight knot of material in an interstellar molecular cloud collapses under its own gravity – poof! flomph! – run as it goes in a process called cloud collapse. As it spins, it washes more material from the cloud surrounding it into a stellar accretion disk.
As mass – and thus gravity – grows, the baby star’s core is squeezed tighter, making it hotter and hotter. Eventually it gets so compressed and hot that the core ignites and thermonuclear fusion begins.
According to our understanding of star formation, once the star has finished growing material, a lot of accretion disk remains. This is what the planets are made of.
Astronomers think that for gas giants such as Jupiter, this process (called silica accretion) begins with small chunks of ice-cold rock and dust in the disk. As they orbit the baby star, these pieces of material begin to collide and stick together with static electricity. Eventually, these growing clumps reach a size large enough – about 10 Earth’s masses – to be able to attract more and more gas from the surrounding disk by gravity.
From that point on, Jupiter gradually grew to its present mass – about 318 times the mass of the Earth and 0.001 times the mass of the Sun. Once it had gobbled up all the available material – quite a distance from the mass needed for hydrogen fusion – it stopped growing.
So Jupiter was never close to being 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 produced the sun.
(NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran / Flickr / CC-BY-2.0)
The real failed stars
There is another class of objects that can be considered ‘failed stars’. These are the brown dwarfs, and they fill that gap between gas giants and stars.
Starting at more than about 13 times the mass of Jupiter, these objects are massive enough to support nuclear fusion – not of normal hydrogen, but deuterium. This is also called ‘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 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 towards hydrogen fusion for stars, as they continue to build up mass. But some objects never reach that mass; these are known as brown dwarfs.
For some time after their existence was confirmed in 1995, it was unknown whether brown dwarfs were underperforming stars or overly ambitious planets; but several studies have shown that, like stars, they are formed by the collapse of the clouds rather than the growth of the core. And some brown dwarfs are indistinguishable from planets even among the masses for deuterium combustion.
Jupiter is right on the lower limit of cloud collapse mass; the smallest mass of a cloud collapse is estimated to be about one Jupiter mass. So if Jupiter was formed by the collapse of a cloud, it could be considered a failed star.
But data from NASA’s Juno probe suggests that Jupiter at least once had a solid core – and that’s more consistent with the method of forming core accretion.
Modeling suggests that the upper limit for a planetary mass formed via nuclear accretion is less than 10 times the mass of Jupiter – just a few Jupiter masses shy of deuterium fusion.
So Jupiter is not a failed star. But when we think about why it is not one, we can better understand how the cosmos works. Plus, Jupiter is a striped, stormy, swirly butterscotch wonder in its own right. And without it we humans might not even have been able to exist.
However, that’s a different story, to be told another time.