Jupiter Is Bigger Than Some Stars, So Why Didn't We Get a Second Sun?

from
https://www.sciencealert.com/why-isn-t-jupiter-a-star

Jupiter Is Bigger Than Some Stars, So Why Didn't We Get a Second Sun?
MICHELLE STARR4 JANUARY 2021
The smallest known main-sequence star in the Milky Way galaxy is a real 
pixie of a thing.

It's called EBLM J0555-57Ab, a red dwarf 600 light-years away. With a 
mean radius of around 59,000 kilometres, it's just a smidge bigger than 
Saturn. That makes it the tiniest known star to support hydrogen fusion 
in its core, the process that keeps stars burning until they run out of 
fuel.

In our Solar System, there are two objects larger than this teeny star. 
One is the Sun, obviously. The other is Jupiter, like a giant scoop of 
ice cream, coming in with a mean radius of 69,911 kilometres.

So why is Jupiter a planet and not a star?

The short answer is simple: Jupiter doesn't 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 any lower, it 
would not 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 know

The gas giant may not be a star, but Jupiter is still a Big Deal. Its 
mass is 2.5 times that of all the other planets combined. It's just 
that, being a gas giant, it has really low density: around 1.33 grams 
per cubic centimetre; Earth's density, at 5.51 grams per cubic 
centimetre, is just over four times higher than that of Jupiter.

But it's interesting to note the similarities between Jupiter and the 
Sun. The Sun's density is 1.41 grams per cubic centimetre. And the two 
objects are very compositionally similar. By mass, the Sun is about 71 
percent hydrogen and 27 percent helium, with the rest being made up of 
trace amounts of other elements. Jupiter by mass is about 73 percent 
hydrogen and 24 percent helium.

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

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

But it's still unlikely that, left to the Solar System's own devices, 
Jupiter would even become close to being a star.

Stars and planets, you see, 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! - spinning as it goes in a process called cloud collapse. As it 
spins, it spools in more material from the cloud around it into a 
stellar accretion disc.

As the mass - and therefore the gravity - grows, the core of the baby 
star is squeezed tighter and tighter, which causes it to grow hotter and 
hotter. Eventually it becomes so compressed and hot, the core ignites 
and thermonuclear fusion kicks off.

According to our understanding of star formation, once the star has 
finished accreting material, a whole lot of accretion disc is left over. 
This is what the planets are made of.

Astronomers think that, for gas giants like Jupiter, this process 
(called pebble accretion) starts with tiny chunks of icy rock and dust 
in the disc. As they orbit the baby star, these bits of material start 
to collide, sticking together with static electricity. Eventually, these 
growing clumps reach a large-enough size - around 10 Earth masses - that 
they can gravitationally attract more and more gas from the surrounding 
disc.

 From that point, Jupiter gradually grew to its current mass - about 318 
times the mass of Earth, and 0.001 times the mass of the Sun. Once it 
had slurped up all the material that was available to it - at quite a 
remove from the mass required for hydrogen fusion - it stopped growing.

So, Jupiter was never even close to growing 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 gave birth to the Sun.

27479980787 682abf79bf cropped
(NASA/SwRI/MSSS/Gerald Eichstädt/Seán Doran/Flickr/CC-BY-2.0)

The true failed stars
There is a different 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 over about 13 times the mass of Jupiter, these objects are 
massive enough to support core fusion - not of normal hydrogen, but 
deuterium. This is also known as 'heavy' hydrogen; it's an isotope of 
hydrogen with a proton and a neutron in the nucleus instead of just a 
single proton. Its fusion temperature and pressure are lower than the 
fusion temperature and pressure of hydrogen.

Because it occurs at a lower mass, temperature and pressure, deuterium 
fusion is an intermediate step on the way to hydrogen fusion for stars, 
as they continue to accrete mass. But some objects never attain that 
mass; these are known as brown dwarfs.

For a while after their existence was confirmed in 1995, it was unknown 
whether brown dwarfs were underachieving stars or overambitious planets; 
but several studies have demonstrated that they form just like stars, 
from cloud collapse rather than core accretion. And some brown dwarfs 
are even below the mass for deuterium burning, indistinguishable from 
planets.

Jupiter is right on the lower mass limit for cloud collapse; the 
smallest mass of a cloud collapse object has been estimated at about one 
Jupiter mass. So if Jupiter had formed from cloud collapse, it could be 
considered a failed star.

But data from NASA's Juno probe suggest that, once upon a time at least, 
Jupiter had a solid core - and that is more consistent with the core 
accretion formation method.

Modelling suggests that the upper limit for a planet mass, forming via 
core accretion, is less than 10 times the mass of Jupiter - just a few 
Jupiter masses shy of deuterium fusion.

So, Jupiter isn't a failed star. But thinking about why it isn't one can 
help us better understand how the cosmos works. In addition, Jupiter is 
a stripy, stormy, swirly butterscotch wonder in its own right. And 
without it, we humans may not have even been able to exist.

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