MadSci Network: Astronomy

Re: Does Jupiter have what it takes to be a star?

Date: Wed Nov 25 20:36:17 1998
Posted By: Aaron Romanowsky, grad student,Harvard-Smithsonian Center for Astrophysics
Area of science: Astronomy
ID: 911245149.As

Nope, not true.  But there is a grain of truth there, since 
Jupiter is sometimes called a "failed star".  Jupiter has essentially 
the same composition as the sun (90% hydrogen and 10% helium), and in
principle has plenty of fuel for nuclear fusion (hydrogen converted 
into helium).  However, the fusion processes require very high
temperatures, which require immense pressures, which require a very 
strong gravitational field.  Jupiter simply doesn't have the gravity 
which is required to permit fusion.  It would have to be about 100 
times as massive (the Sun is 1000 times as massive as Jupiter).  

Now, if by some feat of magical technology, someone "squeezed"
Jupiter, this could increase the central pressure and temperature to
the point where fusion would ignite.  However, the moment that external
pressure was relaxed, the internal pressure would expand Jupiter outward,
relieving the pressure, and "stopping the fire".  (We have the same
problem in our laboratory fusion reactors -- once we get the fire going,
it's impossible to contain, so far; we're trying to use fancy magnetic
fields to provide the containment.) So no, Jupiter could not be coaxed 
into becoming a star without continual outside intervention on a
preposterous scale.

Good question!


p.s.  Jupiter and the other giant planets are actually emitting more
energy than they receive from the Sun (by a factor of about two).
This is because they are in the final stages of their gravitational
collapse from a less dense ball of gas to a denser ball of gas
(with metallic hydrogen at the center).  As I said, this kind of
contraction will increase temperature and give off heat, but in the
case of Jupiter, it will never get hot enough for nuclear reactions,
and the planet simply settles down and cools off over billions of years.
The Earth was similarly hot in its youth, but would have lost its
internal heat long ago, were it not for another source of heat.
This is the radioactive decay of unstable elements, which keep the
Earth's interior hot and partly molten, driving continental drift.
These elements were produced in supernova explosions billions of years
ago (but that's another story).

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