QUESTION:
Is the thermal energy of Jupiter increasing?  If so, how long would it take
to become a protostar?

ANSWER from Scott Sandford on November 7, 1995:
Surprising as it may sound to many people, most planets tend to give off 
heat.  The heat that planets give off is generated by three main processes.  
The first one is fairly obvious and involves the Sun.  The Sun shines on all 
the planets and the energy from the Sun's visible light heats them up.  The 
planets then re-radiate some of this energy back into space as infrared light, 
i.e. heat.  The farther a planet is from the Sun the less light it receives and 
the less heat it releases as a result of this process.

The second source of heat is associated with radioactivity.  This source of 
heating does not occur at the surface of planets but instead occurs inside 
them.  When the planets were originally formed by the accumulation of 
smaller objects, they ended up incorporating all kinds of materials (ice, 
rocks, dust, etc.).  As a result, the planets contain all of the elements in 
the periodic table.  Among these were radioactive elements like uranium.  The 
radioactive elements undergo fission as they age and this process produces a 
lot of energy.  This energy is converted to heat deep inside the planets and 
this heat gradually works its way to the surface where it escapes into space.  
In the case of the Earth, we see this heat escaping in a variety of forms, the 
most spectacular being lava and ash from volcanoes!

Finally, planets can also produce heat by gravitational collapse.  This 
sounds a little complicated but it isn't really.  All objects on a planet have 
energy 'stored' in them in the form of gravitational potential energy.  This 
energy has to do with how far an object is from the center of the planet; the 
farther away it is the more potential energy it has.  The presence of this 
energy is easy to demonstrate.  If you drop an anvil off a table and onto a 
drinking glass the anvil will break the glass (and maybe the floor too!).  The 
energy that broke the glass came from the potential energy in the anvil 
released as it went from being farther away from the planet to closer to it.  
This same kind of thing can happen on planets on a MUCH bigger scale and 
it is happening on Jupiter now.  Jupiter is slowly getting more and more 
compact as its own gravity pulls material closer to its center, especially 
denser materials like rock and metal.  This overall contraction of material is 
very slow but the total amount of material involved is so large that it is 
equivalent to somebody dropping thousands and thousands of anvils over 
and over again.  The result is a lot of heat.  At present, Jupiter is giving 
off heat as a result of all the processes I mentioned here.

There is another important process that can generate heat efficiently, namely 
nuclear fusion.  This is the process by which our Sun and other stars 
generate their heat.  In order for fusion to work, however, the center of the 
star must have VERY high temperatures and pressures.  Despite the fact that 
Jupiter is generating a lot of heat, its center will never get hot enough for 
fusion to start and so it will never become a star on its own.  The only way 
for Jupiter to become a star would be to add more and more material to it.  
The added material would add mass at the surface and result in higher 
pressures in the center.  If enough material was added the pressure and 
temperature could reach a point where fusion would start and Jupiter would 
stop being a planet and start being a star!  (This would take a lot of 
material, however; you would have to add much more material than is already in 
Jupiter.)

A good question!
Scott Sandford, KAO Principal Investigator