Expert: Tom Whiting - 11/1/2010

Question
QUESTION: Hi Tom
I'm just wondering is there a limit to the size and mass of a planet? If so why? Your help is much appreciated.

ANSWER: Hi Tony,
Yes, there is an upper limit, but it's typically not well defined because of other factors, mainly composition. Jupiter came close to being either a brown dwarf star or little red dwarf star. It's estimated that if Jupiter had been just 2 to 5 Jupiter masses larger, it would have been a little red dwarf star. Even at it's current size, Jupiter radiates twice as much energy outward than it absorbs from the sun.
So that's why there has to be an upper limit... a body would get massive enough for it's core to reach fusion temperatures and convert the large
"planet" to a small red dwarf star.  So, one can look at planets as simply "failed stars". They did not accumulate enough mass to become a star. And that's also why that over half (50%) of the stars out there are either doubles or multiple star systems. It seems to be the common outcome of star formation... extra bodies attempting to reach "stardom". And typically in a star-forming cloud of dust and gas, two of them (bodies accreting matter) do, to form a double star system.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA



---------- FOLLOW-UP ----------

QUESTION: Thanks a lot for that Tom that was really helpful.
Just another thing I was wondering, is it possible for a rocky planet to to reach the size of Saturn or Jupiter? Or is it impossible for them to ever get as big as a gas giant?

Cheers.

Answer
I don't know.  I know of nothing that would prevent a rocky planet from obtaining the size of Jupiter, except once the mass becomes too great,
it too would get very hot in the core eventually, and start the fusion process. But since the molten core would already be heavy elements like silicon, iron, and nickel, the fusion temperature required would be much higher than normal hydrogen to helium fusion. This in turn would cause
a very large rocky planet to possibly go "supernova" once the alpha capture fusion reaction internal temperature (around 600 million degrees F) was reached internally. Interesting question, and I really don't know the answer... perhaps we'll have a real answer from the Kepler spacecraft in a few years as to whether a large rocky planet can really form up.
Clear Skies,
Tom