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View Full Version : What's The Smallest A Gas Giant Can Be?



Mr. Milton Banana
2006-May-31, 08:50 PM
Well, we know Neptune's the smallest gas giant we've seen, at approximately 30,000 miles in diameter. I would guess there's a upper level of rocky planet. So, is there a point where a rocky planet would end and a gas giant begins?

antoniseb
2006-May-31, 09:02 PM
It probably depends a lot on temperature and composition. I suspect that If Neptune had been where Mercury is now for the last four billion years, you would not call it a gas giant.

Ilya
2006-Jun-01, 02:16 AM
It also depends on a definition. A lot of astronomers do not consider Neptune and Uranus "gas giants" at all -- they are "ice giants". Hydrogen and helium are about 90% of Jupiter's and Saturn's mass, but a little under 50% of Ice giants' mass.

Romanus
2006-Jun-01, 04:03 PM
<<It probably depends a lot on temperature and composition. I suspect that If Neptune had been where Mercury is now for the last four billion years, you would not call it a gas giant.>>

Ditto. I bet there's quite a bit of overlap between the largest possible terrestrial planets and the smallest possible gas giants/ice giants/etc.

Kullat Nunu
2006-Jun-01, 05:29 PM
Well, we know Neptune's the smallest gas giant we've seen...

Only by diameter... Uranus is less massive (17 vs 14.5 times Earth's mass). Uranus' equatorial radius is only 795 km longer than Neptune's, difference between polar radii is even less, 632 km. Interesting how close they are each other in size (like the Earth/Venus pair).

filrabat
2006-Jun-01, 10:30 PM
"Gas Giant" definitions. I suppose there are situations where you can make a legitimate distinction, based on where the "gas giant" is primarily hydrogen/helium or water/co2/ammonia/methane ices. On the other hand, "Gas Giant" would be a misnomer, not to mention earth-centric (barring water, all these substances are gases at earth pressure/temp combinations. Water would be a gas too on Venus. For this reason, if we have to make the distinction, we should use "hydrogen giants" "water giants" (a favorite John Whatmough [RIP] term).

Me? Unless there's a specific reason to make the distinction based on composition, it's probably best to use the catch-all term "gas giants"

Tim Thompson
2006-Jun-01, 11:21 PM
So, is there a point where a rocky planet would end and a gas giant begins?
There is no precisely calculated answer to the question, and I am sure that in reality, it depends much on the details of the environment in which planet formation takes place. Current models suggest that a core as small as 5 Earth masses could experience runaway accretion, and end up as large as Jupiter (i.e., Thommes & Murray, 2006 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006astro.ph..3068T&amp;db_key=PRE&amp;d ata_type=HTML&amp;format=); Klahr & Bodenheimer, 2006 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006ApJ...639..432K&amp;db_key=AST&amp;d ata_type=HTML&amp;format=); Hubickyj, Bodenheimer & Lissauer, 2005 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005Icar..179..415H&amp;db_key=AST&amp;d ata_type=HTML&amp;format=&amp;high=44739db64313499)). It seems a pretty good bet, at least for now, that anything under 5 Earth masses is unlikely to grow beyond terrestrial planet status, anything over 10 Earth masses is bound to become a gas giant (or "ice giant" if you prefer), and for anything in between, "it depends".

antoniseb
2006-Jun-01, 11:52 PM
It seems a pretty good bet, at least for now, that anything under 5 Earth masses is unlikely to grow beyond terrestrial planet status, anything over 10 Earth masses is bound to become a gas giant (or "ice giant" if you prefer), and for anything in between, "it depends".
Suppose that in all the many star forming regions that ever existed, there was one starless core that had acquired about one Earth mass before a supernova shockwave cleared the collapsing gas and dust from the surrounding environment, leaving a lone, very cold, mostly Hydrogen concentration of mass, with nothing hot nearby to drive the process of evaporation.

I'm not sure what the smallest possible gas giant is, but following this sort of scenario, it could be pretty small.