What is the largest known planet?
What is the largest known planet?
I think the real question is where the boundary is between gas giant and failed stars.
If you're interested in the largest in terms of diameter, then we have a relatively short list of objects for which the radius is know (because they transit the parent star). The current record-holder seems to be HAT-P-1 b, with a diameter 1.36 times jupiter's.
Theory predicts that cool gas giants slightly more massive than Jupiter will show a marginally higher radius (up to around 1.2 times Jupiter for a few Jupiter masses, IIRC), but that cool objects with even higher masses will be compacted down to Jupiter-size.
However, it seems that if objects are heated in some way (either because they're young, or orbit close in, or are tidally heated) they can "inflate" to larger diameters. There's a recent summary of theory here.
Planet WASP-1 b also looks like a big one, perhaps bigger in diameter than HAT-P-1b
According to some accounts, this planet has an alternative designation Garafia-1.
The two new-found planets are like nothing imagined by even the most dedicated Trekkie.
“All the theoretical models tell us that these planets should have dense cloud decks made essentially of “rock snowflakes”, Prof Andrew Collier Cameron told the BBC.
“The sort of chemicals which condense to form clouds at the high temperatures on these planets are things we normally think of on Earth as minerals – olivine, forsterite, all the magnesium silicates.”
Isn't anything over 12 Jupiters a brown dwarf?
According to the general definition, yes.
Both WASP-1 b and HAT-P-1 (it is so much fun typing those names) are less massive than Jupiter.
How large could a rocky planet get before it was so dense it started to shrink again?
That's a good answer. An all ceramic planet would be the widest of all the solid types then...
From conversations on other boards, I have come to understand that the centres of rocky planets are so compressed that the electron degeneracy pressure is overcome to an extent, and so matter becomes denser than it could be in an uncompressed state.
If a planet was massive enough the electron degeneracy pressure would be overcome completely, and the planet would be as dense as a white dwarf and made of neutrons. But in the core of Jupiter, and even in the core of the Earth, matter (iron for example) is compressed more than it is on the surface of a planet, and is therefore more dense.
Exactly what bonkers quantum properties this slight degeneracy would cause I have no idea, but if anyone knows, please tell.
Most brown dwarfs only are capable of fusion for a few million years as far as I understand it,
but even after fusion stops they are much hotter than a planet would be, all other things being equal. They give off heat because of gravitational collapse for billions of years.
The way I understand it, the dividing line breaks down like this:
Up to two Jupiter masses: "classical" planet
2 to 13 Jupiter masses: "degenerate planet": mass increases, but radius stays roughly the same
13 to 75 Jupiter masses: brown dwarf; capable of fusion of duterium for short period
75 Jupiter masses and up: star.
This isn't a particularly useful list thoguh; there are plenty of free-floating objects smaller than 13 Jupiter masses.
In a supernova, iron nuclei are bombarded by neutrons, which are captured, and thus build ever bigger nuclei.