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_DRAGONLORD_
2006-Dec-28, 02:59 PM
What is the largest known planet?

Saluki
2006-Dec-28, 03:16 PM
Depends on how you define "known", and what day it is. They are discovering new extra-solar planets all the time.

This link (http://exoplanet.eu/catalog-all.php)shows one at 21.5 Jupiter masses. It was last updated December 11.

Mister Earl
2006-Dec-28, 03:46 PM
I think the real question is where the boundary is between gas giant and failed stars.

grant hutchison
2006-Dec-28, 04:37 PM
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 (http://fr.arxiv.org/abs/astro-ph/0609369), 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 (http://fr.arxiv.org/abs/astro-ph/0612703).

Grant Hutchison

eburacum45
2006-Dec-29, 12:59 PM
Hat-P-1b is also described here, at t'Wikipedia
http://en.wikipedia.org/wiki/HAT-P-1b

with a diameter of about 190,000 km this planet wants a big, big name, something like Godzilla or Leviathan.

eburacum45
2006-Dec-29, 01:25 PM
Planet WASP-1 b also looks like a big one, perhaps bigger in diameter than HAT-P-1b
http://www.exoplanet.eu/planet.php?p1=WASP-1&p2=b
According to some accounts, this planet has an alternative designation Garafia-1.
From here
http://www.tenerifenews.com/cms/front_content.php?client=1&lang=1&idcat=8&idart=4905

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.”

Rejinx
2006-Dec-31, 04:11 PM
Isn't anything over 12 Jupiters a brown dwarf?

eburacum45
2006-Dec-31, 05:56 PM
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.

publiusr
2007-Jan-19, 10:25 PM
How large could a rocky planet get before it was so dense it started to shrink again?

Doodler
2007-Jan-19, 10:43 PM
Isn't anything over 12 Jupiters a brown dwarf?

13-14, I think. This is in terms of mass, not diameter. Some big brownies are no bigger physically than Jupiter.

Doodler
2007-Jan-19, 10:47 PM
How large could a rocky planet get before it was so dense it started to shrink again?

How would a solid planet shrink? Nickel-iron in the core can only get so dense without being fused into something else.

grant hutchison
2007-Jan-19, 11:05 PM
How large could a rocky planet get before it was so dense it started to shrink again?I can only offer a data point: up to 10 Earth-masses, compression serves only to reduce the power law linking mass to radius: http://arxiv.org/abs/astro-ph/0511150.

Grant Hutchison

publiusr
2007-Jan-19, 11:16 PM
That's a good answer. An all ceramic planet would be the widest of all the solid types then...

Tobin Dax
2007-Jan-19, 11:24 PM
How would a solid planet shrink? Nickel-iron in the core can only get so dense without being fused into something else.

How would iron fuse (on its own)? :eh: The fact that it doesn't is what makes stars go boom. :)

punkrockbong151
2007-Jan-20, 02:00 AM
How large could a rocky planet get before it was so dense it started to shrink again?

planet's can shrink from having a high density?

cjl
2007-Jan-20, 03:52 AM
How would iron fuse (on its own)? :eh: The fact that it doesn't is what makes stars go boom. :)
Iron can and does fuse in large stars. However, in fusing, iron takes more energy than it releases through the fusuion process. So, even though it fuses in the large stars, they still collapse because fusing iron isn't a source of energy. So, basically, it'll fuse just fine, it just won't create a net energy gain in doing so.

eburacum45
2007-Jan-20, 10:53 AM
From conversations on other boards, I have come to understand that the centres of rocky planets are so compressed that the electron degeneracy pressure (http://en.wikipedia.org/wiki/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.

parallaxicality
2007-Jan-20, 11:33 AM
Depends on how you define "known", and what day it is. They are discovering new extra-solar planets all the time.

This link (http://exoplanet.eu/catalog-all.php)shows one at 21.5 Jupiter masses. It was last updated December 11.


21.5 Jupiter masses would be a brown dwarf. A planet cannot ever be capable of fusion, so the largest a planet can be is roughly 13 Jupiter masses.

eburacum45
2007-Jan-20, 03:02 PM
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.

parallaxicality
2007-Jan-20, 03:26 PM
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.

Tim Thompson
2007-Jan-22, 05:34 AM
Iron can and does fuse in large stars.
Iron doesn't fuse under those circumstances. In massive stellar cores, the iron nuclei are torn apart by the high temperature photons, and reform into helium nuclei (alpha particles). The resulting mix of free electrons & alpha particles collapses, and forms a neutron star when the electrons are captured by the protons in the alpha particles to form neutrons.

In a supernova, iron nuclei are bombarded by neutrons, which are captured, and thus build ever bigger nuclei.

Tim Thompson
2007-Jan-22, 05:36 AM
planet's can shrink from having a high density?
No, they shrink because they have higher mass, and therefore higher gravity. The higher density is a consequence of the shrinking (density is mass divided by volume).

cjl
2007-Jan-23, 07:56 AM
Iron doesn't fuse under those circumstances. In massive stellar cores, the iron nuclei are torn apart by the high temperature photons, and reform into helium nuclei (alpha particles). The resulting mix of free electrons & alpha particles collapses, and forms a neutron star when the electrons are captured by the protons in the alpha particles to form neutrons.

In a supernova, iron nuclei are bombarded by neutrons, which are captured, and thus build ever bigger nuclei.

I should have specified "in large stars when they go supernova"