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Spinoza
2013-Oct-04, 02:55 PM
In an astronomy cast episode from earlier this year, Pamela Gay said if Jupiter somehow acquired more mass, it wouldn't get any bigger, just denser. Yet I see a report from Alan Boyle saying that Kepler 7-B, a gaseous exoplanet similar to Jupiter, has a diameter 1.5 times that of Jupiter. Can someone please explain? Thanks.

neilzero
2013-Oct-04, 04:15 PM
My guess is Pamela meant not much bigger. I understand brown dwarfs are only slightly larger with mass up to almost 100 Jupiters. I suppose the average internal temperature also affects the diameter? Neil

Van Rijn
2013-Oct-04, 05:43 PM
In an astronomy cast episode from earlier this year, Pamela Gay said if Jupiter somehow acquired more mass, it wouldn't get any bigger, just denser.


I don't believe that's exactly true, and I do think I remember reading about hot Jupiters with larger diameters.



Yet I see a report from Alan Boyle saying that Kepler 7-B, a gaseous exoplanet similar to Jupiter, has a diameter 1.5 times that of Jupiter. Can someone please explain? Thanks.

My first thought was "hot Jupiter" and it is, but also it only has about half the mass of Jupiter, not more mass. See

https://en.wikipedia.org/wiki/Kepler-7b

quoting:

Kepler-7b is a hot Jupiter that is about half the mass of Jupiter, but is nearly 1.5 times its size; at the time of its discovery, Kepler-7b was the second most diffuse planet known, surpassed only by WASP-17b.

galacsi
2013-Oct-04, 05:57 PM
In an astronomy cast episode from earlier this year, Pamela Gay said if Jupiter somehow acquired more mass, it wouldn't get any bigger, just denser. Yet I see a report from Alan Boyle saying that Kepler 7-B, a gaseous exoplanet similar to Jupiter, has a diameter 1.5 times that of Jupiter. Can someone please explain? Thanks.

Yes it is generally true, because if you add matter to a jupiter type planet ,this matter just get compressed.The diameter does not expand much. But not for this fluffy planet , it is so near the surface of its star that it is extremely hot and then have a very low density.
But all is not clear well understood with this Kepler 7-B . If you go to wikipedia http://en.wikipedia.org/wiki/Kepler-7_b you will read this sentence :
Such low densities are not predicted by current standard theories of planet formation.

Van Rijn
2013-Oct-04, 05:58 PM
Yes, here's a planet more massive and larger than Jupiter, WASP-12b:

http://en.wikipedia.org/wiki/WASP-12b

Mass (m) 1.35 0.14 M☉
Radius (r) 1.57 0.07 R☉
Temperature (T) 2525[2] K

Van Rijn
2013-Oct-04, 09:20 PM
Correction, I was looking at the wrong numbers (I realized that the above mass and radius references were about the parent star, in comparison to the sun).

Here are the right numbers:

Mass 1.39 0.04[1] Mj
Radius 1.8 Rj (approximately)

Jeff Root
2013-Oct-04, 09:23 PM
Van Rijn,

You accidentally copied the mass and radius of the parent star
from the Wikipedia article, rather than the mass and radius of
the planet. I noticed because the symbol at the end of each
line is the symbol for the Sun, not for Jupiter.

The figures you meant to post:

Mass (m) 1.39 0.04 MJ
Radius (r) 1.83 +0.06 −0.07 RJ

The numerical bits are so similar that it faked me out, too.

-- Jeff, in Minneapolis

Jeff Root
2013-Oct-04, 09:23 PM
Well!

:)

-- Jeff

Romanus
2013-Oct-04, 11:25 PM
Jupiter is indeed near the maximum size a gas giant can get; piling on more mass will simply compress the interior more, into something like degenerate matter up until true fusion around 80 solar masses.

Hot Jupiters are theorized to be larger because of their enormous external heat flux, which basically puffs them up.

BigDon
2013-Oct-05, 03:00 AM
Jupiter is indeed near the maximum size a gas giant can get; piling on more mass will simply compress the interior more, into something like degenerate matter up until true fusion around 80 solar masses.

Hot Jupiters are theorized to be larger because of their enormous external heat flux, which basically puffs them up.

Yeah, like a mouse in a deep fryer.

And what was the issue again against Jupiter sized rocky planets that doesn't allow their existence?

Jeff Root
2013-Oct-05, 04:20 AM
I'm pretty sure it's that any planet with enough matter to
have a diameter as large as Jupiter's would hang onto gas
and be a gas giant. If all the gas were blown away by a
nearby star, it would end up much smaller than Jupiter.
If it started out with so much matter that blowing away all
the gas still left it the size of Jupiter, then it wouldn't have
been a planet to begin with, but a brown dwarf star, so it
never would have been rocky and couldn't cool off enough
to become rocky in the time since the first stars formed.

-- Jeff, in Minneapolis