Subaru Telescope: "Massive-Core Hot Planet is Discovere

[Spaceman Spiff]

Intriguing...

A Massive-Core Hot Planet is Discovered (http://subarutelescope.org/Pressrelease ... index.html).

At the bottom of the article:

"HD149026 is a solar-type star with spectral type G0IV, 1.3 times solar mass, visual magnitude 8.15. The planet has 0.36 times Jupiter mass and 0.72 times Jupiter size. The orbit is a circular one with an orbital radius of 0.046AU and inclination of 85 degrees from line of sight.

The relative size and mass data suggest a density equivalent to Jupiter's, yet it's concluded there is 70 Earth masses out of 115 that are rock/metal. Must be necessary to squash the hydrogen and helium into a denser form, else this planet would have been as puffed up as Saturn.

Spiff.

[selden]

Considering how close it is to its primary, I suspect a mostly gaseous planet would be expected to be quite a bit larger than Saturn due to the thermal expansion of its atmosphere.

[julesstoop]

I don't think so. Imagine placing Saturn - as is - in Mercury's orbit: it would simply loose most of itsself and a denser, smaller core would be left.

[eburacum45]

I think it would take a long time for a Saturn-type planet to lose it's atmosphere in Mercury's orbit;
despite the fact that Saturn's 'surface' gravity is similar to Earth's, its escape velocity is three times as high.
it would be on the order of a billion years for a Jupiter mass planet to lose its atmosphere in a much closer orbit, so I think Saturn could hang on to a respectable atmosphere for quite some time.

When I first heard about this planet, I made a wide assed guess at tye surface gravity- perhaps 10 gees.
Trying to work it out is giving me a headache- has anyone got a better estimate?
If it is 10 gees then it should hang on to arespectable atmosphere- a very dense and hot one too, probably in the silicon cloud range.

[eburacum45]

This article is interesting;
http://skyandtelescope.com/news/article_1538_1.asp

there are more 'metals' in this one planet than in all of the worlds in our system, and it orbits a high metallicity star (seems reasonable).

Perhaps there are many planets like this, rocky gas giants.

[Michael Kilderry]

This discovery sounds a bit like a theoretical kind of planet I've discussed on this forum before. "Gas Dwarfs" as I call them are sort of like a cross between a gas giant and a terrestrial planet, with a big atmosphere but with a big core as well that has a radius approximately half that of the planet itself.

Michael

[Spaceman Spiff]

Considering how close it is to its primary, I suspect a mostly gaseous planet would be expected to be quite a bit larger than Saturn due to the thermal expansion of its atmosphere.

Quite, but the thing about this planet is that the mass and radius data show this one is not bloated, but dense. It's a new 'category' of extrasolar giant planet. We have relatively very accurate data for both mass and radius just like the bloated gas giant of HD 209458 as it's also a transiting planet.

Trying to work it out is giving me a headache- has anyone got a better estimate?

OK, here's my effort:

SurfaceGravity = GravitationalConstant ?— Mass / ( Radius ^ 2 )

EscapeVelocity = ( 2 ?— GravitationalConstant ?— Mass / Radius ) ^ ( 1/2 )

For Earth:

Mass = 5.97 ?— 10^24 kg.
Radius = 6,378 km.

For HD 149026 b:

Mass = 0.36 Jupiter = 115 Earth masses = 687 ?— 10^24 kg.
Radius = 0.72 Jupiter = 8.0 Earth radii = 51,120 km

Results:

Code: Select all

Body              Earth       HD 149026 b

Surface Gravity   9.81 m/s??   17.5 m/s?? (1.8 ?— Earth)
Escape Velocity  11.2 km/s    42.3 km/s (3.8 ?— Earth)


The new thing about this planet is that it's proposed as having a core of 70 Earth masses (i.e., 60% of its mass) of rocky/metallic material. Planets like Jupiter and Saturn are thought to have such cores making only 10-15 Earth masses. The article in turn suggests this planet didn't form by disk instability core collapse, but by accretion.

"Gas Dwarfs"

Hmm, like the Red Rock advert: It's not a dwarf, and there's not much gas in it! Time to namesmith again? How about Redrock planet, it's red hot, and there's rocks in it!

Er, seriously though. I think this planet will look just like a gas giant on the outside though - it is modelled as having the usual thick hydrogen/helium atmosphere with liquid hydrogen/helium underneath...

Spiff.

[Michael Kilderry]

Hmm, like the Red Rock advert: It's not a dwarf, and there's not much gas in it! Time to namesmith again? How about Redrock planet, it's red hot, and there's rocks in it!

Spiff.

Yes, but the name "Gas Dwarf" wasn't made for this specific planet, I didn't even make it up myself, it comes off the program StarGen.

How about the name "Hybrid Terrestrial" for this planet?

Michael

[eburacum45]

Trying to work it out is giving me a headache- has anyone got a better estimate?

OK, here's my effort:

SurfaceGravity = GravitationalConstant ?— Mass / ( Radius ^ 2 )

EscapeVelocity = ( 2 ?— GravitationalConstant ?— Mass / Radius ) ^ ( 1/2 )

For Earth:

Mass = 5.97 ?— 10^24 kg.
Radius = 6,378 km.

For HD 149026 b:

Mass = 0.36 Jupiter = 115 Earth masses = 687 ?— 10^24 kg.
Radius = 0.72 Jupiter = 8.0 Earth radii = 51,120 km

Results:

Code: Select all

Body              Earth       HD 149026 b

Surface Gravity   9.81 m/s??   17.5 m/s?? (1.8 ?— Earth)
Escape Velocity  11.2 km/s    42.3 km/s (3.8 ?— Earth)


Spiff.

Thank you; that should be the gravity at the cloud tops (probably silicon clouds).

The gravity at the surface of the rocky core (at the bottom of the oceans of water or hydrogen) would be much higher; I got 5 gees, Sky and Telescope mentioned 10 gees.
I suppose it depends on how dense the core is- it might be nearly all iron, or nearly all silicon/aluminium or even carbon.

[Spaceman Spiff]

The gravity at the surface of the rocky core (at the bottom of the oceans of water or hydrogen) would be much higher; I got 5 gees, Sky and Telescope mentioned 10 gees.

OK, yes. Following the S&T link and looking at the interior density models graph, I read the normalised core radius is 0.33 for a rock-only core. Now, I note from that graph that this is the rocky extreme of a series of rock-ice models: note the ice only model also with a normalised radius of 0.49, which can produce the same radius and average density. So this rock core turns out to be just one interpretation of the average density.

Taking the core to be fully rocky, and at 0.33 normalised radius then noting that all material outside this radius will contribute a cancelled out gravitational pull, we can use the core's mass and radius to get its surface gravity. Same method as above...

For HD 149026 b rocky core:

Mass = 0.60?—0.36 Jupiter masses = 68.7 Earth masses = 410 ?— 10^24 kg.
Radius = 0.33?—0.72 Jupiter radii = 2.6 Earth radii = 16,870 km

Results:

Code: Select all

Body              Earth       HD 149026 b Rocky Core

Surface Gravity   9.81 m/s??   96.2 m/s?? (9.8 ?— Earth)
Escape Velocity  11.2 km/s    Irrelevent


Yes, but the name "Gas Dwarf" wasn't made for this specific planet, I didn't even make it up myself, it comes off the program StarGen.

How about the name "Hybrid Terrestrial" for this planet?

Ah, I see! Actually, the S&T article pointed to be eburacum45 mentioned that Jupiter and Saturn are called Gas Giants because most of their mass is in the form of hydrogen and helium (though these are crushed to liquid) and Uranus and Neptune are called Ice Giants because most of their mass is in the form of water, ammonia, carbon dioxide and methane ices. Following that line of thinking, then if this new planet is 60% rock, it's a Rock Giant, even though it has a Gas Giant type atmosphere (well, so do Uranus and Neptune)... <- Decisions, decisions, decisions ->.

Spiff.

[Michael Kilderry]

Ah, I see! Actually, the S&T article pointed to be eburacum45 mentioned that Jupiter and Saturn are called Gas Giants because most of their mass is in the form of hydrogen and helium (though these are crushed to liquid) and Uranus and Neptune are called Ice Giants because most of their mass is in the form of water, ammonia, carbon dioxide and methane ices. Following that line of thinking, then if this new planet is 60% rock, it's a Rock Giant, even though it has a Gas Giant type atmosphere (well, so do Uranus and Neptune)... <- Decisions, decisions, decisions ->.

Spiff.

Rock Giant sounds like a good name, but if we are going to classify planets like this, the definition of "giant planets" would then just mean planets with a gas giant type appearance from the outside.

Michael

[Ynjevi]

Agreed, rock giant sounds good. Giant planets may all have gaseous envelopes, as it may be unevitable result of higher gravity.

What about Gliese 876 d? Should it be called a very small giant or a very large terrestrial planet? It is likely mostly rocky, but has undoubtly a dense atmosphere. What is the border between giants and dwarf planets? The dividing line probably has to be arbitrary.

[ajtribick]

My personal preference for the dividing line would be if there is continuity of phase in the atmosphere - that is the atmosphere becomes liquid as the pressure is increased, with no definite surface to the liquid. If this occurs, then the planet is a giant planet. However this isn't much help when all you have is the planet's (minimum) mass, and I'm not sure if this system would classify Uranus and Neptune as giants or "dwarfs".

[wcomer]

Does the existance of this planet imply that previous size limits for planets are too low? Granted I don't know how you would have enough heavy lements to form a scaled up version of this planet. But couldn't heavy element giants go well beyond 70 Jupiter masses without undergoing fusion in their 'atmospheres'? Does this imply the existance of a new, albeit extremely rare, class of celestial objects?