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PostPosted: Tue Aug 24, 2010 10:22 pm 
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http://www.cosmicdiary.org/blogs/nasa/f ... his/?p=899

That's a pretty interesting system :)

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PostPosted: Tue Aug 24, 2010 10:38 pm 
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Your the expert with this, with all the neptune sized planet so close together would they even form moons?

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PostPosted: Tue Aug 24, 2010 11:02 pm 
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I have no idea - I'd say "possibly?" ;).

Their Hill Spheres are likely to be small, since they're close to the star - so there's less room for moons to form around them. But I'm not even really sure how moons would form around them - would they still form out of a disc of material around the planet? We've only really got one proper example of a multiple (major) satellite system around a Neptune/Uranus sized body, and that's the one around Uranus (5 satellites). Neptune just has Triton, which is weird because it has a retrograde orbit and is slowly spiralling in towards Neptune. Both have lots of minor satellites too, but a lot of those are probably captured bodies.

I think that the gas giants themselves are probably quite unlike Neptune and Uranus in our system in terms of their internal structure - Neptune and Uranus are essentially "icy giants", but I'd guess that these "Warm Neptunes" might be rockier bodies.

I think they probably could have moons, but whether there'd be a lot or a few of them I can't say. According to theory, the moons should all be pretty small though since the masses of the gas giants is comparatively low and satellite system formation seems to be dependent on the mass of the primary body.


(I also think saying "Neptune sized" is rather biased against Uranus, which is about the same size. Then again, we don't say "Venus-size" when talking about earth-size planets either ;), I guess we ultimately have to pick one or the other!)

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PostPosted: Wed Aug 25, 2010 11:25 pm 
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With big planets ending up close in, could they have captured moons on their way?

How quickly would an ice ball melt when it was falling in below the snow line? On what sort of timescale does insolation heat a planet size body? I recall that the earth gets a big chunk of its heat budget from radioactive decay inside its guts. Enough heat to drive the plate tectonics.

What is thought about all these torch orbit Supiters? Are they all expanding and boiling off? I seem to recall one of them was tracked in its transit as being very expanded.


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PostPosted: Fri Aug 27, 2010 7:40 pm 
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More info about this and the (also announced) Kepler discovery of a possible earth-size planet in another system

http://planetary.org/news/2010/0827_Fro ... e_New.html

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PostPosted: Fri Aug 27, 2010 7:46 pm 
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Oops, I must have missed this earlier, sorry!

JustinInOz wrote:
With big planets ending up close in, could they have captured moons on their way?


Possibly, I guess. Capture is really hard to do though - it's probably more likely that they have any moons that they formed with.

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How quickly would an ice ball melt when it was falling in below the snow line?


Um... can't really answer that accurately beyond saying it'd take a very long time, since it'd start outside the snow line, and then slowly get warmer as it moved inwards, and it has to get hot enough to lose the vapour form of its volatiles (e.g. water vapour, CO2 gas, methane gas), and then it would have to get hotter and hotter to lose it faster. It'd be millions of years, probably hundreds of millions of years, maybe billions of years - depends on the size of the body and the type of volatile it's losing.

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On what sort of timescale does insolation heat a planet size body?


Daily :). Solar insolation is what heats up the Earth on a day to day basis.


Quote:
I recall that the earth gets a big chunk of its heat budget from radioactive decay inside its guts. Enough heat to drive the plate tectonics.


It gets way more energy from the sun than it does from its interior (take the sun away, and earth would freeze pretty quickly. The internal heat wouldn't warm the surface much at all, except at volcanic areas). Internal heat does drive the plate tectonics though - solar heating isn't involved in that at all.

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What is thought about all these torch orbit Supiters? Are they all expanding and boiling off? I seem to recall one of them was tracked in its transit as being very expanded.


A few are. But jovians are massive planets, and most can retain hydrogen even in really hot conditions. When you get that close to the star though I think other processes can also enhance atmosphere loss (stripping by solar wind, for example), more so than they do further out.

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PostPosted: Mon Aug 30, 2010 4:28 am 
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http://xkcd.com/786/
Image

Yeah :).

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PostPosted: Tue Aug 31, 2010 8:19 pm 
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http://lanl.arxiv.org/abs/1008.4141

This is an interesting one - what astronomers thought was a single M V star actually turns out to be a binary... and there's a brown dwarf orbiting one of the stars!

They conclude that the BD actually formed in a star-like manner (so it literally is a 'failed star') rather than as a very big planet around the M V star that it orbits.

The BD itself seems to have an anomalously large radius, though it's not clear if it's because they've calculated the age of the system incorrectly (the radius is fine if the system is about 500 million years old, but from the stellar data they calculate that it's 1-2 Ga old. Plus, if it was that young then the BD should be giving off a detectable amount of heat, which it apparently isn't). So it seems that either some mechanism is increasing the BD radius to larger than it should be or the BD internal structure models that we have at the moment may be flawed, but they need to look at more BDs to find out.

The BD seems to be orbiting star A at a distance of only 0.085 AU though, with an orbital eccentricity of 0.056. Not sure what the separation between the two stars (and and B) are, except that it's "wide". EDIT: Later calculation says that the AB separation is about 12.1 AU apparently.

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PostPosted: Wed Sep 01, 2010 1:40 pm 
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Brown Dwarfs seem to be that object that crosses boundaries between definitions.

I think we will find that BDs can form either like a star or like a planet, but in the end, they are BDs.

I love it when things don't fit into our nice little definition boxes!

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PostPosted: Wed Sep 29, 2010 10:47 pm 
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Here's a really promising one around Gliese 581 (bringing the total to 6 planets in that system!) - this one is a bit bigger than Earth and smack bang in the star's habitable zone, so it's definitely a "potentially habitable" planet.

http://news.ucsc.edu/2010/09/planet.html

EDIT: Full preprint paper with more juicy details - http://www.ucolick.org/~vogt/ms_press-1.pdf

System info (from the paper):
Gliese 581 - M3 V solo star
Radius: 0.29 Rs
Mass: 0.31 Ms
Luminosity: 0.013 Ms
Age: 4.3 Ga (?)

Orbit 1: 0.028 AU, 3.15 days, 1.7 Me
Orbit 2: 0.041 AU, 5.37 days, 15.6 Me
Orbit 3: 0.073 AU, 12.92 days, 5.6 Me
Orbit 4: 0.146 AU, 36.56 days, 3.1 Me (habitable?)
Orbit 5: 0.218 AU, 66.87 days, 5.6 Me
Orbit 6: 0.758 AU, 433 days, 7.0 Me

The interesting thing is that all of these planets are pretty low mass (the star itself is slighly lower metallicity than Sol too, so this isn't too unexpected). But while the planet in orbit 2 might be a small Neptunian jovian, the rest are more likely to be more rocky. They probably have deep atmospheres though, as they're pretty massive. The planet 6 is in the outer zone, 5 is in the middle zone, 4 is in the habitable zone, and 1-3 are in the inner zone. All would be tidelocked. The orbits are likely to be near-circular (like our own system - maybe a bit eccentric, but no crazy orbits).

Sooo.... this is turning out to be an interesting system, for sure!

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