Extra-solar planets

The non amateur stuff. Hawking, black holes, that sort of thing

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Vick
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Extra-solar planets

Post by Vick »

I know there are theories to explain how gas giant planets can end up orbiting within four to five million miles of their parent stars but what I can not work out is why, given surface temperatures measured in thyousands of degrees and, presumably, some pretty violant gravitational effects, these planets don't just evaporate.

Anyone help?
Matthew
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Re: Extra-solar planets

Post by Matthew »

Vick wrote:I know there are theories to explain how gas giant planets can end up orbiting within four to five million miles of their parent stars but what I can not work out is why, given surface temperatures measured in thyousands of degrees and, presumably, some pretty violant gravitational effects, these planets don't just evaporate.

Anyone help?
here is my answer:

The Gaseous giants Probabally consist of a large amount of hydrogen - they act like a star but were never massive enough for nuclear reaction to take place. You could almost classify jupiter as a brown dwarf.

Plus they are too far from the sun - if they were as close as earth then they would just boil away.

hope this helps
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Post by joe »

Matthew, I think there are gas giants much more massive than Jupiter that are orbiting their star more closely than the orbit of Mercury never mind Earth. But in answer to Vick's question (and I'm assuming he is talking about extrasolar planets) your guess is as good as anyone's. The jury is still out. It may even be that our solar system is an exception. Time for a new theory.

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Matthew
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Post by Matthew »

good Point

one point is that the inner planets in OUR solar system are small and rocky - they once could have been large gaseous planets where the gaseous outer has evaporated.

I read a book stating that jupiter possibly has a solid core with much more mass than earth, so this could be a valid argument.
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Spaceman
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Gravity?

Post by Spaceman »

Off the top of my head - I think gravity might well have someting to do with this.

The Sun and Moon exert roughly equal gravitational forces on Earth because gravity falls by the square of the distance. Similarly with exosolar planets. Even over a distance of four to five million miles that is a considerable drop in the force experienced.

Inside a body gravity only falls linearly with distance towards the core so the gravitational pull of the planet on its own outer layers is far stronger.

Finally you have to consider that the quoted density for obejct is always the mean density. Saturn, for example, has mean density low than water (i.e. it would float). This value is made up of the low density gaseous outer layers and a more massive, probably solid core. If the solid core contains a large fraction of the planet's mass it would exert sufficient gravitational force to retain its outer layers.

Having said all that is is interesting to speculate if there is some kind of matter transfer going on in these systems through the Langrange points and whether effects of that could be seen at, for example, x-ray wavelengths.
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Post by joe »

Hi Spaceman,

That is interesting but if I can just comment a little (off the top of my head too, I'm afraid), is it actually true that the Moon and Sun have equal influence? I have always thought that weighing up the distances/masses the Moon comes out on top hence the monthly tides, etc. If the gas giants do have solid cores, and I believe metallic hydrogen has been suggested at least for Jupiter, they are surely tiny compared to the overwhelming mass of the gas that constitutes the bodies of the planets. I have always thought that gravity is measured from a point at the centre of a body. When does it "know" when to act linearly and when to act exponentially? If the atoms of the atmosphere have enough energy to escape the gravitational pull of the planet then they will dissipate into space. The nearer they are to the sun then the more energetic they will be. There is a trade off of course, energy versus gravity. Having said this it still doesn't explain Vick's original question about big gas planets being so near their parent star!

Best wishes,
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Spaceman
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More thoughts....

Post by Spaceman »

Joe,

Interesting reply. To clear a couple of things up:

Moon's gravitational pull = 2x Sun's gravitational pull
Inner core of Jupiter is roughly 15 Earth mass, compared to 318 for the whole planet (about 5%)

And for gravity distributions try this link:
http://www.ac.wwu.edu/~vawter/PhysicsNe ... avity.html

So..why don't the atmospheres evaporate?
Well if we assume that these systems are reasonably evolved, and they must be as we are not considering protostars here, then maybe all the excess material has already been removed. In Vick's scenario it is possible that the process of atmospheric mass loss has already taken place.

Maybe we need to think about this in terms of escape velocity? How much energy would the atoms etc. in the upper atmosphere need to have a velocity that exceeds the escape velocity for the star?

Just a thought. Look forward to the replies.

Chris
Vick
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EXTRA SOLAR PLANETS

Post by Vick »

Some interesting thoughts there chaps. Gives me something to think about.

Could it be possible to have massive rocky planets with the sort of masses we assume belong to very large gas giants?
Cliff
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Post by Cliff »

Dear Vick et al
I am pleased to sit on the fence on this topic.
When I started "seriously" observing planets, which is not very long ago, no one had then ever observed any planets beyond our own solar system.
I am inclined to think it is still a bit early to decide exactly what these exoplanets are.
However, assuming that some of them actually are gas giant planets are closely orbitting other stars, can we be sure they have lived there long enough to have there gases boiled off by now?
I know they suggest boiling an egg for 3 minutes, but how long does it take to boil a gas giant?
Being in a particularly nasty mood, I think it was ironic that if I remember correctly the first distant star found to have an orbitting planet was 51 Pegasus and it was not on the suggested suspect list for observation.
Sorry to be a Grumpy Old Codger as usual but best wishes from Cliff
Spaceman
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Post by Spaceman »

Indeed Cliff fence sitting is the place to be right now on this one.

Most observations of extrasolar planets are by indirect means, such as light curve of parent star being altered, gravitational perturbations etc.

It will be many years before a catalogue of high resolutions images of these systems is produced and until then (at the earliest) it will be very difficult to understand the processes at work.

Chris
Kaustav
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Post by Kaustav »

Firstly lets clear up a few things. There's a distinction between a brown drawf and a gas giant planet. Brown drawfs, so called failed stars, have too little gas mass to properly shine by their own energy. i.e. they do not burn hydrogen in their core and to date, distinctions between a planet and a brown dwarf having the same mass and orbit depends on how the body formed. For example the brown drarf Gliese 229B and Jupiter are very similar in their mass. So anyway, planets grow from dust and gas accreting in a circumstellar disk and stars (even brown drarf) form through the gravitational contraction of a gass clouds. But then this leads us to a more vague question: what exactly is a planet? Until recent we thought we knew. We thought that a planet is an object that does not shine of its own light; it has a small mass and needs to orbit a star. These criteria still hold, by and large, however as I've just said, there are indeed stars, brown dwarfs, that do not produce energy and therefore they don't exactly shine of their own light; they're tiny and could be close in mass to the largest planets found; and they are found orbiting stars such as our own.

Spaceman mentioned that it'll be years before we have the resolution to directly view extrasolar planet... true but... There are three planned missions being launched to detect extra solar planets. The one likely to launch in 2007 is called Kepler, a NASA project. It is a special purpose space mission for detecting terrestrial and larger planets in habitable zones, around other stars. It's going to be equiped with a 0.95 m aperature differential photometer and it'll monitor about 100,000 A-K/M drawfs brighter than 14th mag.

Then we have Darwin, an ESA space mission and Terrestrial Planet Finder, another NASA mission. Drawin's a cluster of space telescopes flying in formation acting as an IR interforometer (actually Darwin will be a nulling interferometer). Darwin will detect and analyse Earth like planets for signatures of life and provide high res images. They're going to throw in a wider scope for Darwin in so much as that it'll also be used to investigate stars, planets, and galaxies as well as black holes. Finally, we have TPF from NASA. They haven't decided yet if it'll use IR interferometry or a visible light coronagraph. It's remit is slightly different to Darwin and more narrow. It will detect earth like planets and measure their surface temperatures and see if they have CO2, CO and H20 in their atmospheres.
Kaustav Bhattacharya
>> http://kaustav.uk.com/unisphere/ - An online magazine about Astronomy, Science, Social Media and Society.
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KendalAstronomer
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Post by KendalAstronomer »

Brown Dwarves do shine of their own light, just not in visible wavelengths. The definition is they are not massive enough to burn hydrogen through fusion. They are massive enough to burn deuterium through fusion though, and even after that, they will shine through gravitational contraction - as Jupiter does, just again, not in visible wavelengths. The dividing line between the two lies in the masses. Gliese 229B is about twenty times the size of Jupiter, well above the lowest theoretical limit of about ten. The difference in spectral lines between the three sets of objects - stars, bd's and Gas Giants - are also well noted. BD's have hydrocarbons and lithium lines, neither of which are common in stars. The hydrocarbon lines are also present in reflection spectra of Jupiter and Saturn. Hydrocarbons break up in the heat of actual stars. Lithium tends to be fused to death early in a hydrogen burning star's life, but not a deuterium burning Brown Dwarf's life.

Bodies such as gas giants and brown dwarves have a number of limits they must adhere to. Initially, their radii grow as their mass grows. Then they get to about Jupiter's limiting radii (thoughone of the latest extrasolar planets disputes this) then their density rather than their radius increases as more gas is put in. Eventually, when the mass is about 10-70 Jovian masses, depending on whether or not extra heat is coming from a companion or the composition is fighting against heating, deuterium begins to burn and a Brown Dwarf is born. At about a tenth the mass of the Sun, hydrogen burning commences and a star enters the equation. At the moment, a figure of about one hundred to one hundred and twenty five solar masses gives the upper limit for stellar formation. Why this is is also a mystery.

But whether Brown Dwarves definately form independantly of other stars and cannot form as a superjupiter is certainly far from settled. As more Brown Dwarves come in and more exoplanets, the debate will become even more interesting.

...and as for the original question - dunno - but we are working on seeing how different initial compositions can save a planet from boiling off. It seems some good infrared emitters exist in the hydrocarbon families in the Jovian atmosphere and some of these can lead to a substantial cooling, just not yet enough to explain the survival of so many exoplanets!
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Post by Cliff »

Dear al
I am very definitely sitting on the fence still, and do not see myself jumping off for a long time. Space probes have now "proved" there is water on Mars ! OR HAVE THEY? Although the the generally accepted conclusions may well be right, I personally am not yet 100 percent conviced. As for the future research with regards exoplanets, I cannot help wondering how much it gets hyped up to keep "us" public interested.
With regards to one of the original conjectures on this topic as to why atmospheres have not boiled off these supposed bigger than Jupiter exoplanets orbitting very close to stars (eg closer in than Mercury orbitting the Sun) I agree with spaceman on this one.
I think the suggestion of boiling off atmospheres tends to give an oversimplified and wrong impression. I do not think planetary atmospheres boil off quite in the same way that we create steam when boiling a pan of water. As Spaceman says the loss of a planetary atmosphere is probably dependant on the planets escape velocity and how fast the various molecules in the planets atmosphere are moving. Even if the exoplanet gets the "hots" because its near to very hot star, if the exoplanets own gravitation is strong enough to result in a very high escape velocity, then the exoplanets atmospheric molecules may not exceed that escape velocity and will tend to be retained.
Incidentally, as Joe suggested I tend to think that the chances of exoplanets harbouring life may be a lot less than many astrobiologists (or whatever they call them) seem to suggest. I think there could well be a lot more to it than needing a planet to be in the "Goldylocks" orbit or whatever they call it. Earth is not only in a suitable orbit arround a nice fairly stable star, but Earth also has a Moon which causes tides. I nearly forgot to mention that the Earth also has water which is nice. Indeed 70% of the Earth's surface is covered with water.Although I just read that 97 percent of Earth's water is salty and not suitable for us to drink.
There is one chap in the USA who is dead keen on terraforming Mars as soon as possible. As things stand at the moment we cannot even agree what to do with the Earth - are humans good custodians of Earth, or are we currently terrorforming it.
OK, there might be billions of exoplanets out there. So what?
Best wishes from the Grumpy Old Codger Cliff
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Post by davep »

Cliff wrote:[SNIP] As for the future research with regards exoplanets, I cannot help wondering how much it gets hyped up to keep "us" public interested. [SNIP] OK, there might be billions of exoplanets out there. So what?
What do you think is actually been "hyped" and what purpose do you think such "hype" would serve.

As to "so what?": For me, at least, Astronomy is one of those hobbies/interests that is something that works as much in the mind as anywhere else. Sure, there's the equipment and the gadgets and the standing in the cold and so on but, more important for me, there's the exercising of the brain. For me the discovery of exoplanets is terribly exciting. When I was a kid (twenty plus years ago or so) books and magazines and the like often spoke in ways that said that there probably are planets around other stars but it wasn't uncommon for them to say that we'd probably never know because it would be so hard to tell. Here we are now with actual evidence. For me personally that's very exciting.

As for the general sense of "so what?", the same line could be written as "OK, there might be thousands of stars and other objects visible in the night sky, so what?". There are people who think like that, I'm glad I'm not one of them because, for me anyway, I'd be missing out on something that's amazing to understand and observe.
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Post by joe »

The gas giant/Brown Dwarf/star scenario is relatively easy to comprehend (well explained KendalAstronomer) but what are the limiting factors, if any, where "rocky" planets are concerned. Do they fail to coalesce after they reach a certain mass, i.e. would the Earth and Venus never be able to join forces to become Venarth?

If Jupiter has a solid core then surely it can't be metallic (in the astronomical sense, i.e. not H or He) as it would not be able to fuse deuterium or hydrogen in the event of it gaining enough mass and density?
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