I have had time lately to indulge my passion for TV shows that offer some science. I was watching one the other day and they were talking about the origin of the solar system. The whole clustering and gravitational collapse and sun birth and planets forming from remnant of the blown off dust and gas. Blown off dust and gas from the fusion reaction that made the sun.

Doesn't a explosion in space kind of go 360 degrees, more or less? So how come the planets are all squeezed into this band about 20 degrees wide. And what is with all of the axis tilts?

David Davis
Toledo, OR 97391

The Sun and planets formed from the same gas cloud which was mostly hydrogen and helium with a smattering of heavier elements and dust.
The cloud had a net angular momentum. As it contracted, it sped up (conservation of angular momentum) and flattened out.
The planets did not form "from remnant of the blown off dust and gas". The Sun and planets formed at the same time from the same cloud. Most of the material formed the Sun.
When the proto-Sun ignited, it blew most of the gas outward where it formed the gas giants (that which did not escape completely), leaving dust in the inner region which coalesced into the inner planets.

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The key is here is collisions.

As Kaptain K mentioned in his reply above, the material in the proto-solar system had some net angular momentum. That means that the gas was spinning around in some particular direction. For simplicity's sake, let's call that "clockwise."

Now, since all the particles were moving around the proto-Sun in the same direction, they would not collide with each other "head-on" very frequently. However, some of the gas molecules had orbits which were tilted relative to others. For example, if one particular molecule had a very steeply inclined orbit, it would fly down through the main body of gas, then back up through the main body of gas, during each full circle around the Sun. It would eventually collide with other particles of gas or dust. Due to the collision, the particle would exchange some of its vertical momentum with the other particle, losing some of its vertical velocity. On the other hand, since both particles were orbiting clockwise, they wouldn't lose much of their "clockwise velocity", and would continue to circle around the proto-Sun with the same speed as before.

Over millions of years, particles would collide with other particles many, many times. The vertical components of velocities would tend to disappear, since roughly equal numbers of particles would collide while moving upward or downward through the main body of the gas. The "clockwise velocities", on the other hand, would remain the same size.

The net result is a system of gas particles which have very little vertical motion, but a great deal of "clockwise" motion ... in other words, a disk.

I've simplified things considerably, but this basic idea -- which is sometimes called "dissipation" -- is at the heart of disk formation.

[quote=StupendousMan;1169997]The key is here is collisions.

StupendousMan. The puzzle lies mostly in the angular momentum. If the spinning proto-solar system gas & dust cloud had angular momentum...it puzzles why most of the mass ends up in the sun, but most of the angular momentum ends up in the planets. Contraction of a spinning disk should put the most of the momentum of the disk in the massive object in the center. ( R.P. Feynman ..Los Alamos Journal of Physics, circa Spring 87, I think, but this is a bit fuzzy)...pete

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[quote=trinitree88;1170125]That angular momentum conundrum was a reason for the rejection of the original Kant-Laplace theory. It should have had the Sun rotating much faster than its actual value. In the meantime, researchers have discovered the electromagnetic effects that can redistribute large amounts of angular momentum away from the center during the contraction process.

Globular clusters are systems of thousands of stars. They have plenty of angular momentum, yet they do not collapse into disks. Why not? Because the particles -- stars in this case, rather than molecules and atoms in a proto-solar-system -- do not collide often enough to dissipate their momentum perpendular to the plane defined by their net angular momentum.

There are many articles written on the dynamics of collisionless systems. You could look some up.

Just because a big cloud of material has some net angular momentum does NOT cause it to turn into a flattened disk.

The hypothesis that is proposed in Encyclopedia Britannica, is that the sun's angular momentum was lost due to the solar wind. (i.e. The sun now has 99% of the solar system's mass and only 1% of its angular mass. The large planet's have 99% of the solar system's current angular momentum.) When the solar system was just formed the sun would have had a rotational period that would have been comparable to the current rotational period of the large planets.

The mechanism noted by StupendousMan would enable a cloud of particles to form a disc.

Isn't a second mechanism required to enable the rotating disc to shed angular momentum to enable it to collapse.

I thought there was a third problem that the disc must be able to dissipate energy, which is easier depending on the metallicity of the particles that form the disc.

Comments:
1) The Encyclopedia Britannica article notes that stars more massive than the sun do not have a solar wind. Any comments. I am going to check that out.
2) This paper by Shaviv hypothesizes that as the sun's rotational period has gradually increased with time, the solar magnetic cycle would have been stronger when the sun has younger due to its faster rotation. The stronger solar magnetic cycle blocks the earth from GCR which is Shaviv's solution to the faint sun paradox (Sun's luminosity was roughly 25% less 4 billion years ago, yet the earth was covered with water. The amounts of CO2 required to maintain a liquid ocean are not reasonable.)

http://arxiv.org/abs/astro-ph/0306477v1

There are a couple of new observations that should cause everyone to reflect upon the most widely accepted theory of solar evolution:

1) The comets that we have observed closely contain clays, pyrenes, and other heavy minerals that were previously assumed to only be found in the inner solar system. (This assumption is driven by the condensation model.)

2) We see evidence in the rocky strata of Saturn's rings that rogue - non-Newtonian orbits are followed by some ring particles.

I'm not going to try to connect the dots here; only point out that we need better explanations for these recent observations.

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Actually, they are expected to be found everywhere in the solar system. However, the ices and lighter materials are also found in the outer solar system.

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On one program they had Mr Petit?? I hope I spelled that name right. He is an astronaut and did a rather simple experiment while on the space shuttle. He took a bag of sand particles and shook them up to see if there would be clumping. There was, but the motion was very random within the bag. I could not see how random bits in space could acquire angular momentum, unless that which started the clumping had a spin to it. Perhaps a piece of spinning stellar matter from a nova?

David Davis
Toledo, OR 97391

Not clays and minerals that require relatively high heats of formation - at least not in any where near the quantity they have been indentified in our cometary assays.

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jwj

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I notice some hypothesizes to explain Close Orbit Large Gas Planets.

One assumed that there was significant disc material left after the star formed. That hypothesis, postulated that the gas planet lost orbital velocity and moved toward the star, by capturing the remaining disc material.

Does anyone have any thoughts concerning this hypothesis?

The following is a link to an article that discuss Pegus’ close orbiting planet. The planet in question is estimated to be roughly ½ the size of Jupiter, with an orbit that is approximately one sixth the distance from Mercury to the sun. (See the scaled drawing that is included in the article.)

http://zebu.uoregon.edu/51peg.html

The following is a paper that discusses simulations to answer, among other questions, whether the existence of a hot Jupiter would rule out the planetary system hosting earth like planets. The conclusion of the analysis was it does not.

http://arxiv.org/abs/astro-ph/0507180

Fogg's simulations even turned out potential Earth-like systems, hot Jupiters and hot Neptunes/super-Earths all in the same simulations. Not all the time, of course, but occasionally. If the underlying theory of planet formation is correct (and there are still holes in it), there could be all kinds of interesting systems out there, once we can see them. The evidence so far seems to support it; most of the things being found already are quite surprising, at least compared to most people's views before we started looking.

The hypothesis used in the above simulations, and probably by most of those studying planetary migration, is that the giant planet clears a gap in its host accretion disc and becomes locked into the viscous evolution time ("type II migration"). Most of the gas is not therefore accreted by the planet but 'pushed away' by horseshoe orbits. See perhaps this review for a (rather brief) summary.

Hi cress,

Thanks for the link.

There are likely a number of initial condition factors (elemental composition of disc, disc mass, relative time and location of first protoplanets, and so forth.) that affect the resultant, as well as fundamental mechanisms that control the process and the resultant.

This is the list of the exoplanets. There is a great deal of variability in the resultant.

http://exoplanets.org/planets.shtml

Do all stars have planets? How is the process and resultant affected by a second companion star?

The graphs in this link include a comparison of the different exoplanet parameters.

http://exoplanets.org/figures.html

William, you might like to look at the extrasolar planets encyclopaedia. Choose a catalogue by detection method, then select the histograms or correlation diagrams at the top. The database is kept up to date and the graphs are drawn on the fly, so they will have more recent stats.

This is a difficult question to answer. (And let's only worry about hydrogen burning stars, not stellar remnants.) The obvious thing to say is "no", but absence of evidence is not evidence of absence; stars that look barren may have planets with masses or on orbits that we simply can't detect yet.

Whether or not a planet is created (or survives) depends very much on the evolution (and presence) of a surrounding accretion disc. Some discs may not be able to produce planets. Such stars may still have debris around them, but not large enough to be called planets. Or a disc may be disrupted by external influences (such as being in a dense stellar nursery) before it can create a planet. Planets may form but be stripped away by another star, leaving nothing significant around the parent star.

I think at the moment the best that can be said is the higher the metallicity, the more likely there is to be a planet. (You can plot metallicity against various planet stats on the site I linked above.) If I remember rightly, about 20% of the G and K class stars surveyed have, to date, turned up planets.


It depends on where the companion is. If it's a close binary, you can form planets from an accretion disc that surrounds both stars (circumbinary), similar to as if it was only a single star at the centre of the system. The binary will make a larger 'hole' at the centre of the disc, and may interact with any resulting planets, but the results are believed to be stable in a variety of situations. (See e.g. http://uk.arxiv.org/abs/0707.2677 and the references in it.)

If it's a wide binary then there's no problem; the planet goes around one star as if the other wasn't there (circumprimary). If the separation was great enough you could have a planetary system around each star.

The interesting scenario is the bit in the middle; a companion far enough to leave an accretion disc, but near enough to interact with it. Too close and it will occupy the space where the disc would be, destroying any chances of having a planet. A bit further out and it will chop off the end of the disc, severely limiting the space (and mass) you have to form a planet with. (Yet the disturbance may somehow trigger/aid planet formation, making up for the deficit.) Until Gamma Cephei b was found, it wasn't obvious whether such planets could exist. What the companion subsequently does to the planet depends on the system's configuration; a blanket statement would be inappropriate.

Fewer binaries have been surveyed than single stars, but that should be partially rectified over the next few years.

Hi Cress,

Thanks for your thoughtful comments. I have additional questions and thoughts.

It is assumed that the planet formation phase is post T Tauri stage for the star in question. During the T Taui stage, the spinning disc of gas and small particles, losses mass as pieces of the circumstellar disc slow down, falling into the proto star. The proto star gains angular moment and reaches a maximum size, based on some set of parameters (say disc composition again, disc density, proto star rotation, and so forth).

Is the period when planets form, the period when accretion into the nascent star has stopped and the circumstellar disc has not dissipated, as discussed in this paper?

"Discovery of Gas Accretion Onto Stars in 13 Myr old h and Persei" by Thayne Currie et al.

http://arxiv.org/abs/0709.1847v2

Planet formation begins the moment the accretion disc begins settling, and continues through the period when accretion onto the star is slowing, which is what the upper quote you gave above is saying. That gives you 5-15 Myr to make a giant planet before the gas is gone; studies like the one above suggest many discs last around 7-10 Myr. Solid accretion continues for a long time after the gas is gone; in the case of the terrestrial planets it's mostly done in about 100 Myr.

Of course, it's not like there's a definite end-point, and you could say it's still going on now. Every time a comet or asteroid hits a planet, that's a little bit more accreted.

This is a good bet. I'd add that the level and location of turbulence in the disc also, probably, influences what you get and where it ends up.