| From: Greg L. ® | 11/12/2000 0:34:30 |
| Subject: The Outer Planets | post id: 184863 |
| As its been a while since any major discussion of the planets has come up in a while, I thought it might be useful to look at the outer planets in some detail. So what makes the outer planets so vastly different from the inner planets, which are made of silicate rocks? Why are Jupiter and Saturn so massive, whilst Uranus and Neptune are considerably smaller? These are fascinating questions that are worth examining in some detail. When the planets formed from the solar nebula some 4.5 billion years ago, there were a crucial number of different mechanisms at work that influence the physics and chemistry of how the planets formed. The inner planets, being close in to the young sun, formed from the accretion of rocky planetisimals. Due to their close proximity to the sun, volatile compounds such as water ice and other ices were not able to condense, but metals and metallic compounds were. This is reflected in meteorites that come from the asteroid belt, that are made of rock or metal but contain few ices and volatiles. Further out in the solar nebula, however, it was a different story. The lower temperatures allowed ices and other more volatile compounds to condense and thus form a significant fraction of the material in the accreting planetismals. The planetisimals aggregated together until they formed a body about 10-15 times the mass of the Earth. Once this stage was reached, the young protoplanet began to capture gas from the nebula in great quantities, in a process known as runaway accretion. The protoplanet, by virtue of its large mass, was able to retain the gases it captured, thus forming a huge envelope of gas around the planet. The planet's major moons would also have accreted from this cloud in a similar manner as the planets themselves did from the nebula surrounding the sun. Jupiter and Saturn are most likely to have formed in this fashion, but what about Uranus and Neptune? In this case there were some differences, which I will look at in my next post. | |
| From: Greg L. ® | 11/12/2000 0:45:35 |
| Subject: re: The Outer Planets | post id: 184867 |
| Uranus and Neptune lie at vastly greater distances from the Sun than either Jupiter of Saturn, and exhibit some considerable differences between the aforementioned planets. Uranus and Neptune, whilst still very large and massive (in comparison to the Earth), are much smaller than Jupiter and Saturn. So why is this the case? The answer again, lies in the distance of the planet from the sun and the conditions under which it originated. At such a vast distance from the sun, the planetesimals at this distance would have been made largely of water, methane and Nitrogen ices mixed with rock. The second important thing to consider is that at this distance from the Sun, there were less light gases such as Hydrogen and Helium from which the infant planets could have gained from the 'runaway accretion' process I mentioned in my last post. Uranus and Neptune are thought to have aggregated from icy planetismals and gas, as Jupiter and Saturn had. However, they accreted less gas, were made more from icy planetisimals, and took longer to accrete than Jupiter and Saturn did from the solar nebula. As a consequence, the planets were of a lower mass than Jupiter and Saturn. The planets also differ somewhat in internal structure and composition. Uranus and Neptune have thinner envelopes of Gas, and have thick 'mantles' of a mixture of methane, water ice, hydrogen and ammonia, with cores of dense material, whilst Jupiter and Saturn tend to have very thick gaseous envelopes, and layers of liquid metallic hydrogen surrounding a dense core of rocky or metallic material. Because of the nature of their formation, Uranus and Neptune are sometimes referred to as the Ice Giants. In my next post I will look at the oddball planet Pluto, and how it fits in with the other outer planets. | |
| From: Greg L. ® | 11/12/2000 0:59:43 |
| Subject: re: The Outer Planets | post id: 184869 |
| Pluto has always presented a bit of a mystery to astronomers until recently. Discovered by Clyde Tombaugh in the 1930's, the planet was revealed to be small, icy and is partnered by a very massive moon known as Charon. Pluto is a small body composed of a mixture of rock and methane, nitrogen and water ices, a composition typical of what is expected at this distance from the sun. Pluto occupies a very odd, highly inclined and eccentric orbit around the sun, unlike the other planets. So how does this little world fit into things? Astronomers have put forward various theories about the origin of Pluto. One popular idea was that Pluto was an former moon of Neptune, ejected after a violent interaction with another satellite. Now however, Pluto seems to fit in quite well with a group of icy bodies lying outside the orbit of Neptune known as the Kupier Belt. The Kuiper belt is a collection of icy bodies similar in nature to Pluto that orbit the sun outside the orbit of Neptune. Originally postulated as a 'reservoir' for short period comets, they are thought to be icy planetisimals that formed in the regions near Uranus and Neptune that were ejected there into more stable orbits by gravitational perturbations by these planets. Observations by astronomers in the early 1990's showed that the belt did indeed exist, and some bodies even share a similar orbit to Pluto. These bodies are known as 'Plutinos' and Pluto now seems to fit in very naturally with the Kuiper Belt Objects. Pluto is most likely the largest 'member' of this belt known, and formed from the aggregation of icy bodies such as this. Charon is most likely the end result of a catastrophic impact with a body a significant fraction of Pluto's size, and impacts with huge bodies and proto-planets were quite common in the early solar system. Pluto is thought to be very similar to Neptune's moon Triton in structure and composition, although many questions still remain about this enigmatic little body. A space mission to Pluto would shed much light on Pluto, its formation, and about the outer planets in general. Sadly, it appears that the best mission to do this (Pluto-Kuiper Express) is about to be cancelled, and it may be many more years before another mission is sent to this planet. | |
| From: Dropbear ® | 11/12/2000 9:46:08 |
| Subject: re: The Outer Planets | post id: 184897 |
| Greg, How does this theory account for the presence of large 'planets' found orbiting very close to other stars recently? How could a Jupiter sized object form so close to its sun? | |
| From: mike h | 11/12/2000 11:03:16 |
| Subject: re: The Outer Planets | post id: 184909 |
| I can remember reading or hearing about a possible new outer planet being discovered - did this turn out to be true, or was it just some grit on a telescope lens? Cheers, mike h | |
| From: B.C. ® | 11/12/2000 12:15:46 |
| Subject: re: The Outer Planets | post id: 184937 |
| Since no one else is answering your question Dropbear, I'll have a go. Maybe these extra-solar, gaseous giants were able to form because at the time that the matter coalesced there parent star might have had less mass, or alternatively been a quite star and had minimal solar wind to blow the gaseous nebulae away. Or maybe the star went through a period in it's early history where it's temperature wasn't as high as is today. Grasping at straws but I haven't heard a proper explanation yet. Another idea has just popped into my head, maybe these close orbital gaseous giants are just a step below brown dwarf stage and may have been forming when the parent star was forming, and just failed to qualify for that status. | |
| From: Greg L. ® | 11/12/2000 12:36:46 |
| Subject: re: The Outer Planets | post id: 184945 |
| There's an excellent Scientific American article that answers your question, Dropbear, but I don't have it with me at the moment. I'll post the detailed explanation in the next few days or so. In short, the gas planets are believed to 'spiral in' closer to the parent star due to drag and friction with the nebula from which they form. | |
| From: Greg L. ® | 20/01/2001 19:40:07 |
| Subject: re: The Outer Planets | post id: 211329 |
| This is a post to answer Dropbear's original question about the bizarre orbits of extrasolar planets. The discovery of extrasolar planets in the past few years has sent theorists back to the drawing board, but they are now beginning to piece together the parts of the puzzle. The details are described in the Scientific American article Giant Planets Orbiting Faraway Stars by Geoff Marcy and R. Butler. What we have learned about the nine planets in our own solar system has constituted the basis for the conventional theory of planet formation. The theory holds that planets form in a flat, spinning disk of gas and dust that bulges out of a star's equatorial plane, much as a pizza dough flattens when it is tossed and spun. This model shows the disk's material orbiting circularly in the same direction and plane as our nine planets do today. Based on this theory, planets cannot form too close to the star, because there is too little disk material, which is also too hot to condense. No do planets clump together extremely far from the star, because the material there is too cold and sparse. Considering what we now know, such expectations about planets in the rest of the universe seem narrow-minded. The planet orbiting the star 47 Ursae Majoris in the Big Dipper constellation stands as the only one resembling what we expected, with a minimum bulk of 2.4 Jupiter masses and a circular orbit with a radius of 2.1 AU. If placed in our solar system, this new planet might appear as Jupiter's big brother. But the remaining planetary companions to other stars baffle us. The two planets with oval orbits have eccentricities of 0.68 and 0.40 (an eccentricity of zero is a perfect circle, whilst an eccentricity of 1 is a long, slender ellipse). In contrast, in our solar system the greatest eccentricities appear in the orbits of Mercury and Pluto, both about 0.2, whilst all other planets show nearly circular orbits. These eccentric orbits have prodded astronomers to scratch their heads and revise their theories. Within two months of the first planet sighting, theorists had hatched new ideas and adjusted the standard planet formation theory. | |
| From: B.C. ® | 20/01/2001 19:50:21 |
| Subject: re: The Outer Planets | post id: 211330 |
| Maybe their part of a binary system, with the other partner a neutron star or BH without an accretion disk, at this stage. | |
| From: B.C. ® | 20/01/2001 19:52:05 |
| Subject: re: The Outer Planets | post id: 211332 |
| That should be binary | |
| From: Greg L. ® | 20/01/2001 19:52:27 |
| Subject: re: The Outer Planets | post id: 211333 |
| For instance, astronomers Pawel Artymowicz of the University of Stockholm and Patrick M. Cassen of NASA Ames Research Centre recalculated the gravitational forces at work when planets emerge from disks of gas and dust seen swirling around young, Sunlike stars. Their calculations show that the gravitational forces exerted by protoplanets-planets in the process of forming-on the gaseous dusty disks create alternating 'spiral density waves.' Resembling the arms of spiral galaxies, these waves exert forces back on the forming planets, driving them from circular motion. Over millions of years, planets can easily wander from circular orbits into eccentric, oval ones. A second theory also accounts for large orbital eccentricities. Suppose, for instance, that Saturn had grown much larger than it already is. Conceivably, all four giant gas planets in our solar system could have swelled into bigger balls had the original protoplanetary disk contained more mass or had existed longer. In this case, the solar system would contain four superplanets, exerting gravitational forces on one another, perturbing one another's orbits and causing them to intersect. Eventually, some of the superplanets might be gravitationally thrust inward, others outward, an unlucky few even ejected from the planetary system. Like balls ricocheting on a billiards table, the scattered giant planets might adopt extremely eccentric orbits, as we now observe for three of the new planets. Interestingly, this billiards model for eccentric planets shows that we should be able to detect the massive planets causing eccentric orbits-planets perhaps orbiting further out from the planets detected thus far. A variation on this theme suggests that a companion star, rather than other planets, might gravitationally scatter planet orbits. | |
| From: Greg L. ® | 20/01/2001 19:53:42 |
| Subject: re: The Outer Planets | post id: 211335 |
| Actually some planets were found around a pulsar in 1991, probably formed from the leftover debris of the supernova itself. Needless to say, they wouldn't be a great place to take your next holiday. | |
| From: Greg L. ® | 20/01/2001 20:15:28 |
| Subject: re: The Outer Planets | post id: 211338 |
| The most bizarre of the new planets are the four so-called 51 Peg Planets, which show orbital periods shorter than 15 days. The four members of this class are 51 Peg itself, Tau Bootis, 554 Cancri and Upsilon Andromedae, which have orbital periods of just 4.2, 3.3, 14.7 and 4.6 days respectively. These orbits are small, with radii less than 1/10th the distance of Earth from the Sun, and less than 1/3 Mercury's distance from the Sun. Yet these planets are as big as or bigger than Jupiter's mass. The 51 Peg planets defy conventional planet formation theory, which predicts that giant planets such as Jupiter or Saturn should form on the cooler outskirts of a protoplanetary disk, at least five times the distance from Earth to the Sun. To account for these planetary oddities, a revised planet formation theory is making the rounds in theorists' circles. Astronomers Douglas N.C. Lin and Peter Bodenheimer, both of the University of California, and Derek C. Richardson at the University of Washington extend the standard model by arguing that a young protoplanet precipitating out of a massive protoplanetary disk will carve a 'groove' in that disk, separating it into outer and inner sections. According to their theory, the inner disk dissipates energy because of dynamical friction, causing the disk material and the protoplanet to spiral inward and eventually plunge into the star. A planet's salvation stems from the young star's rapid rotation, spinning every 5-10 days. Approaching this star, a planet would cause tides on the star to rise, just as the Moon raises tides on the Earth. With the young star rotating faster than the protoplanet orbiting the star, the star would tend to sprout a bulge whose gravity would tug the planet forward. This effect would tend to whip the protoplanet into a larger orbit, halting its deathly inward spiral. In this model, the protoplanet hangs poised in a stable orbit, delicately balanced between the disk's drag and the rotating star's forward tug. Even before the discovery of the 51 Peg planets, Lin predicted that Jupiter should have spiraled into the Sun during its formation. If this were so, why did Jupiter survive? Perhaps our outer solar system contained previous 'Jupiters' that did indeed spiral into the Sun, leaving our Jupiter as the sole survivor. Why, we wonder, does no large 51 Peg-like planet orbit close to our Sun? Perhaps Jupiter formed near the end of our protoplanetary disk's lifetime. Or the protoplanetary disk may have lacked enough gas and dust to exert sufficient tidal drag. Perhaps protoplanetary disks come in a wide range of masses, from a few Jupiter masses to hundreds of Jupiter masses. In that case, the diversity of new planets may correspond to different disk masses or lifetimes, or perhaps even to different environments, including the presence or absence of nearby radiation emitting stars. Source: Giant Planets Orbiting Faraway Stars by Geoff Marcy and R. Butler, The Scientific American Book of the Cosmos, St Martin Press, 2000 | |