A recent anthology of Scientific American reprints included an article from the October 2001 issue, which discussed the GHZ (Galactic Habitable Zone) suitable for the development of life. The article concludes that complex life forms are rare in our galaxy. A solar system too close to the center of the galaxy is not likely to harbor any complex life forms due to orbital instabilities (caused by rapidly moving stars), too much radiation (due to massive stars & activity of the central black hole), and cometary/asteroid impacts (due to more objects in the Oort Cloud & Kuiper belt, as well as more disturbances of those regions caused by nearby stars). A solar system too far from the galactic center will not have sufficient heavy elements, which are necessary to living organisms. Elements other than hydrogen and helium are created by stars which go nova at the end of their life cycles. In particular, type II novae are required for the creation of many of the heavier elements. In the outer fringe of the galaxy, there have not been enough nova events to create the abundance of heavy elements required by life forms. The above considerations limit the GHZ to a narrow ring centered near the position of the solar system (about 28,000 light years from galactic center). Our solar system happens to be in a unique zone of the galaxy. The orbital rotation rate of our solar system around the galactic center closely matches the rotation rate of the galaxy. This results in our staying away from the center of the spiral arms for billions of years. This might be a necessary condition for the development of complex life forms. Near the center of the spiral arms the density of stars is higher, making disturbances of the Oort Cloud and Kuiper belt objects more likely. There is also increased likelihood of possible orbital instabilities due to stars passing even closer than the distance resulting in Oort Cloud disturbances. If being in this special zone is a requirement for the development of complex life forms, the GHZ is very narrow. Note that it took about one billion years for simple life to occur on the Earth and another 2 billion or so years for complex life forms to evolve. It could be that staying away from the spiral arms for 3 billion or so years is required for the development of complex life forms. There is another interesting consideration mentioned by the SciAm article. The required abundance of heavy elements in the GHZ in conjunction with a less dangerous environment less did not occur until about 5 billion years ago, about the time of the start of the gravitational collapse of the dust cloud which formed our solar system. The size of the GHZ restricts the development of complex life forms to a particular region of space. The history of the creation of heavy elements and the lessening of catastrophic events restricts the development of complex life forms to recent times. All of the above strongly suggests that we might be the first intelligent life form in our galaxy, and possibly the only one. The SciAm article also mentioned the Fermi Paradox. A space traveling ET searching for habitable planets and/or other technological civilizations would not roam the galaxy randomly. They would search the GHZ. Since we have no evidence that they have noticed us, we can surmise that they do not exist, or they are not searching, or they have not been searching for a long time.
"A solar system too far from the galactic center will not have sufficient heavy elements, which are necessary to living organisms." i disagree. Through Nuclear fusion, a star can create heavy elements out of hydrogen.
Gonzalez seemed biased in his article. Here is a more recent article by Charles Lineweaver, one of the top astronomers alive today, that appeared in the Jan. 2, 2004 issue of Scientific American: http://www.sciam.com/article.cfm?articleID=00017D43-CFD9-1FF1-8FD983414B7F0000 A copy and paste: "Taking a different approach, astronomers have now identified the most likely places in our galaxy for other inhabited solar systems to exist. The analysis suggests that up to 10 percent of the stars in the Milky Way could offer conditions necessary to support complex life. Charles Lineweaver of the University of New South Wales in Australia and his colleagues modeled the evolution of our galaxy and analyzed the requirements for the so-called galactic habitable zone (GHZ). The team traced the distribution of four prerequisites for life: the presence of a host star, sufficient heavy metals to form terrestrial planets, enough time for biological evolution and a location that is safe from deadly supernovae. The findings, published today in the journal Science, indicate that the GHZ is a slowly spreading region located about 25,000 light-years from the galaxy's center. The stars encompassed by it formed between four billion and eight billion years ago; three quarters of them are a billion years older than the sun."
Only up to Iron... and it's not much good deep inside a star! Although old news, this is still really interesting. Or that any evidence is hidden, of course! Seriously, it's well within reason that an "searching" ETI might deliberately conceal their presence for an extended surveillance period.
Whenever a supernove or hypernova occurs anywhere in our galaxy or any galaxy there's going to be the creation of many heavy elements.
Yes, supernovae produce heavy elements. They can produce those beyond iron. Some of you missed the point of the SciAm article. The novae events producing heavy elements tend to occur nearer to the galactic center. The farther from galactic center, the less frequent are such events. which is why the GHZ, does not extend to the edge of the galaxy. The issue of how many stars in the GHZ can support life is controversial. Most are in the spiral arms. It is not clear that the environment in the spiral arms is suitable for the development of life. Also controversial is the issue of the solar system's orbital speed matching the galactic rotation rate. If this is important, the GHZ is very narrow. At any rate, the older concept that life can exist anywhere in a galaxy is no longer valid.
Here are additional considerations supportive of your view Dinosaur: It is evident from the exoplanets discovered so far that inward migration of gas giants is a commonplace occurence. These will either accrete or eject any nascent terrestrial planets in the system during their migration. If the gas giants are too small, or too distant from the parent star they may not intercept a sufficient number of cometary bodies. As a result catastrophic impacts akin to the KT event will be common. Advanced life will be unable to develop. In contrast it is the occasional intervention of one or other catastrophic mechanism, such as bolids impact, that has enabled bursts of evolution that have led to advanced life forms. The balance between too few or too many such events is likely a fine one. The emergence of life onto the land was doubtless facilitated by tidal effects on littoral ecologies. In the absence of the moon the tides would have been much smaller and possibly ineffective. The moon is an unusual body, being so comparativley large. Its existence is the consequence of a Mars sized body striking the Earth at a particualr angle, and at a particular time. Such a combination of events is probably rare. Had this collision not occured the Earth would have a smaller core and a ligher crust. The smaller core may well have had implications for the maintenance of the magnetic field which is critical for preventing atmospheric loss. (This would also have been accelerated by the Earth being less massive.) The greater proportion of granitic materials would have led to a crust that was continental in character, rather than oceanic and continental. This would very likely have locked the lithospheric plates in place within the first couple of bilion years of Earth history. Without plate tectonics the various stabilising cycles it is part of would have ended and life would have ceased. or been limited to microbial forms. The list could readily be continued. The odds just keep building up against (m)any other intelligent life forms.
Ophiolite: Your mention of Luna, got me thinking off the topic of this thread. Did you know that Luna has a perturbed planetary orbit rather than an Earth satellite orbit? Sol’s gravitational force on Luna is about twice the force due to the Earth. Unlike other moons in the solar system, Luna always moves in a generally clockwise direction.
Just about half the moons in the solar system share that characteristic.(Always movng in the same direction heliocentrically) .BTW, the Moon generally considered as moving in an counterclockwise direction by convention. The characteristic that the Moon's orbit has that does differ from the majority of other Moons in the solar system is the fact that it's orbit is always concave to the Sun.
Dinosaur, can you explain what you mean by a 'perturbed planetary orbit' for Luna? Like most (all?) larger moons in our solar system, our moon has a clockwise orbit (prograde) rather than the retrograde orbits common to captured moons. Captured moons are mostly irregular shaped astroids that were captured by their home planet. There are about 125 moons known in the solar system today, mostly captured astroids. I would also like to see a reference to your assertian that the sun exerts twice the gravitational influence on Luna as Earth. For a stable orbit to exist, the gravitational force exerted by the home planet has to be greater on the moon than the gravitational force due to the sun. This is known as the Hill sphere, the zone of stable orbits around a body.
Fg = GMm/D² Gravitational force between Earth and moon: Fg[earth-moon] = 6.673e-11(6e24)(7.35e22)/(3.84e8)² = 1.99e+20 Newtons Gravitational force between sun and moon Fg[sun-moon] = 6.673e-11(2e30)(7.35e22)/(1.496e11) = 4.38e+20 Newtons Fg Fg[sun-moon]/fg[earth-moon]=4.38e+20/1.99e+20 = 2.2 The Hill sphere describes the region where the planet's hold on the satellite is enough to prevent the tidal force due to the Sun acting across the satellites orbit from pulling the satellite free. A "perturbed Planetary orbit" means that the Moon primarily orbits the Sun (as per the fact I already mentioned that the Moon's orbital path is curved towards the sun(concave) at all points.) The Earth and moon follow sychronized Solar orbits, which due the the perturbations each casues on the other, weave in and out from the Sun. When Dinosaur says the Moon's motion is always prograde he means in it's heliocentric path, or the path it takes with respect to the Sun. About half the Moons in the Solar system have a retrograde motion during portions of their heliocentric path.
My understanding is the moon orbits the Earth. The Earth and moon both orbit around a gravitational 'center', the barycenter of the orbits. This Earth/moon barycenter orbits the sun. An 'unperturbed' planetary orbit is a circular orbit of a planet around the sun. A perturbed planetary orbit is a planetary orbit around the sun with perturbations caused by any other bodies in the solar system, resulting in an elliptical orbit. Obviously, the moon is not a planet. Maybe the moon's orbit can be called a 'perturbed orbit' or a 'perturbed lunar orbit'.
Janus58: Thanx for saving me the time required to research the numbers and do the arithmetic. Luna’s orbit is not always concave toward the sun. If you plot the Earth’s orbit and superimpose a plot of Luna’s path, the Lunar path is suggestive of a sine-wave. It is sometimes inside the Earth’s orbit, at which times it is convex relative to Sol. It is sometimes beyond Earth’s orbit, at which times it is concave relative to Sol. If you superimpose a plot of a true satellite on its planet’s orbit, you see loops caused by the satellite sometimes having a component of motion opposite to the direction of motion of the planet. I do not know of any moons other than Luna which never reverse direction relative to the planets they are satellite to. A perturbed planetary orbit varies significantly from an ellipse or a circle. It is an orbit such as that described above for Luna. In theory you only get a circular or elliptical orbit in a two body system. In practice, most planetary orbits are very close approximations to an ellipse or circle. I once had a physics professor who played a prank on the class. He asked us to derive a formula for a three body system (Star, planet and moon) which indicated the maximum distance at which the planet could hold its moon against the star’s gravitational pull. We all assumed that at the maximum distance, the star’s pull would be equal to the planet’s pull on the moon. Several of us managed to come up with essentially the same formula. The professor then gave us the data for the (Sol, Earth, Luna) system and asked us to apply our formula. I think it came out to be about 160,000 miles. He then pointed out that Luna was about 240,000 miles from Earth, suggesting that our formula was wrong. What was wrong was our assumption about equal gravitational forces. Beyond that distance, the satellite has a perturbed planetary orbit, while inside that distance it has a true satellite orbit.
An 'unperturbed' orbit is any orbit, circular or elliptical, which is not effected by other bodies other than the primary and the satellite. A 'perturbed' orbit is any orbit, circular or elliptical, which is effected by other bodies. Why is the moon obviously not a planet? The Earth-moon system can just as easily be considered a double planet system as an planet-satellite system. In fact, there are some astronomers who would argue that it is better described as a double planet arrangement.
Pete: Since there are Solar eclipses due to Luna, there must be times when Luna is closer to Sol than earth. Since there are Lunar eclipses due to the Earth, there must be times when the Earth is closer to Sol than Luna. The above suggests that my description of the Lunar orbit is likely to be correct.
Yes, it is. Consider that the width of the Moon's orbit is only about 1/200 that of the average distance from Earth to Sun. Also consider that the Moon completes about 12.38 of these "sinewaves" during one Year. In the time it takes the Moon do its swing in and out, the Earth moon system has traveled around 29° of a circle. If you superimpose that sinewave on an arc of that length, it is so shallow that the Arc dominates the curvature at all points, making the path always concave to the Sun. Taking just the inner half of the Sinewave (that part where the Moon's path would be most likely to curve away from the Sun. During this time the Earth moon system travels an arc in its path of 14.5° The height of said arc is 1.496e6 x (1-cos(14.5)) = 1,196,000 km. The inner half of the moon's sinewave path, travels inside this arc touching it at both ends and at its furthest, gets 384,000 km to the inside of it. It is obvious from this that its path never curves outward. Jupiter Orbital velocity around Sun: 13 km/sec Ganymede Orbital velocity around Jupiter: 10.8 km/sec Thus even when Ganymede is traveling in a direction opposite that of Jupiter's orbital velocity it is still traveling in the same direction around the Sun as Jupiter. Since all the moons which orbit further than Ganymede have smaller orbital velocities, they also travel "forward" at all points of their heliocentric orbit.
Interesting thread. - I have learned from post here and now ponder (slightly) questions that never occurred to me before. I did not understand Ophiolite’s point about plate tectonics cycles encouraging advanced life forms nor why he asserts that: “inward migration of gas giants is a commonplace occurrence” Perhaps he will explain more fully. I note that which way they are migrating is not an empirical fact. Only “large planets” that are relatively near their sun can be discovered by current techniques Thus fact the known ones are both relative closer to their sun than Jupiter and larger than Jupiter can not be taken as evidence for this migration. The moon is migrating AWAY from the Earth because of tidal interactions, but this fact may not be a strong counter argument against the Ophiolite’s “towards star migration” as it simplistically appears to be, because I think that under certain conditions, which I can no longer recall, the lesser body can even escape from the greater by tidal interactions, although it may take infinite amount of time to do so as these interact rapidly weaken as the separation increases. Janus 58‘s “perturbed orbit” is any that differ from the ellipse (circle being an ellipse with 0 eccentricity) whereas Dinosaur’s must be “significantly” different from the ellipse. I am inclined to Dinosaurs’s position, because as Janus58 fully understands, no orbit is strictly an ellipse. Much more interesting than this quibble over definition of “perturbed orbit” is the definition as to what is a dual planet system, and is the moon one of the members of one or a satellite of the Earth. No one here has offered a definition of “dual planet system” so I will be bold and offer one: If the orbits of two bodies orbiting their star are both always concave towards the star, but each can sometimes be closer to the star than the other, then they constitute a dual planet system, and neither is a satellite of the other. (The “but…” condition is required to keep, for example, Earth and Mars from being a dual planet system.) If others accept my definition, then we can determine if the Earth and moon is, or some day could be, a dual planet system. Clearly, a NEO satellite is not a is not a “dual planet system” by this definition. Dinosaur has (perhaps unintentionally) offered the following definition of a “true satellite.” “If you superimpose a plot of a true satellite on its planet’s orbit, you see loops…” I do not like this definition. I don’t know, but suspect that some of the objects man has launched into space which still orbit the sun in one Earth year, perhaps even the geo-stationary ones, have no loops in their solar orbits. Thus I will stick with my above definition which defines both “satellite” and “dual planet systems” in terms of the “solar concavity” or not of both their orbits. I will close with two questions, which I can not easily answer: (1) Beyond what constant separation form the Earth, if any, does a satellite become a planet? (2) Has man ever made a planet that with Earth constitutes a dual planet system?
Just for clarification here are two graphical representations of heliocentric paths of moon orbits, the first is of Luna: Please Register or Log in to view the hidden image! This shows the half of the Luna's orbit in which the moon passes inside of the Earth's orbit. (trying to show the whole orbit would make things less distinct. The white arc represents the Earth-Luna system path and the yellow line the Moon's path. Here it is easy to see that the moo'n's path remains concave to the Sun. For comparison, here is Callisto's path: Please Register or Log in to view the hidden image! Here we see that callisto's heliocentric path does curve away from the Sun at points of its path. However it still never travels retrograde in its heliocentric path.
