The Original Solar System ... (planets formed in pairs???)

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The Original Solar System ... (planets formed in pairs???)

Post#1 » Wed Feb 21, 2007 11:11 am ... system.asp

The Original Solar System

Tom Van Flandern, Meta Research [Reprinted from Meta Research Bulletin, Vol. 6 p. 17 -- 97/06/15]

Summary: In the light of the multiple exploded planet hypothesis, evidence in this book that Mercury, Mars, and Pluto are escaped moons rather than original major planets, and the arguments in Chapter 19 in favor of a solar fission origin for the major planets, we re-visit the original solar system. We take note of the six original major planets to occur in "twin" pairs, and of the main asteroid belt and new trans-Neptunian belt to apparently each have two parent bodies as well. If fission is considered as the principal mechanism for all major planet and natural moon formation, then solid planets will tend to form with singlet moons, whereas gaseous parent bodies (including the Sun) will tend to fission off smaller bodies (moons) in nearly-twin pairs. We examine the theory of formation by fission and compare it to the major planets and large, natural moons of the solar system. A very good match is found, including the surprising fulfillment of a prediction of the model regarding the order of the pairings in a previously unrecognized pattern. Using a Titius-Bode law for planetary spacing in its simplest form (where each planet has double the period of the previous one), we infer the existence of twelve original major planets, of which half remain today. Two short-lived gas giant planets may be responsible for the "late heavy bombardment" episode in the early solar system, and for building up the mass of Jupiter.


The solar system presently consists of nine known planets. But the distinction between "planet," "moon," "asteroid," and "comet" is somewhat arbitrary, being based mainly on broad, general appearance. For each category, marginal cases exist that can be argued either way. For example, Pluto is sometimes said to be too small to be a major planet. It was suggested in Chapter 17 that Pluto would be more properly classified as a former moon of Neptune,[183] as a large asteroid, or even as a comet. Indeed, Pluto would probably sprout a coma and tail if it were brought considerably closer to the Sun.

In the inner solar system, Mercury is very likely to be an escaped moon of Venus, a thesis for which a great deal of evidence exists (see Chapter 13). In Chapter 24, the author suggested that Mars is a former moon of “Planet V," the original planet next out from the Earth. Although none of these suggestions can yet be proved, considerable evidence can be brought to bear in support of each case.

If we accept these tentative identifications and exclude the three smallest planetary bodies from consideration, it is interesting to look at what is left by way of true, major planets in the original solar system. First we have Venus and Earth, both rather similar in mass, composition, solar distance, and number of original significant moons (if our premise about Mercury is correct). Following the asteroidal gap, we have the two largest gas giants, Jupiter and Saturn, likewise with similar composition and numerous moons, and with masses and solar distances more similar to one another than to any other planet. Next out we have another pair of twins, Uranus and Neptune, with similar masses, compositions, and solar distances. Their number of original significant moons would likewise have been similar if the conjecture about Pluto and Charon’s origin as former Neptunian moons is correct.

One aspect of this picture is striking: a tendency for these planets to occur in pairs. Two of these pairs are similar enough for the respective planets to occasionally be called “twins." Jupiter and Saturn might well be called “twins” too if Jupiter were a bit less massive and Saturn lacked its beautiful rings. But the rings of Saturn are almost certainly a recent addition to that planet (Kerr, 1996).[184] And as we will see, there is reason to suspect that Jupiter had considerably less mass at the outset than it now does.

Each pair is notably dissimilar to its adjoining pair or pairs. Now there is no particular reason under the “primeval solar nebula” hypothesis of planetary formation why this should be so. The nebula from which the planets allegedly condensed should have been rather homogeneous in most respects, and planet masses should have had a smooth radial gradient with solar distance.

On the other hand, Chapter 19 argued that origin of planets by fission from the Sun should be reconsidered because it elegantly solves several problems the standard model does not. For example, if planets fission from the Sun due to overspin while the Sun is still accreting, this more easily explains how 98% of the solar system’s angular momentum ended up in the planets. That fact has always been considered significant for understanding solar system formation since all the planets combined have less than 0.002 of the mass of the Sun.

The fission hypothesis would also solve the mystery of the dominance of prograde rotation for these original planets, since they would have shared in the Sun’s prograde rotation at the outset. J.J. Lissauer[185] summarizes the latest results on this puzzle for the standard model: “Almost all the previous calculations were wrong … If you accrete planets from a uniform disk of planetesimals, the observed prograde rotation just can’t be explained.”

The Fission Theory for the Origin of Planets and Moons

There are some basic similarities between the solar fission hypothesis for origin of the planets and the more traditional theory of accretion from the primeval solar nebula. In both cases, an extended cloud of gas and dust contracts, with a concentration toward the center eventually becoming dense and hot enough to be classified as a star. Once that happens, the extended cloud of gas and dust, stabilized in size, forms a rapidly rotating disk outside the parts of the proto-Sun where nuclear fusion is taking place. The core collapses gravitationally from the inside out, with internal heat stabilizing the configuration. The disk will tend to continually spin up the central star. But the central star cannot continue to accrete matter from the rapidly rotating disk without flinging a significant fraction of it back out. The mechanism for that is still debated, with some astronomers favoring polar outflow models and others favoring outflows that originate in the nebular disk.[186]

However, it would be incorrect to think of the disk as comprised of numerous discrete globules that can collide and accrete, as the solar nebula hypothesis requires. Recall that two bodies in similar orbits around a central mass will go into a state of libration and avoid collisions. (See Chapter 6.) The Trojan asteroids in Jupiter’s orbit, for example, always avoid collision with Jupiter by librating. The more similar the orbits any two bodies have, the more nearly impossible collision between them becomes.

Meanwhile, the core of the proto-Sun has no such problems accreting mass from the disk through collisions and steadily growing in mass and size. And there is a more natural way for the planet formation process to proceed from there. The spin-up of the proto-Sun will make it first oblate, then prolate as it approaches an overspin state where centrifugal forces exceed gravitational forces. This shape is called a “Maclaurin spheroid." When the star-disk boundary reaches the overspin condition, the two prolate bulges on opposite sides of the proto-Sun break away and form twin proto-planets in low orbits just above the proto-star surface in the inner disk. This tendency to form planets in twin (although generally not identical) pairs is the reason we have chosen to revisit this model in this chapter.

Artist's concept of planetary fission. Click here to view a
short animation (419KB).

The remainder of the process is similar to the proposed formation of the Moon by fission from an over-spinning Earth. (Chapter 14. See Binder, 1984 for a diagram and description of this process as it applies to the fission of the Moon.[187]) The initial spin of the proto-planets, as well as their orbits, would be that of the surface of the proto-Sun, and therefore always prograde. Subsequent tidal evolution will evolve the twin proto-planets outward at slightly different rates, with the more massive of the two evolving outward faster because its tidal forces are stronger. Although such tidal forces are negligible in the solar system today, they would have been substantial during the formation stages for the Sun, called its “T Tauri” phase. (Note: Each doubling of the solar radius increases such tidal forces by roughly two orders of magnitude.) Such orbital evolution is very similar to that of Mercury relative to Venus following the inferred tidal escape of the former from the latter (see Chapter 13).

Evolution of the planets would typically proceed via tides and drag toward the maximum-stability Titius-Bode-Law configuration, wherein each planet has a circular, co-planar orbit with double the orbital period of the next planet in. Once that was achieved, further orbital evolution would cease. All the major features of the planetary system can apparently be derived from the process just outlined, although it is different in important ways from the standard solar nebula picture.

Satellite Formation

Some special cases need to be considered further. First consider the case that the rotating body is solid or has substantial material strength, as for the Earth-Moon system. Then just the weaker of the two globules at either end of the prolate major axis would fission, and the rest of the body would snap back to a smaller, rounder shape with a slower spin. After all, the spinning body has given away a substantial part of its angular momentum to the fissioned globule. So only a single moon results. This would apparently be generally true – gaseous or liquid parent bodies would produce pairs of moons by fissioning, whereas solid bodies would produce singlet moons.

It is usually objected that tidal friction between a proto-planet and a gaseous parent, such as the proto-Sun, ought to be negligible because the gaseous parent can reshape itself so that any tidal bulge has no lag or lead, and therefore transfers no angular momentum to the proto-planet. This argues that frictional forces between Sun and proto-planet will always be negligible, so no significant orbital evolution will take place. However, as explained in Chapter 6, it is not the usual longitudinal tidal forces that are effective in this case, but rather radial tidal forces. The proto-planet causes the proto-Sun to bulge radially outward. But the proto-Sun is rotating differentially with depth, slower toward the center and faster toward the surface, as we saw above. So as the proto-Sun bulges, part of its mass is raised into a layer with faster rotation, thereby causing a leading tidal bulge that would cause the proto-planet that raised it to evolve outward.

Now consider a gaseous proto-planet. It will cool and contract rapidly once away from the proto-Sun. And as it does so, it too will acquire differential rotation, but with the fastest rotation toward its core and the slowest rotation toward its surface – the opposite of the rotation gradient for the proto-Sun. As the proto-planet contracts, it must spin up to conserve angular momentum, so it too can reach an overspin condition. Being gaseous, a pair of moons will be spawned by the fission process. But unlike the solar case, the differential rotation of the parent will cause a tidal bulge lag, which will cause the moons to lose angular momentum and spiral slowly inward.

We are safe in presuming that the parent planet will contract faster from cooling than the tidal forces can operate to bring moons down. If that were not the case, no moons would be likely to survive. The observable major difference this causes with the planet case is that the more massive of any pair of moons should evolve inward faster. For planets, as we saw earlier, the more massive member of each pair should be the outer one.

Application to Planet Formation

It will now be apparent why the appearance of the solar system’s major planets in matching pairs is significant. That fact strongly favors a fission origin over a traditional solar nebula origin. But after each planet pair is formed in this way, it will be some time before the Sun and extended cloud reach another overspin as they continue to contract. By that time the Sun will be hotter and more massive and the extended cloud smaller. So the next pair of planets will fission under rather different conditions, forming another pair of planets similar to one another but dissimilar from all previous pairs.

That is the theory. How does it compare to reality in the solar system? The Venus-Earth and Uranus-Neptune pairs conform to expectations, assuming that some later event tilted the spin axis of Uranus. The more massive planet in each of these “twin” pairs is the outer one, as the theory requires. However, the Jupiter-Saturn pair has the more massive planet (Jupiter) closer to the Sun. Indeed, Jupiter is roughly three times more massive than Saturn, which brings into question whether it may be considered a “twin” at all.

The next area of attention is the large gap between Mars and Jupiter. Readers will by now be familiar with the exploded planet hypothesis, and its inference that two major planets exploded there during the past half billion years. Of these two, “Planet K” in the main asteroid belt was surely the larger, giving rise to C-type asteroids which apparently comprise 80% of all main-belt asteroids. “Planet V” was apparently located roughly where the orbit of Mars is now, and was the parent of Mars prior to its explosion, as well as the source for S-type asteroids.

The two planets hypothesized to have exosted and exploded in that gap, K and V, might be considered another pair of “twins." The more massive of the two is the outermost, as expected by the fission theory. And the two are certainly alike in that they both exploded after roughly four billion years of evolution. Beyond that, we can say little except that most of their mass would have been vaporized by the explosion, and almost all of it that did not escape the solar system would have been swept up by Jupiter. Solar wind and radiation pressure would have ensured that all vaporized material would be driven toward Jupiter until it was swept up by that gas giant planet. So this scenario implies that Jupiter’s mass has been increased from its original value by some unknown but significant amount. This apparent fact makes us pause to consider whether we must regard Jupiter as a contradiction of the fission theory simply because its mass is larger than Saturn’s. Perhaps its original mass was smaller than Saturn’s. Among the supporting evidence for this conjecture we find suggestions that Jupiter’s mass has apparently increased by roughly 40% just since its asteroidal moons were captured, probably within the last billion years.[188]

However, we note that hypothetical Planets K and V were terrestrial planets, judging from the asteroids from them that we observe today. That fact implies generally smaller masses, probably in the 4-10 Earth-mass range. Had they been substantially larger, they would surely have been gas giants without a solid surface capable of producing asteroids. So using our best inference, there is insufficient mass in planets K and V to account for Jupiter’s apparent excess mass under the fission theory.

But that presents a new challenge. There is apparently a huge mass difference between Planet K (perhaps 10 Earth masses) and Jupiter (318 Earth masses). Such a large mass discontinuity would be unexpected in most formation theories, and would seem to require some ad hoc mechanism to explain it. The only alternative is to conjecture that another planet formerly existed between Planet K and Jupiter to provide an intermediate mass stage of planet formation. That planet (or planet pair, if we are guided by the fission theory), like Planets K and V, may have exploded, completely vaporizing in the process. If that explosion were early enough in the solar system’s history, the gap left behind could have been closed by tidal and drag evolution of the remaining planets, since these processes were still occurring in the early solar system.

In support of this conjecture, we offer two additional lines of evidence. The first is direct evidence for the explosion of one or more very large planets in the very early solar system. From studies of lunar rocks it is now known that the Moon, and presumably the entire solar system with it, underwent a “late heavy bombardment” of unknown origin not long after the major planets formed. The following are relevant descriptions of the event:[189]

“[The late heavy bombardment] occurs relatively late in the accretionary history of the terrestrial planets, at a time when the vast majority of that zone’s planetesimals are already expected to have either impacted on the protoplanets, or been dynamically ejected from the inner planets region.”

“It appears that a flux of impactors flooded the terrestrial planets region at this point in the solar system’s history, and is preserved in the cratering record of the heavily cratered terrain on each planet.”

“An essential requirement of any explanation for the late heavy bombardment is that the impactors be ‘stored’ somewhere in the solar system until they are suddenly unleashed about 4.0 Gyr ago.”

“A plausible explanation for the late heavy bombardment remains something of a mystery.”

“ seems likely that the late heavy bombardment is not the tail-off of planetary accretion but rather is a late pulse superimposed on the tail-off. Nor is there any reason to suppose that it was the only such pulse; it may have been preceded by several others which are not easily discernible from it in the cratering record.”

In short, the late heavy bombardment, a real solar system event, sounds like just the sort of early planetary explosion event that is suggested by the line of conjecture we were pursuing.

A second, somewhat weaker, line of evidence arises from use of the dynamical constraint that the most stable orbital configuration is the 2-to-1 resonance. If the solar system originally evolved to such a configuration wherein each planet had half the orbital period of the next planet out, then there appears to be room for one or two additional planets that may no longer exist. In particular, if we assume that Venus must have originated at least as close to the Sun as Mercury is now, as indicated by the Van Flandern and Harrington scenario (Chapter 13), there is room for precisely two additional planets between Earth and Uranus. At least one of those planets, perhaps both, was surely in the zone between Planet K and Jupiter, and was responsible for major accretion of mass in Jupiter. We designate this hypothetical body as “Planet A."

We will then call the other hypothetical body “Planet B." It may have been the “twin” of Planet A, also located just inside Jupiter’s orbit, in which case Jupiter was originally much smaller and was Saturn’s twin counterpart. We call this possibility “Planet B1." Or it may have been Saturn’s twin, located between Jupiter and Saturn in a region were comets often have outbursts, which suggests impacts by numerous small planetesimals in that region. We call this latter possibility “Planet B2," which would imply that Jupiter was Planet A’s twin.

A summary of these conjectured original planets is shown in Table 1, which adopts the first possibility for Planet B.

Planet Original Distance (au) Recent Distance
(au) Original Period
(yr) Original Mass
(x Å) Present Mass
(x Å)
Venus 0.5 0.7 0.35 0.8 0.82
Earth (x Å) 0.8 1.0 0.7 1.0 1.00
V 1.3 1.6 1.4 8 ---
K 2.0 2.8 2.8 10 ---
A 3.2 --- 5.6 120 ---
B 5.0 --- 11 150 ---
Jupiter 7.9 5.2 22 65 318
Saturn 13 9.5 45 80 95
Uranus 20 19.2 90 14 14.6
Neptune 32 30.0 180 17 17.3
T 50 --- 360 2 ---
X 80 --- 720 3 ---

Table 1. The Original solar system as inferred from the planetary fission theory. Original periods all have 2-to-1 ratios. [revised 2004/03/15 to reflect new information about trans-Neptunian objects, and that "Planet X" is a suitable parent for the TNO named Sedna]

Looking beyond Neptune, we note what may be another asteroid belt, possibly the remnants of an exploded planet in the outer solar system, in the form of tens of thousands of large fragments in Pluto-like orbits. This is often referred to as the “Kuiper Belt," although it apparently has little or nothing to do with the comets that either Kuiper or more recent astronomers predicted. We designate the hypothetical original parent planet as “Planet T," since we prefer to call the asteroids in that region TNOs (for Trans-Neptunian Objects).

That leaves the hypothetical Planet X (Chapter 18), the source of unmodeled perturbations on outer planets and certain comets, still undiscovered. We are of course free to presume that T and X were likewise twins at the outset, although we have no means yet to verify that presumption. But it would fill out the original solar system to the distance beyond which passing stars would make planet orbits unstable in the long run.

Of course, it should be noted here that Planet X may now itself be an asteroid belt, long since exploded. The perturbations in the outer planets and comets were mildly inconsistent with a single perturbing ring. But as we have remarked on earlier occasions, if there is more than one significant cause of the remaining perturbations, such as two rings, sorting that out by dynamical analysis of the observations alone will not succeed. If Planet X is now exploded, then the T and X pair would apparently be similar to the V and K pair. They would have produced two overlapping asteroid belts with presumably compositionally distinct asteroids, similar to the S-type and C-type asteroids in the inner and outer main asteroid belt. Certainly, the prediction of a second planetesimal belt beyond Neptune, if fulfilled, would be a strong point in favor of the fission theory for the origin of planets.

In summary, our view of the original solar system from the perspective of the fission theory is rather different from the planetary system we are familiar with today. We expect that originally there were six pairs of “twin” planets: Venus/Earth, V/K, A/B, Jupiter/Saturn, Uranus/Neptune, T/X. Or it is possible that A/Jupiter was a pair, and B2/Saturn was another. It is sobering to realize that if our deductions are valid, fully half of the solar system’s original planets may have perished in explosions over the past 4.5 billion years.

The strong similarity of pair members compared to the differences among pairs suggests a common origin of pair members. In the solar fission theory, when the Sun reaches overspin, two planets would form simultaneously on opposite sides of the Sun. The larger of the two masses would evolve outward by tidal friction faster than the other, as observed for each pair except Jupiter/Saturn, where Jupiter is over-massive. But the biggest jump in mass is at this location too. This may have resulted from a missing pair, both of which perhaps exploded in the very early solar system, producing the period of heavy bombardment, and accreting mass to Jupiter. The remnant TNOs and the hypothetical Planet X may represent yet another pair of original planets.

Application to Satellite Formation

The theory predicts that planetary moons have originated through the same process, with the exception that tidal forces would cause moons of gas giant planets to evolve inward. This predicts that the large, regular moons of the gas giant planets will occur in pairs, with the more massive always being the inner of the two. How does that prediction compare to reality? The results are in Table 2. Taking masses in units of 10-5 of the primary’s mass, and we include all moons with mass > 1. Distances are in multiples of the primary’s radius. We have included Pluto and Charon as if the conjecture that they are escaped former moons of Neptune is true, as proposed in Chapter 17.

The table points up some interesting patterns among these major planetary satellites. They do indeed tend to occur in pairs, and the inner member of each pair is always the more massive, just as the fission theory predicts. This alternating sequence of satellite masses has not been previously recognized, to this author’s knowledge, much less considered significant.

Primary Moon Mass Distance
Jupiter Io 4.7 5.9
Jupiter Europa 2.5 9.4
Jupiter Ganymede 7.8 15.0
Jupiter Callisto 5.7 26.3
Saturn Titan 23.8 20.3
Uranus Ariel 1.6 7.5
Uranus Umbriel 1.4 10.4
Uranus Titania 4.1 17.1
Uranus Oberon 3.5 22.8
Neptune Triton 20.9 14.3
Neptune Pluto 14.6 ?
Neptune Charon 3.2 ?

Table 2. Moons of gas giant planets with mass at least 10-5 of the parent planet's mass.

Jupiter and Uranus have the most regular and apparently undisturbed large satellite systems: circular and co-planar orbits, orbit-synchronized spins, with orbital periods each roughly double that of the next moon in. Correspondingly, their patterns contain no exceptions to the requirements of the fission theory. Neptune, of course, has a highly disrupted satellite system. But the close resemblance between Pluto and Triton has been noted by many astronomers. They surely qualify as “twins” as well as any pair of solar system bodies. The fission theory tells us that Pluto (with smaller mass) must have been exterior to Triton in the original configuration, which is consistent with results obtained nearly two decades ago and reviewed in Chapter 17.

Nereid has only 2% of the mass it would need to qualify as a large satellite by the criterion adopted here, so it is not a member of a satellite pair with Charon. Charon’s presumed original partner has most likely been ejected into independent solar orbit, very much the way Pluto was, where it awaits discovery as probably the largest of the undiscovered TNOs. Alternatively, it may have transferred to Planet X.

Among the gas giant planets, Saturn is the main surprise. Its many moons have rather unevenly spaced orbits with several huge gaps, interspersed with rings of material. It seemed evident that the Saturnian moons were not in their original orbits well before this analysis. Now we see yet another criterion that underscores that disturbed condition: Of Saturn’s eight original, presumably non-asteroidal moons, only Titan is as large as 10-5 of Saturn’s mass. Titan weighs in at 23.8 x 10-5 Saturn, making it the most massive moon in the solar system. The next largest Saturnian moon, Rhea, is roughly 50 times smaller in mass. Most of the others range from a few times 10-6 to a few times 10-8 of Saturn’s mass. One is tempted to conjecture about the nature of the disruption event, possibly including the formation of Saturn’s spectacular icy rings. But it is difficult to see evidence directing us toward a unique cause.


If we make allowance for special cases that have most probably been altered from their original condition since the solar system’s beginning, as judged by lines of evidence existing before this analysis began, we may conclude that the undisturbed solar system members provide a spectacularly good match to the predictions of the tidal fission theory. That includes major planets and large, regular moons.

At one point I began to wonder about the inference in Table 1 that the Earth was much closer to the Sun in the early solar system than it is now. Would Earth at that distance have been too hot to have oceans? Then I opened the May 23rd (1997) issue of Science magazine and found an article on “the early faint Sun paradox," trying to figure out what kept the Earth from freezing four billion years ago, when the Sun had 25%-30% less luminosity than it does today (Sagan and Chyba, 1997).[190] A good theory should always provide pleasant surprises, not new mysteries; and this one had just produced a very pleasant one--a solution to the early faint Sun paradox.

But to be a scientific theory, a model must be falsifiable; and to be useful it must make successful predictions. So we conclude with an important prediction, the failure of which will falsify the hypothesis. The astronomy news has been filled over the past two years with announcements of discoveries of planets orbiting other stars. The fission theory predicts that such planets will tend to occur in twin pairs, with some exceptions, as we have seen in our solar system. However, extra-solar planets cannot be viewed directly, even with the Hubble Space Telescope. Their existence must be inferred by indirect means, such as looking for a periodic wobble in the position of a visible parent star.

If extra-solar planets do occur as twins, that will not be immediately evident in the earliest observations because it is difficult to separate out periods for bodies of similar mass that are either close to the same value or are in resonance with one another. The first data will reveal just a single member of each pair. Observations over a longer time span will make it appear that the orbit is highly eccentric, when in reality the wobble of the star reflects the beating of two near-resonance periods. But with a still longer time span of data, the dual nature of the planets will be revealed. We predict that many of the discoveries of extra-solar planets recently announced will follow that course as the span of observations lengthens in the coming years.


[183] Van Flandern, T., “Worlds apart," a Focal Point debate between W.B. McKinnon and T. Van Flandern over the origins of Pluto, Charon, and Neptune's moons, Sky&Tel. 82, 340-341 (1991).

[184] Kerr, R.A., “Slow leak seen in Saturn’s rings," Science 274, 1468 (1996).

[185] Kerr, R.A., “Theoreticians are putting a new spin on the planets," Science 258, 548 (1992).

[186] Shu, F.H., “Protostars, theory," in Astron.&Astroph.Encycl., S. Maran, ed., Van Nostrand Reinhold, New York, 562-563 (1992).

[187] Binder, A.R., “The binary fission origin of the Moon," in Origin of the Moon, W.K. Hartmann, R.J. Phillips and G.J. Taylor, eds., Lunar and Planetary Institute, Houston, 499-516 (1984).

[188] Byl, J. and Ovenden, M.W., “On the satellite capture problem," Mon.Not.Roy.Astron.Soc. 173, 579-584 (1975).

[189] Weissman, P.R., “The impact history of the solar system: implications for the origin of atmospheres," in Origin and Evolution of Planetary and Satellite Atmospheres, S.K. Atreya, J.B. Pollack, and M.S. Matthews, eds., Univ. of Arizona Press, Tucson, pp. 247-249 (1989).

[190] Sagan, C. and Chyba, C., “The early faint Sun paradox: organic shielding of ultraviolet-labile greenhouse gases," Science 276, 1217-1221 (1997).

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