Immanuel Velikovsky argued famously, based on his interpretation of ancient sources, that Venus had emerged from Jupiter as a comet, interacted with the Earth and Mars in the second and first millennia B.C., and then finally settled into a nearly circular orbit of the Sun.
Here are three new lines of reasoning that tend to support this theory:
1. Instead of the various unpersuasive suggestions that Velikovsky and others have made for how a cometary Venus could have emerged from Jupiter, we should consider the possible consequences of the immense gravitational field of Jupiter, which pulls into the giant planet a stream of asteroids and comets such as Shoemaker-Levy 9 in 1994.
A plausible scenario would have an initially dark, cold proto-Venus, pulled by Jupiter’s gravity from the outer solar system shortly before 2500 B.C., pass close to Jupiter, then manage to escape its gravitational field. We can term this the Peripheral Passage (PP) of Jupiter by Venus, and we can assume that extreme tidal forces from Jupiter’s gravitational field created tremendous heating and released a great deal of light that would have been visible to observers on Earth. This could account for the curious stories of the ancient Greeks, that Athena (Venus–eventually Aphrodite replaced Athena in this role) was born from the head of Zeus (Jupiter), and of the ancient Hindus, that Shukra (Venus) emerged from the mouth of Shiva (Guru or Jupiter).
According to Greek myth, Zeus swallowed the pregnant Metis (the story that he tricked her into turning into a fly, which he then swallowed, appears to be a modern invention). She gave birth to Athena inside of him, and then Athena emerged from his head—on the opposite side from his mouth. In effect, at least one ancient Greek observer spotted proto-Venus when it approached Jupiter as tidal friction heated it up to incandescence, and the Greeks called it Metis.
Velikovsky seems to have been unaware of the Metis myth and thus he misinterpreted the birth of Athena from the head of Zeus myth to mean that Venus had been explosively expelled from Jupiter itself. His scientific critics rightly considered that bizarre and unacceptable, so his reputation suffered and his other valuable contributions were ignored.
This proto-Venus would have been large and dense enough to maintain its integrity in the gravitational, magnetic, and thermal environment of Jupiter, though its surface would melt and some would be shorn off and become its giant tail, and the entire planet could have become molten. This Venus would possess a set of elements different from those of Jupiter itself. This scenario would also resolve the paradox that Venus is old (the proto-Venus) and yet its surface features appear young (shaped by its PP experience). A similar pattern may be associated with Jupiter’s moon Ganymede, whose surface and interior were remodeled by intense heat, presumably from its own PP of Jupiter–but Ganymede lacked sufficient velocity to escape from Jupiter’s gravity and so ended up orbiting the gas giant.
The Peripheral Passage explanation would overcome the three chief objections–escape velocity, elemental composition, and heat generation–to the assertion of Velikovsky that Venus emerged as a comet from Jupiter. The issue of the Jovian origin of Venus has been one of the main criticisms of Velikovsky’s theories in general (it was #1 on astronomer Carl Sagan’s list) as well as a key reason for denying validity to ancient accounts, many in mythic form, as sources of astronomical, climatic, and geological information. These objections held particular importance because the whole theory depended on the reported emergence of a comet-like Venus from Jupiter, which seemed outlandish and not credible to critics. Now that this emergence can be seen to have a commonsensical, scientifically plausible explanation, the credibility of the ancient observers–who after all were eyewitnesses–must rise accordingly, and therefore their other stories must be examined more carefully as potential sources of important information. The story of Metis constitutes precious evidence that, correctly interpreted, could have significantly changed the course and outcome of the Velikovsky controversy. It also ties into a more general theory of the terrestrial planets of which an appropriately modified version of Velikovsky’s theory forms part.
Curiously, PP could also account for the two specific roles that Jupiter played in ancient myth: as father of the gods (such passages of various initially invisible celestial bodies near or touching Jupiter could have been viewed as births) and as hurler of thunderbolts (the energy generated by Jupiter’s active processes could have caused gigantic lightning bolts visible from Earth to leap onto objects passing nearby).
2. It is possible that the Black Drop (see 2004 photo below) observed during the transits of Venus across the solar disk is in fact the residual tail of the comet/planet. On ingress from the solar limb, the Black Drop stretches out behind Venus; on egress, it appears in front of the planet. During transit, it is not visible. A residual comet’s tail on Venus could be shifted by the solar wind from trailing the planet at ingress, to standing away from the sun during transit (thus only the disk of Venus would appear), and finally to preceding the planet at egress.
The Black Drop is usually ascribed to various optical effects. One 18th-century observer said it made the planet look “like a nine pin”. Drawings by observers make the Black Drop appear exactly like a small tail. The effect was originally thought to be related to the atmosphere of Venus, but the Black Drop was subsequently found to be too large. Mercury, which has no atmosphere, also has a Black Drop when transiting the sun; and it is clearly visible from a space-based telescope, so this effect is not an artifact of the Earth’s atmosphere. This suggests that either the Black Drops of both planets are caused by some extra-atmospheric optical effect or that both planets possess remnants of comet tails. Presumably, the motion of the planet would make the tail (Black Drop) shorter at egress than at ingress, which could be measured; it is possible that skewing of the tail could be detected in transits that cut very obliquely across the Sun; and there might be some way to detect the tail as it stands away from the surface of the planet during transit. Or a sensitive telescope might detect a residual tail even when Venus is not transiting the Sun.
Venus also possesses a much-studied gas tail that extends as far as the Earth’s orbit, so this also needs to be investigated in reference to the Black Drop.
A recent explanation of the Black Drop based on a Mercury transit observed from an Earth-orbiting telescope is that the effect is caused by the combination of the point-spread function of the telescope and “solar limb darkening”.
According to the comet tail hypothesis, the solar limb darkening is not a cause of the Black Drop but an effect of the residual comet tail.
In 2012 Venus will transit the Sun for the last time in this century, affording an important opportunity to study the Black Drop to determine whether it actually constitutes a mini-tail of the sort associated with comets.
Findings from an investigation of the Black Drop may also help resolve the question of the Ashen Light, faint luminescence on the night side of Venus first reported in 1643. Since the Ashen Light appears in roughly the same location opposite the incoming solar wind as would a comet tail, it may be caused by chemical reactions within a residual tail of Venus. Presumably the strong glare of the solar orb prevents us from seeing the Ashen Light during transit by making the Black Drop appear blacker than it would otherwise be. Both the Black Drop and the Ashen Light appear to have become harder to detect over the years since the early telescope astronomers spotted them. This would be consistent with the diminution over several centuries of a residual comet tail.
3. Lastly, a new interpretation of iconographic evidence also deserves attention.  In the photograph from Abu Simbel of Pharaoh Ramses II and his consort Nefertari appear what look like the comet Venus and its two-pronged tail in Nefertari’s headdress, and the smaller Mars with its two imperfectly round moons and its own tail, presumably of dust stirred up by an encounter with Venus or borrowed from Venus’ tail, in the headdress of Ramses II. What is the age of this sculpture? How could the ancient Egyptians have seen the moons of Mars with the naked eye? How could these headdresses be explained other than as depictions of Venus and Mars during approaches to the Earth? Why should this evidence not be considered probative? 
1. H. Gruenwaldt et al. Venus tail ray observation near Earth. Geophysical Research Letters. Vol. 24, No. 10, May 15, 1997, pp. 1163-6
2. Jay M. Pasachoff, Glenn Schneider, and Leon Golub. The black-drop effect explained. In: D.W. Kurtz, ed. Transits of Venus. New Views of the Solar System and Galaxy. Proceedings IAU Colloquium No. 196, 2004, pp. 242-53