A central argument of theorist Immanuel Velikovsky, based on his interpretation of ancient astronomy, was that Venus had emerged from Jupiter as a comet with a tail, 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 notes that tend to support that argument:
1. Critics have used the very different spectrographic profiles of Venus and Jupiter to argue that Venus could not possibly have emerged from Jupiter. However, the inner regions of Jupiter contain a set of elements that is quite unlike the hydrogen and helium gases of its surface. Jupiter's mighty gravitational and magnetic fields pull into it a stream of asteroids and comets, such as the comet Shoemaker-Levy 9 in 1994. (One can see the effect on the heavily cratered Jovian moon Callisto, which presumably has taken hits from many objects attracted by Jupiter. Callisto is commonly reported to be very ancient, but another explanation for why its surface is so pockmarked with craters compared to the surfaces of Jupiter's other moons could be that their surfaces have more geological activity that would obliterate craters. The other moons may also be quite young. Callisto is so saturated by craters that many more might have been covered over by newer ones.) As a result, Jupiter's interior must contain a wide array of elements that churn around and can form the basis of objects that subsequently emerge from the surface as comets. This concept can be termed Jovian Recycling; and it can help 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). And, of course, it would account for the widely held view that Jupiter was the father of the gods.
2. The presence of some 1,000 craters on Venus has been used to suggest that Venus was of much older origin than Velikovsky, according to his reading of ancient astronomy, had claimed. However, if, according to Velikovsky, Venus had emerged from Jupiter sometime in the several millennia before ~1500 B.C., Venus then presumably passed through the asteroid belt where it could have been bombarded by asteroids. Subsequently, for many centuries, the comet/planet Venus followed a highly elliptical orbit that brought it into contact with Mars, and presumably extended somewhat beyond Mars, i.e., again into the asteroid belt. This exposure to the asteroid belt could very plausibly account for the roughly 1,000 craters on Venus, a number unknown to Velikovsky. Indeed, this would appear to be a simpler, more economical explanation than a putative sweeping clean of older craters on Venus via geological activity at a convenient point during a 4-billion year history, as is posited by some of his critics.
3. 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. A recent explanation 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" (Pasachoff et al., 2004). 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.
Note
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