How the giant planets protect Earth from massive impacts

An asteroid hitting Jupiter

In Chapter 14 of The Afterlife Revolution, Whitley Strieber writes:

The planet is shielded from strikes from cosmic debris by the gravity fields of the gas giants in the outer solar system and by the closeness of the moon. Jupiter and Saturn will take the first hits and if anything gets closer, the moon will work as a shield. The result of this is that large asteroid strikes are much rarer on Earth than they are on the other planets. Earth isn't pockmarked with craters because the moon is.

As a longtime reader of Strieber (who is a horror novelist and philosopher, not a scientist), I know he keeps informed about science -- reads the major journals and all that -- but also that his scientific knowledge is broad rather than deep and does not rest on any very strong foundation of basic scientific literacy. (I remember reading in one of his books that during the Mesozoic the earth was dominated by a single animal species and being completely baffled as to which species he might have in mind -- only to read on and find that he meant "the dinosaur"!) So when I read something like this, I know that it is almost certainly based on some real scientific research reported in the press, and also that he has almost certainly misunderstood it.

I'm a relatively clueless layman, too, but I won't let that stop me from taking a cursory stab at making sense of this.

Look up at Jupiter in the night sky. See how big it is -- not even a single arcminute in diameter? Assuming "cosmic debris" could come at Earth from any direction, what unimaginably tiny percentage of it would be blocked from hitting Earth because it hit Jupiter first? Even taking into account Jupiter's gravitational field, it would surely still only cover a negligible proportion of the sky and offer negligible protection.

As for the Moon's working as a shield, it is only from a naive geocentric perspective that the Moon is "up" and therefore stands between Earth and any incoming debris. There's no reason at all to assume that debris approaching the Earth-Moon system would tend to hit the Moon rather than Earth; on the contrary, the opposite must be true, as Earth is both physically larger than the Moon and exerts a stronger gravitational pull. Of course some sizable percentage of incoming debris will hit the Moon rather than Earth, and so the Moon probably is a "shield" in that sense, but to say that "Earth isn't pockmarked with craters because the moon is" is to betray a fundamental misunderstanding. Earth is less cratered than the Moon because it has an atmosphere -- burning up most meteors before they hit the surface, and eroding away the craters left by those that do -- not because meteors are somehow drawn to the Moon rather than to Earth.

Coming back to Jupiter and Saturn,  research by NASA's Tom Barclay and his colleagues, reported here about a year before the publication of The Afterlife Revolution, is presumably what Strieber is referring to. (His books consistently make reference to very recent science news, as documented by the pseudonymous Heinrich Moltke here.) Barclay's team did computer simulations of the early solar system "after Mars-size planet embryos had already formed in the system, and looked at cases with and without giant planets on the outer perimeter." The article reports their conclusions thus:

The researchers found that, with giant planets around, the remaining small solar system bodies were either ejected out of the system more quickly — because of the angular momentum the gas giants add to the system, Barclay said — or became a part of the existing planets sooner.

Without the influence of giant planets, the fragments formed a large, dangerous cloud orbiting close within the system that took much longer to disperse.

Elsewhere, the article quotes Barclay directly:

"If you have giant planets, your last giant impact happens somewhere between 10 and 100 million years [after planet formation], which is pretty fine — it's like what happened on Earth," Barclay said. "If you don't have giant planets, the last giant impact can happen hundreds [of millions] to billions of years in. This really is a risk to habitability."

While I am of course skeptical of any conclusions based on an oversimplified computer model of an imperfectly known situation, Barclay's theory at least has a prima facie plausibility that Strieber's version does not. The idea is not that any given object would tend to hit Jupiter rather than Earth, but rather that the presence of giant planets would, over a period of millions of years, tend to clear out much of the debris in the solar system so that there would later be much less of it around to hit Earth or any other planet.