YEC Best Evidence 8: short-lived comets

Posted at 10:00 on 18 December 2017

The eighth of AIG's ten best evidences for a young earth is the existence of short-lived comets. Comets lose mass as they come close to the sun, when ice evaporates and dust gets dislodged by the solar wind.

The author of this article, Danny Faulkner, tells us that this means we can easily calculate an upper limit to their ages. He doesn't tell us how to do so, nor does he quote any figures for this, but he is correct in saying that comets don't last all that long once they venture into the inner Solar System. For example, Halley's Comet is only expected to last another 10,000 years or so.

He tells us that comets can get kicked out of the Solar System by gravitational interactions with the planets. What he doesn't tell us is that comets can also get captured into the Solar System by exactly the same interactions. In fact, NASA ran some calculations on Comet 67P/Churyumov–Gerasimenko (the one that was visited by Rosetta and the Philae lander) and found that this was exactly what happened to it:

Analysis of the comet's orbital evolution indicates that until the mid-19th century, the closest it got to the Sun was 4.0 AU (about 373 million miles or 600 million kilometers), which is roughly two-thirds of the way from Mars' orbit to that of Jupiter. That far from the Sun's heat, it would not sprout a coma or tails, so it was invisible from Earth.

But scientists calculate that in 1840, a fairly close encounter with Jupiter must have sent the comet flying deeper into the inner solar system, down to about 3.0 AU (about 280 million miles or 450 million kilometers) from the Sun. Churyumov-Gerasimenko's perihelion (closest approach to the Sun) drifted a bit closer to the Sun over the next century, and then Jupiter gave the comet another gravitational kick in 1959. Since then, the comet's perihelion has stood at about 1.3 AU, which is about 27 million miles (43 million kilometers) outside Earth's orbit.

So it's not uncommon for long-lived comets to come in from the outer Solar System, be captured by one of the planets, and turned into short-lived comets. The Oort Cloud and the Kuiper Belt are believed to be the source of these long-period comets in the first place.

Yet Faulkner dismisses the Oort Cloud as a "rescuing device" concocted by "evolutionary astronomers" (as a reminder: there's no such thing as an "evolutionary astronomer") saying that "there is no evidence for the supposed Oort cloud, and there likely never will be." He also cites the large size of the Kuiper Belt asteroids and their composition as evidence that the Kuiper Belt can not be the source of these comets.

"Pluto's surface is young!"

In July 2015, the New Horizons space probe flew past Pluto and sent back pictures of a surface that looked much more pristine and smooth than what we see on other rocky planets and moons in the Solar System. The same author, Danny Faulkner, wrote this in response to these pictures:

Compounding this problem for a 4.5-billion-year age for the solar system is the fact that Pluto is located in a particularly crowded part of the solar system. Pluto orbits the sun in a region with many other large objects that are too small to be planets and are also orbiting the sun. Presumably, thus far we have found only the larger members of this second asteroid belt, the first belt being mainly between the orbits of the planets Mars and Jupiter. We would expect that for each of these bodies in this second asteroid belt there would be many more much smaller bodies. Therefore, Pluto ought to be undergoing impacts today at a higher rate than most other objects in other portions of the solar system.

So ... on the one hand, we are told that the Kuiper Belt is too sparsely populated to be able to provide a steady supply of short-term comets, yet on the other hand, we are being told that it is so densely populated that it must be pulverising Pluto's surface to smithereens! Which is it?

Absence of evidence is not evidence of absence.

The reason for the large size of known KBOs is that the smaller ones are too hard to see, not that they don't exist. Comet-sized objects (about 10-20 km) in particular are right at the resolution limits of our telescopes at that distance, and looking for them is complex and expensive. We also know that objects in both the asteroid belt (Gladman et al, 2009) and the Kuiper Belt (Fraser & Kavelaars, 2008) follow a power law distribution in terms of size, with smaller ones being far, far more common than larger ones. Consequently, Faulkner's claim that the Kuiper Belt objects are all too large and sparse to account for the origin of comets is completely out of touch with reality.

After New Horizons visited Pluto, NASA conducted a search for Kuiper Belt objects for it to visit as a follow-up. The potential candidates had to meet some fairly stringent criteria in terms of size, distance and position in the sky, and the fact that they managed to find three possible targets is further evidence that there is no shortage of material in the Kuiper Belt.

Ultimately, this argument boils down to "absence of evidence is evidence of absence." While absence of evidence may be evidence of absence if it is something that we expect to see, such as sequenceable DNA in 6,000 year old dinosaur fossils, it most certainly is not evidence of absence if it is something that we don't, such as small objects beyond the resolution limits of our telescopes.

The Oort Cloud may not have been directly observed, but it has good explanatory power, and its existence has never been falsified. Besides, which is more plausible an explanation -- that the Oort Cloud exists, or that hundreds of thousands of independent, high-precision measurements whose commercial incentives strongly favour correctness over ideological convenience are all consistently in error by up to a factor of a million?

Featured image: Comet Hyakutake, by Bill Ingalls/NASA (source: Wikipedia)