Where do comets come from?

I’ve been thinking about comets a lot lately, trying to image C/2009 G1, reading “The Hunt for Planet X”, wondering why Galileo was so wrong about them and recently reading a Young Earth creationist blog post on them. The latter referred to a very interesting pre-publication article. And I’d like to discuss this article, as this illuminates not only the origins of comets but also how science is done.

Where do comets come from? They scream in from the outer dark, briefly flash across the skies to our wonder then return to the dark. Actually, there are multiple answers to this question. We can divide comets into two basic types, long period comets and short period comets. Long period comets have their origin deep within the outer reaches of the solar system, they will return to the warm embrace of the Sun on geological time scales, if at all. Short period comets come from within the solar system, and can be further divided into Jupiter Family comets, with an orbital period of 20 years or less whose orbits extend not much further than that of Jupiter, and Halley Family comets, with periods of between 20 to 200 years.

Source Hubble Space Telescope:

But this is only part of the answer to “where do comets come from?” Comets are basically dirty snowballs, and as the Jupiter Family comets swing close to the Sun, a little bit of them evaporates. The spectacular (or not so spectacular) tails that awe us are composed of comet material boiling away into space [1]. It’s obvious that comets can’t last forever, and eventually the ice will evaporate away, leaving an inert core of rubble and dust which can no longer be called a comet (or in extreme cases disintegrate into a stream of rubble).

We have examples of comet death in the spectacular decay of comets such as 73P/Schwassmann-Wachmann 3 and the asteroid/comets with feeble tails. The time for a Jupiter family comet to melt away is much shorter than the age of the solar system (around 300,000 years compared to the age of the solar system, around 5 billion years, Earth is around 4.5 billion years old). Any comets present in short period orbits at the formation of the solar system will be long gone by now, but we obviously have active short period comets. Where do they come from?

Source: Wikipedia

One of the first proposals was that the short period comets, like the long period comets, come from the icy dark that surrounds the solar system, and represent long period comets captured into short period orbits by interactions with the planets. However, while some short period comets may come from this source, it’s unlikely that most of them do.

Firstly the short period comets are more or less confined to the orbital plane of the solar system (but not as much as the planets), where as long period comets come from all over. Secondly, simulations showed that too few comets could be captured this way to explain the number of comets we see.

On the basis of dynamic simulations, it was predicted that there was a band of icy objects lying beyond the orbit of Neptune, and comets were the results of small icy bodies being bounced into the inner solar system by gravitational interactions with the planets and other icy bodies. A hunt for these objects soon found them [2], and the discovery of the Kuiper Belt a spectacular vindication of cometary theory. Now, cometary objects at that size are too small to be seen, even by the Hubble telescope, but counts of larger objects found that the size distribution followed a power law (with of course, more objects at smaller sizes than at larger sizes), and extrapolation of the best surveys indicated there was more than enough objects in the Kuiper Belt to account for the number of comets we see today.

Which brings us to this paper by Volk and Malhotra. They have done one of the most exhaustive simulations of cometary dynamics to date, to estimate the necessary population of Kuiper Belt objects needed to produce the number of comets we see today, with some of the best estimates of the actually Kuiper Belt population.

And they come up with a shortfall, there appears to be too few small icy objects in the Kupier Belt to account for the number of comets we see. Now, I’ve been simplifying things a bit. The cometary group they were studying is a subset of the short period comets, the Jupiter family comets. These are the most numerous short period comets. Also, the Kuiper belt is not homogeneous, being made up of the classic Kuiper Belt, the Scattered Disk and various bodies in orbital resonance, such as the Plutinos. The work of Volk and Malhotra refers to the origin of Jupiter Family Comets from the Scattered Disk. Anyway, they concluded that there is over two order of magnitude fewer objects in the scattered disk than is needed to provide the number of comets we see today. The Young Earth creationists have seized upon this, if we have too many comets they say, and comets cannot survive a long time, therefore the soar system must be young. Lets leave aside the numerous problems with this argument and look at the actual paper for a moment.

Source: NASA and A. Feild

The authors calculate that around 1 x108 objects are required to provide the number of comets we see today, but they estimate that there are only 3x105 comet-sized objects in the Scattered Disk of the Kuiper Belt. This is a fairly large discrepancy, but then we come to the confidence interval. The 95% confidence interval, that is the region where we are statistically confident there is a 95% chance the true number of comet sized objects lies in that region, runs from 1x105 to 2x108.


In other words, our degree of uncertainly about the true number of Kuiper belt objects is so large, that it may very well be sufficient to explain the number of comets we see. Now, I am not a professional astronomer, nor do I play one on TV. But I do do a heck of a lot of curve fitting and statistics, so I am qualified to comment on this.

Why it’s hard to count faint Kuiper belt objects Source: Fig1 Bernsetin et al., 2004.

Why is there such a large error range? As I mentioned before, we can’t actually see comet sized objects in the Kuiper Belt, even with Hubble. We have to estimate their number by extrapolating from the size distribution of objects we can see. Early estimates gave lots of objects, later surveys, looking at dimmer objects, gave a smaller estimate. The Volk and Malhotra article uses a Hubble survey estimate, which looked at the faintest objects yet. However, it is very easy to miss objects, as the history of astronomy shows when we have missed very large icy objects (automated systems may miss faint objects, the survey may be looking where objects aren’t etc.). So the Hubble estimates come with very large error bars. Other deep surveys (although not as deep as Hubble) find more objects.

Estimates of small Kuiper belt objects from one of the most recent surveys ( Source: Fig6, Fraser et al., 2008). Note the large error bars.

The very best we can say is that we do not have enough data to make a definitive estimate of the number of comet-like Kupier objects, and we will have to wait until better, deeper surveys are done.

The creationist says

“The simplest explanation as to why we can still see short-period comets is that the solar system is young.”

No. Making the solar system young does not solve this (non)issue. Apart from requiring physics (the kind that allows our digital watches to run, mobile phones and car GPS’s to work) to be badly broken (we would have notice d by now if physics was that badly wrong), it explains nothing about comets that we would like to know. Why is the median age of Jupiter family comets 300,000 years, (not possible in a 6-10,000 year old solar system) and where do the nearly exhausted comets come from if the solar system is much younger than the median comet lifetime? Why are there 2:3 and other resonances in the Kuiper belt, resonances that will take millions of years to form? If the solar syetm is only 6-10,000 years old, why are comets in the inner solar system at all (and why do comets have the chemical composition of Kuiper Belt objects?)

There are a number of ways we can resolve the comet “problem”. Better estimates of Kuiper belt objects may show there are enough objects, other parts of the Kuiper Belt may supply comets, comet fragmentation (as seen with 73P/Schwassmann-Wachmann 3), as the objects move from the Kuiper Belt to the inner solar system, may provide more objects. There are a number of different ways which we can resolve this, none of which break normal physics, and you can bet that astronomers will investigate them.

Now, let’s have a look at how science operates. The Kuiper belt was a prediction based on our knowledge of comets, and that prediction was spectacularly confirmed. Instead of resting on their laurels, astronomers tried to estimate the numbers of Kuiper Belt objects, to see if it fit with their theories. Despite early agreement, they kept on refining their estimates. And when their estimates seemed problematical, did they hide them? No, they are published in open forums so that people can understand and work on the problem.

On the other hand, creationists only prediction was that the Kupier Belt was a figment of “evolutionists” imagination, when found, they did nothing but carp about it. Creationism has yet to give up on any of its theories, does no original work, and basically acts as a reaction to the findings of researchers.

Whatever the resolution to the Kuiper Belt conundrum, you can bet it will be scientists, not creationists, who will work it out (and that creationists won’t like the answers)

[1] It doesn’t actually boil, but rather sublimes, vigorously.
[2] “soon” involving hundreds of person hours, years of patient watching and poring over plates and digital media, but “soon” in the context of how long it took to discover Pluto, for example.