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New Members of the Solar System

During the past three years more than two dozen objects of a new type have been discovered orbiting the Sun beyond Neptune. These bodies are too small to be classified as planets, but are also unlike most asteroids in that they inhabit exclusively the outer solar system, and unlike most comets in that their orbits are not very eccentric. They are otherwise too faint for much to be known other than that they tend to be unusually red in color. Nonetheless, astronomers agree that, since so many objects showed up in several deep searches of very small areas of the sky, there are estimated to be perhaps 35,000 such objects (2-5 per square degree), each at least 100 km across, between 30 and 50 astronomical units from the Sun.

Are They Comets?

The distinction between comets and asteroids has become rather blurred over the past decade because no single characteristic uniquely identifies a body as either with certainty. For example, debate still rages over whether the Jupiter impactor, Shoemaker-Levy 9, was a comet or an asteroid. Despite having very different origins in conventional models, comets and asteroids have continued to display similar reflectance spectra, albedos, size ranges, etc. Comas and tails are the best guides we have to indicate that an object is a comet. But many comets display no tails; indeed, virtually all of them beyond Jupiter do not. And some comets have lost their comas, becoming completely asteroidal in appearance, while some asteroids have suddenly begun to exhibit comet-like activity, including the surprise appearance of a tail in one case.

Highly elongated orbits used to be associated exclusively with comets until the discovery of some unusual objects without coma or tail that therefore appear to be asteroids, yet moving in highly elongated, planet-crossing orbits. Pholus is one such object. The object Chiron is another example of the dilemma, since it was first assumed to be a large asteroid in a planet-like orbit crossing the orbits of Saturn and Uranus. When it was later found to have brightness variability, many astronomers began to call it a comet.

The trans-Neptunian objects (shall we call them TNOs?) show neither coma nor tail, and are therefore completely asteroidal in appearance. But it is argued that, at such great distances from the Sun and at such faint magnitudes, comas are likely to be invisible. However, all objects designated as comets heretofore have displayed visible comas or magnitude variability, even at solar distances beyond Saturn, since the coma is generally far brighter than the nucleus. (For example, the new Comet Hale-Bopp was photographed pre-discovery in 1993 while still near the orbit of Uranus, yet had a visible coma.) But the TNOs show only point-like nuclei.

The matter of comet versus asteroid would be more difficult to resolve if the trans-Neptunian objects had spectra like other comets. But they do not. TNOs are quite red. So the only known objects they resemble are the asteroids on highly elongated Jupiter-crossing orbits such as Pholus, which likewise happen to be highly reddened. Indeed, since those highly elongated asteroid orbits are unstable (those that cross Jupiter’s orbit will be eliminated within 100,000 years), it is a reasonable conjecture that they were recently perturbed into their present orbits from the trans-Neptunian population by a close encounter with Neptune. Subsequent perturbations by Jupiter would tend to quickly eliminate the Neptune-crossing character of the orbit, making that origin slightly less obvious today. Chiron, for example, might have originated beyond Neptune, but had its orbit subsequently modified by encounters with first Neptune, and later Saturn.

Pholus is an interesting case in point. It has a perihelion distance of 8.7 au and aphelion of 32.3 au. (Neptune is at 30.1 au.) And it had the reddest spectrum of any object in the solar system prior to the discovery of 1992 QB1, the first trans-Neptunian object. It seems safe to bet that Neptune modified Pholus’s orbit from Pluto-like to its present condition within the past 100,000 years; and that Pholus is a sample of the trans-Neptunian population we can examine at relatively close proximity. If so, then Pholus gives no indication of being a comet.

The argument that TNOs are members of the predicted "Kuiper belt" of comets is entirely spurious. For example, it is often argued that short-period comets have small inclinations to the ecliptic plane, in contrast to new comets from the "Oort cloud" that have fairly random inclinations. And this is taken to indicate a different source for the short-period comets. However, this suggestion overlooks the consideration that capture of new comets into short-period orbits by Jupiter is far more likely for comets of low inclination because they spend more time near the ecliptic, and therefore potentially close to Jupiter. The technical details of the extensive arguments against the existence of a Kuiper belt of comets may be found in "The Kuiper belt of comets does not exist" by T. Van Flandern in Periodic Comets, J.A. Fernandez and H. Rickman, eds., Universidad de la Republic, Montevideo, 75-80 (1992).

Therefore, from what we now know, it seems a likely conjecture that TNOs are not comets, but rather are a new class of unusually red asteroids, a few of which have been perturbed into temporary orbits that can reach the inner solar system.

Another Exploded Planet?

If the trans-Neptunian objects are debris from an exploded planet, as a great deal of evidence suggests is the case for main-belt asteroids, then their initial orbits would have to exhibit a wide range of eccentricities, inclinations, and mean distances from the Sun. However, although data is sparse, the objects now known are apparently confined to a modest range of eccentricity and inclination. But the process used for search and discovery strongly biases the discovered objects toward low inclination; and the eccentricities of many orbits are not yet well-determined. So one must be cautious about drawing the conclusion this early that an exploded planet does not fit the data.

There is also a tendency for TNOs to segregate into two ranges of mean distance at discovery, one of which is close to the 3-to-2 resonance with Neptune that Pluto and its moon Charon occupy. See Table 1. The other group may be near that resonance as well if the orbital eccentricities turn out just right. For comparison, some outer planet data appears in Table 2. Again, the significance of this early data must be judged with caution, both because of strong observational selection effects, and because the long-term gravitational effects of Neptune are not yet well known. The evolution of Pluto’s orbit, for example, has been difficult to study because of its resonance with Neptune, and because it can apparently make close approaches to Neptune if one looks far enough into the past, of order one billion years. Indeed, it has been proposed that only orbits in a 3-to-2 resonance will have long-term stability. [Icarus 116, 180-185 (1995).]

There are apparently far more "asteroids" over 100 km in diameter beyond Neptune than in the main asteroid belt. If the trans-Neptunian objects are from a parent body that exploded, the 35,000 number mentioned earlier can be combined with a mean diameter of, say, 150 km each to imply a parent body at least 5000 km in diameter, the size of Saturn’s moon Titan. But that would require a gentle breakup with little mass loss. If most of the mass was blown out of the solar system, then the parent body must have been a major planet. One possible conjecture is that the parent body was the hypothetical Planet X, a trans-Neptunian planet possibly still awaiting discovery that may have had a close encounter with Neptune in the distant past [see Dark Matter, Missing Planets and New Comets, North Atlantic Books (1993)]. Perhaps the encounter somehow destabilized the planet’s interior, leading eventually to the explosion. Indeed, this same encounter/destabilization scenario can readily be visualized as possibly applicable to the nearest planets to Jupiter, the parent bodies of the main asteroid belt, as well.

A Former Neptunian Ring?

In Dark Matter, …, we described evidence indicating that Pluto & Charon and the disruption of Neptune’s satellite system may have resulted from a past encounter with "Planet X". But in that study, the late Robert Harrington and I did not investigate the effect such an encounter might have had on any natural rings around Neptune at the time. However, since rings are made up of individual bodies that behave dynamically like individual satellites, it seems clear that a Neptunian ring could have met the same fate as other Neptunian moons – being stripped away from Neptune into an independent solar orbit that remains Neptune-crossing. Such is the present condition for the solar orbit of Pluto and its large moon Charon.

The principal arguments against this are the size of the trans-Neptunian objects (too large to be ring pieces), the implied size of the parent body (much larger than any existing Neptunian moon), and weak evidence for some low-eccentricity TNOs that do not come close to crossing Neptune’s orbit and therefore could not have originated in this way.

But whatever the origin of the curious new objects, they occupy a volume of space so vast that all the 200,000,000,000 stars in our galaxy could fit within that volume without touching! This means that the rate of collisions of smaller bodies with the larger TNOs is so small that origin by accretion is ruled out for a belt with present densities. It therefore may be concluded with certainty that something fundamental is missing in conventional models suggesting accretion of these objects from a primeval solar nebula. [AJ 110, 856-868 (1995).]

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