Into The Gap

I’m not quite sure how this post is going to go. It’s not going to be the same as the others. Then again, neither was the one about the name of the seventh planet from the Sun. It is, however, somewhat similar to the one about the space between the Sun and Mercury.

Up until now, I’ve mainly posted about the planets, moons and asteroids of the Solar System in one way or another, but one of the most impressive things about the system is not so much the worlds it contains as the enormity of its scale, which is of course itself dwarfed by interstellar space. In a way, the Solar System is a rather irregularly-shaped space centred on the Sun with a radius of at least a light year in every direction. Beyond those points, the gravity of other bodies, usually stars, becomes more significant than that of the Sun. This is a significant feature of the system because out there in the darkness lurk countless bodies, perhaps future comets, and although the distances between them are huge, it can’t be said that this space is entirely empty. However, that isn’t what most people think of as the Solar System, and that’s what I want to consider here.

Most people probably think of this thing as the Sun plus eight or nine planets with the occasional comet. Whereas popular perception may not be the best way to go with science, the system isn’t just about science. The Apollo astronauts walking on the nearest celestial body, for example, weren’t merely “science”, and the discoveries about the Martian atmosphere which helped end the Cold War were very significant for all of us. Looking at Saturn through a telescope for the first time isn’t about science either, but awe and a sense of beauty. Nonetheless, science comes into this.

Saturn is something over ten times Earth’s distance from the Sun. Its orbit circumscribed the known Universe for thousands of years. Within that ellipse, with a little latitude either side of its orbital plane, is a rather busy and light community of stars, asteroids and moons, the largest of which have been visited more than once by space probes. In fact, the largest object never to have been subject to a mission at all in this region is the asteroid Hygeia, with a diameter of 450 kilometres. Outside this region are the seventh and eighth planets plus Pluto and the distance between Neptune and the next planet in is a minimum of almost eleven times that between Earth and the Sun. In other words, the entire distance between the Sun and Saturn is smaller than the width of this gap. It’s also fair, if technically inaccurate, to consider the orbit of Neptune as the true border of the Solar System. It isn’t of course, but we can think of it in that way if the system is considered the set of official planets orbiting the Sun.

Voyager 2 took eight years to travel from Earth to Uranus (sigh). It then took only three more years to cross this enormous gulf, slowing as it went. This is a bit of a distortion because there’s a huge gap between the sixth and seventh planets too, about the same as that between Earth and Saturn at their closest to each other, so in other words a comparable distance to the next gap. However, beyond Saturn lie most of the centaurs, the relatively huge satellite system of that planet and also its magnetic field, and Saturn is also large and able to exert a considerable gravitational influence on the region. This gap is different. The two planets marking its edges are smaller and less massive and in fact roughly the same size. This means there will be a moving point halfway between them where their gravity is balanced, at around two dozen AU from the Sun but varying and sometimes being dominated by the forces of other bodies. The situation is actually quite like that between Venus and Earth due to their similar masses but on a much larger scale. Although the gravitational forces involved are much lower, there’s also a lot less to perturb these forces in such an uncluttered region.

Many objects orbit on either side of this gap. The centaurs are substantially found further towards the Sun and the realm of the Kuiper Belt is beyond Neptune. This raises a couple of complementary questions. One is whether the gap contains such objects and the other is, if it doesn’t, why? Clearly the further out from the Sun one goes, the less stuff there is. However, in this case there’s a difference between the objects on either side, with small, comet-like objects within Hamlet’s orbit and often relatively large, planet-like objects beyond Neptune’s. What, then, is present between them, if anything?

One very definite and obvious object spending time in the gap is of course Pluto. It was there between 1979 and 1999 CE. Pluto does not, however, cross Neptune’s orbit, at least most of the time. It’s inclined at seventeen degrees to the ecliptic compared to Neptune’s one degree and four dozen minutes. Like all planets, however, both Pluto’s and Neptune’s orbits gradually “spirograph” around the Sun over a long period of time, and therefore theoretically they could collide at some point. They never have, or Pluto would no longer exist, or perhaps it’d be a moon of Neptune like Triton is, along with its own companions. Pluto ventures twenty-three million kilometres closer to the Sun than Neptune does, although that doesn’t mean it ever gets as close as that to Neptune due to the angle of the orbit. In fact, even at their closest approach the two can be as far apart as Earth and Saturn, which gives a sense of the scale involved at that distance. Pluto’s tilt also doesn’t really need an explanation. It’s simply that the gravitational influence of the Sun is weaker out there, and Pluto is not very massive and therefore has less momentum than the likes of Neptune.

It’s been calculated that the region from twenty-four to twenty-seven AU from the Sun is a haven for objects with stable orbits over the age of the Solar System to date, so there are some reasons for supposing there will be something there. This is actually quite exceptional and doesn’t apply, for instance, to centaurs, whose orbits are unstable over a period of a few million years. An object around that distance in a roughly circular orbit will never be captured by a planet, dragged into the inner system like a comet or ejected far out of it. In fact, this is the largest region where there are potentially stable orbits all the way from the Sun to Pluto’s aphelion. This belt is divided into two particularly stable regions around 24.6 and 25.6 AU. These calculations also predict that there are around three hundred objects at least fifty kilometres in diameter in this belt, which can be compared to the Kuiper Belt, some of whose occupants are much larger than that. The reason they haven’t been detected is likely due to their darkness and the dimness of solar illumination at that distance. They would be hard to spot. If they do turn out not to be there, it suggests that an event such as a planet shifting its position through that region has cleared them from it. That planet might actually be Neptune because it’s in the wrong place. Neptune alone among the planets does not obey Bode’s Law. Another possibility, and I say this as a naïve amateur, since to my knowledge this has never been suggested, is that this apparently clear space is the result of a “mini-Neptune” moving through it before being ejected from the system. The most common type of planet detected orbiting other suns is a planet about midway between Earth and Neptune in mass, so maybe this is where it used to be. It’s unlikely because it would again violate the Titius-Bode Series.

Suppose, then, that a typical cis Neptunian object has a diameter of fifty kilometres and has a roughly circular orbit near the ecliptic plane at a mean distance of 25.6 AU. What features would it be likely to have? Well, it would take 129 years to orbit the Sun, which at that distance would be six hundred times dimmer than from Earth. It would also have a fairly dark reddish surface, moderately cratered, and probably hasn’t been geologically active for a very long time if at all. Surface temperature would be around -200°C and it would be composed of an undifferentiated mixture of ice and rock. In fact, it would probably very closely resemble some of the outer moons of the two ice giants and that may be where those came from in the first place.

Another thing about this region is that the centaurs and Kuiper Belt each impinge on it, from opposite sides. There are a considerable number of trans-Uranian centaurs. One of these sounds very much like the predicted type of object: 2005RL₄₃. This is 24.6 AU from the Sun, has negligible eccentricity and has a diameter of 143 kilometres. Another is Nessus, although that has a very elliptical orbit. On the other side, clearly at least one plutino moves into the gap so the question arises of whether there are any others. The definition of a plutino is that it orbits twice for three orbits of Neptune, which is what Pluto itself does. This does put their mean orbit at a distance of 39 AU, about the same as Pluto’s and also in accordance with the Titius-Bode series, but there’s no reason a body beyond Neptune wouldn’t be within its orbit for part of its year, and this would put them in the gap. I don’t want to spend too long on this because at some point I want to talk about Kuiper Belt objects in their own right, but it would be remiss of me not to mention them here too. Some of them dip into the stable region at their perihelia. A few of them even approach the Sun more closely than Saturn and there’s one which not only does this but also recedes to a maximum of more than thrice Pluto’s distance. Plutinos are not the only class of trans-Neptunian object. Cubewanos stay beyond it and twotinos orbit once for every two orbits of Neptune. There are two known twotinos whose perihelia are within Neptune’s orbit, just barely. In other words, their orbits are less eccentric than those of the plutinos.

There are some comets whose aphelia are lower than Neptune’s. Each gas giant has an associated cometary family, whose aphelia are close to those of the planets. Jupiter’s is largest, followed by Neptune’s. That of the seventh planet is particularly small. There isn’t too much more to say about it than that, except that as well as these two families, other comets move through this region on their way in and out of the inner Solar System. The gas giants attract and steer comets into these orbits, and this happens with the two ice giants. Neptune is closest to the Oort cloud, so it’s particularly significant in this respect.

If the orbits of the planets marking the edges of the gap are projected onto the ecliptic, the area of this region is just over 1500 square AU, which is almost five hundred times the area of Earth’s orbit and 175 times the area of the inner system. Space has more than two dimensions of course, and the bodies occupying and defining this region don’t orbit in the same plane, particularly the moons of Hamlet. Features characterising it are quietness, coldness and dimness.

I just thought I should mention it because it’s easy to ignore the space between things.