There’s been a lot of speculation of late about a putative ‘Planet Nine’.
No, Pluto hasn’t been restored to proper place – this one is a new idea, proposed in 2014 by Chad Trujillo and Scott S. Sheppard on the basis of the orbits of the trans-Neptunian objects Sedna and 2012VP (which hasn’t been given a formal name but is classified as a ‘sednoid’). The idea was reinforced in 2016 by Konstantin Batygin and Michael E. Brown who suggested that six trans-Neptunian objects had particular orbits due to the influence of an unknown ninth planet ‘out there’ in the Oort Cloud, the belt of planetismals and debris of which Pluto and other worlds are a part.
The problem is that nobody’s actually seen the proposed planet, and other explanations for those orbital patterns have been proposed, the latest just a few weeks ago. So in some ways the jury’s out. It’s understandable. At the expected distance from the sun, Planet 9 wouldn’t be well lit and the reflected light coming back to us would give it a magnitude of less than 22, which is dim indeed. Add to that the fact that time on the big telescopes able to pick that up is heavily booked for other and equally interesting work, and it’s a challenge indeed. A crowd-sourcing project using WISE data to search for various faint objects in or near the solar system hasn’t come up with anything yet, either.
All of this, to me, highlights how much we don’t know, even about our own solar system. In the past couple of generations our view has changed in all sorts of ways. Back in the early-twentieth century it was a simple setup: eight planets (with maybe a ninth beyond, which Clyde Tombaugh shortly found and which was named Pluto) and an asteroid belt, plus a lot of comets. They orbited at distances which seemed to obey a ‘law’ invented by Johann Daniel Titius and Johann Elert Bode in the mid-eighteenth century (it had been noted by others before, though). Nobody had any evidence whatsoever of any other planetary systems, and exactly how all this had formed wasn’t clear, although the possibility of a grazing near-miss by another star was considered – drawing off matter from the Sun which formed the planets. Another theory, which turned out to be the actual process, involved the planets condensing from a nebula along with the Sun, but exactly how that happened wasn’t known then either.
By the early-mid twentieth century, however, there was growing evidence that the outer solar system, particularly, was more complex than anybody imagined. The possibility that Pluto might not be alone out there was raised by Frederick C. Leonard soon after Tombaugh’s discovery was announced. Then in 1943, British astronomer Kenneth Edgeworth proposed that the dust nebula from which the system probably formed might have been too small on the edges to condense into larger planets, implying there was a host of smaller objects out beyond Neptune. Then in 1951 Gerard Kuiper then proposed that such a belt might have formed early in the solar system’s life, but dispersed. Proof that it was still there, however, did not come until the early 1980s when David Jewitt and Jane Luu discovered the first of what became a flood of trans-Neptunian objects – TNO’s. Pluto remains the largest of them. The belt was named the Kuiper Belt after the great astronomer.
Meanwhile, in 1950, Dutch astronomer Jan Hendrick Oort proposed a vast cloud of small icy objects also existed, well beyond any Pluto-distance type objects, and was a source of comets.
Into that mix, from the mid-1990s, arrived evidence of solar systems other than our own, which posed some curious questions because none of them were anything like ours. Part of that was due to the detection methods of the day, which worked best for Jupiter-plus sized planets orbiting close to their primary. However, since then we’ve detected a flood of other systems and only a few approximate ours.
What those discoveries did was throw light on the way our own solar system might have formed and evolved – and what an extraordinary place it’s turned out to be. The major gas giants have all migrated in or out – exactly how is still being disentangled. During the formation of the system from a cloud of gas and dust there was potentially another giant planet that was thrown out of the system altogether. Meanwhile the smaller worlds were playing cosmic pinball with each other before it settled down.
All that happened over 4 billion years ago, but was that the end of the adventure? Of course not. Since then, relatively close encounters with other stars (each with their own Oort-style clouds extending maybe a light year) have altered the outer solar system – there is speculation that some of the odd orbits of the larger Kuiper Belt objects may be the result of such encounters. Sedna, for example, has a ridiculous orbit – as shown in the diagrams above – which takes it from 76 times Earth’s distance from the Sun at perehelion (which it is approaching now) to about 936 times Earth’s distance from the sun. That’s 0.015 light years. And it takes about 11,400 years to complete an orbit. We’re lucky it was found near perehelion: discovery distance in 2003 was 89.6 times Earth’s distance from the Sun. If it was further out, Sedna would have been too dim to easily spot. So you can imagine that there is potential for a lot of similar bodies to be out there, wa-a-a-a-y off in the distance, that we haven’t found yet.
And now there’s the potential for another and much larger body to be out there too, again not so much hiding as just hard to spot. That in turn raises other questions – did it form there? If not, where did it come from? Is it a ‘capture’ from a stellar near-miss? What? And that’s quite apart from the discoveries of more Kuiper belt objects, which are bound to be there.
All of this points to a pretty exciting time coming up in planetary and solar system science, which is very cool.
Copyright © Matthew Wright 2018