Planet Nine from Outer Space

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.

Wright_Solar System w Voyager 2
Here’s the solar system, a picture I made with Celestia. The large oval is the orbit of Sedna. Everything else – including all the known planets, dwarf planets and asteroids, is jammed in on the left. The red line is the orbit of Voyager 2.

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.

Wright_Solar System w Voyager 1
Another picture I made with Celestia. The large brown oval is the orbit of Sedna, and the red line is the orbit of Voyager 2. Yah…

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.

Diagram of the Oort cloud and Kuiper Belt. Public domain, NASA, via Wikipedia.

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


8 thoughts on “Planet Nine from Outer Space

  1. Great article, Matthew. I’ve done a tiny amount of research, seriously tiny, but during the course of it, I came across the idea that our solar system might actually be part of a very distant binary. Or perhaps that it formed as part of a binary. If that binary no longer exists for whatever reason [is that even possible?] could that explain some of the anomalies?

    1. Thanks – glad you liked it! The issue of a potential ‘hard-to-see sun companion’ has certainly been discussed, including as a way of explaining the apparently regular showers of long-period comets to which the inner solar system is subjected (about every 27 million years or so, potentially provoking mass-extinctions on Earth). As far as I can tell it was first seriously proposed by Richard Muller of Ucal in 1984, and suggestions have included the idea that something as as ‘small’ as a brown dwarf, several times the mass of Jupiter (but, because of the way gravity vs non-solid objects works, about the same physical size) might do it.

      However, the Two Micron All Sky Survey looked for just such an object, in the infrared, and found 173 brown dwarfs (apologies to Tolkien…) elsewhere, but none near our solar system. Nor did the Wide-field Infrared Survey Explorer discover anything nearby. A putative ‘invisible companion star’ should have been found by these. That, in part, is where the ‘Planet 9’ idea came from – because something’s been disrupting the outer solar system, and if it isn’t a star then it has to be something else.

      Another possibility is that a passing star might also do this – the Oort cloud extends out to a light year, and stars do periodically come that close. The last one to do so was Scholz’s Star, discovered in 2013, which whisked past the Oort Cloud around 70,000 years ago – I blogged about it in 2015, here: Probably it dislodged some comets, but they won’t have reached us… yet… (and might not for a long while – also, Jupiter is handy at knocking such things into other orbits before they get to us).

      1. Oh wow…I admit I was kind of expecting you to say it was all just a fantasy. So…if not a companion binary star then possibly something else that we haven’t come across yet. Exciting. It’s a bit like deducting the existence of something only from its shadow.

        1. That’s pretty much exactly what science is doing – and it is absolutely exciting! I’d say ‘watch this space’ but I suspect the actual phrase should be ‘watch outer space’…

        1. Sorry about that. As far as I can tell it’s because WordPress generated a query by way of a link – all of them are on my blog, but the particular post’s buried. Here’s a shortlink which I’ve generated from the post’s ‘share’ tab, hopefully that’ll work:

          1. Found it! And I’m amazed. So that red Dwarf could have been a part of Earth’s sky for thousands? of generations [assuming there were people to see it]. Not a binary but…so exciting!

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