Planet 9?

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Another space blog post today, complete with some thoughts about life out there, and an extract from my work-in-progress The Liminal Zone.

First, though, the elusive Planet 9. For some time now, astronomers and space scientists have been speculating that an additional planet, of considerable size, lies out beyond Pluto. The evidence is indirect, in that such a planet has not been observed via telescope. Hence the matter is currently unresolved. But a recent paper argues that its presence would solve several unexplained issues, while its absence would create several more.

Orbital resonance in the moons of Jupiter (Wiki)
Orbital resonance in the moons of Jupiter (Wiki)

So what are the problems? Essentially, they come down to the logic of orbital dynamics, which says that you can’t just put a bunch of planets in random orbits around a star and expect them to be stable. Even though the gravitational attraction between two planets is small, it nevertheless exerts a steady regularising influence on the two paths around the sun. So the orbits of our sister planets show all kinds of patterns of ratios which at first sight seem remarkable (they’re still remarkable when you take gravity into account, but in a different way). And the more patterns that you see, the more you can infer about things you can’t see.

This, for example, is how the outer planets beyond Saturn were deduced before they were observed. The planets from Saturn inwards have been known since prehistory. But when careful observations with a telescope could be made, small but noticeable perturbations in their tracks were found. These pointed to the existence of unknown planets further out. The same principle explains why the orbits of Neptune and Pluto are synchronised – two of Pluto’s orbits match 3 of Neptunes. So, although Pluto dips inside Neptune’s orbit for a couple of decades every 248 years (one Pluto year), they are never at risk of colliding. These synchronisations happen all over the place – for example within the moon systems of Jupiter and Saturn, within the asteroid belt, or forming the delicate internal patterns of Saturn’s rings.

Now, Pluto is the first major body in the Kuiper Belt, a disc of space outside Neptune which we now know contains a decent number of small asteroids and similar objects. So it starts around 30AU from the Sun (AU = Astronomical Units, the distance between Earth and Sun). But it then Belt stops, quite abruptly, around 50AU. Why should this be? Why not feather off gradually?

Trans-Neptunian Object orbits (LIve Science / ESO)
Trans-Neptunian Object orbits (LIve Science / ESO)

Additionally, as we have built up a catalogue of these asteroids, a picture is emerging in which a surprising number have orbits around the sun which are aligned with each other. The simplest way to explain this is to suppose that some sizeable, but as yet unknown, object is synchronising them.

So, why has it not yet been found? Well, first of all, as Douglas Adams said, space is “vastly, hugely, mind-bogglingly big” (Hitchhiker’s Guide to the Galaxy, chapter 8). So although the potential planet is several times larger than the Earth, it is on average 20 times further from the sun than Neptune is – 600AU – with an orbit that is quite noticeably elliptical rather than circular. That means that there is a lot of space to search in, and also that it is dark and cold out there. There is not a lot for optical or infrared telescopes to detect. But each new discovery helps narrow the search window down, and some lucky group of astronomers may well announce a discovery soon.

Or, of course, not. It may be that the apparent alignment we see will be eroded by more observations. Which would be a bit of a shame, in that it is always nice to have unknown things to discover. It would also leave several other problems unresolved. Other things being equal, I’d like Planet 9 to be found!

Artist's impression, Planet 9 (Live Science / JPL-CalTech)
Artist’s impression, Planet 9 (Live Science / JPL-CalTech)

So, what might it be like to live there? For one thing, cold and dark. Our sun is still the nearest and brightest star by a huge margin. But at 20 times further away than Pluto, it gets just 1/400 of the solar radiation of any kind. Or if you like, 0.0003% of what we enjoy on Earth. You’d want to know you had reliable sources of heat and light, if you went there. And it will take a long time to get there. It is not a place for a quick jaunt. For reference, Voyager 1 is a little over 100AU from Earth and has spent about 40 years getting there.

Could there be indigenous life out there? Well, life as we know it depends on liquid water, and the surface of Planet 9 is way too cold for that. But possibly, there could be subsurface heat turning ice into water at some depth? Or perhaps, there might be a moon which would be subject to gravitational flexing, just as happens to the inner moons of Jupiter and Saturn. This could – maybe – provide enough heat to give us water. We’ll have to wait and see.

I haven’t yet written anything going that far out from the sun. In the universe of Far from the Spaceports, an Earth-Mars trip takes a couple of weeks. An Earth-Pluto trip takes a few months. An Earth-Planet 9 trip would take anywhere from seven or eight months up to just over a year, depending on whereabouts in its orbit it happens to be. Not a journey you’d make lightly.

The Liminal Zone (temporary cover)
The Liminal Zone (temporary cover)

The Liminal Zone takes place on Charon, the main moon of Pluto. The New Horizons probe returned some fascinatingly detailed pictures to us of these two, transforming them from hazy blobs to detailed worlds. New Horizons is currently en route to an object further out in the Kuiper Belt, 2014 MU69, popularly known as Ultima Thule, and is due to arrive early next year. Finding a second destination more-or-less on the flight path after Pluto was a remarkable thing in itself, as objects are so exceedingly thinly spread out there. Anyway, The Liminal Zone is not a financial fraud book like Far from the Spaceports or Timing – it’s more of a voyage of discovery, both personally for the main character, Nina, and more generally for the society she is part of. So here is a short extract – Nina is talking to Percy, one of the Charon residents, about events surrounding an emergency several years ago…


Something about his expression made Nina stop.
“But you didn’t actually see anything?”
He drew back a little.
“Seeing’s not everything. Haven’t you ever just known something for sure?”
His eyes held hers, suddenly very intense, and she felt a little internal quaver run through her body. She had hoped it wouldn’t show, but then she saw the trace of a smile cross his eyes.
“I’ve got Welsh blood, you know. It helps me comprehend things which maybe can’t be seen with the naked eye. And what about you, Nina? Where do you come from?”
She went blank.
“I grew up in Lacus Gaudii. On the Moon.”
He shook his head.
“Not that recent. Go back a few generations. Where did your family live? Before they came up to settle in that lunar lake of yours.”
The noise of the kettle was maddening. She withdrew inside herself, trying to escape the pressure.
“I… I don’t know. I suppose I could find out. It’s never mattered.”
He looked away, letting the moment pass.
“Ah, but it just might make a difference here.”
She took a long breath and tried again.
“But did you actually see anything?”


I’ll be posting more on progress into The Liminal Zone as it comes along…

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