Oh dear – we’re going to have to rewrite Solar System Science Fiction

For those of you who are already eagerly keeping up with new discoveries, this news is already a few days old. But we may have an extra couple of planets lurking on the outer edges of our Solar System. See here for details.


So how far out from the Sun could they be?

Well, a couple of clever chaps came up with the Titus-Boyd law, which basically says that a planet should exist at

a=0.4 + 0.3*2**m where m = -∞, 0, 1, 2, …

(** means to the power of)

where a is the distance from the Sun in astronomical units, an astronomical unit being the distance of the Earth from the Sun.

So we have

a = 0.4 – Mercury

a = 0.7 – Venus

a = 1 – Earth

a = 1.6 – Mars

a = 2.8 – the asteroid belt, which you can count as a broken up planet

a = 5.2 – Jupiter

a = 10.0 – Saturn

a = 19.6 – Uranus

This is where it gets tricky. The next value of a is 38.8. This happens to be Pluto. So what has happened to Neptune? Well, Neptune and Pluto are in orbital resonance of Pluto doing 2 orbits for Neptune’s every 3 orbits. So in one sense they occupy the same orbital parking lot if I can use such a phrase.

So the tenth planet, if it really exists should be in the parking lot of about 77.2 astronomical units from the Sun. And the eleventh planet is at 154 astronomical units from the Sun. Both these putative planets are very far away. If they are the size of Pluto, it can’t come as a surprise that they have not been found.

Um… hold on a minute… Eris (yes I know it has a very eccentric orbit – I should have perhaps explained, that a planetary orbit is basically an ellipse defined by its semi-major axis and eccentricity, and that the distances quoted are the semi-majot axes)…. now where was I?…. Yes, Eris is 67.8 astronomical units from the Sun… Oops! This is spot on (in astronomical terms) for the tenth planet.

Of course like the Neptune-Pluto parking-lot pairing, there may other such Trans-Neptunian Objects (TNOs) sneaking around that particular parking lot to keep Eris company. And this could be the missing tenth planet.

However, there are several thing that science fiction can learn from the above we simple exercise in science.

  1. If you are designing your own Solar System, there’s a simple relationship between the distances of the planets.
  2. Those science fiction stories that have been written at what we knew to be the previous edge of the Solar System, are very likely to be dated now that the newer information is available. Writing ‘near term’ science fiction is always likely to be superseded by new discoveries. It’s the stories that stay in the mind. So if you are writing ‘near term’ make sure that there is some other interest in the story line e.g. a new way or idea of getting things done or how a character reacts to a hitherto unthought of situation.
  3. Of course those stories that turn out to have made the right predictions will be remembered. But there is something else important here. It’s finding out why they got it right. Sometimes it was just a lucky guess and nothing more can be gained from investigating further. other times, it my lead to a better understanding of our real world, and this is something that should be welcomed. Just like Titus-Boyd did all those centuries ago… yes, I know they did not do it via science fiction, but the genre can help in this respect.

4 thoughts on “Oh dear – we’re going to have to rewrite Solar System Science Fiction

  1. I understand your explanation about distance making them difficult to find, but if they exist I am surprised that we don’t appear to have even the merest hint of their existence at the moment.

    I guess as with everything, time will tell

    1. Ever since the discovery of Neptune in the 1840s, planets and indeed, other celestial objects, have mainly been found by observing perturbations of nearby objects away from their expected orbits / paths. In this case the perturbations would in astronomical terms be very small and therefore also difficult to measure accurately.

      It all boils down to resolution of the telescopes we have got, which are not as good as we would like them.

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