This week’s (Jan 20, 2016) Astrophysical Journal has a paper by two researchers (K. Bayugin and M. Brown) announcing the possible discovery of another planet in the solar system. I say possible, because the planet’s existence is deduced from the orbits of several dwarf planets, beyond the orbit of Pluto (Kuiper Belt objects or KBOs). They argue that the orbits of those bodies are best explained by a large (Neptunish sized) planet in the far reaches of the solar system, which is perturbing the orbits of the smaller dwarf planets.
This is a well-established planet hunter technique, going back to the discovery of Neptune, which was based on orbital anomalies in the orbit of Uranus. The search for Pluto was also motivated by gravitational arguments, though it turns out to be too small to have the effects thought to have been seen in the orbit of Neptune. Mostly, its discovery was fortuitous – gravitational arguments just happened to lead Claude Tombaugh to look in the general area where Pluto happened to be. However, many extra-solar planets have also been discovered via gravitational means, usually the effect of large planets close in to their star, which creates a sort of jitter in the stars motion, which can be detected from Doppler shifts in the star’s spectrum.
I am calling this planet X9, as a bit of an astrophysics/science fiction gag on my part. The X can stand for “unknown”. But it can also stand for 10, as in Roman numerals, since it would be the 10th planet under the old system, which had Pluto as the 9th. But, since Pluto has been demoted to “dwarf planet”, the new planet is really number 9. Thus, via a sort of planet naming mash-up, I will call it Planet X9. Plus it reminds one of “Plan 9 from Outer Space”, one of the all-time best good-bad science fiction movies.
I will do my best to interpret their paper in the text below, though with only a humble B.Sc. in physics, I am no expert on orbital mechanics (or automobile mechanics, for that matter).
One reason for thinking there may be another planet out there, is evident in the following picture, from their paper:
The image can be thought of as looking at the solar system from a view above the plane of the ecliptic (the common plane in which the planets orbit around the sun), rather like looking at the Earth from far above the north pole might be. The ellipses (ovals) show the orbits of some large KBOs. As you can see, their perihelia (the closest point of the orbit to the sun) are all clustering on the same side of the sun, in a fairly narrow range. The authors of the paper have calculated that this is a very unlikely configuration, an almost certainly not based on pure chance, so there must be something behind the process. They think that a large planet in the far reaches of the solar system would produce this result.
There is some possibility that this result is due to observational bias (the tendency to see more things in one area of the sky because that’s where you spend the most time looking), but the authors say that is unlikely – though some bias might be present due to there being an observational bias towards the plane of the ecliptic, that doesn’t explain the clustering in ecliptic longitude that is seen.
The paper then goes into both an analytical analysis and a numeric analysis of what gravitationally perturbing bodies might account for the behaviour of the KBOs. The former involves a lot equations based on standard gravitational theory, which demonstrate that the observed behaviour can be produced by the presence of an unseen planetary mass. However, they note that the analytical solution has many simplifying assumptions built in to it, so some skepticism is warranted.
They then offer a numeric solution via computer simulations - in other words direct N-body simulations of the solar system. With this method, they could vary inputs to the model, see how the computer simulation evolved, then compare that to the known observations. They could then derive such parameters as the distance to the perturbing planet, the planets orbital eccentricity and relationship to the KBOS, and planetary mass. The surprising result of this analysis, is that the perturbing planet is favoured to be opposite to the KBOs, as this image from a CBC story shows:
They tried simulations using various masses for the unseen planet. Though an Earth size mass could work, a larger mass planet (10 Earth masses) fit the simulations much better and is therefore considered a more realistic choice.
Simulations also show that the perihelion (closest approach to the sun) of such a planet would be very distant – something on the order of 250 AU (astronomical units, equal to the distance of the Earth’s orbit from the sun). For comparison, Pluto’s perihelion is about 30 AU, so this planet would be about 8 times farther away.
Finally, the simulations indicate that the unknown planet’s orbit may be inclined to the plane of the ecliptic, but probably not by a very large amount. In other words, it may be orbiting in a different plane than the Earth but not radically different (within a few tens of degrees). Again for comparison, Pluto is inclined to the plane of the ecliptic by about 17 degrees.
The paper notes that this theory has some testable implications, as more KBOs are discovered, in terms of their inclinations, and so forth. It also notes that the ultimate test of the theory is to actually observe the possible planet. The paper acknowledges that the search space is still very wide, based on the theory developed. But, who knows, once the search starts in earnest, we may have a result sooner than we expect.
As for the origin of such a large planet at this great distance from the sun, they speculate that it may be that this is a gas giant that was ejected from the inner solar system (inner relative to 250+ AU) during the early stages of its evolution.
Here's a link to the article:
There are now thought to be potentially billions of rogue planets in the galaxy, based on micro-lensing findings. Could this be one of these? The paper doesn’t speculate about that, but it seems to me that's a possibility worth thinking about.
One would expect a world so far from the sun to be extremely cold, but some of the theoretical work on rogue planets has indicated that they could in fact have warm interiors under a layer of ice, perhaps even liquid oceans. Maybe this planet is the same, if it exists. But it would take an awfully long time for a space probe to get there and do some close-in measurements, should we ever pin its location down.
And here’s a science fiction novel which includes plenty of action on an unexplored planet, in the far reaches of the galaxy. It's only $3.99 :)