The two moons of Mars have always presented planetary scientists with something of a mystery. These tiny moons, Phobos and Deimos, whizz around Mars at no great height at all: Phobos whips around the red planet in less than 8 hours, at a height of only 3,700 miles – the closest of any moon to its parent planet. I say ‘parent’ advisedly because a new theory of the origin of these peculiar little moons suggests that they emerged from a major impact between mars and a dwarf planet. It has generally been assumed that they were captured asteroids, but the relative circularity of their orbits argued against such a capture. Work on the possibility of a catastrophic origin was carried out last year by two separate teams of researchers, after decades of battling intense scepticism within the scientific community (1). An important finding of the modelling at that time was that the resultant debris would circulate around the red planet at a relatively low altitude, which is in keeping with the orbits of the two extant moons.
More recently, further computer modelling of various impact scenarios carried out by one of those teams has narrowed down the range of masses of an impactor to about the size of Pluto. The resultant debris field was initially far more extensive than the two moons left today:
Academic papers aimed at further constraining the parameters of the purported ‘Planet Nine’ body continue to emerge from various quarters, many from researchers with long-term interests in outer solar system anomalies. Fairly quickly after Brown and Batygin’s announcement about Planet Nine (1), a paper was published by A. Fienga et al examining the astrometry of Saturn through Cassini’s radio ranging data (2). This work served to constrain the possible locations of Planet Nine, which were wide ranging to say the least. This is because if one can establish the very precise positioning of outer planets over time, then this can provide clues to any slight gravitational effect, or perturbation, the planet might be experiencing from an undiscovered distant substantial Planet X body (3). However, given that Planet Nine is thought to have a highly elliptical orbit, then if it is located at the further end of that ellipse, its effect upon the outer planets gravitationally becomes vanishingly small. It turns out then, as one might predict, that we can rule out its current location being in the nearer half of its elliptical path, according to the Cassini data about Saturn. Which is more or less common sense, anyway.
One of the pair of Caltech scientists who announced in January that there was a very high probability that a ‘super-Earth’, dubbed ‘Planet Nine’, exists beyond Neptune (1,2), has noted that a newly discovered eccentric Kuiper Belt Object cuts down the possibility that they were wrong still further.
“The object [uo3L91] shares some of the same behavior as the other six Kuiper Belt bodies, suggesting it has also been pushed by a large planet that is between 200 and 1,200 times the distance from the Sun to Earth. The object was discovered by the Canada France Hawaii Telescope, which is conducting the Outer Solar System Origins Survey (OSSOS); information about its movements were presented recently by astronomer Michele Bannister at the SETI Institute.” (3)
A week on from Caltech’s announcement, Dr Mike Brown and Dr Konstantin Batygin, the two astrophysicists proposing the existence of their ‘Planet Nine’, sketched out the range of orbits which their object might be moving through, including its all-important approximate perihelion and aphelion positions. Essentially, Brown and Batygin consider the perihelion position of the Planet Nine body to be in a broad region around the zodiacal constellations Scorpius/Sagittarius (R.A. = ~16hrs), whilst the aphelion positionof Planet Nine is likely in the equally broad Orion/Taurus area (R.A. = ~4hrs) (1,2).
Image credit: Caltech
We seem to be getting very close now to a discovery of a massive Planet X in the outer solar system. I heard this report on the evening BBC news, a slot which indicates the seriousness with which this subject is now being taken by the scientific community:
“American astronomers say they have strong evidence that there is a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto. The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet. Rather, the scientists make the claim based on the way other far-flung objects are seen to move. But if correct, the putative planet would have 10 times the mass of Earth.