Free Astronomy Magazine

DWARF PLANETS The argument of perihelion is the angular distance between the perihelion point (closest to the Sun) and the ecliptic plane (the imaginary plane contain- ing the Earth's orbit). If the or- bits of the two dwarf planets were randomly distributed, that near coincidence of values would be highly unlikely. But that’s not all. Those values are al- so shared, with good approximation, by various other trans- Neptunian objects whose orbits have semi-major axes ex- ceeding 150 AU. Whichever mecha- nism may have led to the formation of the inner Oort Cloud, its members’ orbits should have today ran- domly distributed argu- ments of perihelion. This though is not so: which means that there must be a perturbing body whose con- siderable mass affects that distri- bution, acting like a shepherd herd- ing his sheep at a certain hour of the evening. According to Trujillo and Shep- pard, a planet of about 10 Earth-masses or- biting at 200-250 AU from the Sun could account for the coincidences detected. But the same thing would also be true for more massive planets located at greater distances. Very recent Luhman and Kirkpatrick’s works, based mainly on the survey by the WISE telescope, place stringent upper mass limits to a possible trans-Neptunian planet, but do not exclude the existence of a super-Earth. This would, however, be beyond the reach of the most powerful instruments currently available to astronomers, given its temper- ature of only few tens of kelvin. The fact that the hypothetical perturbing planet is not observable, does not automatically mean that it really exists, in fact, the proba- bilities that it is there are rather limited. As pointed out by Hal Levison (Southwest Re- C urious repre- sentation of the Oort Cloud, highlighting the inner and outer regions. The dis- tance scale gives an idea of their sizes, even though for graphic rea- sons the planetary region has been greatly oversized. [Andrew Z. Colvin] search Institute), if that planet has been ejected from our solar system (hypothesis more plausible than others) it would have had only a 2% probability of settling into an orbit consistent with Trujillo and Shep- pard’s hypothesis. This means that either we are dealing with a rare event, or that the planets ejected from the solar system were numerous (but this seems highly un- feasible). To understand how their orbits are perturbed we will just have to wait for further discoveries of dwarf planets similar to Sedna and 2012 VP 113 . At that point it should be possible to determine the mass, distance and – above all – the position of the perturbing body. n