LEWIS DARTNELL
We're taking the first steps in narrowing down where the hypothetical planet may be hiding away
The argument that there may be an undiscovered planet lurking far in the outer Solar System has been gathering momentum of late. This mysterious world has been dubbed Planet Nine, and several different studies have recently rallied statistical evidence that indicates its existence. Most notably, Konstantin Batygin and Michael Brown claimed last year that clustering of the most distant Kuiper Belt objects (KBOs) is caused by the gravitational nudging of an unseen but massive outer planet. Batygin and Brown estimated that this potential super-Earth world would follow a highly eccentric orbit with a semi-major axis of around 700 AU; around 20 times farther from the Sun than Neptune.
Other astronomers have since argued that this potential planet might explain the 6° tilt of the Sun relative to the ecliptic, or the population of trans-Neptunian objects with highly inclined orbits. The major concern now is to try to actually observe this remote planet. The question is, where in the sky should we target our telescope searches?
Sarah Millholland and Gregory Laughlin, both at the astronomy department of Yale University, have tried to further constrain the range of orbits that this Planet Nine would need to have in order to perturb distant KBOs in the pattern we see. They worked from the assumption that Planet Nine interacts with some of the KBOs in a mean motion resonance – where a celestial body experiences a regular gravitational tug because its orbit is in an exact ratio with another. This can occur with multiple bodies at the same time: the orbits of the Jovian moons Ganymede, Europa and Io, for example, are locked in a 1:2:4 mean motion resonance. If Planet Nine does indeed share an orbital resonance with some of the KBOs, it would really help constrain not only its orbital path, but where exactly along that ellipse it currently is – crucial for pinning down where in the night sky it can be observed.
Using the orbits of 11 KBOs, Millholland and Laughlin ran a series of orbital simulations over the past billion years of Solar System history to determine the orbital parameters of an object that could be gravitationally perturbing these KBOs. They calculate that Planet Nine, if it does exist, is most probably in an orbit with a semi-major axis of 654 AU, and is six to 12 times more massive than the Earth. They were also able to estimate the parameters of the elliptical orbit and the planet’s current position. Millholland and Laughlin predict that Planet Nine should be found in the region of the night sky with a right ascension of between 30° and 50°, and a declination of between –20° and 20° – an area roughly centred on the constellation of Cetus, the Whale.
This helps narrow down the search area for our planet-hunting telescopes to hopefully discover Planet Nine. If it is found to exist, a further question is how such a large planet came to form so far from the Sun. One intriguing possibility is that it may have been captured from another star in the sun’s birth cluster – Planet Nine could well be an interstellar migrant!
LEWIS DARTNELL was reading… Constraints on Planet Nine’s orbit and sky position within a framework of mean motion resonances by Sarah Millholland and Gregory Laughlin Read it online at https://arxiv.org/abs/1612.07774
“An orbital resonance would really help constrain not only its orbital path, but where exactly along that ellipse it currently is”