Australian Sky & Telescope — November-December 2017

http://www.skyandtelescope.com.au 19

PABLO MCLOUD / SUBARU TELESCOPE / NAOJ; DIAGRAM: LEAH TISCIONE /

S&T

, SOURCE: S. S. SHEPPARD

Signpost for an unknown planet?

The object, now called 2012 VP 113 , has the most distant

perihelion (its closest approach to the Sun) of any object

known in the Solar System. It’s one of a handful of bodies

found with very distant orbits and large perihelia — extreme

objects that spend most of their time well beyond the known

mass in the Solar System and thus should be minimally

gravitationally disturbed by the known planets.

When we started to look at the orbits of these extreme

objects, we noticed a strange resemblance: Although they

have orbits wildly tilted out of the ecliptic plane, all their

perihelia lie near where their orbits cross the ecliptic

moving from south to north. The technical way to describe

this situation is to say they all have similar arguments of

perihelion. The argument of perihelion is an angle that

defines the orientation of an orbit’s perihelion with respect

to the ecliptic plane, somewhat like a latitude angle on

Earth defines an orientation with respect to the equator. The

arguments of perihelion for these objects should have been

random, yet they clearly clustered around 0°.

We first thought this cannot be; it must be some

observational bias. Certain angles might be more easily

found over others when looking at a small range of ecliptic

latitudes. But most of the extreme objects were discovered

through surveys that looked at a range of latitudes, making

observational biases unlikely. Furthermore, our numerical

simulations confirmed that, if the known planets were the

only significant influences on these objects, the arguments of

perihelion should have been random: Even if these extreme

objects started with nearly the same orbital angles, their

paths would randomise in much less than the age of the

Solar System. The only way to create and keep this orbital

clustering stable is if whatever is causing it still exists. The

best answer? The influence of a massive, unknown planet.

SUBARU Two teams hunting for Planet X are using the 8.2-metre Subaru telescope, shown here on Mauna Kea in Hawai‘i.

Neptune at time

of Pluto perihelion

Neptune at time of

next Pluto perihelion

Pluto

Neptune

Pluto

Neptune

SIN RESONANCE Pluto and Neptune occupy a 2:3 orbital resonance:

For every two loops Pluto completes around the Sun, Neptune

completes three. Because of this, Pluto and Neptune never come close

to each other, even though Pluto crosses Neptune’s orbit. Similarly, an

unknown planet could exist in resonance with the extreme objects.