Australian Sky & Telescope — November-December 2017

http://www.skyandtelescope.com.au 21

LEAH TISCIONE /

S&T

, SOURCE:

CAMBRIDGE DICTIONARY OF ASTRONOMY

Our basic simulations showed this planet would need to lie

beyond a couple of hundred a.u. and be larger than a few Earth

masses to shepherd the extreme objects. Its strong gravitational

influence would produce some sort of dynamical resonance

behaviour, similar to that between Pluto and Neptune that

forces Pluto to go around the Sun twice for every three orbits

Neptune completes. This 2:3 resonance also keeps Pluto and

Neptune safely separated, even though their orbits cross. Even

if we never saw Neptune, we would still know it exists because

of the hundreds of known objects, called plutinos, with very

similar orbits to Pluto’s. This is also true with the Jupiter and

Neptune Trojans (which orbit ahead of and behind the planets),

which are in a 1:1 resonance with their respective planets.

We also saw hints of a second trend, which Konstantin

Batygin and Michael Brown (both at Caltech) confirmed two

years later in 2016: The orbital planes of the most extreme

objects are also aligned in ecliptic longitude. Just as locations

on Earth can have the same latitude but different longitudes,

orbits with similar arguments of perihelion can still have

various orientations with respect to each other. But if the

orbital planes have similar latitudes and longitudes, then they

line up in physical space (see facing page).

Using this alignment, the team was able to calculate a

rudimentary orbit for a distant, massive planet that would

cause these two geometric clusterings. The planet needed to

have a highly elongated orbit, with a perihelion that lies on

the opposite side of the Sun as the extreme objects’ perihelia.

This arrangement allows the planet to spend most of its time

well away from the other, smaller objects and thus keeps

their orbits stable over the age of the Solar System, somewhat

analogous to how Pluto stays away from Neptune.

Although statistically it was still unclear if the longitude

alignment was real, this work took me from some 70% sure

in 2014 to 80% sure that a massive planet much larger than

Earth exists beyond a couple of hundred a.u.

Since then, Renu Malhotra (University of Arizona), Sarah

Millholland and Gregory Laughlin (both at Yale), and others

have shown that some of the extreme objects could be in

orbit period resonances with the much larger world. An orbital

period for the planet of around 17,000 years seems to work well.

Curiouser and curiouser

I’m now nearing the 90% confidence level that this massive,

distant planet exists. Not only does our continuing survey now

cover most areas of ecliptic longitude on the sky, but we have

also discovered two more extreme objects, called 2013 FT 28

and 2014 SR 349. Amazingly, 2014 SR 349 has all the same orbital

characteristics as the previously known extreme objects.

But while 2013 FT 28 has a somewhat similar argument of

perihelion, its perihelion lies on the same side of the Sun as

the mystery planet’s presumed perihelion, a 180° longitude

difference from the others. The existence of 2013 FT 28

suggests that two clusters of objects exist, not just one, with

the first group on the opposite side as the unseen, massive

world and the second on the same side, aligned with the

Solar System orbits? Elementary.

Name Definition and Function

Semimajor axis Distance from elliptical orbit’s

centre to edge along the longest

axis. Defines the mean size of

the orbit.

Eccentricity Distance between the ellipse’s

centre and one of its foci, divided

bya. Defines how elongated an

orbit is.

Inclination Angle between the orbital plane

and the ecliptic plane.

Longitude of Angle drawn in the ecliptic plane

ascending node between the vernal equinox

direction and the line where the

orbit intersects the ecliptic with

the object moving south to north.

Defines the direction in space of

the line where the orbit intersects

the ecliptic plane.

Argument of The ascending node–Sun–

perihelion perihelion angle, measured in

the orbital plane in the object’s

direction of motion. Defines the

orbit’s orientation with respect

to the ecliptic.

Time of perihelion When the object reaches

passage perihelion. Sets the reference

frame for the orbital motion.

Direction of

vernal equinox

Plane of

ecliptic

e=c a

c i

P

P = perihelion

N

t

1

Orbital

plane

a

Astronomers describe the shape, orientation and timing of orbital

motion using a set of parameters called orbital elements. Six orbital

elements — a, e, i, 1 , t and T — specify everything you need

to know to locate an object in an elliptical orbit at any time. The

ascending node, N, denotes where the body passes from below

the ecliptic to above it. Dotted lines belong to the underlying plane.

Orbital basics

Symbol

a e i 1 t T