Australian Sky & Telescope — November-December 2017

18 AUSTRALIAN SKY & TELESCOPE November | December 2017

SCOTT S. SHEPPARD / CARNEGIE INSTITUTION FOR SCIENCE

We were going cross-eyed searching for faint, slow-

moving points of light among the steady state of background

stars and galaxies. When Chad Trujillo (Northern Arizona

University) and I started the search over a decade ago, we

had in mind a simple question: What lies beyond the known

observable Solar System? In the survey that ensued we

detected a few comets, thousands of asteroids and Kuiper Belt

objects, and even some of the first known Trojan asteroids

shadowing Neptune — but nothing that was way, way out

there, far beyond Pluto and the Kuiper Belt.

In late 2012 the game changed with the new Dark

Energy Camera on the Blanco 4-metre telescope in Chile.

Astronomers use this camera to search for signs of dark

energy, by mapping the locations of hundreds of millions of

galaxies to study patterns in cosmic structure. To do so, the

camera takes huge images, each of which is large enough to

fit about 14 full Moons. These images cover some 10 times

the sky area possible with the telescope’s previous camera.

Compared to the new camera’s view, it was like we had

previously been looking through a straw.

Time on the Blanco was also available for other projects

— so why not ours? We realised one night’s observations

with DECam would cover as much sky as we had searched in

all the previous years. “We don’t know what we will find,”

we wrote in our proposal for telescope time. Nobody had

searched to the faint depths and across the large sky area that

we envisioned doing.

The hunt for Planet X

Evidence is building that a large world lurks far beyond Pluto

and the Kuiper Belt. The race to find it is on.

Taking the images themselves posed a challenge, however.

During commissioning, the revitalised telescope had trouble

pointing and tracking with the camera. To prevent smearing

we had to take several short, 90-second exposures, then shift

their positions by the bad tracking rate before adding them

together to get our required deep, 6-minute exposure time.

The pointing of the telescope was so far off in 2012 that

the automated computer algorithm we normally used to flag

possible moving objects didn’t work yet on the misaligned

images. In order to find and re-observe new objects before

we lost them, I looked through the data by eye, without the

help of any algorithm. It was similar to how Clyde Tombaugh

manually blinked photographic plates to find Pluto.

I was looking at one of our worst-aligned images, my eyes

glossing over, when I came across the slowest object I had ever

seen. Based on its slow motion the object appeared to be about

80 astronomical units (a.u., the average distance between the

Sun and the Earth) from the Sun, almost 2½ times farther

than Pluto is currently. But we didn’t know much else. A

couple of months had passed since the images had been taken,

and the new object was in danger of being lost.

In early 2013 we re-observed the object using the

Magellan Baade 6.5-m telescope and found it near the image

edge, almost lost to oblivion. The orbit we derived showed the

object never comes closer to the Sun than it currently is. It

was then we knew we’d found something special that could

shed light on the fringes of our Solar System.

SPLANETARY HERALD These discovery images show the distant object 2012 VP 113 (arrows), taken about 2 hours apart on November 5, 2012, with

the Dark Energy Camera. The author found the object thanks to 2012 VP 113 ’s motion against the background of stars and galaxies.

PLANET QUEST by Scott S. Sheppard