Sky and Telescope - October 2017 - 16

SOLAR SYSTEM QUEST by Scott S. Sheppard

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 revitalized 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.) 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 at the Magellan Baade 6.5-m telescope (not far away at Las Campanas
Observatory) 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.

p PLANETARY HERALD These discovery images show the distant object 2012 VP113 (arrows), taken about 2 hours apart on November 5, 2012, with
the Dark Energy Camera. The author found the object thanks to 2012 VP113's motion against the background of stars and galaxies.

O C T O B E R 2 0 17 * S K Y & T E L E S C O P E

SCOT T S. SHEPPA RD / CA R NEGIE INSTIT U TION 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-meter 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 realized 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.

Sky and Telescope - October 2017

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