Australian Sky & Telescope — November-December 2017

http://www.skyandtelescope.com.au 23

LEAH TISCIONE /

S&T

, SOURCE: S. S. SHEPPARD

To conduct the hunt, we need both a medium-to-large

aperture and a wide field of view. Most large telescopes

(including the Hubble Space Telescope) have small fields of

view. They’re good for looking at known objects but are not

adept at searching for new ones. One of the few exceptions is

Japan’s 8.2-metre Subaru telescope in Hawai‘i, arguably the

most powerful survey telescope in the world. Subaru’s Hyper

Suprime-Camera, commissioned in 2014, can cover some

9 full Moons in one image. Both our group, which includes

Chad Trujillo and David Tholen (University of Hawai‘i),

and the Caltech group working with the Japanese, are using

Subaru to hunt for the hypothesised planet. With this

telescope we’re focusing on the Eridanus and Orion region;

the other group is looking in or near Taurus and Orion.

The next most powerful survey telescope is the 4-metre

Blanco and its Dark Energy Camera. The DECam’s field

of view is bigger than Subaru’s, but the telescope’s mirror

has only a fourth as much collecting area, meaning it’s

less sensitive to faint objects. Our group is also using this

telescope to survey regions farther south, in Cetus and

Eridanus. Images from the Dark Energy Survey consortium’s

work, though not optimised for planet hunting, might also

serendipitously find the planet, as their field locations include

predicted aphelion locations for the planet.

Uranus, Neptune and Pluto were all seen or imaged before

astronomers realised what they were. Observations of this

distant, massive planet could likewise already be in some

archived data, whether at optical wavelengths or infrared. To

actually detect the planet will require at least two very deep

images separated in time long enough to see it shift against the

background stars. These two criteria are hard to satisfy, because

so little of the sky has been imaged to the depths required and

with the time-base of days necessary to see movement.

Back in the saddle

With the arrival of September, the prime search areas are

now up. If the weather behaves, some 30% to 70% of this real

estate will be covered in the next few months. We could be

in for the first planet discovery in our Solar System in about a

century or two, depending on how you’re counting.

It’s this potential revelation that powers our breakneck

pace. At almost every new Moon, when the sky is dark, our

survey continues searching for any objects at the fringe of our

known Solar System. Just as I get back on a normal sleep cycle,

I’m usually off again to Chile or Hawai‘i to start the sleepless

nights all over again. We can capture almost a terabyte of data

per night, but we must find the objects within a few weeks in

order to re-observe them before they are lost. I start looking at

the images while on the plane home from the telescope run.

I always wonder what the person sitting next to me thinks

I’m doing. Blinking through the new images, marked up with

a bunch of green circles our algorithm has drawn around

moving dots, might just look like the latest video game craze.

It’s no game, but it is just as fun. In each new image you never

know what you are going to get. Game on.

„SCOTT S. SHEPPARD is an astronomer in the Department

of Terrestrial Magnetism, Carnegie Institution for Science in

Washington, D.C. If the Guinness Book of World Records had

a category for moon discoveries, Sheppard would hold it.

STHE PLAYING FIELDBased on an approximate estimate for its orbit (yellow shaded region), astronomers think that a massive but unseen planet

might be lurking in the September to January sky. They have narrowed in on the boxed region for the 2017 campaign. Unfortunately, this race is

beyond amateurs’ equipment abilities — telescopes with apertures less than 4 metres need not apply.

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