Meet DeeDee. At about three times Pluto’s current distance from the sun, DeeDee is the second most distant known trans-Neptunian object (TNO) with a known orbit. Only the dwarf planet Eris lies further. Both circle the sun in the scattered disk, a sparse ring of bodies that lies even farther than the Kuiper Belt, an outer asteroid belt beyond Neptune. The Kuiper Belt is related to the more familiar rocky asteroid belt between Mars and Jupiter, but far larger and home to asteroids made primarily of water, ammonia and methane ice instead of rock. It extends from Neptune to about 50 times the distance of the Earth from the sun or nearly 5 billion miles.
The scattered disk overlaps the Kuiper Belt and extends well beyond 10 billion miles. It’s the tossed salad of the solar system. Asteroids were scattered here by the gravitational influence of the giant outer planets; their orbits are very elongated (more like comets) and instead of going around the sun in the plane of the planets, they follow paths that tilt up to 40°. Astronomers believe there are tens of thousands of Kuiper Belt and scattered disk asteroids of which we know of about 1,200. DeeDee is one of them.
Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of astronomers have recently begun putting a face on DeeDee, the informal name for 2014 UZ224, a scattered disk asteroid discovered in 2014 and located about three times Pluto’s current distance from the sun or some 9 billion miles. At this enormous distance, DeeDee takes more than 1,100 years to go around the sun, and its light 13 hours to reach Earth.
The new ALMA data reveal that DeeDee is roughly 395 miles (635 km) across, or about two-thirds the diameter of the dwarf planet Ceres, the largest member of the inner asteroid belt. At this size, DeeDee should have enough mass to crunch itself into a spherical shape, the criteria necessary for astronomers to consider it a dwarf planet, though it has yet to receive that official designation.
There are currently five dwarf planets: Pluto, Ceres, Eris, Haumea and Makemake. To be eligible, an object has to travel in an orbit around the sun and have enough mass to crush itself a nearly-round ball. Planet status requires one more criterion — that the object be large enough with a correspondingly strong gravitational pull to clear its orbit of other smaller asteroids and comets.
“Far beyond Pluto is a region surprisingly rich with planetary bodies. Some are quite small but others have sizes to rival Pluto, and could possibly be much larger,” said David Gerdes, a scientist with the University of Michigan and leader of the team that discovered DeeDee in 2014. They found it using the 4-meter (156-inch) Blanco telescope in Chile while participating in the Dark Energy Survey (DES), an international effort to map hundreds of millions of galaxies and detect thousands of supernovae to understand dark energy, the mysterious force that’s accelerating the expansion of the universe.
The DES has produced thousands of images of hundreds of millions of stars and galaxies including a tiny fraction of closer-by objects that appear to move very slowly across the sky — telltale signs of TNOs. One such object was identified on 12 separate images. Astronomers dubbed it DeeDee, short for Distant Dwarf. After further study to determine its orbit and distance, the team announced the news in 2016.
Gerdes and company wanted to learn more about DeeDee including its size, but it was just too far away to show as anything but a point of light. DeeDee might be a large and dark object or small with a bright, reflective surface. Either would look the same to the telescope’s eye. But how to tell them apart? That’s where ALMA came to the rescue.
ALMA can detect heat — in the form of millimeter-wavelength light — emitted naturally by cold objects in space. Even the coldest things emit some heat, so long as they’re warmer than absolute zero (–459.7° F / –273.1° C), the temperature at which all atoms stop moving. Turns out the heat signature from a distant solar system object is directly proportional to its size. DeeDee’s surface temperature turned out to be incredibly frigid, about 405° below zero Fahrenheit, but warm enough for ALMA to get a fix on its size. Given the asteroid’s temperature, distance and the amount of light it reflects, astronomers could calculate the object’s size. 395 miles is big as asteroids go, big enough even to earn dwarf planet pin.
While it seems strange to spend hours digging up information on an object only as bright as a candle seen halfway to the moon, we study objects like DeeDee because they’re leftovers from the formation of our solar system that were never incorporated into planets. Their orbits and physical properties reveal clues about how the planets, including Earth, formed. Gerdes points out that ALMA may also help us one day find and measure the hypothesized “Planet Nine” that may reside far beyond DeeDee and Eris.