Researchers using Atacama Large Millimeter/submillimeter Array (ALMA) have uncovered new clues about the recently-discovered planetary body UZ224. The results will appear in the Astrophysical Journal Letters (arXiv.org preprint).
Artist concept of the planetary body 2014 UZ224. Image credit: Alexandra Angelich / NRAO / AUI / NSF.
Also known as DeeDee (short for ‘Distant Dwarf’), UZ224 is 395 miles (635 km) across, or about two-thirds the diameter of the dwarf planet Ceres.
At this size, UZ224 (the official full designation is 2014 UZ224) should have enough mass to be spherical, the criteria necessary for astronomers to consider it a dwarf planet, though it has yet to receive that official designation.
“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 lead author David Gerdes, an Arthur F. Thurnau Professor in the Departments of Physics and Astronomy at the University of Michigan.
“Because these objects are so distant and dim, it’s incredibly difficult to even detect them, let alone study them in any detail.”
Currently, UZ224 is about 92 AU (astronomical units) from the Sun — that is almost three times farther from the Sun than the dwarf planet Pluto is.
At this tremendous distance, it takes this planetary body about 1,140 years to complete one orbit. Light from UZ224 takes roughly 13 hours to reach our planet.
UZ224 is currently the second-most distant known trans-Neptunian object with reported orbital elements, surpassed in distance only by the dwarf planet Eris.
Orbits of objects in the Solar System, showing the current location of UZ224. Image credit: Alexandra Angelich / NRAO / AUI / NSF.
Size comparisons of objects in the Solar System, including UZ224. Image credit: Alexandra Angelich / NRAO / AUI / NSF.
Prof. Gerdes and his colleagues announced the discovery of this object in 2016.
They found it using the Victor Blanco 4-m telescope at the Cerro Tololo Inter-American Observatory in Chile as part of the Dark Energy Survey (DES).
The optical data from the Victor Blanco telescope enabled the team to measure UZ224’s distance and orbital properties, but they were unable to determine its size or other physical characteristics.
“2014 UZ224 was originally detected at a heliocentric distance of 92.5 AU in seven linked observations on four nights between September 27 and October 28, 2014, with an r-band magnitude of 23.0 and an ecliptic latitude of minus 10.3 degrees,” the scientists explained.
“The object was detected in six more DES survey images between August 19, 2014, and January 8, 2015, and was recovered in a targeted DECam observation on July 18, 2016.”
ALMA image of the faint millimeter-wavelength ‘glow’ from UZ224. Image credit: ALMA / ESO / NAOJ / NRAO.
Since ALMA observes the cold, dark Universe, it is able to detect the heat emitted naturally by cold objects in space. The heat signature from a distant solar system object would be directly proportional to its size.
“We calculated that this object would be incredibly cold, only about 30 degrees Kelvin, just a little above absolute zero,” Prof. Gerdes said.
While the reflected visible light from UZ224 is only about as bright as a candle seen halfway the distance to the moon, ALMA was able to quickly home in on the planetary body’s heat signature and measure its brightness in millimeter-wavelength light.
This allowed astronomers to determine that it reflects only about 13% of the sunlight that hits it.
By comparing ALMA observations to the earlier optical data, Prof. Gerdes and colleagues had the information necessary to calculate UZ224’s size.
“Objects like UZ224 are cosmic leftovers from the formation of the Solar System. Their orbits and physical properties reveal important details about the formation of planets, including Earth,” the researchers said.
“This discovery is exciting because it shows that it is possible to detect very distant, slowly moving objects in our own Solar System.”
“The same techniques could be used to detect the hypothesized Planet Nine that may reside far beyond UZ224 and Eris,” they added.
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David Gerdes et al. 2017. Discovery and Physical Characterization of a Large Scattered Disk Object at 92 AU. ApJL, in press; arXiv: 1702.00731
