Meet Didi, a distant and dim planetary body in our solar system

Using the Alma telescope array in Chile, astronomers have revealed extraordinary details about a recently discovered distant friend of our solar system. The planetary body 2014 UZ224 was officially named DeeDee

Image Caption: An artist's illustration of the planetary body 2014 UZ224 known by its official name Didi. The Alma telescope was able to distinguish a faint light with a wavelength of less than a millimeter, confirming that it is approximately 635 km in diameter. For this size, Didi should have enough mass to be spherical and this is one of the criteria required for astronomers to consider it a dwarf planet, although it has not yet received the official classification. Illustration: Alexandra Angelique (NRAO / AUI / NSF)

Astronomers observing the Alma submillimeter telescope array in Chile have revealed extraordinary details about a recently discovered planetary body far away in our solar system, the planetary body 2014 UZ224 more formally known as 'DeeDee'.

Didi's distance is approximately 3 times Pluto's current distance from the Sun and is the trans-Neptunian object whose orbit has been confirmed, only slightly less distant than the orbit of the dwarf planet Eris. Astronomers estimate that there are tens of thousands more such icy bodies in the outer solar system beyond the orbit of Neptune.

The new ALMA data reveal for the first time that Didi's diameter is about 635 km or about two-thirds the diameter of the dwarf planet Ceres, the largest member of the asteroid belt that orbits between the orbits of Mars and Jupiter. At this size, Didi should have enough mass to be spherical - a criterion required for astronomers to consider it a dwarf planet, although it has not yet received the official classification.

"The region beyond Pluto is an incredibly rich region of planetary bodies, some quite small but others large enough to compete with Pluto and even be larger than it," said David Gardes, a scientist at the University of Michigan and lead author of the study appearing in the Astrophysical Journal. "Since these objects are so distant and dim, it is very difficult to identify them, let alone investigate them in detail, but Alma has unique abilities that have allowed us to learn exciting details about the distant worlds."

Currently, Didi is about 92 AU from the Sun. An astronomical unit is the average distance between the Earth and the Sun, or about 150 million km. At this enormous distance, it takes Didi more than 1,100 years to complete one orbit around the Sun. Measured light takes almost 13 hours to reach the earth.

Gerdes and his team announced the discovery of Didi in the fall of 2016. They found it using the four-meter-diameter Blanco telescope at the Inter-American Cerro Tulolo Observatory in Chile as part of ongoing observations of the Dark Energy Survey, an optical survey of about 12% of the sky with the goal of understanding the mysterious force theorized The conventional wisdom is that he is responsible for accelerating the expansion of the universe.

The Dark Energy Survey produces a huge variety of astronomical images, giving astronomers the opportunity to look for even distant objects in the solar system. In the initial search, which includes almost 15,000 images, more than 1.1 billion candidate objects were identified. The vast majority of these turned out to be background stars or even even more distant galaxies. However, a small number of them appear to be slowly receding across the sky in subsequent observations, which is a sign that it is actually trans-Neptunian.

Such an object was identified in 12 separate images. The astronomers unofficially named it Didi, short for 'Distant Dwarf'. The optical data from the Blanco telescope allowed astronomers to measure Didi's distance and optical properties but they were unable to determine its size or other physical properties. This could have been due to the fact that Didi is a relatively small object in the solar system, or alternatively large and unusually dark, due to the fact that it reflects only a tiny fraction of the weak sunlight that reaches it. Both scenarios may produce identical optical data.

Because Alma observes cold and dark objects in the universe, it is able to detect the heat - in the form of millimeter wave light - that is naturally emitted from cold objects in space. The heat signature from a distant solar system object will be directly proportional to its size. "We thought this object would be very cold, only 30 degrees Kelvin, just above absolute zero," Gerdes said.

While Didi's reflected visible light is only as bright as a candle halfway to the Moon, Alma was able to measure the infrared signature indicating the heat of the planetary body and measure its brightness at a millimeter wavelength. This allowed astronomers to determine that it reflects only about 13% of the sunlight that hits it as dry dirt.

By comparing these Alma observations to the previous optical data, astronomers had the information needed to calculate the size of the object. "Alma did it relatively easily," Gerdes said. "Therefore, we were able to resolve the ambiguity we had with the optical data alone."

Objects like Didi are cosmic remnants from the formation of the solar system. Their orbits and physical properties reveal important details about the formation of planets, including Earth.

This discovery is also exciting because it shows that very distant, slowly moving objects in our solar system can be detected. The researchers note that these techniques can be used to identify the supposed "ninth planet", which may orbit the Sun beyond the orbits of Didi and Eris and which may be as large as one of the largest planets in the solar system.

"There are still new worlds to discover in our cosmic backyard," Gerdes concludes. "The solar system is a rich and complex place."

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