The diagram at left shows the orbits of the newly-discovered dwarf planet 2014 UZ224, together with the present positions of Uranus, Neptune, and Pluto. The dot indicates the present position of the 2014 UZ224 in its orbit. (Image credit: JPL Horizons / Sky and Telescope)

What is it?

• 2014 UZ224 is a distant dwarf planet discovered by the Dark Energy Survey (DES). It's located in the region of the solar system beyond Neptune known as the Kuiper Belt, and is presently more than 90 astronomical units (AU), or nearly 14 billion km, from the sun. The earth is 1 AU from the sun, so this is one of the most distant solar system objects we know about. Among known solar system objects with well-determined orbits, only the dwarf planet Eris is currently more distant. It's more than twice as far away as Pluto; light from 2014 UZ224 takes 12.5 hours to reach us.



What are its vital statistics?

• • Orbital period: 1136 years
  • Semi-major axis: 109 AU
  • Eccentricty: 0.65
  • Closest approach to the sun (perihelion): 38 AU (5.7 billion km), about Pluto's typical distance)
  • Farthest distance from the sun (aphelion): 180 AU (27 billion km, about 4.5 times Pluto's distance)
  • Inclination: 26.8 degrees
  • Next perihelion date: January 2, 2142 (give or take 70 days)
  • Observed magnitude: 23.2
  • Absolute magnitude: 3.5
  For a complete listing of its properties and a link to the optical observations, visit 2014 UZ224's web page at the Minor Planet Center.



How was it discovered?

• We looked in nearly 16,000 images collected by the Dark Energy Survey between 2013-2016. DES is surveying 1/8 of the sky over 5 years using the Dark Energy Camera (DECam) on the 4-meter Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. The main purpose of DES is to collect images of hundreds of millions of galaxies and use them to understand why the expansion rate of the universe is accelerating. It was not designed specifically to look for solar system objects. But the same combination of survey area and depth that make DES a state-of-the-art cosmological survey also make it a powerful tool for discoveries in our own cosmic backyard. The stars and galaxies in our images are stationary, but objects in our solar system appear in different places on different nights. We call an object that changes position or brightness from night to night a "transient". To identify transients, we used a technique known as "difference imaging". When we take a new image, we subtract from it an image of the same area of the sky taken on a different night. Objects that don't change disappear in this subraction, and we're left with only the transients. This technique was originally developed to search for Type Ia supernovae in small regions of our survey. Our collaborators at the University of Pennsylvania led by Professor Masao Sako made the key breakthroughs that allowed us to perform difference imaging over the entire survey area. This is a VERY computing-intensive process: thousands of computers at Fermilab were needed to process hundreds of terabytes of data. On a single computer, it would have taken 300 years! This process yields millions of transients, but only about 0.1% of them turn out to be distant minor planets. To find them, we must "connect the dots" and determine which transients are actually the same thing in different positions on different nights. There are many dots and MANY more possible ways to connect them. This is much too difficult to do by eye, so we wrote computer programs to do it. This process takes months of computing time.



What does it look like?

• Even with a powerful instrument like DECam, distant minor planets look like points. Click on the images below for larger versions.

  

  

  

Who was involved in the discovery?

• The Dark Energy survey is a large international collaboration, and it took the work of many people to produce data of this quality: the people who built the camera, the DES Data Management team, the scientific and technical staff of Cerro Tololo Inter-American Observatory in Chile, the observing crews who take great data every night, and the funding agencies in the US and around the world that make this work possible. See our full acknowledgement below. The search for solar system objects with DES is being carried out by groups at the University of Michigan, the University of Pennsylvania, and Fermilab. The analysis team includes senior scientists, postdocs, and graduate and undergraduate students.



How big is it?

• DeeDee is far too distant for us to resolve its size and shape directly. Even the Hubble Space Telescope can't do it. Our optical images tell us how much sunlight it reflects, but we didn't know if it's large and dark or small and shiny. In order to get a better measurement of its size, we need complementary data. In August 2016 we were able to obtain an image of 2014 UZ224 with the Atacama Large Millimeter / sub-millimeter Array (ALMA) in Chile. This telescope detects far-infrared radiation and can pick up the faint flicker of heat given off by the object. When we pointed ALMA at DeeDee, here's what we saw:

  

  Using these data in combination with our optical measurements we determined that DeeDee is about 635 km (395 miles) in diameter and reflects about 13% of the sunlight that shines on it. This is about the same fraction of sunlight that's reflected by bare, dry dirt. Based on this measurement we think that DeeDee has a mixed ice-rock composition.



So is it really a dwarf planet?

• Probably, but we don't know for sure. According to the official IAU guidelines, a dwarf planet must satisfy four criteria. It must a) orbit the sun (check!), b) not be a satellite (check!) c) not have cleared the neighborhood around its orbit (check!) and d) have enough mass to be round. It's this last item that's uncertain, and the only way for sure is to get a picture that's detailed enough to actually see its shape. Nevertheless, an object over 400 km in diameter is likely to be round. Our ALMA data indicate that it's 635 km in diameter. This strongly suggests, but does not conclusively prove, that DeeDee is round.



How faint is it? Can I see it in an amateur telescope?

• You'd need a really big telescope. This object is as faint as a candle 100,000 miles away, almost halfway to the moon! The Blanco Telescope used by the Dark Energy Survey has a 4-meter primary mirror.



Do you have a scientific paper on this object?

• We submitted a paper to Astrophysical Journal Letters in Feb. 2017. The accepted version of this paper is available on the arXiv: Discovery and Physical Characterization of a Large Scattered Disk Object at 92 AU.



Does this object have anything to do with Planet 9?

• Planet 9 is a hypothesized planet with a mass about 10 times greater than the earth. It would be hundreds of times more distant from the sun than earth is, maybe even over 1000 times more distant. Unfortunately, although 2014 UZ224 takes over 1100 years to orbit the sun, it is not distant enough to be influenced by Planet 9. So the orbit of our new object doesn't tell us anything one way or the other about the existence or characteristics of Planet 9.



What is the significance of this discovery?

• It's an interesting object in its own right -- distant objects like this are "cosmic leftovers" from the primordial disk that gave birth to the solar system. By studying them and learning more about their distribution, orbital characteristics, sizes, and surface properties, we can learn more about the processes that gave birth to the solar system and ultimately to us. But beyond its intrinsic interest, the discovery of an object at 92 AU shows that the Dark Energy Survey is a powerful tool for making discoveries in the distant reaches of our own solar system. The fact that we can pull an object like 2014 UZ224 out of data collected for a completely different purpose illustrates the power and scientific richness of DES data. And it means that if there are more objects like this in our data, including Planet 9, our analysis is capable of finding them.



Do you get to name it?

• 2014 UZ224 isn't a very catchy name, is it? Under rules established by the International Astronomical Union, the discoverer of a minor planet can propose a name to the IAU once the object's orbit is sufficently well-determined. It will probably take several more years of observations for 2014 UZ224 to reach this point. But then we can't choose just anything. Trans-Neptunian minor planets like this one must be named after mythological creation deities. In the meantime, we've been referring to this object informally as "DeeDee", which is short for "Distant Dwarf."



Has DES discovered any other solar system objects?

• We've discovered about 50 trans-Neptunian minor planets so far. The complete list is here -- we're adding to it all the time. One of the objects we've discovered, 2013 RF98, has a period of nearly 6000 years and is one of the "extreme TNOs" whose orbital alignment with other objects in its class is part of the evidence for Planet 9.



Acknowledgements:

• We are grateful for the extraordinary contributions of our CTIO colleagues and the DES Camera, Commissioning and Science Verification teams for achieving excellent instrument and telescope conditions that have made this work possible. The success of this project also relies critically on the expertise and dedication of the DES Data Management organization. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Edinburgh, the University of Illinois at Urbana-Champaign, the Institut de Ciencies de l'Espai (IEEC/CSIC), the Institut de Fisica d'Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität and the associated Excellence Cluster Universe,
  the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, The Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University.

The search for distant solar system objects with DES is funded by the National Science Foundation and by NASA.