Dark Energy Survey discovers potential new dwarf planet

Thanks to scientists on the Dark Energy Survey (DES), the solar system just got another member.

DES scientists recently reported the discovery of a potential dwarf planet located 92 times farther from the sun than the Earth is, more than twice as distant as Pluto. The new dwarf planet was discovered using the Dark Energy Camera, a scientific instrument built at Fermilab to probe the mystery of dark energy. But as scientists on the DES collaboration can attest, DECam turns out to be a powerful tool for astronomy as well as cosmology.

The newly discovered object, which the team has nicknamed DeeDee (for “distant dwarf”), is for now known as 2014 UZ224. DeeDee takes more than 1,100 years to complete one orbit around our sun and is currently the second-most distant known object in the solar system. Light from DeeDee takes 12-and-a-half hours to reach us.

DeeDee is one of many small icy worlds that lie beyond the most distant planet in the solar system, Neptune. Such celestial bodies are called trans-Neptunian objects, or TNOs, the most famous of which is the dwarf planet Pluto. TNOs are “cosmic leftovers” from the formation 4 billion years ago of the giant planets, such as Jupiter and Neptune, and scientists study them to learn more about the history of our solar system.

David Gerdes and his students at the University of Michigan first spotted DeeDee as a moving spot of light that appeared in just 14 of the tens of thousands of pictures taken by the Dark Energy Survey.

The DES collaboration uses the state-of-the-art Dark Energy Camera on a telescope in Chile to map distant galaxies, to find supernovae and to search for patterns in the cosmic structure. DES began observing the sky in 2013 with the goal of shining light on dark energy, the mysterious substance that is accelerating the expansion of the universe, and collaboration scientists are primarily engaged in that task. Trans-Neptunian objects are not part of DES’ main science interests since they don’t tell us about the universe’s expansion.

The DES supernova search, which takes pictures of the same part of the sky every week, sparked a bright idea in Gerdes: Instead of searching for spots that change their brightness over time, his students would search for spots whose positions change over time. Although DES looks at faraway galaxies, the backyard that is our own solar system is part of every picture the telescope takes. A dwarf planet could be captured in the DES data — one just had to look for it in the right way.

“I wanted a self-contained project for my summer students that would be fun and achievable in 10 weeks,” Gerdes said. “Most topics using DES data are parts of long and complex analyses that are not manageable in such a short time frame.”

Gerdes and his collaborators Masao Sako and Gary Bernstein at the University of Pennsylvania employed a technique developed for DES supernova searches and adjusted it to find slow-moving objects.

“So far we’ve discovered over 50 new TNOs in our data,” Gerdes said. “DeeDee is the largest and most distant one.”

For DeeDee to be a dwarf planet, it has to fulfill four criteria: First, it must orbit the sun. Second, it cannot be a planet’s satellite, such as our moon. Third, it can’t have attracted other objects along its orbit to become its satellites, nor can it have forced their orbits out of its way. This is the major difference between a dwarf planet and a full-fledged planet. Since Pluto’s orbit is tied to Neptune’s, by this criterion Pluto was demoted to dwarf planet status.

And last but not least, it has to have enough mass so that its own gravitational force compacts it into a spherical shape. DeeDee easily checks the first three qualifications, but its shape is not yet confirmed.

The team speculates that DeeDee is round because it has a diameter of about 350 miles, which means that it likely has enough mass, and therefore enough gravitational force, to be spherical. Gerdes and his team are currently analyzing additional data from a radio telescope to determine its size.

So far DeeDee’s chances of joining the elite group of dwarf planets are good. It might even earn its own mythological name, such as the dwarf planets Eris and Haumea, named after the ancient Greek goddess of discord and strife and the Hawaiian goddess of childbirth and fertility, respectively.

Scouting for more

DES uses the Dark Energy Camera to take its awe-inspiring pictures of the cosmos. The camera is mounted on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes mountains. Fermilab, with the support of DOE’s Office of Science, led its construction and plays a major role in the DES data analysis, with a focus on illuminating the dark universe.

“The DES data set is a very rich astronomical data set, and one critical step toward its discoveries is the calibration of the data,” said William Wester, Fermilab scientist involved in DES analysis. “The calibration helps determine the brightness of an object. In DeeDee’s case, this hints to its size.”

Not every bright dot is actually a star or a galaxy, or even a TNO. It could also be an artifact or a reflection of light created by the camera.

“You need to know what you are searching for, then you can formulate your question correctly for the data at hand and pull out from the multitude a sensible and manageable number of candidates,” said Jim Annis, Fermilab senior scientist.

The number of possible objects in the DES data set easily approaches a billion, so thorough and reliable data sorting is critical to find promising candidates. Wester and Annis are well-practiced in similar exercises, having been involved in many different searches across the DES collaboration.

DeeDee’s discovery is more than just that — it is another step on the way to a greater possible discovery: Planet 9. Planet 9 is a hypothetical ninth planet at the edge of our solar system with 10 times the mass of Earth. Otherwise unexplained patterns in the orbits of the largest-orbit TNOs hint at its existence. This opens the possibility that Planet 9 itself could be captured in the DES data, as in DeeDee’s case.

The scientists of the DES collaboration, both at Fermilab and at its other 24 partner institutions, continue to mine the three years’ worth of data they’ve already collected and will gather more data through its conclusion in 2018. DeeDee is just one more of many discoveries to come.

Shining aluminum panels hang like heavy curtains on each side of the particle detector MicroBooNE at Fermilab. Thin wires run along the sides of each panel and are bundled together similar to a curtain with a cord.

Even though the heavy panels now block the view of the large detector in its solitary pit with their eye-catching sheen, they are not just decorative. They serve a critical purpose: spying on particles coming from cosmic rays before they hit the actual detector.

The signals of cosmic rays

Cosmic rays are a constant rain of particles that are created in our sun or faraway stars and travel through space to our planet.

They’re subjects of many important physics studies, but for MicroBooNE’s research, they simply get in the way. That’s because MicroBooNE scientists are looking for something else — abundant, subtle particles called neutrinos.

Unlocking the secrets neutrinos hold could help us understand the evolution of our universe, but they’re exceedingly difficult to measure. Fleeting neutrinos are rarely captured, even as they sail through detectors built for that purpose.

Add to that the fact that their interactions are potentially drowned in a sea of cosmic rays rushing through the same detector, and you get a sense of the formidable challenge that neutrinos represent.

The MicroBooNE experiment starts with Fermilab’s powerful accelerators, which create neutrino beams that are then propelled through the MicroBooNE detector.

“The neutrino beam here at the lab gives us the right conditions to study neutrinos,” said Elena Gramellini, a Yale University graduate student on the MicroBooNE experiment. “Our challenge is to pick out neutrinos from many cosmic rays passing through the detector.”

Since cosmic rays are made of some of the same particles produced when a neutrino interacts with matter, they leave signals in the MicroBooNE detector that are often similar to the sought-after neutrino signals. Scientists need to be able to sort the cosmic rays in the MicroBooNE data from the neutrino signals.

 

Tagging and sorting

Even several feet of concrete enclosure would not completely block cosmic rays from hitting a detector such as MicroBooNE, not to mention that such a structure would be inconvenient and expensive. Instead, MicroBooNE uses the aforementioned panels, called a cosmic ray tagger, or CRT. While the panels don’t block cosmic rays, they do detect them.

Each CRT panel has particle-detecting components – strips of scintillator – that lie beneath its shiny aluminum enclosure. Cosmic ray particles can easily pass through aluminum and the scintillator — a clear, plastic-like material — on their way toward the MicroBooNE detector.

The cosmic ray particles deposit energy in the plastic scintillator, which then emits light. An optical fiber buried inside the scintillator captures the emitted light and transmits it to devices that generate the digital information that tells scientists where and when the cosmic ray struck.

“With our current layout of scintillator strips in each panel, we are able to tell precisely where the cosmic ray enters the MicroBooNE detector after it left the panel,” said Igor Kreslo, professor at the University of Bern who designed the CRT panels for MicroBooNE. “Our design effort really paid off and now ensures thorough cosmic ray tracking.“

So why the shiny aluminum shell? It blocks unwanted light from the detector’s immediate surroundings so that only light created by cosmic rays inside a CRT panel reaches the optical fiber and is detected.

 

Putting up panels

The 49 rectangular CRT panels are the contribution of the University of Bern in Switzerland, one of the 28 institutions collaborating on MicroBooNE worldwide. They produced the panels last winter and shipped them to Fermilab during the spring.

“This was a large project for us, and it took everyone in Bern to finish everything in time,” said Martin Auger, scientist at the University of Bern who planned the arrangement of the CRT panels. “A key moment was the test of the CRT panels after the long journey to Fermilab. All the panels arrived in good shape!”

The installation team overcame a number of challenges —including the tight space in which MicroBooNE stands — to successfully place the panels around the detector.

“The installation crew is a crack team of veteran Fermilab employees,” said John Voirin, who leads experiment installations at the laboratory. “In the end we have a very elegant, safe operating product that is a valuable asset to the experiment.”

Later this year the group will complete the installation by placing the final layer on top of the MicroBooNE detector. Even without it, the CRT already greatly enhances the capabilities of the experiment.

“We started taking data just in time for the first neutrinos delivered to the experiment,” Gramellini said.

 

This article is dedicated to our dear friend and colleague Gino Bolla.

The Dark Energy Spectroscopic Instrument, called DESI, has an ambitious goal: to scan more than 35 million galaxies in the night sky to track the expansion of our universe and the growth of its large-scale structure over the last 10 billion years. Using DESI — a project led by Lawrence Berkeley National Laboratory — scientists hope to create a 3-D map of a third of the night sky that is more accurate and precise than any other.

A precise map requires that DESI itself be built and assembled with micrometer precision. Fermilab, a Department of Energy national laboratory, is contributing a key piece of the instrument: a large, barrel-shaped device that will hold optical lenses to collect the light from millions of distant galaxies. The smallest deviation in lens alignment could lead to the instrument being permanently out of focus. Every piece of the barrel must be perfectly placed, so the Fermilab team is currently taking every measure to ensure its precise assembly.

The process involves a special machine, meticulous handling and a healthy dose of patience.

Precision assembly

The lens-holding device is a roughly 8-foot-long and 4-foot-wide segmented cylinder — about the size of a small elevator. Once the hulking steel barrel is complete, it will be installed at the Mayall four-meter telescope at the Kitt Peak National Observatory, southwest of Tucson, Arizona.

The lenses will collect the light reflected from the telescope’s mirror and focus it into 5,000 optical fibers, through which the light is transported to special detectors, called spectrographs. With the help of 10 such spectrographs, scientists can measure the distance of the galaxies.

In May, a team of specialists at Fermilab began assembling the barrel’s five segments carefully, checking that each nut and bolt was perfectly situated. But a nuts-and-bolts-level fit isn’t enough. To achieve the precision scientists are aiming for, the DESI barrel and its inner structure must be assembled accurately to within an incredibly tight 20 micrometers. That’s one 10th of the thickness of a sheet of paper.

To achieve the required fit, the team has been making small, critical adjustments to the assembled barrel.

 

Accurate alignment

The barrel adjustments take place in a vacant area the size of a small bedroom. Four tall pillars – nearly seven feet high – stand at the corners of the space.

Above their heads, a rail, similar to train tracks, connects the tops of the two pillars on one side. A second rail connects the other two. A moveable carriage track spans the gap – like a high bridge spans a river – connecting the two rails. The carriage itself glides along the track.

The team guides the carriage so that it stops just above the barrel. The carriage carries a mechanical arm that points towards the floor. It can rotate in all directions in the space within the pillars. At the end of the arm is a highly sensitive and precise sensor, fixed to an articulating motorized probe.

The arm with the sensor comes to life: It reaches down to the barrel and starts feeling for its surfaces. It searches for specific points on the barrel – a corner, an edge, another significant surface marker. When it finds them, it measures the coordinates in the designated space. Very carefully and with tiny movements, it moves over the whole surface of the barrel, measuring up, down and around the surface. As it does, it records the measurement data and saves it for further analysis. Jorge Montes, one of the team members, strategically places markers on the barrel’s surface to assist their alignment efforts.

After making the measurement, the scientists return the barrel to an outside area. There they disassemble it, realign all the parts, relying on the previously placed markers. They then reassemble it. With great care they bring the once more fully assembled barrel into the empty space and measure anew the precision of their assembly.

Comparing their performance with their previous assembly, they learn which pieces, if any, are misaligned — even slightly — and where they improved the alignment.

A magic machine

The precise, slow-moving measuring machine that points out the misalignments is called a coordinate measuring machine, or CMM. The group making these point-by-point measurements, led by Fermilab engineering physicist Michael Roman, uses it to ensure the DESI barrel’s perfect assembly.

With the help of the CMM, they repeat the whole procedure of assembly, measurement and disassembly again and again, always comparing their performance against previous tries. When they reach their alignment within 10 micrometers — about a 10th the width of a human hair — in a certain number of tries, they are satisfied.

“From early on we knew that the barrel needed high-precision measurements for the assembly and that it would be too large for any of the CMMs at Fermilab to perform such measurements,” Roman said.

“In strong support of DESI, Fermilab bought a machine for the dedicated measurements on the barrel,” said scientist Gaston Gutierrez, who is one of the DESI project leads at Fermilab.

 

Steady and stable

To ensure that the CMM’s measurements are as precise as they need to be, the CMM is set up in an air-conditioned room, where scientists monitor and control the temperature 24 hours a day. Materials expand when they get warm, affecting the accuracy of CMM’s measurements.

So scientists worked out the right control settings for the environmental control system to ensure that the temperature never varied more than one degree from 20 degrees Celsius.

Even the eventual effect of heavy weights on the DESI barrel, including the lenses, can be measured with the new CMM. Scientists place the DESI barrel in the machine and measure it, then add test weights on its sides and remeasure. The team can see how the barrel shrinks or bends, if at all, and determine whether the lenses will hold steady when the telescope is in motion.

The Fermilab team expects to finish all CMM measurements by early 2017. Then they will disassemble the DESI barrel and send it to University College London. In London, their colleagues will install the lenses in the support structures. Once the lenses are installed, the barrel will start its journey to its future home in Arizona.

 

Measuring the expansion of the universe

Scientists have discovered that our universe is growing bigger and bigger — without any end in sight. Like raisins in a rising loaf of bread, the universe’s galaxies are being pushed apart from each other.

­

From previous measurements, scientists have a kind of cosmic ruler, a standard length that goes back to the universe’s early beginning. Using this ruler together with the high-precision DESI map, scientists will be able to tell how far galaxies have moved apart and how much our universe has grown throughout its history.

“With the DESI experiment, we want to follow the growing steps of our universe,” Gutierrez said. “We start from today and go backwards in time to measure how much the universe has expanded since its early days.”

The fabrication, assembly and operation of DESI are small but highly important steps toward precisely understanding the universe.