October 17, 2016
Astronomers at the University of Michigan and their colleagues on the Dark Energy Survey have discovered a new dwarf planet that's more than 90 times farther from the sun than Earth, making it the second-most distant minor planet in the known solar system.
The Dark Energy Survey, which uses a powerful digital camera called DECam on a 4-meter telescope in Chile, was designed to capture images of distant galaxies to understand why the expansion of the universe is accelerating.
"But the same sensitivity that makes this a state-of-the-art survey of the distant universe also makes it a powerful tool to look for new objects in our own cosmic backyard," said David Gerdes, Arthur F. Thurnau Professor and professor of physics who led the planet-finding team.
The DES images are sensitive enough to detect the reflected sunlight from the new object, which is as faint as a single candle 100,000 miles away.
The researchers say the discovery of the icy, faraway world shows that their technique is a promising approach for finding "Planet Nine" — a massive body hypothesized to reside around 600 times farther from the sun than Earth. The existence of Planet Nine would explain the elongated, aligned orbits of a group of distant minor planets similar to, but not including, this newly discovered one.
The U-M team submitted observations of the object to the Minor Planet Center, an international organization that designates and tracks minor planets, comets and moons. The center gave it a designation — 2014 QZ224 — but after its orbit has been refined for several more years, the researchers can propose an official name. In the meantime, they've dubbed it DeeDee, short for distant dwarf.
At this point in its orbit, DeeDee is more than 8.5 billion miles from the sun, or 92 astronomical units. One astronomical unit is the distance from the sun to Earth. Only the Pluto-sized dwarf planet Eris is currently more distant, though other minor planets with off-centered orbits spend most of their time even further out. On DeeDee, the sun would look like a very bright star.
The data indicate that DeeDee is between 200 and 800 miles in diameter, meaning it is probably large enough to qualify as a dwarf planet. The researchers expect to obtain a much better estimate of its size from an image they recently obtained with the Atacama Large Millimeter/submillimeter Array telescope in Chile. The researchers expect to complete this analysis and publish the results by mid-November.
To identify DeeDee, the researchers looked through thousands of images to find moving objects in orbit around the sun, against the background of millions of stars and galaxies that remain in the same place from night to night. Gerdes likens this to "finding a really small needle in a really big haystack."
The researchers don't look for planets with their eyes. Thousands of computers at Fermilab were used to analyze hundreds of terabytes of data, a process that would have taken more than 300 years on a single computer.
"Every image taken by DECam is subtracted from every other image from the same piece of the sky. That way, we can find moving solar system objects even if they happen to lie right in front of a background galaxy or star," said Masao Sako, a physicist at the University of Pennsylvania who was also involved in the search.
This analysis still left the researchers with millions of "dots," and many more possible ways to connect them. Computer programs developed by the U-M team took several more months to perform that task. Their code identified DeeDee this summer.
Michigan undergraduates played important roles on the team.
"I'm so grateful to have been given the opportunity to work with Professor Gerdes and contribute to this project," said Tali Khain, a sophomore majoring in physics and math whose work for this project involves analyzing the long-term behavior of minor planets beyond Neptune. "It's extremely exciting."
Even more exciting would be identifying the elusive Planet Nine. Its existence was recently suggested as a way to explain the off-kilter orbit of the dwarf planet Sedna, which crosses through the plane of the solar system, but swings far out of it. More objects with similar orbits have been discovered since.
"The discovery of DeeDee is a promising sign of our ability to find distant new worlds," Gerdes said. "If more things like this are in our data, the tools we've built will find them."
But on its own, the new discovery tells us a bit more about where we came from.
"All the bodies that make up our solar system came from the same cloud of gas and dust that began to collapse over 4 billion years ago," said Stephanie Hamilton, a doctoral student in physics who was involved in this discovery.
"The smallest bodies in the solar system are the ones that preserve its history. They have been knocked around and strewn about through interactions with larger planets, and by studying lots of them we can try to learn how that happened."