Carnegie Science | Spring 2019 10 dynamic trio—Carnegie’s Scott Sheppard, Northern Arizona University’s Chad Trujillo, and the University of Hawaii’s David Tholen— are redefining the Solar System’s edge. They recently announced a discovery of a new extremely distant object far beyond Pluto with an orbit that supports the presence of a more distant Super-Earth or larger Planet X. They then announced the most distant body ever observed in our Solar System, the first known object that that is more than 100 times farther than Earth is from the Sun. Searching for even more distant objects is in full swing. New Object Points to More Distant Super-Earth or Larger Planet X The International Astronomical Union’s Minor Planet Center announced the trio’s discovery of an object called 2015 TG387 in October 2018. The object is about 80 astronomical units (AU) from the Sun. An AU is the Sun-Earth distance. For comparison, Pluto is about 34 AU. The new object, nicknamed “The Goblin,” is on a very elongated orbit and never comes closer to the Sun, a point called perihelion, than about 65 AU. Only objects 2012 VP113 and Sedna, at 80 and 76 AU respectively, have more-distant perihelia than 2015 TG387. Though 2015 TG387 has the third most distant perihelion, it travels much farther from the Sun than the others. At its farthest orbital point, about 2,300 AU, 2015 TG387 is one of the few known objects that never comes close enough to the giant planets to significantly interact gravitationally. “These so-called Inner Oort Cloud objects like 2015 TG387, 2012 VP113, and Sedna are isolated from most of the Solar System’s known mass, which makes them immensely interesting,” Sheppard explained. “They can be used as probes to understand what is happening at the edge of our Solar System.” The object with the most distant orbit at perihelion, 2012 VP113, was also discovered by Sheppard and Trujillo and announced in 2014. That discovery led the two to notice similarities of the orbits of several extremely distant Solar System objects. They proposed the presence of an unknown planet several times larger than Earth, sometimes called Planet X or Planet 9, orbiting the Sun well beyond Pluto at hundreds of AUs. “We think there could be thousands of small bodies like 2015 TG387 on the Solar System’s fringes, but their distance makes finding them very difficult,” Tholen said. “Currently we would only detect 2015 TG387 when it is near its closest approach to the Sun. For some 99% of its 40,000-year orbit, it would be too faint to see.” The object was discovered as part of the team’s ongoing hunt for unknown dwarf planets and Planet X. It is the largest and deepest survey ever conducted for distant Solar System objects. “These distant objects are like breadcrumbs leading us to Planet X. The more of them we can find, the better we can understand the outer Solar System and the possible planet that we think is shaping their orbits—a discovery that would redefine our knowledge of the Solar System’s evolution,” Sheppard added. Years of Data Collection Because 2015 TG387 moves so slowly with a vast orbital period, the team took a few years to observe its orbit. They first observed it in October 2015 at the Japanese Subaru 8-meter telescope atop Mauna Kea in Hawaii. They conducted follow-up observations at the Magellan telescope at Carnegie’s Las Campanas Observatory in Chile and the Discovery Channel Telescope in Arizona in 2015, 2016, 2017, and 2018. 2015 TG387 is likely on the small end of dwarf planets; it has a diameter near 300 kilometers (185 miles). 2015 TG387’s perihelion location is similar to 2012 VP113, Sedna, and most other extremely distant objects with average orbits beyond Neptune, suggesting that something is pushing them into these similar orbits. Carnegie’s Scott Sheppard (left), the University of Hawaii’s David Tholen (middle), and Northern Arizona University’s Chad Trujillo (right) are at the Japanese Subaru 8-meter telescope on Mauna Kea in Hawaii. Image courtesy Scott Sheppard and Chad Trujillo PUSHING THE LIMITS of the Outer Solar System A 10 Carnegie Science | Spring 2019