Scientists and amateur astronomers have long been fascinated by the possibility of a "Planet X" at the edge of the solar system that may explain some apparent anomalies in the orbits of planets such as Neptune and Uranus. However, in recent years, astronomers have largely ruled out the possibility of a large, unseen planet far beyond the orbit of Pluto.
Research groups from Sweden and Mexico have now submitted pre-prints of two research papers to arXiv (here and here) that claim to have discovered a massive object at the edge of the solar system. Using observations from the Atacama Large Millimeter/submillimeter Array in Chile during 2014 and 2015 the astronomers spied "a new blackbody point source" that appears to be moving in conjunction with the Alpha Centauri star system, about 4.3 light years from Earth.
The authors do not believe the new object is part of the Alpha Centuari system, however, because if it were that far away, such a star would have been bright enough to be seen before. Rather, they offer several explanations for the object, which one of the research teams named "Gna." Perhaps most notably, they suggest a "Super Earth" at a distance of about 300 astronomical units (AU) from the Sun, or about six times further than Pluto is at its aphelion. Another explanation is a "super-cool" brown dwarf (too big to be a planet, too small to be a star) at about 20,000 AU from the Sun.