Three relatively bright space rocks recently found in Pluto's neighborhood may be new members of the dwarf planet family, astronomers say.
The objects were discovered in a little studied section of the Kuiper belt, a region of the solar system that starts beyond the orbit of Neptune and extends 5.1 billion miles (8.2 billion kilometers) from the sun.
The region of the Kuiper belt visible from Earth's Northern Hemisphere has been fairly well studied. But until recently, a lack of instruments prohibited searches from the Southern Hemisphere.
The latest survey turned up 14 new Kuiper belt objects, three of which are probably big enough to join Pluto, Eris, Ceres, Haumea, and Makemake in dwarf-planet status, the study authors say. (Related: "Pluto Gets 14 New Neighbors.")
"I'm glad someone finally did it. It needed to happen," Mike Brown, a Caltech astronomer who was not involved in the study, said of the southern-sky survey.
New Bodies Are Planetary Pipsqueaks
The International Astronomical Union defines a dwarf planet as something orbiting the sun that's so massive its own gravity molds the object into a rough sphere. Such worlds also haven't cleared their neighborhoods of other small, planet-like bodies.
(Related: "New Moon Discovered Orbiting Pluto.")
By estimating the objects' reflectivity, Sheppard and colleagues could derive their sizes: The largest object is likely 384 miles (620 kilometers) across. The worlds are massive enough to be round, but they're still pipsqueaks, even by dwarf-planet standards.
By contrast, Pluto and Eris are both 1,450 miles (2,333 kilometers) or so wide. Ceres, the smallest of the confirmed dwarfs, is about 303 miles (487 kilometers) across.
Because the newfound objects are so small and so far away, astronomers can't yet say for sure whether they are in fact spherical and therefore worthy of being named dwarf planets.
Pluto Has No Southern Twins?
The Kuiper belt is full of objects about the size—or at least the brightness—of the three new bodies, Brown added. In fact, there are 37 other objects in the Kuiper belt at least as bright as the newly discovered candidate dwarfs.
Still, the new work does help fill in gaps in the known population of the solar system.
"By determining how many large objects are in the Kuiper belt, we now know how much stuff is there," study leader Sheppard said. "The point was to complete [our knowledge of] the Kuiper belt."
For instance, the study closes the possibility of finding a larger, Pluto-size object in the southern skies.
But it's always possible that "we will find more objects out in the next region [beyond the Kuiper belt], where Sedna is," Caltech's Brown said, referring to another candidate dwarf planet, which orbits at an average of 8.9 billion miles (14.3 billion kilometers) from the sun.
The paper describing the three possible new dwarfs has been accepted for publication in the Astronomical Journal.