The neighborhood beyond Neptune is becoming ever more crowded, with astronomers announcing this week the discovery of another likely dwarf planet.
A survey at the Canada-France-Hawaii Telescope on Hawaii has been tracking more than 600 bodies in a ring of icy debris known as the Kuiper belt. One of them turned out to be the likely dwarf planet.
“This is a big fish among a whole lot of small ones we’re working with,” said Michele Bannister, a postdoctoral fellow at the University of Victoria in British Columbia who is working on the survey.
In the year since NASA’s New Horizons spacecraft flew past Pluto, planetary astronomers continue to make discoveries in the Kuiper belt. The study of these objects also offers hints about the formation and migration of the gas giant planets: Jupiter, Saturn, Uranus and Neptune.
Even if the newly found world is a dwarf planet, however, it will probably be years before it might earn official designation — part of the confusion of definitions that followed the International Astronomical Union’s decision in 2006 to demote Pluto and reduce the solar system to eight planets from nine.
More than 100 bodies in the solar system, all but one located along the ring of icy debris beyond Neptune, appear to meet the definition of a dwarf planet, a category that the astronomical union created to describe Pluto as well as Ceres, the largest asteroid, and Eris, a Kuiper belt object slightly smaller than Pluto. (A full-statured planet has an additional requirement: It must have “cleared the neighborhood” of smaller debris.)
If dwarf planets were to be reclassified as planets, as advocates for restoring Pluto to full planethood status hope to do, forget about trying to devise a workable mnemonic device.
The new object, designated 2015 RR245, was first spotted in February as the astronomers looked through images taken five months earlier. Further observations a few weeks ago confirmed the object’s 700-year loping path around the sun.
The astronomers cannot directly measure the object’s size. Rather, from its brightness, how far away it is and an assumption of how reflective its surface is — most Kuiper belt objects are roughly the darkness of coal — they estimated the diameter to be 370 to 500 miles.
They also cannot directly tell if 2015 RR245 is round. A key requirement in the definition of a dwarf planet is that the gravity is strong enough to pull the body into the shape of a ball.
Mimas, an 250-mile-wide icy moon of Saturn, is round, and it is likely that the much larger 2015 RR245 is also round.
The astronomical union has been slow to designate new dwarf planets, adding just two since 2006: Haumea and Makemake. But there are a slew of additional Kuiper belt objects larger than Mimas.
If its estimated diameter is accurate, 2015 RR245 would rank as just the 19th largest potential dwarf planet. Larger objects include Quaoar, Orcus, Salacia and still-unnamed objects with temporary designations like “2007 OR10” and “2002 MS4.”
“I hate to say any Kuiper belt object is uninteresting, but it’s a typical Kuiper belt object that is in the top 20 biggest ones,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology who discovered Eris and most of the larger Kuiper belt objects through a sky survey a decade ago. “This one is no more or less bizarre than most of them.”
Brown’s computer keeps track of large Kuiper belt objects, and 96 of them appear to be larger than Mimas and thus likely to be round dwarf planets. Another 300 are smaller but possibly could still be large enough to be round.
Dwarf planets are “not a rare class of objects in the outer solar system,” Brown said.
Brown and a colleague, Konstantin Batygin, further upended the field this year when they proposed the existence of a new planet, somewhere between the size of Earth and Neptune, in an orbit far beyond Pluto. They made their prediction based on the orbits of distant objects that all appeared to be aligned in roughly the same direction, nudged by the gravitational force of the unseen planet, which they are calling Planet Nine.
Bannister’s dwarf planet is not distant enough to be affected by Planet Nine, but at least one of the 600 objects tracked by the survey is. She declined to give details but has described it in talks, including one attended by Brown.
“I know that it’s going to fit in at least with most of the story,” Brown said. “It’s exactly in the direction it should be for Planet Nine.”
Brown said Planet Nine, if it exists, could be confirmed in two to three years.
S. Alan Stern, the principal investigator of the New Horizons mission and one of Pluto’s most vocal defenders, said the ninth planet was discovered long ago.
“My opinion about Planet Nine, which is Pluto, is just the same,” he said. “It’s a planet.”
He agrees with Brown’s assertion that many objects as small as 2015 RR245 or smaller are almost certainly dwarf planets, and he thinks they should all be planets.
Stern has even gone a step farther, arguing that round moons like Earth’s moon should also be counted as planets.
“There are a lot of little guys,” he said. “We were just wrong to think that there were just a few planets and they were all close and they were all big like Jupiter or big like the Earth.”