A new dwarf planet an estimated 700 kilometers (435 miles) across has been detected orbiting far out past Pluto, adding to the five we know of already. The discovery was made with the Canada-France-Hawaii Telescope, and announced by the International Astronomical Union (IAU) yesterday.
Until a suitably mythological name has been chosen, the newly discovered object will be referred to as RR245. It was found in February in images taken by the Outer Solar System Origins Survey (OSSOS) in September last year.
"There it was on the screen – this dot of light moving so slowly that it had to be at least twice as far as Neptune from the Sun,” said Dr Michele Bannister of the University of Victoria, British Columbia in a statement. The time since has been used confirming that the find is real, and establishing its orbit.
At this distance, we know very little about RR245. "It's either small and shiny, or large and dull," said Bannister. Since brightness tells us something about a world's surface chemistry, astronomers are keen to know which is correct. For the moment the OSSOS is using an estimate of 700 kilometers in diameter, which would make it smaller than Ceres. "The dwarf planet Haumea is covered in water ice, and is very reflective," Bannister told IFLScience. "That can be due to a past history of massive collisions. The size of [an object] also affects what icy materials it can keep around on its surface: bigger ones are able to keep molecules like methane, where smaller ones will lose that over billions of years due to the physics of volatile escape."
Bannister said collaborators will be looking for a time when RR245 passes in front of a star, allowing them to measure its size.
The orbit is better understood. For most of its 700-year orbit RR245 spends its time out beyond the other known objects of the Solar System, reaching a distance of 19 billion kilometers (12 billion miles) or 128 Astronomical Units (AU, 1 AU is the distance from Earth to the Sun). However, in 2092 it will reach the closest part of its orbit, a mere 34 AU, not far outside the orbit of Neptune and closer than Pluto's average distance from the Sun.
Being a third as wide as Pluto, RR245 may lack the fascinating geology revealed by the New Horizons mission. Nevertheless, prior to New Horizons, Pluto was expected to be much less active than it turned out to be, so RR245 may surprise us. If like many of the other known dwarf planets of the outer Solar System it turns out to be accompanied by a moon, we may not only learn its mass, but have better prospects of finding something really interesting going on.
OSSOS has discovered more than 500 objects beyond Neptune, but RR245 is the largest and only dwarf planet so far. The discoveries have been made possible by the wide field imager on the giant CFH Telescope.
The orbits of other objects orbiting at great distances from the Sun have shown signs of herding, which has been taken as evidence for the existence of an as-yet unseen Planet Nine. Both Bannister and Michael Brown, the most prominent proposer of the Planet Nine theory, told IFLScience that RR245's orbit is too close to the Sun to be affected by Planet Nine, and consequently neither provides evidence for or against its existence. Bannister wrote, "The orbit of RR245, while very distant, is dominated by the gravitational influence of Neptune."
Gif in text: RR245 is the dark dot at the center right. OSSOS