Data from the New Horizons spacecraft that flew past Pluto earlier this year shows the orbits of the dwarf planet's four tiny outer moons are chaotic, making them "the strangest moons in the solar system," scientists say.
The tiny moons — Hydra, Nix, Kerberos and Styx — are behaving like spinning tops, one mission scientists says.
"The way I would describe this system is not just chaos, but pandemonium," says Mark Showalter, New Horizons co-investigator at the SETI Institute in Mountain View, California.
"We honestly have not seen anything like this before, and we still don't know what to make of it," he said during a meeting of the Division for Planetary Sciences of the American Astronomical Society.
Some of the tiny moons are spinning much faster than expected and are tilted on their sides as they travel around Pluto.
Additionally, one is even spinning backwards against its orbital direction, the astronomers say.
"If you lived on one of Pluto's moons, you might have a hard time determining when, or from which direction, the sun will rise each day," NASA says on one of its websites.
While almost every other moon in the solar system, including our own, is in synchronous rotation with its planet — keeping one face aimed at the planet — Pluto's small moons are not, mission scientists report.
For example, they say, the most distant moon — Hydra — spins a full 89 times for every single orbit it makes around Pluto. Any faster, Showalter says, and material would fly off of its surface from the centrifugal force.
The four small moons may be spinning at varying rates under the influence of Pluto and its much larger moon Charon, as the combined gravitational effect on the much tinier moons keeps them from settling into synchronous rotation, scientists suggest.
In addition to their odd orbital behavior, the shape of several of them suggests they could have been formed during mergers of two or more moons, Showalter says.
"We suspect from this that Pluto had more moons in the past, in the aftermath of the big impact that also created Charon," he explains.
Charon is large for a moon in relation to its planet — its diameter is fully half that of Pluto — and scientists often refer to the two bodies as a "binary planet" revolving around a point in space located between them.
Charon was discovered in 1978; Pluto's four smaller moons weren't discovered until much later, between 2005 and 2013.
