Third largest dwarf planet found to have moon
Dwarf planet 2007 OR10, the third largest of nine known dwarf planets in the Solar System, is orbited by a small moon, scientists discovered using current and archival images gathered by three separate observatories.
Smaller than only two other dwarf planets, Pluto and Eris, 2007 OR10, approximately 950 miles (1,530 kilometers) in diameter, was discovered a decade ago by a team of researchers using the Samuel Oschin Telescope at the Palomar Observatory in California. It is currently three times further from the Sun than Pluto and has an eccentric orbit.
The moon of 2007 OR10 was first seen in images captured by NASA’s Kepler Space Telescope, which searches for planets orbiting stars other than the Sun, known as exoplanets.
Kepler data revealed that 2007 OR10 rotates on its axis once every 45 hours – a rotation rate much slower than the “[t]ypical rotation periods for Kuiper Belt Objects [of] under 24 hours”.*
Csaba Kiss of the Konkoly Observatory in Budapest, Hungary, the lead author of a study on the discovery published in The Astrophysical Journal Letters, said that researchers suspected the dwarf planet’s slow rotation period was caused by the gravitational pull of an orbiting moon.
They examined archival images of 2007 OR10 taken by NASA’s Hubble Space Telescope, but, initially, they did not see the moon because it is so small and faint.
However, in perusing subsequent Hubble images captured by the telescope’s Wide Field Camera 3 approximately one year apart, the researchers spotted the moon moving with the dwarf planet against background stars, indicating it is gravitationally bound to 2007 OR10.
“Ironically, because we don’t know the orbit, the link between the satellite and the slow rotation rate is unclear,” said team member John Stansberry of the Space Telescope Science Institute in Baltimore, Maryland.
Using the Herschel Space Observatory, which observes in the far-infrared, the researchers determined the diameter of both the dwarf planet and small moon. The latter ranges from 150 to 250 miles (241 to 402 kilometers) wide.
Most Kuiper Belt dwarf planets with diameters larger than 600 miles (965 kilometers) are now known to have one or more moons. Scientists believe this is due to collisions between outer Solar System objects when the system was forming more than four billion years ago.
“The discovery of satellites around all of the known large dwarf planets – except for Sedna – means that at the time these bodies formed billions of years ago, collisions must have been more frequent, and that’s a constraint on the formation models,” Kiss noted. “If there were frequent collisions, then it was quite easy to form these satellites.”
Collisions likely occurred frequently in the Kuiper Belt, a region of icy bodies between 30 and 50 astronomical units (AU, with one AU equal to the average Earth-Sun distance: 93 million miles / 150 million kilometers) from the Sun because of the sheer number of objects there.
“There must have been a fairly high density of objects, and some of them were massive bodies that were perturbing the orbits of smaller bodies. This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions,” Stansberry explained.
To create satellites, those collisions would have to have occurred at very specific speeds. If the impacts were too fast, they would have produced an enormous amount of debris that could have escaped the Solar System entirely. If the impacts were too slow, their effects would be limited to craters on the impacted objects.
It is believed that Earth’s Moon was created in a similar event. According to HubbleSite: “Based on [moonrock] samples from NASA’s Apollo mission, astronomers believe that Earth’s only natural satellite was born out of a collision with a Mars-sized object 4.4 billion years ago.”*
*Updated on May 22, 2017, at 16:40 EDT to reflect actual quotes from the HubbleSite.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.