2003 EL61 Family

Ann Feild, STScI,
ESA, NASA






Larger illustration.






2003 EL61 is one
of the largest
known planetary
bodies found
orbiting the Sun
beyond Neptune
(more).

2003 EL61 is one of the brightest objects in the Solar System beyond the orbit of Neptune, surpassed only by Pluto and 2005 FY9. According to astronomer Michael Brown's team web page on 2003 EL61, the object is one of the largest in the Edgeworth-Kuiper Belt. It is currently located about 51 AUs from the Sun in an elliptical orbit (e= 0.189) with an inclination of 23.19 degrees but has an average orbital distance (semi-major axis) of about 43.34 AUs. It has two known moons (more below).

The EKO is as much as 930 miles (1,500 kilometers or km) wide -- compared with Pluto's diameter of about 1,400 miles (2,300 km). It is one of over two dozen EKOs that may have formed closer to the Sun but subsequently were scattered by Uranus and Neptune (called "Centaurs and scattered disk objects") that have been discovered in highly inclined, elliptical orbits extending as far out as 200 AUs.

Brown Team's Eris Page


Larger illustration


Along elongated and
slanted, the orbits of
2003 EL61 and 2005
FY9 objects are still
considerably less
eccentric than that
of dwarf planet Eris.

With a spin period of just 3.9 hours, 2003 EL61 is the fastest rotating known body in our Solar System larger than 100 kilometers (60 miles) across (David Tytell, Sky and Telescope, April 20, 2006; and (Brown et al, 2005 preprint). Shaped like a rugby or American football because of its fast rotation, 2003 EL61 must be as dense as basalt and be made almost entirely of rock or else its very fast spin would have stretched it out even more than it is already. This suggests it has lost most of its low-density icy mantle -- unlike many other known EKOs -- probably due to an ancient cataclysmic collision which could have broken up 2003 EL61's ice mantle, set it reeling, and produced it's two small moons. Probably spherical and 20 percent larger before the collision, EL61 was about about 1,000 miles wide (almost 1,610 km), or about two-thirds the width of Pluto, and made of roughly half ice and half rock, like other EKOs. It is thought to have collided with an object that was perhaps 700 miles wide (roughly 1,130 km) and traveling at nearly 7,000 miles per hour (over 11,000 km per hour). The impact produced at least seven other large icy objects with diameters ranging from 6 to 250 miles (10 to 400 km), and its discovery team have grouped the scattered objects into a family based on their matching gray color and evidence of surface water ice derived from spectral analyses.

The bright pure-ice surface of 2003 EL61 and its moons was unexpected. Observations of the primary body made by astronomer Chadwick A. Trujillo and his colleagues revealed the strong spectral signature of crystalline water ice. While crystalline ice forms at temperatures above 110° Kelvins (-163° Celsius), the ambient temperature of space around 2003 EL61 is much colder, at below 50° Kelvins. In addition, since crystalline water ice should turn dark and ruddy in less than 100 million years from cosmic rays and micrometeoroid impacts in a process known as space weathering. Given the orbital spread of the EL61 family of fragments, however, the collision that produced them must have taken place billions of years ago. Hence, the object may have experienced resurfacing, perhaps by micrometeorite impacts that convert surface ices to crystalline form by flash-heating.

Spectra of EL61's outer satellite obtained by astronomers Kristina M. Barkume, Michael E. Brown, and Emily L. Schaller also reveal the signature of almost pure water ice (Barkume et al, 2006). While the observations were too low in resolution to distinguish the type of water ice, it seems that nearly all of the moon is coated in highly reflective frost, like the three moons of Pluto which may also have formed from a violent collision. Hence, the moons of the largest KBOs may differ in origin from those of ordinary KBOs, possibly because their satellite systems formed from the remains of violent impacts instead of by delicate gravitational capture (Noll et al, 2007; Stern et al, 2006 and 2005; and Brown et al, 2005).

José Luis Ortiz Moreno, an astronomer at the Sierra Nevada Observatory in Spain, and colleagues Francisco José Aceituno Castro and Pablo Santos-Sanz claimed to have discovered the object on July 25, 2005. After re-analyzing observations they had made on March 7, 2003 and scouring older archives (a process known as "precovery"), they found the object in images dating back to 1955. Ortiz's team announced their discovery on July 27, 2005, and it was published two days later by the Minor Planet Center (MPC) operated by the International Astronomical Union (IAU), the clearinghouse for such discoveries.

After sending congratulations, Brown's team subsequently found that it had inadvertently published the internal code designation (of K40506A) for their discovery of Santa on July 20, 2005. Apparently, typing this code designation into internet search engines allowed other EKO searchers to find the observation logs of Brown's team (including Santa's observed positions) at a web site containing the observing logs of the SMARTS system, where Professor Brown and his colleagues were using a SMARTS telescope at the Cerro Tololo Inter-American Observatory in Chile to track the object. Dr. Richard W. Pogge (of Ohio State University), who maintains the SMARTS web site, subsequently used third-party web server logs to determine that the web page in question had been accessed eight times from July 26 to 28, 2005 by an IP address used by computers at the Instituto de Astrofísica de Andalucía where Ortiz's team worked, and which the team used to notify the MPC of its discovery of 2003 EL61 (see "electronic trail").

Dr. Ortiz subsequently confirmed that he and a student accessed SMARTS telescope logs through the internet and downloaded the relevant information a day before making his announcement for the purpose of confirming whether Santa was the same object. Denying any wrongdoing, Ortiz is said by one account to have conceded that Brown's team discovered 2003 EL61 first. On August 14, 2005, Professor Brown wrote an email to Brian G. Marsden, director of the IAU's MPC, which accused Ortiz's team of a serious breach of scientific ethics and asked that the MPC to strip it of discovery status and that the IAU issue a statement condemning their actions (more discussion of the discovery controversy from Dennis Overbye, New York Times, September 13, 2005 and at Wikipedia's page on 2003 EL61).

These findings suggest that the five smaller bodies are fragments of the icy mantle of EL61 that were ejected after an ancient collision. While 35 collisional families of objects have been found in the Main Asteroid Belt, this family of objects is the first to be found in the Edgeworth-Kuiper Belt. Since each of the fragments have surfaces that may have once been internal regions of the original object, astronomers hope to ascertain the internal structure of the original colliding bodies by analyzing the subtle differences in composition between the fragments (Barkume et al, 2006; and New Scientist, March 14, 2006). Although only five fragments had been found by March 2007, the total number of objects eventually found in the EL61 collisional family may rise to the hundreds or even thousands in coming decades.

Spectroscopic measurements (with the 10-meter Keck II Telescope at Mauna Kea, Hawaii) have revealed that the five objects found thus far -- the largest of which is around 250 miles or 400 km across -- have the same unusual surface properties as 2003 EL61, consisting of nearly pure water ice crystals. In total, the five bodies appear to have around one tenth of the total amount of material blasted off proto-EL61, and while some of the rest probably evaporated into space, smaller fragments are probably still in the vicinity awaiting discovery.