(136108) 2003 EL61

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(136108) 2003 EL₆₁

Artist's conception of (136108) 2003 EL61
Discovery
Discovered by:	Ortiz et al. / Brown et al.
Discovery date:	December 28, 2004
MPC designation:	(136108) 2003 EL61
Alternative names:	none
Minor planet category:	TNO (cubewano)
Orbital characteristics
Epoch 2005-08-18 (JD 2453600.5)
Aphelion distance:	7708 Gm (51.526 AU)
Perihelion distance:	5260 Gm (35.164 AU)
Semi-major axis:	6484 Gm (43.335 AU)
Eccentricity:	0.18874
Orbital period:	104,234 d (285.4 a)
Avg. orbital speed:	4.484 km/s
Mean anomaly:	198.07°
Inclination:	28.19°
Longitude of ascending node:	121.90°
Argument of perihelion:	239.51°
Satellites:	2
Physical characteristics
Dimensions:	~1960×1518×996 km (~1500 km)
Mass:	(4.2±0.1)×1021 kg
Mean density:	2.6–3.3 g/cm³
Equatorial surface gravity:	0.44 m/s²
Escape velocity:	0.84 km/s
Sidereal rotation period:	0.16314±0.00001 d (3.9154±0.0002 h)
Albedo:	0.7±0.1
Temperature:	32±3 K
Spectral type:	?
Absolute magnitude:	0.1

(136108) 2003 EL₆₁ (also written (136108) 2003 EL61), is a large (roughly one-third the mass of Pluto) Kuiper belt object discovered by J. L. Ortiz et al. of the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain, and Mike Brown's group at Caltech in the United States. The MPC currently gives formal discovery credit to the group of Ortiz et al.

Its two moons, rapid rotation, and high albedo due to crystalline water ice on the surface, make it exceptional among the known cubewanos. The object is also thought to be the largest member of a collisional family, created in a single break-up event.

Before the discovery of the object was published and designated, the Caltech team used the nickname "Santa", which stems from its discovery just after Christmas, on December 28, 2004, although the Caltech team had acquired images of it starting May 6, 2004. (136108) 2003 EL₆₁ will not be named "Santa"; following IAU guidelines, the object should be named after a deity related to a creation myth.

[edit] Discovery controversy

(136108) 2003 EL₆₁ is circled in red

José Luis Ortiz Moreno, an astronomer at the Sierra Nevada Observatory in Spain, and colleagues Francisco José Aceituno Castro and Pablo Santos-Sanz announced the discovery of the object on July 25, 2005, when they re-analysed observations they had made on March 7, 2003. They then scoured older archives (a process known as precovery) and found the object in images dating back to 1955. Ortiz's group announced their discovery on July 27, 2005, and it was published two days later by the MPC.

A Caltech team consisting of Michael E. Brown, Chad Trujillo, and David Rabinowitz had been observing the object for half a year with the 1.3 m SMARTS Telescope, but had not yet made the data public. Brown and his collaborators initially supported giving Ortiz and his group credit for the discovery, but withdrew support when they found reason to suspect that Ortiz may have used discovery data from Brown's team, which had inadvertently been made publicly available on the web.

A week before Ortiz's discovery announcement, on July 20, Brown's team had published an abstract of a report they intended to use to announce the discovery, in which the object was referred to by the internal code name K40506A. Typing this code into internet search engines allowed anyone to find the observation logs of Brown's group, including the observed positions of the object. Third-party web server logs indicated that the page in question had been accessed by an IP address used by computers at the Instituto de Astrofísica de Andalucía where Ortiz's group worked.^[1] Brown's group accused Ortiz's group of a serious breach of scientific ethics and asked the Minor Planet Center to strip them of discovery status.^[2]

Ortiz later admitted he accessed the internet telescope logs with the relevant information a day before making his announcement, but denied any wrongdoing.^[3] According to him they did not use the data, other than checking them out of curiosity whether it could be the same object they had found in their 2003 images that same month. This after they realized the object in the abstract by Brown et al. seemed to be an object with similar characteristics. Googling the informal designation mentioned in the abstract, they ended up at the telescope log.

The ambiguity in who discovered the object stems from the fact that the Caltech group of Brown did not submit their discovery to the Minor Planet Center for a year after detecting it in their images. Standing protocol is that the one who first does submit a report to the MPC with enough positional data for a decent orbit determination, gets discovery credit. This is what Ortiz' group did, thus following correct protocol, using their 2003 imagery, 2005 follow-up imagery, and "precovery" positions from historic archives.

The Minor Planet Center's discovery circumstances database gives the group of Ortiz et al. as the formal discoverers of the object.

On July 29, 2005, shortly after the Ortiz discovery announcement, Brown's group announced the discovery of another Kuiper belt object, Eris, which is more distant and is thought to be larger than the dwarf planet Pluto. The announcement was made earlier than planned, at the urging of the Minor Planet Center, to forestall the possibility of that discovery leaking out as well.

[edit] Size and composition

(136108) 2003 EL₆₁ compared to Eris, Pluto, (136472) 2005 FY₉, Sedna, Orcus, Quaoar, Varuna, and Earth.

The only method to estimate the size of a small trans-Neptunian object is its magnitude assuming a value for the albedo. For larger objects, thermal emission can provide a size directly. (136108) 2003 EL₆₁ is exceptional because its two moons provide the means to determine directly the mass of the system from Kepler's third law. The estimated mass is 4.2 × 10²¹ kg, 28% the mass of the Plutonian system.^[4] Because (136108) 2003 EL₆₁ rotates roughly once every four hours, faster than any other known body in the solar system larger than 100 km in diameter, it should be distorted into a triaxial ellipsoid. (136108) 2003 EL₆₁ displays large fluctuations in brightness. Although these fluctuations could be due to a mottled surface, it is thought that this fluctuation is due to an elongated shape. Rapid rotation and elongated shape result in constraints on the density (the denser the object, the less elongated), estimated at 2.6–3.3 g/cm³, suggesting substantial non-ice content (compare with Pluto's density of 2.0 g/cm³ and Moon's density of 3.3 g/cm³). These limits on the density, together with the known mass, give another way to constrain the dimensions of the object.^[5] (136108) 2003 EL₆₁ has approximately the diameter of Pluto along its longest dimension, and half that along its shortest. This would make it one of the largest trans-Neptunian objects discovered so far; possibly fourth after Eris, Pluto and arguably (136472) 2005 FY₉, larger than Sedna, Orcus, and Quaoar.

The short rotation period of (136108) 2003 EL₆₁ may have been caused by a giant impact, which also created its satellites. (136108) 2003 EL₆₁ may not be the only elongated, rapidly rotating, large object in the Kuiper Belt. In 2002, Jewitt and Sheppard suggested that Varuna should be elongate, based on its rapid rotation (see the references there).

2003 EL61, 2002 TX₃₀₀ and four smaller Kuiper belt objects are traveling in similar orbits and all have a similar color and proportion of water ice to it. Mike Brown and his team have postulated that they are the remnants of a past impact and their surfaces were once ejected from the mantle of the original object.^[6] (See Collisional family below).

[edit] Surface

Gemini telescope obtained spectra of (136108) 2003 EL₆₁, which show strong water ice features similar to the surface of Pluto's moon Charon. Trujillo, Brown, et al. report crystalline water ice.^[7]

Water ice has been reported on many trans-Neptunian objects but typically in the form of amorphous ice. Crystalline ice is unstable on timescales of 10 million years under conditions in the Kuiper Belt. This discovery hints at resurfacing processes producing fresh ice. As surprising as the crystalline form is the inferred amount of ice. Following the report, the surface of (136108) 2003 EL₆₁ appears to be ⅔ to ⅘ pure ice, with the remainder of the surface material of unknown composition.

(136108) 2003 EL₆₁ has an albedo approaching that of pure snow, consistent with crystalline ice on the surface. This very high albedo does not appear to be unique among large TNOs. Recent measurements of Eris imply an even higher (inferred) albedo (0.86) for that object.

[edit] Orbit

Orbits of 2003 EL₆₁ (yellow) and Pluto (red).

(136108) 2003 EL₆₁ is classified as a classical trans-Neptunian object with an orbit common for large cubewanos: the perihelion is close to 35 AU and significantly inclined. The diagram shows a view of its orbit in yellow, (Pluto in red, Neptune in grey) and position (as of April 2006). The object passed its aphelion (Q) in 1991, and is currently more than 50 AU from the Sun.

The inclination of its orbit (~28° to compare with 17° for Pluto) and its current position, far from the ecliptic where most of the early surveys took place, combined with a slow mean motion explain why (136108) 2003 EL₆₁ was only discovered recently, in spite of its magnitude.

[edit] Moons

Main article: 2003 EL61's natural satellites

Two small satellites have been discovered orbiting (136108) 2003 EL₆₁.

[edit] S/2005 (136108) 1

S/2005 (2003 EL₆₁) 1 (provisional designation; nicknamed "Rudolph" by the Caltech team), renamed S/2005 (136108) 1 once its primary was numbered, was the first satellite discovered around (136108) 2003 EL₆₁. It orbits once every 49.12 ± 0.03 days with semimajor axis 49,500 ± 400 km and eccentricity 0.050 ± 0.003[1]. Mutual occultations of the moon and the primary, as seen from Earth, occurred in 1999 and will not occur again until 2138.

Only the total mass of the system is known, but assuming the moon has the same density and albedo as the primary, the mass of the satellite is 1% of the mass of (136108) 2003 EL₆₁ and it has a diameter of ~310 km.^[8]

Strong absorption features at 1.5 and 2 micrometres discovered in the infrared spectrum are consistent with absorption due to water ice. Their depth suggests that much of the satellite’s surface is covered with ice.^[9]

[edit] S/2005 (136108) 2

S/2005 (2003 EL₆₁) 2 (provisional designation), later renamed S/2005 (136108) 2, is the smaller inner satellite of (136108) 2003 EL₆₁. The object has been nicknamed "Blitzen".^{[citation needed]}

Its discovery was announced on November 29, 2005. It was found 39,300 km away and, with the assumption of a circular orbit, it orbits the primary in 34.7 ± 0.1 days, and is inclined 39 ± 6° from the larger moon.

The measured brightness implies a diameter 12% that of (136108) 2003 EL₆₁, ~170 km, assuming similar albedo.

[edit] Collisional family

The collisional family of 2003 EL₆₁ (in green), other classical KBO (blue), Plutinos and other resonant objects (red) and SDO (grey). Radius is semi-major axis, angle orbital inclination.

EL61 is the largest member of a collisional family, similar to asteroid families: a group of objects with similar orbital parameters and common physical characteristics, presumably with a common origin in a disruptive impact of the progenitor object of EL61.^[10]

The family, the first to be identified among TNOs, includes EL61 and its moons, 2002 TX₃₀₀, (24835) 1995 SM₅₅, (19308) 1996 TO₆₆, (120178) 2003 OP₃₂ and (145453) 2005 RR₄₃. The dispersion of the proper orbital elements of the members is a few percent or less (5% for semi-major axis, 1.4° for the inclination and 0.08 for the eccentricity). The diagram illustrates the orbital elements of the members of the family in relation to other TNOs.

The objects' common physical characteristics include neutral colours and deep infrared absorption features (at 1.5 and 2.0 μm) typical of water ice.^[11]

Collisional formation of the family requires a progenitor some 1660 km in diameter, with a density of ~2.0 g/cm³, similar to Pluto and Eris. During the formational collision, EL61 lost roughly 20% of its mass, mostly ice, and became denser.^[10]

The current orbits of the members of the family cannot be accounted for by the formational collision alone. To explain the spread of the orbital elements, an initial velocity dispersion of ~400 m/s is required, but such a velocity spread should have dispersed the fragments much further. This problem applies only to 2003 EL₆₁ itself; the orbital elements of all the other objects in the family require an initial velocity dispersion of ~140 m/s. To explain this mis-match in the required velocity dispersion, Brown et al. suggest that 2003 EL₆₁ initially had orbital elements closer to those of the other members of the family and its orbit (especially the orbital eccentricity), changed after the collision. Unlike the other members of the family, EL61 is in a chaotic orbit, near the 7:12 resonance with Neptune, which would increase EL61's eccentricity to its current value.^[10]

[edit] References

[edit] External links

• NASA visualization of the orbit
• Updated data
• MPEC listing for 2003 EL₆₁
• Space.com story: Large new world discovered beyond Neptune
• New Scientist story: New world found in outer solar system
• BBC story: Distant object found orbiting Sun
• Astronomers Discover "10th Planet" - Sky & Telescope article describing the discovery of (136108) 2003 EL₆₁ and 136199 Eris.
• Michael Brown's webpage
• New Scientist story: Distant solar system body may be cigar-shaped
• BBC story: Record spin rate for cosmic body
• news @ nature.com story: Cosmic 'cigar' spins at astonishing pace; Pluto's neighbour has a very unexpected shape, 9 September 2005.
• New York Times story: "One find, two astronomers", September 13, 2005.
• S.C. Tegler, W. Grundy, W. Romanishin, G. Consolmagno, K. Mogren, F. Vilas: Optical Spectroscopy of the Large Kuiper Belt Objects 136472 (2005 FY9) and 136108 (2003 EL61). Preprint