The Haumea family is the only identified trans-Neptunian collisional family

Collisional family

In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact . They have similar compositions, and most share similar orbital elements....

; that is, the only group of trans-Neptunian objects (TNOs) with similar orbital parameters and spectra (nearly pure water-ice) that suggest they originated in the disruptive impact of a progenitor body. Calculations indicate that it is probably the only trans-Neptunian collisional family.

Characteristics

The dwarf planet

Dwarf planet

A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

  is the largest member of the family, and the core of the differentiated progenitor; other identified members are the moons of Haumea

Moons of Haumea

The outer Solar System dwarf planet Haumea has two known moons, Hiiaka and Namaka, named after Hawaiian goddesses. These small moons were discovered in 2005, from observations of Haumea made at the large telescopes of the W. M. Keck Observatory in Hawaii....

 and the Kuiper belt objects , , , , , , , , and , all with an ejection velocity from Haumea of less than 150 m/s. The brightest Haumeids have absolute magnitudes (H) bright enough to suggest a size between 400 and 700 km in diameter, and so make them dwarf-planet candidates; though it is important to note that the suggested high albedos of these objects may prevent them from qualifying as dwarf planets. The dispersion of the proper orbital elements

Proper orbital elements

The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale...

 of the members is a few percent or less (5% for semi-major axis

Semi-major axis

The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

, 1.4° for the inclination and 0.08 for the eccentricity

Orbital eccentricity

The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

). The diagram illustrates the orbital elements of the members of the family in relation to other TNO

Trans-Neptunian object

A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

s.

The objects' common physical characteristics include neutral colours

Color index

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

 and deep infrared absorption features (at 1.5 and 2.0 μm) typical of water ice.

Formation and evolution

Collisional formation of the family requires a progenitor some 1660 km in diameter, with a density of ~2.0 g/cm³, similar to Pluto

Pluto

Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris

Eris (dwarf planet)

Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

. During the formational collision, Haumea lost roughly 20% of its mass, mostly ice, and became denser.

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 Haumea 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 Haumea 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, Haumea is in a chaotic orbit, near the 7:12 resonance

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with Neptune, which would increase Haumea's eccentricity to its current value.

A second proposal suggests a more complicated origin for the family: that the material ejected in the initial collision instead coalesced into a large moon of Haumea, which gradually increased its distance from Haumea through tidal evolution

Tidal acceleration

Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

, and was then later shattered in a second collision, dispersing its shards outwards. This second scenario produces a velocity dispersion of ~190 m/s, considerably closer to the measured ~140 m/s velocity dispersion of the family members; it also avoids the difficulty of the observed ~140 m/s dispersion being much less than the ~900 m/s escape velocity of Haumea.

Haumea may not be the only elongated, rapidly rotating, large object in the Kuiper belt

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

. In 2002, Jewitt and Sheppard suggested that should be elongate, based on its rapid rotation. In the early history of the Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, the trans-Neptunian region would have contained many more objects than it does at present, increasing the likelihood of collisions between objects. Gravitational interaction with Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 has since shifted many objects from the Kuiper belt to the more distant scattered disc

Scattered disc

The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

.

The presence of the collisional family hints that Haumea and its "offspring" might have originated in the scattered disc

Scattered disc

The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

. In today's sparsely populated Kuiper belt, the chance of such a collision occurring over the age of the Solar System is less than 0.1 percent. The family could not have formed in the denser primordial Kuiper belt because such a close-knit group would have been disrupted by Neptune's subsequent migration into the belt, which is believed to have been the cause of its current low density. Therefore it appears likely that the dynamic scattered disc region, in which the possibility of such a collision is far higher, is the place of origin for the object which would become Haumea and its kin. Simulations suggest the probability of one such family in the Solar System is approximately 50%, so it is possible that the Haumea family is unique.
Because it would have taken at least a billion years for the group to have diffused as far as it has, the collision which created the Haumea family is believed to have occurred very early in the Solar System's history. This conflicts with the findings of Rabinowitz et al. who found in their studies of the group that their surfaces were remarkably bright; their colour suggests that they have recently (i.e. within the last 100 million years) been resurfaced by fresh ice. Over a timescale as long as a billion years, energy from the Sun would have reddened and darkened their surfaces, and no plausible explanation has been found to account for their apparent youth.

However, more detailed studies of the visible and near infrared spectrum of Haumea
show it is an homogeneous surface covered by an intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics. This high amount of amorphous ice on the surface confirms that the collisional event must have happened more than 100 million years ago. This result agrees with the dynamical studies and discards the assumption that the surfaces of these objects are young.

External links

• http://news.softpedia.com/news/New-Body-Parts-From-Kuiper-Belt-039-s-Haumea-95833.shtml