Trans-Neptunian Objects broken into pieces

Mike Brown and collaborators discovered the first colisional family of Trans-Neptunian Objects, also known as Kuiper Belt Objects. The results had been announced by K. Barkume, Mike Brown's PhD student, in October 2006, at the Division for Planetary Sciences meeting, in Pasadena, U.S.A.

The article just came out in Nature (Brown et al. 2007, Nature, 446, 294-296). The work identifies a group of Trans-Neptunian Objects (TNOs) as the "debris" of a violent collision between two bodies. This group of TNOs was characterized by the presence of strong water ice absorption bands on its surface but with no evidence for methane ice. Note that 3 of the largest TNOs, Pluto (2320 km in diameter), Eris (2400 km) and 2005FY9 (about 1600-2000 km) possess methane ice on its surface. TNOs without methane normally show little or no water ice (we are talking about surfaces here, the interiors are another question). However, the objects: 2003EL61 (about 1500 km), 1995SM55, 1996TO66, 2002TX300, 2003OP32, 2005RR43 and S/2005(2003EL61)1 - the brightest satellite of 2003EL61 -, possess strong absorption bands due to water ice without showing methane. This fact called attention to these objects.

An orbital dynamics analysis showed that all these objects possess orbits similar to the one of 2003EL61, leading to the conclusion they are, in fact, the "debris" of the collision between the proto-2003EL61 (i.e. the old 2003EL61) and another object. The simulations suggest that 20% of the proto-2003EL61's mass can be thrown into Space if it collides with one object of 60% of its size, at about 3 km/s (10000 km/h).

The fact that 2003EL61 has a rugby ball shape, possess two satellites, a relatively high mass density and one "day" of only 4 hours indicated already that, in the past, this object had suffered a violent collision that ejected its original mantle of ices leaving it with two satellites and a stretched shape.

To get an idea of the difficulty of this discovery, let us imagine a "party" in a china store where each person has a baseball bat... In the next day someone will take a look at the pieces and to try to find out if there was a Franklin Mint Princess Diana plate there and where. The comparison is not exaggerated.

The study of Trans-Neptunian Objects, done at the sensitivity limits of the largest telescopes, continues to surprise us.

Colision of Comets at the Helix Nebula

[Image:NASA/JPL-Caltech/Univ. Arizona]

Kate Su (University of Arizona, U.S.A.) and collaborators, find evidence for a high number of collisions between comets at the Helix Nebula, using the
Spitzer Space Telescope (NASA).
The Helix Nebula is at about 700 light-years of distance from the Earth and it is the final phase of a star similar to the Sun: a white dwarf surrounded by a distant cloud of gas and dust.
The Spitzer Telescope operates in the infra-red, hence it does not get images in the visible like the Hubble Telescope does. However, it is capable to detect the thermal radiation from tiny close objects or from very distant objects. The different intensities of the detected radiation are transformed into false colors to create an image.
Kate Su's team detected an excess of "thermal brightness" between 35 and 150 Astronomical Units (1 AU = Earth-Sun Distance = 150 000 000 km) of distance from the white dwarf at the center of the Helix Nebula. Most probably due to a dust disk. One did not expect to find dust at such distances around this type of star. However, in our Solar System, at the same distance from the Sun, we have the Kuiper Belt. And, in reality, it is a large reservoir of comets which, simplifying, are no more than "dirty ice balls". It is then most likely that that the detected dust was released by a large number of collisions between comets that gravitate, or gravitated, around the nebula's central star.

The Spitzer press-release is available on-line.

The article's abstract: Su et al., 2007, Debris Disk around the Central Star of the Helix Nebula?, Astrophysical Journal Letters, Vol. 657, L41-L46, is also available.