Planck and Herschel Exclusive Interviews

Thomas Mueller

(Max Planck Institute for Extraterrestrial Physics, Garching)

Principal Investigator for the 'TNOs are Cool: A Survey of the Transneptunian Region' Herschel Open Time Project

TNOs of our own Solar System, which reside beyond the orbit of Neptune. Images from NASA.

What is the current state in terms of our understanding of the characteristics and number of TNOs and how will you use Herschel to learn more about these objects?
Transneptunian Objects (TNOs) are believed to represent one of the most primordial populations in the Solar System. The TNOs are the frozen leftovers from the formation period of the outer Solar System. The current total mass in the Kuiper Belt is estimated to be around 0.03-0.3 Earth masses, but there is evidence that a much larger mass (10-40 MEarth) was originally present at the time of formation. About 1200 TNOs have been detected so far and, as detailed hereafter, new studies have started to reveal a richness of orbital and physical properties. These TNOs represent only a few percent of the estimated 30 000 TNOs bigger than nearly 50 km.

You use the term 'dynamical architecture' in your summary. Could you explain what this means and how this relates to the evolution of the circumstellar disc?

TNOs can be grouped in several dynamical families according to their orbital properties. Two core populations exist in the main Kuiper Belt: objects in orbits resonant with Neptune (e.g. the Plutinos in the 3:2 resonance, that is whereas Neptune orbits the Sun 3 times, the Plutinos orbit the Sun 2 times) and the classical disc objects in non-resonant orbits between 40 and 48AU from the Sun. Interestingly, the classical population has a clustering sub-population, the objects in dynamically “cold” orbits (inclination i<5, eccentricity e<0.1) and a wide halo of objects in dynamically “hot” orbits (i>5, e>0.1). It is likely that these two sub-populations were formed in different regions of the planetary disc, but the precise formation regions remain uncertain. Beyond the main Kuiper Belt, “scattered disc” objects follow eccentric and sometimes highly-inclined orbits with perihelion distances close to Neptune’s orbit, indicating that they are scattered from the main Kuiper Belt by gravitational interaction with Neptune. The “detached” objects (e.g. Sedna) orbit the Sun far outside the Kuiper Belt and are clearly disconnected from immediate resonant or scattering interaction with the outer planets. They may have been stranded in their orbits by past gravitational interactions with passing stars or planetary embryos.
 
What is the significance of the large albedo diversity already observed?

Determining the size of a TNO is one of the most difficult measurements to obtain. The visible magnitude (e.g. the absolute H-magnitude, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit from both the Sun and the observer and at a phase angle of zero degrees) gives only a crude estimate of the size due to the unknown geometric albedo. For example, a factor of 5 uncertainty in albedo leads to a factor 2.2 uncertainty in the diameter and a factor of 11 in the volume. Apart from 3 TNOs (including Pluto) with disc-resolved observations, size and albedo determinations are based on combined measurements of the reflected light and thermal emission flux. Spectroscopy provides semi-qualitative information on the presence of surface compounds, a detailed modelling of surface spectra requires knowledge of the absolute albedo. On the other hand, pure ices are usually bright and a low albedo is indicative of a dark surface material (e.g. tholin, dark carbon) which is otherwise spectrally neutral.

Statistical study of TNO albedos is limited by the small number of measurements, but useful hints are beginning to emerge. Contrary to expectation, convincing evidence is not seen for dependence of albedo on object size for diameters spanning an order of magnitude. Similarly, no clear correlation of color with albedo is observed. A wide range of albedos is evident among Scattered, Resonant, and Classical objects, as well as among both small and large, and gray and red objects. Models of size-dependent surface processes such as impact erosion and volatile loss being proposed to interpret TNO colors need to accommodate the existence of bright and dark objects of diverse sizes, colors, and dynamical classes.

Herschel will be used to study the debris discs around other stars, which in our own Solar System corresponds to the Kuiper Belt, the rocky debris strewn beyond the orbit of Neptune.

How have you selected the 141 objects you have proposed, and does this represent a good sampling of the range of TNO characteristics that are already known?

We propose Herschel observations for a total of 141 TNOs, 25 of which are binary or multiple systems. Our target list also includes Saturn’s satellite Phoebe, with its well-established physical properties, for calibration purposes. The motivations are: (i) determining the size of 100+ objects will provide a clue to the original size distribution in the Kuiper Belt. Objects detectable with Herschel will be typically 200 km or more in diameter. These larger TNOs should reflect the primordial-size distribution, in contrast to objects smaller than 100 km which are slowly eroded by collisions. (ii) While spectroscopy provides semi-qualitative information on the presence of surface compounds, a detailed modelling of surface spectra requires a knowledge of the absolute albedo and finally (iii) we anticipate that correlations between size, albedo, colour, composition, and orbital parameters will be diagnostic of evolution processes. For example, red objects may be expected to be dark, as the red colour is thought to be associated with space weathering, which also darkens surfaces if acting for long enough.

How important is it to understand the Solar System's own debris disc before being able to understand extra-solar planetary systems or do you think it works both ways, in that  observing extra-solar discs will help us learn more about our own?
Herschel is set up to make a key contribution to the study of dusty debris discs around other stars. We propose here a Key Programme focused on the study of our own debris disc, the only case in which the parent bodies are accessible to direct observations.
The Transneptunian population is the Solar System analogue of the debris discs observed around several other, 10-500 Myr old stars. This analogy is bolstered by similarities in sizes and observed masses (typically 30-300AU and 0.01-0.1MEarth for the “exodiscs”), with the important difference that the detected mass in extra-solar debris discs is in the form of 10-1000 μm, short-lived, dust particles. The vast majority of the mass in such discs is invisible to us, and in the form of kilometre (or more)-sized bodies, resembling Transneptunian objects. It is these objects which serve as parent bodies, from which the dust is generated by collisions.