Planetary Systems being Formed

Can we satisfactorily construct models of the formation of planetary systems based on a study of just our solar system? A decisive step forward has been taken thanks to high angular resolution techniques capable of surveying planetary systems being formed around stars other than our Sun.

In 2005 the JENAM (Joint European and National Astronomy Meeting), which brings together astrophysicists from all over the world, took place at the University of Liège. There the astronomers Olivier Absil and Dimitri Mawet, from the ULg’s Extragalactic astrophysics and space observation group, presented an overview of the interferometers used in astrophysics and thus attracted the attention of the editors of the Astronomy and Astrophysics Review. It was the birth of an ambitious editorial project which has just been published (1) and in which our two Liège researchers take stock of 10 years of observing planetary systems being formed, using different high angular resolution techniques such as interferometry or coronagraphy.

Angular resolution is the capacity of an instrument to distinguish two objects which appear very close to each other in the sky. In effect for one and the same physical separation two celestial bodies will appear all the more closer to one another the more distant they are from the observer. Thus the further away and/or smaller the astrophysical sources are, the more the instruments which observe them must have available high angular resolution power. The techniques which allow access to high angular resolution are relatively recent: they only started to supply results at the beginning of the years 2000, but the harvest is already an abundant one...

The solar system, for the lack of anything else

Up until the end of the 1990s, our Sun and its cortege of planets was (practically) the only observable and observed planetary system. It was thus on this model that leaned the theoretical models of planetary formation whose physical foundations are still accepted today, at least in terms of the broad outlines: a planetary system is formed around a young star, in the remnants of the protostellar nebula at the origins of the star. By rotating on itself this residue of gas and dust flattens out to the point where it forms a disk of sufficient density to allow collisions between different dust grains and thus enables their clustering together. It is the distance from the star which determines the type of planet formed: a planet which forms close to its star will be small and rocky, whilst if it is further out it will be a giant and gaseous. ‘If this model has not dated,’ specifies Olivier Absil, ‘it has been refined since the beginning of the years 2000, thanks to the discovery of several hundreds of exoplanets. We now know that the planets do not necessarily stay in the place where they were formed and that they can migrate:  a massive planet formed far from its star can come closer to it (hot Jupiters) and the opposite is also true.’

(1) Absil O., Mawet D., 2010. Formation and evolution of planetary systems : the impact of high angular resolution optical techniques, A & A Review 18, 317-382. DOI 10.1007/s00159-009-0028-y

Such was the knowledge astronomers had of the formation of planetary systems when high angular resolution instruments first made their appearance. These tools opened the door to the study of protoplanetary disks (or circumstellar disks), but also to the different stages of planetary formation or the architecture of a planetary system. Two main research fields took than shape: determining the overall architecture of protoplanetary disks and gaining an understanding of the evolution of the dust grains within them, in other words the large scale structures and the small scale interactions.

From the physics of protoplanetary disks...


To take the large scale structure first of all, few things were known about protoplanetary disks before the advent of high angular resolution techniques: lines of silicate in stellar spectra already betrayed their presence, but they did not allow the degeneracy which contaminated the theoretical models to be lifted. It was in particular impossible to simultaneously know the distance to the star and the temperature of these silicate grains: grains of different types can have the same temperature without necessarily being the same distance from the star. Without these distances it was impossible to gain a representation of the disk.

High angular resolution imaging has lifted this degeneracy to the point of enabling the characteristics of the grains to be determined, such as their colour, size and composition. It has also revealed the architecture of these disks, which are today classified into two large categories: disks which are opened out or flared and relatively flat disks. Several more exotic details have also brightened up the observational data: certain disks have spinal arms, holes or gaps which could indicate the presence of objects which are being formed.

Interferometry occupies a privileged place amongst the high angular resolution techniques. Its angular precision allows one to zoom in on the spectrum of the different parts of a protoplanetary disk, whilst a regular spectrum results from an integration of the whole of the disk. Amongst the ‘astronomical’ fruits of interferometry let us highlight one result in particular: the study of the central section of these disks has enabled adjustments to be made to the modelling of this hitherto little known region: ‘On the basis of these interferometric measurements,’ describes Olivier Absil, ‘it turns out that the radius of this region is greater that the models predicted. We can only explain it by supposing that the gas which fills this region is less opaque than anticipated, and thus it does not form an obstacle to the star’s light (cf: see the diagram opposite). It thus very strongly heats the disk at this frontier. There the disk takes on thickness, forming like a wall which will protect the part of the disk behind it from the star’s direct light. Interferometry thus allows us not only to check out the type of dust in the central region but also to more precisely characterise the protoplanetary disk., which is divided into three regions: a very hot central area, an intermediary one which is cooler, and an exterior one which is once against hot.’

If the large scale study of protoplanetary disks has made giant steps forwards with the advent of high angular resolution techniques, there remain questions which are yet to be answered.  For example: ‘after ten million years, most of these disks have disappeared,’ declares our astronomer. ‘It’s as if they have evaporated. We still do not know through what processes, but it is typically this type of question which could in the future be tackled by high angular resolution techniques as they can see which part of the disk is the first to disappear and thus provide clues as to the physics which hides behind it.’

...to the dynamics of planetary systems

If the techniques of high angular resolution have provided general representations of protoplanetary disks they have also enabled studies at the very smallest scale to be carried out, indispensable to an understanding of the formation of planets. In effect it is through the collision of grains that larger and larger agglomerates form, up to the point of giving birth to the planets. Thus, when a disk dissipates it gives place to planets, but also to a multitude of other bodies (comets, asteroids, etc.) which, in colliding with each other, produce second generation dust which forms a disk of debris.  ‘The study of several of these debris disks has led to the appearance of strange structures, such as a star surrounded by a disk which ends abruptly, instead of fading out gradually,’ recounts Olivier Absil. ‘The dynamics of the dust could only explain it by the presence of a companion planet. Observations in 2004, repeated in 2006, brought to light the theoretically predicted intruder. It’s a very great result which brings together observation, modelling and prediction. It was the subject of a publication in Nature in 2008.’

Amongst the crop of high angular resolution observations let us mention once again the case of β-Pictoris, a nearby young star seen from the Southern hemisphere. This case is interesting as the star has a warp in its disk, which has been known of since 2001: the disk’s inclination is not the same in its interior and its exterior, as if it had two disks. This warp has been interpreted as the sign of the presence of a disruptive planet which is attracting towards itself material from the primary disk, thus indirectly forming a second. This companion was finally observed last year, exactly in the place predicted.

Maturation expected

The technique used in this case was not interferometry, but coronagraphy. In effect interferometry is a powerful tool for studying, amongst others, the formation and development of planetary systems. It has been made much use of to probe protoplanetary disks. But as far as detecting exoplanets goes, even if it has been envisaged for 15 years, the technique in its current state has reached its limits, as Olivier Absil has demonstrated in a recent study (2). ‘With the aid of three of the VLT’s auxiliary telescopes and their AMBER instrument we used interferometry to observe β-Pictoris. We knew in advance that it was sheltering a companion as it had been observed via coronagraphy. But we were looking to detect it through interferometry. To do so we intensively observed the star for three successive nights, but in vain. This non-detection of the nevertheless avowed companion allowed us to estimate the sensitivity of our instrument: for a companion orbiting between 0.1 and 3 astronomical units, we have a 90% chance of detecting a body with a mass 50 times greater than Jupiter’s mass (MJ), in other words 1500 times greater the mass of our planet. This probability falls to 50% for a body of 35 MJ. We can thus detect companions which are brown dwarfs, but not planets. To enter the domain of planets (less than 13 MJ) we still have to increase our sensitivity by a factor of 5. The technique of interferometry is a complicated one, but it should reach maturity with the new instruments installed at the VLTI (Very Large Telescope Interferometer) in Chile or at the CHARA network (Center for High Angular Resolution Astronomy) at Mount Wilson.’