Observations with the instrument SPHERE at ESO's Very Large Telescope have produced an unprecedented gallery of "debris disks" in exoplanetary systems. Gaël Chauvin (Max Planck Institute for Astronomy), project scientist of SPHERE and co-author on the paper publishing the results, says: "This data set is an astronomical treasure. It provides exceptional insights into the properties of debris disks, and allows for deductions of smaller bodies like asteroids and comets in these systems, which are impossible to observe directly."
In our own solar system, once you look beyond the Sun, the planets, and dwarf planets like Pluto, there is a bewildering array of smaller ("minor") bodies. Of particular interest are the larger small bodies, with diameters between about a kilometer and several hundred kilometers. We call those objects comets if they put on (at least occasionally) a display of losing gas and dust to form distinctive visible structures like a tail, and asteroids when they don't. Small bodies provide a glimpse of the earliest history of the solar system: In the evolution from dust grains to full-size planets, small bodies called planetesimals are a transitional stage, and the asteroids and comets are remnants from that stage – planetesimals that did not manage to evolve into larger planets. Small bodies are (somewhat) modified remnants of the building material for planets like our Earth!
Small bodies around stars other than the Sun?
So far, astronomers have detected more than 6000 exoplanets (that is, planets orbiting stars other than the Sun), giving us a much better idea of the diversity of planets out there, and of the place of our solar system within this teeming population. Taking actual images of such planets is a considerable challenge, though. At this time, there are less than 100 exoplanets that astronomers have been able to image, and even giant planets are no more than a structureless little blob on such images. "Finding any direct clues about the small bodies in a distant planetary system from images seems downright impossible. The other indirect methods used to detect exoplanets are no help, either" says Dr Julien Milli, astronomer at the University Grenoble Alpes and co-author of the study.
The solution, ironically, comes from stuff that is even smaller, by orders of magnitude. In particular in younger planetary systems, planetesimals will regularly collide – sometimes to stick together to form a larger body, sometimes to go their separate ways. These collisions create copious amounts of new dust, and the dust, it turns out, can be observed over large distances, given suitable instruments: Whenever you divide an object into smaller components, the total volume remains the same, but the total surface area increases. Divide an asteroid with a diameter of one kilometer into dust grains with diameters of one micrometer (= millionth of a meter), and you increase the overall surface by a factor of a billion! That is, in large part, why it is possible to observe debris disks around young stars by the starlight they reflect. Observe the dust, and you can glean information about the planetary system's small bodies.
Observing debris disks
Over time, such a debris disk will fade. Collisions will become less frequent. Dust will be blown out of the system by radiation pressure, caught by planetesimals or planets, or ends up in the central star. Our own solar system provides an example of what is left after billions of years: In this case, there are two remaining planetesimal belts, namely the asteroid belt between Mars and Jupiter, and a reservoir of comets outside the orbits of the giant planets in what is known as the Kuiper belt. There is also dust in our solar system's main orbital light, known as zodiacal dust. Under a very dark sky, you will be able to see light reflected by that dust with the naked eye shortly after sunset or shortly before sunset, the so-called zodiacal light.
This configuration would be difficult to detect for alien astronomers studying our solar system from afar. But as the present study has shown, with the best current telescopes and instruments, for not-too-far-away systems, the dust should be observable for about the first 50 million years of the debris disk's life. Which is not to say that such observations are not a considerable technical challenge! Imaging a debris disk is like taking a picture of a puff of cigarette smoke, but the smoke is hovering next to a bright stadium floodlight, and you are trying to take the picture from a distance of several kilometers. This is where suitable instrumentation makes all the difference, and it is where the SPHERE instrument, which began operating at one of ESO's Very Large Telescopes (VLT) in the spring of 2014, excels.
Blocking out starlight
At the heart of SPHERE is a very simple concept. If in everyday life, we want to look at something and the Sun in the background is making this difficult, we put up a hand to block out the sunlight. When SPHERE observes an exoplanet or debris disk, it uses a coronagraph to block out the star's light – in effect, a little disk inserted in the optical pathway that removes most of the starlight before the image is taken. The catch is that unless imaging is very precise and stable, this simple recipe cannot work in practice!
To meet the stringent requirements, SPHERE utilizes an extreme version of adaptive optics, where the unavoidable perturbations caused by the light passing through Earth's atmosphere are analyzed and largely compensated for in real time through the use of a deformable mirror. Another, optional part of SPHERE filters out light with specific properties ("polarised light") that are characteristic for light reflected by something like dust particles, as opposed to starlight, setting the stage for particularly sensitive debris disk images.
An unprecedented gallery of debris disk images
The new publication presents an unprecedented collection of debris disk images, produced with SPHERE from starlight reflected by small dust particles in these systems. "To obtain this collection, we processed data from observations of 161 nearby young stars whose infrared emission strongly indicates the presence of a debris disk," says Natalia Engler (ETH Zurich), the lead author of the study. "The resulting images show 51 debris disks with a variety of properties — some smaller, some larger, some seen from the side and some nearly face-on – and a considerable diversity of disk structures. Four of the disks had never been imaged before."
Comparisons within a larger sample are crucial for discovering the systematics behind object properties. In this case, an analysis of the 51 debris disks and their stars confirmed several systematic trends: When a young star is more massive, its debris disk tends to have more mass as well. The same is true for debris disks where the majority of the material is located at a greater distance from the central star.
Finding asteroid belts and Kuiper belts in other systems
Arguably the most interesting feature of the SPHERE debris disks are the structures within the disks themselves. In many of the images, disks have a concentric ring- or band-like structure, with disk material predominantly found at specific distances from the central star. The distribution of small bodies in our own solar system has a similar structure, with small bodies concentrated in the asteroid belt (asteroids) and the Kuiper belt (comets).
All of these belt structures appear to be associated with the presence of planets, specifically of giant planets, clearing their neighbourhoods of smaller bodies. Some of the giant planets had been observed already. In some of the SPHERE images, features like sharp inner edges or disk asymmetries give tantalizing hints of as-yet unobserved planets. In this way, the SPHERE disk collection sets interesting targets for future observations: the JWST, or the Extremely Large Telescope (ELT) currently under construction by ESO should allow astronomers to produce images of the planets that create these structures.
Background information
The results described here have been published as Natalia Engler et al., "Characterization of debris disks observed with SPHERE," in the journal Astronomy and Astrophysics. DOI: 10.1051/0004-6361/202554953
The MPIA researchers involved are Gaël Chauvin, Thomas Henning, Samantha Brown, Matthias Samland, and Markus Feldt, in collaboration with Natalia Engler (ETH Zürich), Julien Milli (CNRS, IPAG, Université Grenoble Alpes), Nicole Pawellek (University of Vienna), Johan Olofsson (ESO), Anne-Lise Maire (CNRS, IPAG, Université Grenoble Alpes), and others
