Interview with Olivier Absil, astrophysicist, FNRS Senior Research associate, Head of the PSILab (Planetary and Stellar systems Imaging Laboratory) at the STAR Institute of the University of Liege.
What are the great mysteries of the universe that astrophysicists hope to unravel with JWST?
Beyond the study of the first galaxies and the first stars that appeared at the cosmic dawn of our universe, one of the main goals of JWST is to study the formation and evolution of planetary systems. Over the past decade, ground-based observatories such as ALMA (an array of antennas in Chile operating in the millimeter range) and optical telescopes equipped with high-contrast imaging instruments have revealed the presence of structures in the dust disks around forming stars, such as spirals, rings, cavities, etc. But in most cases, the source of these structures remains unobservable. With JWST, it should be possible to confirm that these structures are caused by young planets in the process of formation, which will become detectable thanks to the unparalleled sensitivity of JWST at infrared wavelengths, where these planets are brightest.
What science project(s) based on JWST data are you involved in? What do they consist of?
My laboratory is involved in two major observing programs with the Mid-Infrared Instrument (MIRI), a mid-infrared spectro-imager. These two programs are part of a larger program of early scientific exploitation of the MIRI instrument, organized by the consortium of European laboratories that led the design and construction of the instrument. These two programs, called "guaranteed time" because they correspond to the reward given to the consortium for their efforts, aim to study respectively a number of circumstellar disks –around young stars, and of extrasolar planets with MIRI.
They each represent about 100 hours of observations with JWST, which is both huge, considering the pressure on the use of the telescope, and very small, considering the number of potential targets and the different observing modes of JWST. Through these guaranteed time programs, we will therefore have to focus on a small number of iconic targets that will allow us to demonstrate the capabilities of JWST to answer the questions at hand.
One of your projects will focus on the study of giant exoplanets located far from their star, objects that are generally less known. What is the interest of these new targets? What can they teach us?
These giant planets detected from the ground by direct imaging methods have the common point of being relatively young, with ages typically ranging from a few tens to a few hundred million years. Not only do they allow us to study the architecture of planetary systems on a large scale - which is usually impossible with other exoplanet detection techniques that focus mainly on the inner regions of planetary systems - but also, thanks to their young age, they have the potential to teach us more about the early phases of planetary evolution. And, in particular, to help us better understand the products of planetary formation in terms of mass, radius, temperature, internal energy, etc., of giant planets. This is still a relatively poorly understood field, where several theories make rather different predictions. These young planets can also bring direct information about the conditions that prevailed around the stars during their formation, which offers a direct connection with the guaranteed time program on circumstellar disks.
What imaging methods will you use, and with which JWST instrument?
To choose is to renounce. The MIRI instrument has the advantage of being able to use the two most popular techniques of observation in astronomy at the same time: imaging and spectroscopy. More precisely, MIRI is an integral field spectro-imager, which means that it allows to obtain an image where each pixel is itself decomposed into a spectrum of infrared light. This very powerful observing technique is increasingly used in ground-based observatories, in particular for the study of exoplanets by direct imaging, because it allows in a single step to spatially separate the light of the planet from that of the star (essential to extract the part of the light of interest), and to obtain the spectrum of the planet (essential to characterize the composition of planetary atmospheres). JWST will be the first to use this technique for infrared astronomical observations from space. Thanks to its complete coverage of the infrared range, it will give access to measurements impossible from the ground, e.g. the abundance of certain molecules like ammonia.
Ammonia has a great importance in the thermal structure of the atmosphere of the giant planets of our solar system. In particular, it determines the effective temperature measured for the planet by an external observer. Ammonia is expected to have a similar role in exoplanetary atmospheres. Detecting it would allow us to better understand the thermal structure of the atmospheres of giant exoplanets (especially planets detected by direct imaging).
What is a circumstellar disk? What does this object bring as information on the formation of planets and our universe?
During the formation of a star, a disk of gas and dust forms around it. This disk not only supplies the star with “fuel” during the earliest phases of the system, but is also the cradle in which planets can form. Planetary formation takes place during the first ten million years, after which the disk is almost completely emptied of its gas by the intense radiation of the newly formed star. It is in these disks that we see today, thanks to ground-based observatories, traces of (sometimes intense) planetary formation activity, such as formation of spiral arms, cavities, rings of variable density, etc. But today, it is still impossible in most cases to connect the observed structures with one or more planetary mass objects, which still escape our ground-based instruments. Thanks to JWST, we will finally be able to understand which type of planet is at the origin of which type of structure, and thus refine our theories on planetary formation in general.
You already work with data from many ground-based telescopes (VLT in particular). What data will JWST be able to detect that ground-based observatories (and other space-based telescopes) cannot?
Two specific ingredients of JWST will be decisive compared to ground-based observatories in the study of planetary systems: its incomparable sensitivity, and its complete coverage of the infrared range. These features will give us access to the observation of planets of lower mass, and cooler than those that can be detected from the ground, which should allow us to see planets in the process of being born, which remains totally exceptional from the ground. And these planets will be characterized with an unprecedented precision in terms of physical properties and atmospheric composition.
How will the JWST targets be defined? Is it the ground-based telescopes that will guide the choice of what will be observed by JWST?
The observing time with JWST is so precious that we cannot afford to go looking for new planets "blindly", except in very special cases where we can demonstrate that the chances of success are very high (such as around some forming stars surrounded by highly structured disks). Most of the targets observed by the JWST will thus first have to be detected from the ground, either in terms of detection of potentially interesting disks, or in terms of detection of planets for systems where planetary formation is already completed (we already know of a few dozen). In the case of circumstellar disks, the advantage is that, even if no planet is detected, the data collected by JWST will remain very useful to study the disk itself. The detection of planets in these disks can thus be considered as the icing on the cake!
Does the JWST have the capacity to image potentially habitable planets?
Unfortunately, this will not be possible. Although JWST has in principle the sensitivity to detect an Earth-like planet around a nearby star, it will lack, in spite of its large mirror, the necessary resolving power to directly access the habitable zone, where planets have the appropriate temperature to harbor water in liquid form.
This type of detection should become possible within ten years for a handful of stars thanks to future giant ground-based telescopes (more than 30 meters in diameter), but their capacity to characterize these planets will be limited. To study in detail real twins of our Earth, it will probably take until the following decade (2040) and the advent of the new generation of space observatories (large telescopes working at very high contrast in the visible range, or interferometers working in the infrared).
