The MIRI instrument installed on the James Webb telescope will allow to directly point some of these known exoplanets in order to analyze their atmosphere. Whether it is for exoplanets in very close orbit around their star, or those evolving at several tens of astronomical units, the spectral range covered by MIRI is completely new for these objects in astrophysics, and we hope to reach unequalled sensitivities.
The Webb telescope and the MIRI instrument.
Source : LESIA, Observatoire de Paris-PSL.
Indeed, the mid-infrared is a domain that allows access to the thermal emission of exoplanet atmospheres that contains molecular signatures such as methane, ammonia or water vapor for the most widespread, which allows in particular to better understand the formation of exoplanets.
Exoplanets, terra incognita of the XXIᵉ century
The discovery of the first exoplanet around a solar-type star, has caused a stir in the scientific community. It is a result that was expected for a long time : the existence of exoplanets had been formulated since antiquity.
The discoverers of the exoplanet 51 Pegasi b were in fact awarded the 2019 Nobel Prize in Physics. And yet it took a little time to accept the idea that a planet the size of Jupiter could orbit at only 5 hundredths of an astronomical unit (an astronomical unit is 150 million kilometers, or approximately the distance Earth-Sun) from a solar-type star. For comparison, Mercury, the closest terrestrial planet to our Sun, is separated from it by about 4 tenths of an astronomical unit, and it is clearly smaller than 51 Peg b which is a gas giant. 51 Peg b was the proof that planets could form at great distances from their star and then migrate towards the interior of the systems.
As early as 1992, three planets had been detected around a pulsar, but without being well accepted by the community, because a pulsar is a very dense star that rotates very quickly on itself, residue of the explosion of a star. How could planets orbiting pulsars have survived the final destructive phase of a massive star ? Would they have reformed from the remains of the supernova ? If the existence of these planets is now confirmed, these questions remain.
Almost 5000 exoplanets are now known, and they have very diverse properties.
Their mass can vary from the mass of the Moon to about 10 times the mass of Jupiter, but those of a few Earth masses are the most abundant. Some orbit their star in only a few days, or even less than one Earth day. The majority of them are between 0.05 and 3 astronomical units, but some are detected up to several hundred astronomical units from their star.
How was detected 51 Pegasi b ?
We can measure the effect of a planet on its star without "seeing" the planet - these are indirect detection methods.
51 Pegasi b was the first "hot Jupiter" (i.e. a large gas planet very close to its star) detected by the radial velocity method.
In this method, the fact that the star and a planet orbit around their center of mass causes the star to move in space. This results in a periodic variation of the star’s velocity, detectable with very high resolution spectrographs, capable of seeing the stars "move" on their line of sight, thanks to the Doppler effect, with accuracies of about 1 meter per second. This allows us to determine the orbit of the planet as well as an estimate of its mass. To take the example of Jupiter, it changes the speed of the Sun by 13 meters per second. The closer and more massive the planet, the higher this speed.
The discovery of CoRoT-7b and the seven planets of the Trappist-1 system
When a planet passes in front of its star, it eclipses its light a little. The "transits" method allows to detect a periodic decrease of the luminosity of the star if the plane of the orbit is aligned with the observer. This is a rare condition : to improve statistics, satellites scan hundreds of thousands of stars.
The decrease in brightness of the star is related to the radius of the planet and it is necessary to be able to measure photometric variations of the order of 1 per 10000. For this, space instruments, which are not subject to the influence of the atmosphere, are necessary.
The planet CoRoT-7b is one of the first telluric planets discovered with this method of transits, with the CoRoT satellite in 2009. It belongs to the category of "super-Earths", planets whose mass is between 1 and 10 times the mass of the Earth. It is so close to its star that it goes around it in 0.85 Earth days and its surface is probably molten rock.
The Trappist-1 system, emblematic of the transits technique, contains seven planets, all are probably telluric and some are located in the famous habitable zone : where water, if present on their surface, could be liquid. Be careful not to confuse Trappist-1 with the solar system. Its star is an M dwarf barely larger than Jupiter and the notions of habitability could be very different from those encountered on Earth.
Analyzing the atmosphere of exoplanets with the James Webb Space Telescope
Trappist-1 is a prime target for MIRI, which aims to analyze the atmosphere of exoplanets.
For this, it is necessary to "see" the planets directly, i.e. to measure their light. The choice of the observation wavelength determines the type of information obtained on the atmosphere, the infrared giving access to molecular absorptions.
MIRI uses two direct detection techniques, each adapted to a specific type of exoplanet.
The most obvious one consists in forming an image where we can distinguish the light of the star and that of the planet, which is often problematic because of the phenomenon of light diffraction, which "widens" the images : the image formed by the telescope of a planet and its star tend to mix and it is difficult to detect such a faint object next to such a bright object.
The solution then consists in suppressing the light of the star, using an optical system : the coronograph. Initially invented by Bernard Lyot in 1930 at the Meudon Observatory to observe the solar corona, the coronograph has evolved into a stellar version.
The MIRI coronographs use an interferometric principle. No exoplanet has yet been imaged in mid-infrared, and MIRI opens the field of infrared coronography.
In this regime, the contrast between the star and its planet is more favorable, and the MIRI coronographs are designed to measure the physico-chemical properties of young giant exoplanets, in particular their temperature and the presence of certain molecules like ammonia or methane.
On the other hand, in the infrared, it is more difficult to distinguish two close objects : the exoplanets observed by MIRI will necessarily be distant from their star, typically beyond 10 astronomical units (about the distance of Saturn in the solar system).
The HR 8799 system contains 4 planets whose orbits last decades, even centuries. Here an animation made with seven infrared images from the Keck telescope. Source : Wang/Marois.
For example, MIRI will observe the HR 8799 system which contains four giant planets, located between 15 and 70 astronomical units, with masses between about 7 and 10 Jupiter masses.
When eclipses between the planet and its star allow to study the atmosphere
The method of transits also allows the study of exoplanetary atmospheres.
When the planet passes in front of the star, the light of the star crosses the atmosphere of the exoplanet and its spectrum is modified : we measure the absorption due to the atmosphere of the planet.
Conversely, when the planet passes behind the star, we measure (by subtraction) the thermal emission of the atmosphere of the planet, that is to say the photons emitted directly by the planet in connection with its temperature (black body law).
These two measurements are complementary and can in some cases be obtained for the same exoplanet. The low resolution spectrograph of MIRI will determine the molecules present, their abundance, and the pressure and temperature structure of the atmosphere. The giant planets will be the privileged targets of this method of "transit spectroscopy", but we also hope to probe for the first time in mid-infrared the atmosphere of telluric planets, in particular that of the famous Trapp