James Webb Space Telescope to map the atmosphere of exoplanets

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Illustration of a "super Neptune," TOI-674 b, with an atmosphere that, according to a recent study, includes water vapor. Image Credit: NASA/JPL-Caltech

Exoplanets, planets orbiting stars other than the Sun, are found at distances far from Earth. For example, the closest exoplanet to us, Proxima Centauri b, is 4.2 light-years away, or 265,000 times the distance between the Earth and the Sun.

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Illustration of a "super Neptune," TOI-674 b, with an atmosphere that, according to a recent study, includes water vapor. Image Credit: NASA/JPL-Caltech

Louis-Philippe Coulombe

To the naked eye, the planets of the solar system appear as bright spots. However, using a telescope, these dots stand out against the stars and reveal structures such as Jupiter's Great Red Spot, Saturn's rings, or the ice caps of Mars.

Although the presence of these phenomena on exoplanets is expected, their distance from Earth prevents us from directly resolving their surfaces. However, there are ways to better understand the structure of their atmospheres and to map them.

I am a PhD student in astrophysics at the University of Montreal. My work is related to the characterization of exoplanet atmospheres. More specifically, my research focuses on the development of tools to map the atmosphere of exoplanets using observations from the James Webb Space Telescope.

The telescope, launched on December 25, 2021, is expected to revolutionize the field of exoplanetary science.


Detection and characterization of exoplanets

Apart from a few special cases where the light from a planet can be observed directly, most exoplanets are detected by indirect methods. An indirect method consists of observing the effect of the planet's presence on the light emitted by its star.

The transit method is the one that has led to the largest number of exoplanet detections. Transit occurs when, from our perspective, an exoplanet passes in front of its host star. During the transit, the light from the star dims as the star's surface is partially obscured by the planet.

Light is divided into a spectrum of wavelengths corresponding to different colors. When a transit is observed at various wavelengths, it is possible to measure the atmospheric composition of the exoplanet. For example, water molecules strongly absorb light at infrared wavelengths, which makes the planet appear larger because its atmosphere blocks a larger fraction of the light from its star. Similarly, it is also possible to measure the temperature of the atmosphere and detect the presence of clouds.

In addition, a transiting planet can also pass behind its star. This phenomenon, in which only the light from the star is observed, is called a secondary eclipse. By observing it, it is possible to isolate the light coming only from the planet and thus obtain additional information about its atmosphere.

The transit method is more sensitive to the presence of clouds, while the secondary eclipse method provides more information about the temperature of the atmosphere.



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Mapping methods

Although it is impossible to directly observe the surface of an exoplanet, it is possible to measure the spatial variation of the atmosphere by two methods: phase curve analysis and secondary eclipse mapping.

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Diagram of a planet around its star and the light coming from the system according to its position (ESA).

The phase curve is the variation of the light of the star-planet system during a period of revolution. As the planet rotates around itself during its orbit, different sections of its atmosphere are successively visible to us. From this signal, it is possible to map the intensity of the light emitted by the planet in longitude. In the case of hot Jupiters, whose diurnal face is generally hotter, the maximum of the planet's light is found near the secondary eclipse. Similarly, the minimum of the curve is near the transit, since it is then the night side that is observed.

In secondary eclipse mapping, the day side of the exoplanet is resolved. As the planet moves behind its star from our viewpoint, sections of it are occulted, allowing us to isolate the light emitted by a given section of its atmosphere. By measuring the amount of light emitted by each individual section, it is then possible to map the daytime side of the atmosphere as a function of longitude and latitude.

The arrival of the James Webb Space Telescope

To date, phase curve analysis has been applied to several planets using space telescopes, such as the Hubble, Kepler and TESS space telescopes. Secondary eclipse mapping has only been applied to one exoplanet, the hot Jupiter HD189733 b, from observations with the Spitzer space telescope. However, these observations are usually made at a single wavelength and do not provide a complete picture of the atmospheric processes occurring on these exoplanets.

With a 6.5-meter mirror, compared to Hubble's 2.4-meter mirror, the Webb telescope will provide observations of unprecedented precision over a wide range of wavelengths. Four instruments, including Canada's NIRISS (Near-infrared Imager and Slitless Spectrograph), will observe in the infrared range and characterize the atmospheres of a multitude of exoplanets.

With the Webb telescope, it will be possible to apply our existing mapping methods to measure the three-dimensional variation of exoplanet atmospheres. These measurements will allow us to deepen our understanding of atmospheric processes.

As technology and instruments continue to advance, it may even be possible to map an Earth-like exoplanet in the future.