Exoplanets, planets that orbit stars other than the sun, are found at distances very 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.
To the naked eye, the planets in the solar system appear as bright spots. However, using a telescope, these dots stand out from 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 such phenomena is expected on exoplanets, their distance from the Earth prevents us from directly resolving their surfaces. Nevertheless, there are ways to learn more about the structure of their atmospheres and 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 Dec. 25, 2021, is expected to revolutionize the field of exoplanetary science.
Detecting and characterizing exoplanets
Apart from a few special cases where light from a planet can be observed directly, the majority of exoplanets are detected using 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 has led to the greatest number of exoplanet detections. A transit occurs when, from our perspective, an exoplanet passes in front of its host star. During the transit, the light from the star decreases as the star’s surface is partially obscured by the planet.
Light is divided into a spectrum of wavelengths that correspond to different colours. When a transit is observed at several wavelengths, it is possible to measure the atmospheric composition of the exoplanet. For example, water molecules strongly absorb light in the infrared wavelengths, making the planet appear larger, since its atmosphere blocks a larger fraction of the light from its star. In a similar way, it is also possible to measure the temperature of the atmosphere and to 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 secondary eclipse. By observing this, 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.
In general, the atmosphere of an exoplanet is considered a one-dimensional object when analyzing it. That is, its composition and temperature are considered to vary only with altitude and not with its position in longitude and latitude. To take these three dimensions into account simultaneously would require complex models as well as a high degree of observational accuracy. However, solely considering altitude may produce approximations that are not valid. On Earth, for example, the temperature at the equator is much higher than at the poles.
Some exoplanets also have strong spatial variation in their atmospheres. Hot Jupiters, similar in size to Jupiter, orbit very close to their host star and can thus reach temperatures of several thousand degrees Celsius.
In addition, these planets generally revolve around themselves at the same speed as they do around their star. This means that on these planets, a day and a year are the same length. In the same way that we can only see one side of the Moon from Earth, only one side of a hot Jupiter constantly faces its star. This phenomenon can lead to a large temperature difference between the day side, which is illuminated by the star, and the night side, which is perpetually in darkness.