The James Webb Space Telescope will map the atmospheres of exoplanets
Artist's depiction of some exoplanets discovered by NASA's Kepler mission. (Image source: NASA/JPL-Caltech)
Exoplanets orbit non-Solar stars very far from Earth. For example, proxima Centauri b, the closest exoplanet to Earth, is 4.2 light-years away, or light travels between the Sun and Earth repeatedly 265,000 times.
To the naked eye, the planets in the solar system appear as bright points of light. But if you use a telescope, the points of light will be extracted from the brilliant starry sky and reveal their own structure. Similar situations include Jupiter's Great Red Spot, Saturn's rings, and Martian ice.
Although the above phenomena may occur on exoplanets, the distance from Earth prevents humans from directly analyzing their surface layers. Either way, there are ways to learn more about the atmospheres of exoplanets and map them.
"I am a PhD in astrophysics at the University of Montreal. My work is related to describing the properties of exoplanet atmospheres. More specifically, my research focuses on advancing equipment for mapping exoplanets for the observatory work of the James Webb Space Telescope. ”
The telescope, launched on December 25, 2021, is expected to revolutionize the field of exoplanet science.
Detect and characterize exoplanets
With the exception of a few special cases where light from planets can be observed directly, most exoplanets are detected using indirect methods. Indirect methods include observing the effects of a planet's presence on the light emitted by its star.
The lingxing method is most common in exoplanet detection. From our point of view, a transit occurs when an exoplanet passes in front of its host star. During transit, light from the star decreases because the surface of the star is partially obscured by the planet.
Light is divided into wavelength spectra corresponding to different colors. When transits are observed at several wavelengths, it is possible to measure the atmospheric composition of exoplanets. For example, water molecules absorb infrared wavelengths of light sufficiently, making the planet appear larger because its atmosphere blocks most of the light coming from the star. In a similar way, it is also possible to measure atmospheric temperatures and detect the presence of clouds.
In addition, a transiting planet can also pass behind its star. This phenomenon of only observing light from stars is called subphasic. By looking at this, it may be possible to isolate light that comes only from a planet, thus obtaining additional information about its atmosphere.
The lingxing method is more sensitive to the presence of clouds, while the secondary eclipse law provides more information about the temperature of the atmosphere.
In general, when we analyze the atmospheres of exoplanets, they are considered one-dimensional objects. That is, its composition and temperature are thought to vary only with altitude, not with its position at longitude and latitude. Considering these three dimensions at the same time requires complex models and a high degree of observational accuracy. However, considering height alone may produce an invalid approximation. On Earth, for example, the equator is much warmer than the poles.
Some exoplanets also have strong spatial variations in their atmospheres. Hot Jupiters are similar in size to Jupiter, and their orbits are very close to their host star, so they can reach temperatures of several thousand degrees Celsius.
In addition, these planets typically rotate around themselves at the same speed as they do around stars. This means that on these planets, the length of a day and a year is the same. Just as we can only see one side of the moon from Earth, hot Jupiter has only one side facing its star all the time. This phenomenon can lead to a large temperature difference between the starry day and the night when it is always dark.
Drawing method
While it is not possible to directly observe the surface of an exoplanet, there are two methods that can be used to measure spatial changes in the atmosphere: phase curve analysis and secondary eclipse mapping.
The phase curve is the change in light from a star-planet system during rotation. Since the planet rotates in its orbit, we can see different parts of its atmosphere in turn. From this signal, it is possible to plot the intensity of the light emitted by the planet in longitude. In the case of hot Jupiters , its daytime days are usually hotter , with the maximum amount of light coming from the planet near the next eclipse. Similarly , the minimum value of the curve is near the transit star , since it was nighttime observations.
In the secondary eclipse mapping, the daytime side of the exoplanet is resolved. From our perspective, when the planet moves in and out from behind its star, some parts of it are hidden, allowing us to isolate the light emitted by a given part of its atmosphere. By measuring the amount of light emitted by each individual section, the daytime side of the atmosphere can be plotted according to longitude and latitude.
Arrival of the James Webb Space Telescope
To date, phase curve analysis has been applied to surveys of several planets with the help of space telescopes, including the Hubble, Kepler and Tess space telescopes. Through observations by the Spitzer Space Telescope, the secondary eclipse mapping was applied only to the survey of HD189733 b, an exoplanet hot Jupiter. However, these observations are usually made at a single wavelength and do not provide a complete picture of how the atmospheres in these exoplanets are moving.
Compared to Hubble's 2.4-meter mirror, the Webb telescope has a 6.5-meter mirror that will provide unprecedentedly accurate observations over a wide wavelength range. Four instruments, including Canada's NIRISS (Near-Infrared Imager and Slit-Free Spectrometer), will observe and characterize the atmospheres of a large number of exoplanets in the infrared range.
With the Webb telescope, we can apply the available mapping methods to measure three-dimensional changes in the atmosphere of exoplanets. These measurements will give us a better understanding of how the atmosphere works.
With the continuous advancement of technology and instrumentation, it is even possible to map the earth-like exoplanets in the future.
BY： Louis-Philippe Coulombe
FY：Astronomical volunteer team
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