The James Webb Space Telescope’s mirror segments are aligned and its scientific instruments are undergoing calibration. Currently, it is just weeks away from full operation. NASA plans to reveal the first observations this summer, after which, Webb will begin with in-depth scientific observations. Among the investigations that are planned for the telescope’s first year are two exoplanets classified as “super-Earths”: the lava covered 55 Cancri e and the atmosphere-less LHS 3844 b.
They are classified as super-Earths due to their size and rocky composition. Scientists will train Webb’s high-precision spectrographs on these two planets to understand more about the geologic diversity of planets across the galaxy, and also to understand the evolution of rocky planets like the one we live on.
55 Cancri e: Super-hot super-Earth
55 Cancri e is an exoplanet that orbits less than 1.5 million miles from its star, which is 4 per cent of the distance between Mercury and the sun. This means that the planet completes an entire revolution around its star in less than 18 hours. Basically, a year on 55 Cancri e is equivalent to 18 Earth hours.
Such planets which orbit so close to their stars are assumed to be tidally locked; with one side facing the star at all times. This should mean that the hottest spot on the planet should be the one that is constantly facing the star; and that the amount of heat coming from the dayside should not change much over time.
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But curiously, this doesn’t seem to be the case with 55 Cancri e: observations of the planet made by NASA’s Spitzer Space Telescope suggest that the hottest region on the exoplanet is offset from its dayside. It also shows that the total amount of heat detected from the dayside varies.
Scientists have come up with multiple explanations for this: One is that the planet has a dynamic atmosphere that moves heat around. This atmosphere could be a thick one dominated by oxygen or nitrogen, according to scientists who will be using Webb’s near-infrared camera (NIRCam) and Mid-Infrared Instrument (MIRI) to capture the thermal emission spectrum of the dayside of the planet. If the planet does have an atmosphere, Webb should have the sensitivity and wavelength range to detect it.
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Another intriguing explanation is that 55 Cancri e is not tidally locked and that instead, it may be like Mercury; rotating three times for every two orbits, giving the planet a day-night cycle. This could also explain why the hottest part of the planet is shifted; just like on Earth, it could take time for a surface to heat up and the hottest time of the day would be in the afternoon and not right at noon. If that is true, researchers plan to test the hypothesis by using NIRCam to measure the heat emitted from the lit side of 55 Cancri e during four different orbits.
LHS 3844 b: Literally cooler
Unlike 55 Cancri e, LHS 3844 b will offer a unique opportunity to analyse solid rock on an exoplanet surface. But just like the former, LHS 3844 b orbits extremely close to its star; completing a full orbit in 11 hours. But since its star is relatively small and cool, the exoplanet’s surface is not hot enough for the surface to be molten. Spitzer observations indicate that the planet is very unlikely to have a substantial atmosphere.
It is not possible to image the surface of LHS 3844 b directly with Webb but it is possible to study its surface with spectroscopy due to the lack of an obscuring atmosphere. Just like how our eyes can see the difference in colour between rocks due to the visible light they reflect, there are similar differences in the infrared light that rocks give off.
Researchers will use Webb’s Mid-Infrared Instrument (MIRI) to capture the thermal emission spectrum of the dayside of LHS 3844 b to compare it to spectra of known rocks so that its composition can be determined. The spectrum could