Free Astronomy Magazine May-June 2021

23 MAY-JUNE 2021 ASTRO PUBLISHING T his plot shows the spectrum of the atmosphere of an Earth sized rocky exoplanet, GJ 1132 b, which is overlaid on an artist’s impression of the planet. The orange line represents the model spectrum. In comparison, the observed spectrum is shown as blue dots representing averaged data points, along with their error bars. This analysis is consistent with GJ 1132 b being predominantly a hydrogen atmosphere with a mix of methane and hydrogen cyanide. The planet also has aerosols which cause scattering of light. This is the first time a so-called “secondary atmosphere,” which was replenished after the planet lost its primordial atmosphere, has been detected on a world outside of our solar system. [NASA, ESA, and P. Jeffries (STScI)] Swain. “This system is special be- cause it has the opportunity for quite a lot of tidal heating.” The phenomenon of tidal heating occurs through friction, when en- ergy from a planet’s orbit and rota- tion is dispersed as heat inside the planet. GJ 1132 b is in an elliptical orbit, and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. At least one other planet in the host star’s system also exerts a gravita- tional pull on the planet. The conse- quences are that the planet is squeezed or stretched by this gravi- tational “pumping.” That tidal heat- ing keeps the mantle liquid for a long time. A nearby example in our own Solar System is the Jovian moon, Io, which has continuous volcanism as a result of a tidal tug-of-war be- tween Jupiter and the neighbouring Jovian moons. The team believes the crust of GJ 1132 b is extremely thin, perhaps only hundreds of feet thick. That’s much too feeble to support any- thing resembling volcanic moun- tains. Its flat terrain may also be cracked like an eggshell by tidal flex- ing. Hydrogen and other gases could be released through such cracks. “This atmosphere, if it’s thin — meaning if it has a surface pressure similar to Earth — probably means you can see right down to the ground at infrared wavelengths. That means that if astronomers use the James Webb Space Telescope to observe this planet, there’s a possi- bility that they will see not the spec- trum of the atmosphere, but rather the spectrum of the surface,” ex- plained Swain. “And if there are magma pools or volcanism going on, those areas will be hotter. That will generate more emission, and so they’ll potentially be looking at the actual geological activity — which is exciting!” “This result is significant because it gives exoplanet scientists a way to figure out something about a planet’s geology from its atmos- phere,” added Rimmer. “It is also im- portant for understanding where the rocky planets in our own Solar System—Mercury, Venus, Earth and Mars, fit into the bigger picture of comparative planetology, in terms of the availability of hydrogen ver- sus oxygen in the atmosphere.” !