Astronomers have constructed the first “weather map” of the exoplanet WASP-127b, and the forecast there is brutal. Winds roar around its equator at speeds as high as 33 000 km/hr, far exceeding anything found in our own solar system. Its poles are cooler than the rest of its surface, though “cool” is a relative term on a planet where temperatures routinely exceed 1000 °C. And its atmosphere contains water vapour, so rain – albeit not in the form we’re accustomed to on Earth – can’t be ruled out.
Astronomers have been studying WASP-127b since its discovery in 2016. A gas giant exoplanet located over 500 light-years from Earth, it is slightly larger than Jupiter but much less dense, and it orbits its host – a G-type star like our own Sun – in just 4.18 Earth days. To probe its atmosphere, astronomers record the light transmitted as it passes in front of its host star according to our line of sight. During such passes, or transits, some starlight gets filtered though the planet’s upper atmosphere and is “imprinted” with the characteristic pattern of absorption lines found in the atoms and molecules present there.
Observing the planet during a transit event
On the night of 24/25 March 2022, astronomers used the CRyogenic InfraRed Echelle Spectrograph (CRIRES+) on the European Southern Observatory’s Very Large Telescope to observe WASP-127b at wavelengths of 1972‒2452 nm during a transit event lasting 6.6 hours. The data they collected show that the planet is home to supersonic winds travelling at speeds nearly six times faster than its own rotation – something that has never been observed before. By comparison, the fastest wind speeds measured in our solar system were on Neptune, where they top out at “just” 1800 km/hr, or 0.5 km/s.
Such strong winds – the fastest ever observed on a planet – would be hellish to experience. But for the astronomers, they were crucial for mapping WASP-127b’s weather.
“The light we measure still looks to us as if it all came from one point in space, because we cannot resolve the planet optically/spatially like we can do for planets in our own solar system,” explains Lisa Nortmann, an astronomer at the University of Göttingen, Germany and the lead author of a Astronomy and Astrophysics paper describing the measurements. However, Nortmann continues, “the unexpectedly fast velocities measured in this planet’s atmosphere have allowed us to investigate different regions on the planet, as it causes their signals to shift to different parts of the light spectrum. This meant we could reconstruct a rough weather map of the planet, even though we cannot resolve these different regions optically.”
The astronomers also used the transit data to study the composition of WASP-127b’s atmosphere. They detected both water vapour and carbon monoxide. In addition, they found that the temperature was lower at the planet’s poles than elsewhere.
Removing unwanted signals
According to Nortmann, one of the challenges in the study was removing signals from Earth’s atmosphere and WASP-127b’s host star so as to focus on the planet itself. She notes that the work will have implications for researchers working on theoretical models that aim to predict wind patterns on exoplanets.
“They will now have to try to see if their models can recreate the winds speeds we have observed,” she tells Physics World. “The results also really highlight that when we investigate this and other planets, we have to take the 3D structure of winds into account when interpreting our results.”
The astronomers say they are now planning further observations of WASP-127b to find out whether its weather patterns are stable or change over time. “We would also like to investigate molecules on the planet other than H2O and CO,” Nortmann says. “This could possibly allow us to probe the wind at different altitudes in the planet’s atmosphere and understand the conditions there even better.”
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