The planets in our solar system experience a wide variety of weather, from Jupiter’s famous Great Red Spot through Mars’s dust devils to Saturn’s hexagonal north polar storm. But planets around other stars are too distant for us to directly discern their short-term weather, such as changes in clouds or wind.
Now, David Armstrong at the University of Warwick, UK, and colleagues scrutinised four years of data from the Kepler satellite, and noticed that the brightness of a planet called HAT-P-7b changed over time.
“With this four-year timeline, you can really start to look in depth at these planets,” says Hannah Wakeford, who studies exoplanet atmospheres at the NASA Goddard Space Flight Center in Greenbelt, Maryland, and was not involved in the work. “Our full understanding of these planets and the clouds in their atmospheres is just beginning.”
The planet is about 40 per cent larger than Jupiter and is baked to a searing 2200 degrees kelvin (1927 °C), in part because it’s so close to its star – it completes an orbit every two days. Armstrong and colleagues found the brightest areas on the planet moved around with time, which they say is due to changes in cloud coverage around the world.
The planet is locked in position, so it shows the same side to the star, the way the moon always displays the same face to Earth. As a result, the planet’s day side is much hotter than its night side. Clouds could condense on the cooler night side, and the temperature difference would create winds that send the clouds streaming around the planet.
“The winds transport clouds from the night side, so the cloud bank stretches some way into the day side before finally evaporating,” Armstrong says. As the clouds evaporate, the planet absorbs more light and warms up, strengthening the winds.
More powerful telescopes like the James Webb Space Telescope and the European Space Agency’s PLATO telescope will be able to study those clouds, and potentially search for signs of life in exoplanet atmospheres. But Wakeford says there’s plenty of exoplanet meteorology to do in the meantime.
An exoplanet reflects and emits light as it orbits its host star, thus forming a distinctive phase curve. By observing this light, we can study the atmosphere and surface of distant planets. The planets in our Solar System show a wide range of atmospheric phenomena, including stable wind patterns, changing storms and evolving anomalies. Brown dwarfs also exhibit atmospheric variability. Such temporal variability in the atmosphere of a giant exoplanet has not yet been observed. HAT-P-7 b is an exoplanet with a known offset in the peak of its phase curve . The peak brightness repeatedly shifts from one side of the planet’s substellar point to the other. The variability occurs on a timescale of tens to hundreds of days. These shifts in brightness are indicative of variability in the planet’s atmosphere, and result from a changing balance of thermal emission and reflected flux from the planet’s dayside. We suggest that variation in wind speed in the planetary atmosphere, leading to variable cloud coverage on the dayside and a changing energy balance, is capable of explaining the observed variation.
HAT-P-7 b is a hot Jupiter of radius 1.4 R J that transits its host star with a period of 2.20 days. It is extremely hot, with a dayside brightness temperature of 2,860 K and an equilibrium temperature of 2,200 K6 . It was continuously observed for four years by the Kepler satellite at optical wavelengths. HAT-P-7 b has also been intensively observed at infrared wavelengths with the Spitzer satellite.