More than 3,000 exoplanets, which orbit stars other than our sun, are now known to exist. But just knowing that a planet exists is one thing – what we’d really like to know is what the conditions on that planet are like. Does it have an atmosphere, and does that atmosphere have weather?
We expect gas giants to have strong winds and violent storms, like the Great Red Spot on Jupiter. But to date, observing these things on a giant planet in another solar system has been impossible. That’s because detecting a planet’s atmosphere is very hard. We need bright targets, and even then the light from the parent star overwhelms that from the planet.
To study weather on an exoplanet we need not just to see the atmosphere, but how it changes over time. This has only been done once before, in relation to the “super-Earth” planet 55 Cancri e. Now we have for the first time observed the weather on a gas giant planet outside our own solar system. The results have been published in Nature Astronomy.
To study weather patterns on an exoplanet we can look at the light it reflects. Light from the parent star heats the planet, but some of it is reflected, either by the planet’s surface or its atmosphere – especially if the exoplanet is cloudy. On top of this the planet emits its own light, getting brighter the hotter it is. By observing the planet as it orbits, we can see changes in the planet’s light and so create a map of the brightness of the planet’s surface. If we observe multiple orbits of the planet, we can see how that brightness changes each orbit, and so work out how the planet’s atmosphere changes over time.
The weather on HAT-P-7b
We used NASA’s Kepler satellite to track such changes on HAT-P-7b, an exoplanet about 16 times larger than Earth and more than 1,000 light-years away in the constellation Cygnus. The gas giant – similar to Jupiter, but bigger – orbits very close to its star. In particular we looked at cloud formations coming and going for four entire years, giving us the most detailed determination yet of weather on an exoplanet.
NASA Ames/W Stenzel
HAT-P-7b is tidally locked: one side of the planet is always in daylight facing the star, with the other side facing the cold of space. This makes the day side much hotter than the night side. Strong temperature differences like this typically cause powerful winds, blowing around the planet in an equatorial jet.