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.
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.
Because the night side of the planet is colder, we expect clouds to form there but rapidly evaporate when they reach the hot day side. But that’s not the whole story. The strong winds can transport the clouds, moving a bank of them to the “morning” part on the day side, where they stay for a while before evaporating. Clouds reflect light, so they make this region of the planet absorb less energy from the sun and cool down. A cooler day-side means weaker winds, and so less clouds moving there. But with fewer clouds, the planet can absorb more energy, allowing the day side to heat up and the winds to transport clouds again – forming a weather cycle.
When the winds are strong, the planet is brightest where the clouds are, and we see mostly reflected light. When the clouds are gone and the winds are weak, they still transport heat, meaning the brightest point is on the opposite side and we see mostly light emitted from the planet. By tracking changes in the brightest point on the planet we were able to see the winds changing and so observe weather in an exoplanet’s atmosphere.
But these clouds are like nothing seen on Earth. When we talk about temperature on a planet like HAT-P-7b, hot means 2,100°C and cold about 1,300°C. What could form a cloud that only evaporates in the middle “morning side”, at 1,700°C – hot enough to melt iron? While we don’t know for sure what the clouds are made of, one plausible answer is corundum. Corundum is a mineral better known as ruby or sapphire, so perhaps clouds of ruby are moving around this planet, appearing and disappearing in a stunning display – from a distance.
Finding more examples will be a goal of future satellite missions such as NASA’s James Webb space telescope and ESA’s PLATO. Our study helps tell these missions know what to look for, although we will certainly find the unexpected too.
Planets are diverse: we know of Earth-like planets, super-Earths, Neptunes and Jupiters, many at temperatures far exceeding anything in the solar system, and in planetary systems unlike anything we’ve seen before. Exoplanet atmospheres will be just as diverse, and a whole range of atmospheres and weather are waiting to be uncovered. While we studied a hot giant planet here, the same techniques may one day be used to look at planets like the Earth. To find out if such a planet is habitable, we need to understand its atmosphere and better yet, its weather.