Astronomers have observed Jupiter for centuries. But a study that looks at the gas giant as if it were an exoplanet could help to make more reliable interpretations of the atmospheres of bodies orbiting stars hundreds of light years away1. The results largely confirm the conventional picture of Jupiter, but also reveal some surprises — including clouds of ice crystals previously unheard of on the planet.
The hundreds of planets now known to orbit stars other than our own are almost never directly visible in telescopes. In a handful of cases, however, astronomers have been able to learn about their make-ups, by interpreting how starlight filters through their atmospheres as it skirts the planets while they cross between their parent stars and Earth.
But low resolution and experimental noise mean that such results, which are usually only possible for large planets orbiting bright stars, are often disputed2. “The models give us answers, but we don’t always know if we can believe those answers or not,” says Tyler Robinson, an astronomer at NASA’s Ames Research Center near Mountain View, California.
Now astrophysicist Pilar Montañés-Rodríguez at the Astrophysics Institute of the Canary Islands in Tenerife and her colleagues have devised a way to apply the idea to studying Jupiter. The technique used for exoplanet atmospheres does not immediately translate to Jupiter, because its orbit never takes it between Earth and the Sun. So, instead of looking directly at sunlight filtered through Jupiter’s atmosphere, the team analysed light reflected back from the Jovian moon Ganymede when Jupiter passed between it and the Sun — in other words, when the planet partially eclipsed the Sun as seen from Ganymede.
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