Using Jupiter as an Alien World Analog

Owing to its size, fascinating chemistry and system of varied moons, Jupiter is one of the most studied planets in the solar system, though many mysteries remain. But a new study has taken a look at the gas giant from a whole different perspective — as an alien, living far beyond the solar system, would see it.

At first it may not seem obvious what a team of astrophysicists at the Astrophysics Institute of the Canary Islands in Tenerife are doing. As Jupiter passed between the sun and its largest moon Ganymede, the researchers, headed by Pilar Montañés-Rodríguez, studied the faint light reflecting off the moon’s surface.

When Jupiter blocks the sunlight from Ganymede’s perspective, some of that sunlight is filtered and scattered through the gas giant’s atmosphere. This scattered light therefore carries some information about what Jupiter’s atmosphere contains.

Now, if we were an alien in a neighboring star system and we saw Jupiter pass in front of the sun, we might want to zoom in on Jupiter to measure the scattered light so we can understand what Jupiter is made of. But from Earth’s perspective, Jupiter never passes between us and the sun, so we can never hope to study the scattered light passing through Jupiter’s upper atmosphere. (Indeed, the only planet with an atmosphere that passes between the sun and Earth is Venus, but the next Venus transit doesn’t occur until the year 2125.)

This is where Ganymede comes in.

During partial eclipse events, as Jupiter blocks sunlight from directly hitting Ganeymede, Montañés-Rodríguez’s team zoomed in on the moon and detected the very faint reflected light that had already passed through Jupiter’s atmosphere and bounced off Ganymede. They are basically using Ganymede as a reflector, allowing us a view of Jupiter we wouldn’t otherwise have ever seen.

Using the European Southern Observatory’s Very Large Telescope (VLT) in Paranal, Chile, and the William Herschel Telescope at the La Palma Observatory in Spain’s Canary Islands, the researchers were able to tease-out valuable spectroscopic information from this reflected light, building a picture of Jupiter’s atmospheric composition. And even though Jupiter may be pretty well studied, the team made an unexpected discovery about the solar system’s largest world.

Within the spectroscopic data appears to be some evidence of water ice, a factor that may prove controversial, says Montañés-Rodríguez, as Jupiter is thought to contain very little water. Perhaps this discovery suggests that cometary impacts on Jupiter deposited a layer of water ice that has, until now, evaded detection.

But key to this research is to use Jupiter as an analog exoplanet. As we know — more or less — what the planet is made of, its chemical composition and atmospheric stratification, the researchers hope to use the reflected light from Ganymede during Jovian eclipse events to build a profile of what information the scattered sunlight through a massive planet’s atmosphere contains. By comparing the Jupiter data with exoplanet transit events, we can better understand the signal of refracted and scattered starlight through these alien worlds’ atmospheres.

“It’s an extremely clever idea and spectacularly carried out,” said astronomer Sara Seager, of the Massachusetts Institute of Technology in Cambridge.

Read more at Discovery News