Imagine a Sunset on a Hot Exomoon

The entire universe in blog form
March 26 2014 7:45 AM

Spectacular Video: The Sky on Fire

Sunset on Perth ... or perhaps an alien world.

Photo by Colin Legg, from the video

We’ve been discovering exoplanets left and right the past few years, and many orbit their stars very close in, closer than Mercury orbits the Sun. These planets tend to be “hot Jupiters,”, as massive as Jupiter or more.

Those planets are certainly uninhabitable … but if any have moons, and alien life exists there, then I would imagine this video by Colin Legg is what sunset must look like for these overly warm creatures.


Legg filmed this on a beach in Perth, Australia, facing west (duh) where he could see the Rottnest Island lighthouse 23 kilometers (14 miles) away using his telescope and video camera setup. As you look to the horizon a number of optical effects come into play, as the layers and turbulence in Earth’s air distorts the image of the Sun. It gets flattened, for one, squashed like someone sitting on a beachball. The colors at the top of the Sun can also be separated, generating flashes of green and blue that are difficult to observe and treasured by photographers and scientists alike.

It’s fun to wonder if this scene is also possible on alien worlds … but sadly, my scenario involving moons of hot Jupiters is unlikely. A planet that close to its star would have a hard time holding on to its moon; over time the effects of the star’s gravity could likely pry the satellite loose from the planet. It depends on the size of the planet, the star, and how far the moon is from the planet. Even then, the close proximity of the star would probably fry the moon clean of the potential for life.

Still, it’s fun to think about. And if the planet is farther out from its star, then an Earth-like exomoon isn’t so far-fetched. It would be lovely to find one, even if the sunsets wouldn’t be quite so spectacular as in Legg’s video. But just knowing such a world existed would be an ample trade-off.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


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