Lava planets

Earth-like planets that orbit too close to their stars are probably completely covered in lava.

Artist's impression of a lava planet
Artist’s impression of a lava planet
ESO/L. Calçada, CC BY 4.0, via Wikimedia Commons

Astronomers have hypothesized about exoplanets – planets outside our solar system – for a long time. But the first ever definitive confirmation of an exoplanet came just thirty years ago. Since then, we’ve detected more than five thousand such planets. Based on their orbits and detectable physical characteristics, we can infer a lot… but we still know very little about them.

Take TrES-2b, for example. Discovered in 2006, this exoplanet is larger than Jupiter and darker than coal. I mean that literally – less than one percent of the light that hits its surface is reflected back. And we don’t know how or why.

When CoRoT-7b was detected in 2009, it was the smallest exoplanet yet found, less than twice the diameter of Earth. It’s close to its sun, absurdly close, and that means it orbits faster than any previously detected exoplanets: CoRoT-7b has a 20-hour year.

Put all of these characteristics together, and astronomers came to an interesting conclusion. Although it’s most likely a big ball of rock, like Earth, the sheer heat combined with the tidal forces of its star would turn the surface of this planet into an ocean of molten magma. CoRoT-7b is probably a lava planet.

How did this happen? Well, CoRoT-7b could be just a plain old terrestrial planet like ours, but caught in a devastating orbit. There’s another possibility, though. This planet could have once been a gas giant, like Jupiter or Saturn.

The technical term for this kind of exoplanet is a chthonian planet. (“Chthonian,” by the way, is my favourite adjective. So evocative!) Gas giants are thought to have solid metallic or rocky cores. If a gas giant gets too close to a star, its atmosphere is stripped off by solar winds. The solid core is all that remains – and that may be what happened to CoRoT-7b. To quote Byron, it becomes the burning wreck of a demolished world, a wandering hell in the eternal space.

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