Lava Planet

In Exoplanets by Brian Koberlein0 Comments

When it comes to exoplanetary systems, the 55 Cancri system is one we understand fairly well. The star is similar in mass to our Sun, and has a Jupiter-type planet at roughly the same distance as our giant gas planet. But there the similarities stop, because 55 Cancri also has three other gas planets, all closer to the star than Venus is to the Sun. Two of them are closer than Mercury. If that’s n0t surprising enough, there is also a super-Earth even closer to the star. Known as 55 Cancri e, this terrestrial world orbits at a distance less than 1/20th that of Mercury. It is a very strange world indeed. 

Since the planet is so close to its star, it is tidally locked. In other words, it rotates on its axis at the same rate that it orbits the star, so that the same side of the planet always faces the star. Our own Moon is tidally locked to the Earth, which is why we always see the same features.

At such a close distance, you might imagine the planet would be stripped of any atmosphere, and yet studies have found it actually has an atmosphere. It’s a toxic atmosphere with significant amounts of hydrogen cyanide (HCN). It likely has a crust of graphite, diamond and carbonate minerals. But despite its atmosphere, the side facing the star is always much hotter than the side away from the star. Recent observations have shown it truly is a planet of extremes.

Map of the light emitted by 55 Cancri e, from which the surface temperature can be determined. Credit: Demory, et al.

Map of the light emitted by 55 Cancri e, from which the surface temperature can be determined. Credit: Demory, et al.

A team recently mapped the surface temperature of the planet, and found the dark side of the planet has a surface temperature of about 1,300 K, while the star-facing side has a temperature of about 2,700 K. Even the “cool” side of the planet is hotter than the surface of any planet in our solar system, but the hot side is so hot that the surface is likely molten. The planet is likely a dual world, with a molten side facing the star and a hot but solid dark side. Clearly its atmosphere isn’t able to spread out the heat to a more uniform temperature.

It’s a good example of why “super-Earth” is a bit of a misleading term. Though this particular world is only about twice the size of our own planet, it’s surface and atmosphere are nothing like our comfortable home.

Paper: Brice-Olivier Demory, et al. A map of the large day–night temperature gradient of a super-Earth exoplanet. Nature doi:10.1038/nature17169 (2016)

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