Water World or Diamond Planet

Figure 1: Earth and 55 Cancri e shown to scale.

55 Cancri e is a planet orbiting a Sun-like star 41 light years from Earth. This planet takes just 17 hours 41 minutes to orbit its host star and its distance from its host star is only 1/20th the distance of Mercury from the Sun. Being so close to its parent star, the dayside of 55 Cancri e is scorched to a temperature of over 2000 °C. 55 Cancri e is in a class of exoplanets known as “super-Earths”. It has about 8 times the mass of Earth and a diameter just over twice that of Earth. Knowing both the size and mass of 55 Cancri e allows the planet’s internal composition to be predicted. When 55 Cancri e is being plotted onto a mass-radius relationship diagram, it seems that the planet is too large for its mass to be made up of just rock material.

A study done by M. Gillon et al. (2012) suggests that 55 Cancri e has a rocky interior with a thick overlying envelop of water comprising ~30 percent of the planet’s total mass. Such a layer of water is expected to be a few thousand kilometres thick. Because of the stupendous temperatures and pressures, this ocean of water is neither liquid nor gas. Instead, the water is in a supercritical phase where distinct liquid and gas phases do not exist. Beginning from the edge-of-space, a descend down into the depths of this ocean will be unusual because an “ocean surface” will not be crossed. The density of supercritical water will simply increase with depth, from a gas-like density until it exceeds the density of liquid water on Earth, down within the great depths of the ocean. As a result, there will be no clear boundary between sea and sky for this ferociously hot ocean of supercritical water on 55 Cancri e.

Figure 2: 55 Cancri e as plotted in a mass-radius relationship diagram. Credit: M. Gillon et al. (2012).

Another study done by M. Nikku et al. (2012) reports that the mass and diameter of 55 Cancri e can be explained by a carbon-rich interior comprised of iron, carbon, silicon carbide and other silicates. Such a carbon-rich interior will not require the planet to have a thick water envelop to account for its size. The high pressures in the planet’s interior can crush carbon into diamond and this makes 55 Cancri e a candidate for a “diamond planet”. “This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” says lead researcher Nikku Madhusudhan, a postdoctoral researcher at Yale University.

Figure 3: An illustration of 55 Cancri e shows a surface of mostly graphite surrounding a thick layer of diamond. Credit: Haven Giguere, Yale University.

References:

1. M. Gillon et al. (2012), “Improved precision on the radius of the nearby super-Earth 55 Cnc e”, arXiv:1110.4783 [astro-ph.EP]

2. M. Nikku et al. (2012), “A Possible Carbon-rich Interior in Super-Earth 55 Cancri e”, arXiv:1210.2720 [astro-ph.EP]