Earth’s Nearest Exoplanet, Proxima Centauri b, May Be a Water World Too

Proxima b, Time for New Artist's Impression Credit: ESO

Proxima b, Time for New Artist’s Impression
Credit: ESO

In August, the European Southern Observatory announced the discovery of an Earth sized planet orbiting in the habitable, or Goldilocks Zone, of the red dwarf star Proxima Centauri. Given its location just 4.22 light years away, the newfound world is as close to Earth as a potentially life-supporting planet could plausibly be, raising interesting philosophical questions as well as making an obvious target for future exploration, both robotic and perhaps one day crewed.

Regardless of who or what gets there first, it could be a wet landing. According to a new study released by CNRS, the Marseille Astrophysics Laboratory, computer modeling suggests the planet, which orbits very close to its parent star, may consist of a rocky core encircled by a global ocean some 200 km deep.

From the press release (translated from French)

“The study shows that Proxima b has a minimum radius of 5990 km, and the only way to get this value is to have a very dense planet, consisting of a metal core with a mass equal to 65% of the the planet, the rest being rocky mantle (formed silicate) until this surface. The boundary between these two materials is then located about 1500 km depth. With such a composition, Proxima b is very close to the planet Mercury, which also has a very solid metal core. This first case does not exclude the presence of water on the surface of the planet, as on Earth where the water body does not exceed 0.05% of the mass of the planet. In contrast, Proxima b can also have a radius of 8920 km, provided that it is composed of 50% of rocks surrounded by 50% water. In this case, Proxima b would be covered by a single liquid ocean 200 kilometers deep. Below, the pressure would be so strong that liquid water would turn high-pressure ice before reaching the boundary with the mantle to 3100 km depth. In these extreme cases, a thin gas atmosphere could cover the planet, as on Earth, making Proxima b potentially habitable.”

Clearly, there is a wide degree of latitude in the possible forms Proxima b could take, but it is a mystery which may not last for too long. The planet is rapidly emerging as one of the most desirable targets for imaging by the James Webb Space Telescope when it launches in two years. While JWST is an infrared telescope that will operate outside the bounds of visible light, researchers believe it may still be able to pull the cover off Proxima b by revealing whether or not it has an atmosphere which is conveying heat from one side of the planet to the other. Given its orbit at .05 AU, or 1/10 the distance between Mercury and the Sun, it is almost certainly tidally locked, meaning it has a “hot” and “cold” side. A regular fluctuation in output would be consistent with an atmosphere, and possibly an ocean.

In the mid 2020’s, a new generation of Earth-based Extremely Large Telescopes with apertures greater than 30 meters will be coming online, providing what will be the first opportunity to directly image the planet which may one day provide out first stepping stone to the stars.

Posted in: Exo Planets

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