Encompassing Universe

March 8, 2011

The Earth’s crust is made mostly out of oxygen and silicon, but that need not be the case for terrestrial planets. Terrestrial planets can be iron-rich, carbon-rich, water-rich, or silicate-rich. As terrestrial Earth-type planets go, any planet with a significant amount of mass will accumulate an atmosphere, but if the planet gets too massive, it will take on too much atmosphere and become a gas giant more akin to Neptune or Uranus. If a planet is too small, it won’t accumulate much of an atmosphere at all and that will prevent liquid water from accumulating on the surface making the surface of the planet dry and frozen like Mars. A smaller planet will have its liquid outer core and mantle solidify faster, so volcanism and the planet’s magnetic field will shut down much quicker than on Earth. With no volcanism to replenish the atmosphere, no magnetic field to keep solar wind at bay, and a generally smaller gravitational field that can’t hold on to as much atmosphere, smaller planets are less habitable than Earth-mass planets and greater and aren’t habitable for as long, either.

A terrestrial planet more massive than Earth but less than about 10 times the mass of the Earth is considered a super earth. Anything more than 13 times the mass of the Earth would cause the planet’s gravity to hold on to too much gas and the thick envelope of a gas giant’s atmosphere would form. One astronomer suggested that a gas giant could be stripped of its atmosphere and may became a chthonian planet if a nearby massive star goes nova and tears the atmosphere off the planet, which you would expect to find in a galactic area with high-metallicity and many nearby aging stars. Super earths can be classified by their composition and internal structure and they come in two major varieties; water-rich and rocky super earths.

Iron-rich planets would form closer to the protoplanetary disk of the star they orbit, where metal content is highest. Planets rich in iron would cool quicker than silicate-based planets and that means volcanism, plate tectonics and a magnetic field would halt much sooner on a planet that cools that quickly. Mercury in our solar system is most similar to this; Mercury’s lighter silicate crust could have been boiled away, leaving behind the iron core, which makes up a greater proportion of the planet’s mass.

Our Sun has a carbon: oxygen ratio of about 0.5, so CO2 is common in the atmosphere of planets with silicate crusts. Bur for super earths that would accumulate much more carbon than a planet like Earth, there would be less CO2 in the atmosphere and the crust would be made predominantly of silicon carbide and graphite, and a layer of diamonds would be present deeper within the crust as graphite gets squeezed by heat and pressure to form diamonds. During volcanic eruptions, molten diamonds would gush from the volcano along with silicon carbide.

Planets covered by ocean are called water worlds, and because of the pressure of the atmosphere, this water would form a layer of ice VII over the entire surface of the planet. Ice VII is a truly alien form of water that would be crushed into a solid form at near-boiling temperatures. Water worlds resemble planets like Uranus or Neptune that would have migrated closer to their star and melted. These planets would be composed of a volatile content identical to the ice-bearing comets where their water would have come from. Rocky-type super earths might have the amount of water comparable to what one might find on Earth, but because the planet has a much bigger radius, oceans would straddle less of the planet’s surface, like it does on Areios. In fact, the amount of volatile content like water that gets captured by a planet might vary on an order of magnitude of about 1,000. This means that a planet could wind up with next to no water on its surface, or it might be flooded with water all over its surface. While a water world may be habitable to life, space faring intelligent life can’t arise on a water world because if a species can’t even build fire, these creatures certainly couldn’t discover rocketry, radio telescopes or even metallurgy. This means that unless we build a rocket and fly to one of these water worlds, we may never come in contact with an intelligence that dwells there.

A silicate-rich planet would resemble the terrestrial planets in our solar system; the crust would be made of silicon dioxide mostly, and plate tectonics would control the amount of carbon dioxide in the atmosphere by virtue of subduction. Areios is a silicate-rich planet like our Earth, but because of its more massive size, volcanism wipes the atmosphere clean of carbon dioxide just as fast as volcanoes can spurt it out. The same volcanic processes on Earth appear on Areios, but at a much faster pace. The crust on Areios is the same thickness as on earth, yet with a larger mantle and more gravity pushing down on the crust; plate tectonics operate in the same mechanism as they would on Earth, with the denser basalt plates getting driven beneath the lighter continental crust.

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