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Super-Earth Properties

Imagine a planet that is larger and more massive than Earth but smaller than Uranus. Would a planet like this have deep water oceans - a true water world - or would it be a dry planet with huge volcanoes billowing smoke high into a thin atmosphere? Our Solar System offers no clues. Uranus is almost 15 times more massive than Earth, which is a huge jump in mass. The gap separates the rocky planets and the gas giants into two completely different types of planets.

The existence of Super-Earth planets was first proposed around the year 2000. The first Super-Earth was discovered just five years later, in 2005. The working definition of a Super-Earth is a planet that has a mass between 1 and 10 times the Earth’s mass. NASA's Kepler team adopted this mass range, and further defined the size of a Super-Earth as having a radius smaller than 2.0 times the Earth's radius. This definition reflects our current knowledge about the physical properties of planets in this size range. Continuing exploration and discovery will help us to refine the definition of a Super Earth.

Super-Earths are likely composed of rocks and water, with a range of possible water fractions. Perhaps they have continents, oceans and atmospheres. We imagine them to be a large and diverse family, with different compositions and environments. Earth is a junior member of that family. Now we know, thanks to the NASA Kepler mission, that Super-Earths are among the most numerous type of planet in the universe!

The percentage of water on Super Earths varies. The evidence suggests that many of them contain vast amounts of water - up to 50-70% by mass. For comparison, all the water on Earth is barely 0.02% by mass! Such water worlds (see previous section on Composition) would be still solid planets in bulk, because of the curious properties of H2O under high pressure. As we know from the beginning of this course, H2O is a polar molecule - hydrogen bonds form between the positively charged hydrogen ends and the negatively charged oxygen ends. Each hydrogen bond is weak, but many together form strong structures, like in the case of low temperature when common water ice forms. Under high pressure the molecules are forced closer, the bond angles are bent, rings of molecules interpenetrate - the structure becomes denser than liquid H2O and solid as ice, even at very high temperatures, say of 1000 K.

On Earth we know of these exotic forms of water only from the lab, where we can make those "ices," also known as Ice VII and Ice X. These solid forms of ices require very high pressures -the pressure at the bottom of Earth's oceans is not nearly high enough to compress it so. In fact, the ocean would have to be almost 10 times deeper than the deepest points on Earth today for liquid water to turn into Ice VII. However, a typical Super-Earth with 10% or more water by mass would have an ocean deeper than 100 km and at that depth the bottom of the ocean would be solid water (Ice VII). The water Ice VII is a cubic crystal with two interpenetrating lattices and is about 65% denser than liquid water. Unlike common water ice, Ice VII does not float on water, though it is still less dense than rock. The Super-Earth would still have a rocky core well beneath the solid water, and gases like CO2 and CH4 would still be expected to slowly make their way up, through the solid, but dynamic and malleable, water mantle. Eventually these gases would emerge into the atmosphere.

The Super Earth GJ1214b, provides evidence that some water world contain significant amounts of light gases above the thick layer of water. Super-Earth often have enough mass to muster the gravitational pull needed to keep their atmospheres from evaporating away (see interactive tool in section on Atmospheres). On occasion a Super-Earth orbits so close to its star, like planet CoRoT-7b, that even its high mass is not high enough to keep any gases from

blowing off, due to the very intense stellar heat. But as a general rule, and in orbits away from the star, we expect Super-Earths to have atmospheres.

The bulk composition of these Super-Earth planets could still be very exotic. We know of rocky Super-Earths that must be richer in iron (and heavy metals like iron) than our Earth or neighboring rocky planets Venus and Mars. We call them Super-Mercuries, because they resemble the bulk composition of planet Mercury with a core of almost 70% iron (as opposed to 30% by mass for Earth). Others might be Super-Moons, similar to Earth's Moon which does not have a noticeable iron core.

Super-Earths are a diverse family that we are still exploring, and many surprises are probably awaiting us.

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