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Why Super-Earths

Is Earth the ideal planet for life - in mass and in size? From all we know today, super-Earths are better. Earth is simply at the small end of the family of super-Earths. If confirmed by future exploration, this fact is great news because it points to many more places with habitable potential in our Galaxy.

What makes super-Earths so suitable for life to emerge and sustain itself?

First, by being more massive than Earth, super-Earths have an increased gravitational pull which makes it easier to retain their atmospheres and keep water from evaporating. Everything else being equal, small planets like Mars, have a narrow window of opportunity when it comes to retaining an atmosphere. Review the previous section on Atmospheres and the web tool for atmospheric loss.

Without an atmosphere dense enough to allow water to be in a liquid phase, even a warmer planet than Mars will be inhospitable to any form of life based on molecules that require water. While it is difficult to establish a precise cutoff, it is clear that planets slightly smaller than Earth are vulnerable to massive evaporation and other drastic changes in their atmospheres, especially when their orbit is closer to the inner edge of the habitable zone. At the same time, there are no known restrictions on atmospheric stability when the mass and radius of a planet exceed that of Earth.

Tectonic activity is the second factor that makes super-Earths more prone to developing an atmosphere. Rocky planets are dynamic, despite their short-term appearance of static durability. Volcanoes erupt releasing gases, tectonic plates break up the crust, move about and differentiate and recycle surface minerals and volatile compounds. Mars has ample evidence of volcanic activity early in its history - in fact, it is the planet with the largest volcano in the Solar System. However, unlike Earth, Mars cooled very quickly, and because it never had moving tectonic plates, its crust became so thick that even the largest volcanoes became extinct.

A planet more massive than Mars will cool more slowly, and by most accounts, will perpetuate a long-lasting cycle of plate tectonic activity. Earth's crust is very thin, proportionally similar to the skin on an apple. We expect that the crusts of super-Earths are relatively thin for the duration of their existence over billions of years, allowing them to continue tectonically activity.

Gases replenishing the atmosphere, subduction of crustal plates and the recycling of minerals inside the planet's mantle all make the geochemistry of a planet dynamic, but also relatively stable. Under such conditions geochemical cycles often dominate the exchange of gases and compounds and establish long-term stability and surface conditions, including climate.

For example, on Earth this exchange occurs through the global CO2 cycle, also known as the "carbonate-silicate cycle". This cycle is independent of life, though life has long ago become an active and important part of it. On a lifeless planet the CO2 cycle begins with volcanoes and vents releasing CO2 gas into the atmosphere from inside the mantle. In the atmosphere the gas is absorbed into water droplets which rain down upon the rocky surface, eroding silicate rocks and soils and depositing their sediments into the oceans where carbonate rocks, such as limestone, are formed. Tectonic plate activity brings the seafloor rocks back into the mantle, where they become molten, mixed, and the gas is recycled back into the atmosphere.

The CO2 cycle does more than just recycling gas and minerals - it acts as a global "thermostat" for the planet's surface and makes the climate and surface temperature relatively stable over millions and even billions of years. This is accomplished through a feedback mechanism: if the global atmospheric temperature gets too high, increased evaporation increases atmospheric humidity and rain then removes more of the greenhouse CO2 gas from the atmosphere, allowing the atmospheric temperature to drop (less greenhouse warming); if atmospheric temperature drops too low, reduced evaporation lowers atmospheric humidity, allowing more CO2 to remain in the atmosphere, causing the greenhouse effect to warm up the atmosphere. This thermostat cycle occurs slowly, with a typical timescale of half-a-million years, but it has kept Earth safe from deleterious climate swings over its history. This process should work even better on super-Earths.

Being smaller, Earth is vulnerable to any number of cosmic accidents and drastic changes. Super-Earths offer more stability, combined with dynamic and renewing surface conditions and chemistry. What little we know of life's emergence and nature, such environments seem best fit for it.

Солнечная система и ее тайны