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ALIENS are surely AMONG US: Average star has TWO potentially Earth-like worlds

Simon Brown

Two earth-like worlds? Not sure how they define that... my prediction is that evolved life in the universe will be far rarer than a lot of people are suggesting for two reasons that make life in THIS solar system a lot more likely.

All the goldilocks zone does is give you liquid water. That's a start but it's only a start. You also need a magnetosphere that's sufficiently powerful enough to prevent your host star from frying the planet. You need a large moon for several functions. It acts as a galactic vacuum cleaner, protecting the biosphere for 4.5bn years.

Firstly in this solar system we have large gas giants in long orbits. That's pretty rare. In most of the extrasolar systems we've observed so far, big gas giants tend to be pretty close in - and they're very obvious when they transit. With this solar system they're a long way out. This means that rocky planets inside the system are able to survive without being swept up by a gas giant AND it means that anything coming in from the edge or outside the solar system (comets, asteroids) along the plane of the system, is likely to hit the gravitational field of one of the gas giants first or at least at some point during its orbit of the sun. Over the space of billions of years, this gives Jupiter, Saturn, Uranus and Neptune the chance to swallow up most of the big stuff that gets in and doesn't go straight into the sun.

Secondly right at the beginning of the solar system, before the current orbits had settled and before the gas giants had much chance to clean up, a Mars-sized planet collided with the proto-earth. In doing so it will have vaporised and caused most of the earth to melt (again). Most of the combined material that WASN'T ejected into space became the earth but a substantial blob of the combined combined material became the moon. The moon is ENORMOUS - for the earth. Satellites THAT big around a planet this small are impossible under normal planet-forming conditions. The ONLY way to form a satellite that big is to have the collision we had.

The moon plays a number of interesting roles, not just in the mixing of surface liquids on earth. As it swings round the earth every 28 days or so, over the space of millions and millions of years, it creates tidal movement within the fluids of the earth's mantle which are thought to have been part of the process which triggered the start of plate tectonics - the other part of the process is the role of water which at high temperatures and pressures acts as a lubricant. It is this water coming to the surface that has allowed life on earth to begin - in waters that were mixed by the movement of the moon, acting like a cosmic wooden spoon, mixing the ingredients back and forth - the simple carbon, nitrogen, sulphur, oxygen, hydrogen, iron molecules, heating them up, cooling them down, mixing them around.

Without the gravitational tidal action of the moon, none of that can happen. It's also quite possible that the gravitational action of the moon plays a role in helping to establish the stability of the earth's electromagnetic field. Remember that without the EM field, we all fry. So what we need to be looking for, to find evolved life outside the solar system, is a very stable main-sequence star with the capacity to last for 10 bn years, a rocky planet in the goldilocks zone, with an abnormally large moon due to some random process early in its lifespan, a strong and stable electromagnetic field and large (but not too large) gas giants on the outer edges of that solar system. Simples.

Alternatively viable conditions might exist on satellites around a gas giant in orbit around a main sequence star but the planet would get bathed with regular sweeps of EM blasts from the gas giant so that might effectively sterilize the planet.

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