OK, was just watching the Science channel on solar flares and it made me remember a question I wanted to ask.
Astronomers are finding super earth-like planets, with the goal of - not sure - perhaps proving that earth-like planets possibly with life exist in the universe.
But it's not enough to find an earth-sized planet in the Goldilocks zone, is it? It has to have an atmosphere of nitrogen/oxygen, liquid water, temperate temperatures, an ozone layer and according to this program, it has to have an iron core and a rotating magma sea to create a magnetic field to protect the planet from too much radiation from solar flares.
Isn't that correct?
Reason I'd been thinking about it as I was watching Star Wars the Phantom Menace and the planet Naboo is all but a paradise...But is doesn't have an iron core or plate tectonics or magma. The whole planet is hollow with an ocean you can traverse the planet through.
And I began to wonder if such a habitable planet was possible.
Anyone?
Liquid water is almost certainly a requirement for 'life as we know it'; Carbon is far and away the best candidate as the primary element for building complex molecules*, and the most interesting chemistry needs the mobility of a fluid, with the stability of low energy interactions; ruling out all states of matter other than liquid. So you need a solvent that can dissolve a lot of different compounds, and liquid water is the best candidate. I guess other solvents could work, but given the abundance of light elements in the universe, water is the best of the ones that are likely to be common. As a crude rule of thumb, you can have as much hydrogen as you like; and the rest of the elements are available in rapidly declining proportion to their mass, so Oxygen + 2 Hydrogen is a much more commonplace solvent than, for example, Carbon + 4 Chlorine, even before we look at the range of compounds that dissolve in each.
A Nitrogen/Oxygen atmosphere isn't a prerequisite for life; On Earth, we know that our atmosphere is the result of life acting on a primordial atmosphere that had almost zero free Oxygen, with almost all of the Oxygen tied up in Silicates, Water, and Carbon Dioxide (and the rest in other Oxygen containing cations like Carbonates and Sulphates). Free Oxygen is very reactive, and was very toxic to early life; The free Oxygen in the atmosphere first arose in two steps (known as the Great Oxygenation Event and the less well understood Neoproterozoic Oxygenation Event); these two events were biological in nature, and represented the evolution of sufficient Oxygen tolerance for photosynthetic organisms to survive exposure to high concentrations of their own waste-product. Life on Earth initially evolved beneath an Ammonia/Nitrogen/Carbon Dioxide/Water atmosphere, probably with plenty of Methane, Hydrogen Sulphide and other (to us) noxious compounds. These nasties couldn't survive the generation of large volumes of free Oxygen from Carbon Dioxide, and have long since been oxidised to Nitrates and Sulphates - or in the case of Methane, to Carbon Dioxide and Water. Life arose in highly reducing conditions; The current highly oxidising environment is an artefact of life itself - and may be a good way to detect life on exoplanets - If a planet can be shown to have an atmosphere containing free oxygen, it would be hard to explain this without hypothesising the presence of life.
Temperature is apparently a constraint only insofar as liquid water is only compatible with a fairly small range of temperatures and pressures; It seems likely that the lower end of the liquid water range is most suitable for life, as a wider range of complex molecules are stable at such temperatures, but even boiling water on Earth can harbour life, so it seems that life has adapted to the Earth's temperature, rather than the particular temperature of the Earth being a prerequisite for life. Likewise radiation/magnetic field/ozone issues; life can (and observably has) evolved to survive high radiation environments, so it seems more that the majority of life on earth can't tolerate boiling or hard radiation because it evolved in a world where those things were uncommon, rather than because those things are fundamentally inimical to any kind of life. Certainly an Ozone layer cannot be a prerequisite for life, as the earth didn't have one until after the Oxygenation of the atmosphere; However all life prior to that was aquatic, so an ozone layer (and therefore and Oxygen containing atmosphere) might perhaps be a requirement for terrestrial (as opposed to aquatic) life.
The other big question that remains open is whether a large Moon is a prerequisite for (or major booster to the chances of) life. Having our big moon tends to stabilise the Earth, leading to fewer and less marked shifts in the axis of rotation relative to the plane of the ecliptic; this leads to long periods of relative stability in climate, which may be essential for life to get started. As we only have one datum, it is very hard to say whether this (or any other) characteristic is necessary for life.
The huge number of exoplanets found so far, and the significant numbers of those that are in the 'goldilocks zones' of other stars, render it highly likely that other planets very similar to Earth exist in huge numbers in our galaxy alone. It seems very unreasonable to assume that life is unique to Earth; but the galaxy is a big place, and right now we can only look very locally, so it's not surprising we have yet to see other life. The odds of intelligent life, close enough for us to communicate within a human lifespan, seem very remote; but the size of the galaxy makes other intelligent life somewhere out there seem to me to be almost certain.
A planet with liquid water seems very likely to support life, without any other factors being necessary (although we really don't know for sure if any other things are needed, given our current sample size of 'one'). If we could detect a planet with free oxygen in its atmosphere, that would be good enough, at least for for me, to be very confident indeed that other life certainly exists - not because Oxygen is necessary for life, but because life is almost certainly necessary for atmospheric free Oxygen.
* Carbon has a valance of four, which makes it great for building big, branching chains with other interesting stuff tacked on; Silicon, which also has a valence of four, is less good for this purpose, for several reasons - it doesn't exhibit p-Shell bonding, so it is less 'versatile'; and it is heavier, and therefore less abundant in the universe as a whole - both effects render Silicon likely to be less abundant and less chemically available at a planetary surface than Carbon; Carbonates are more soluble in water than Silicates, and Silicon Oxides are solids in conditions where water is liquid, while Carbon Oxides are (soluble) gasses. The Sci-fi staple of 'Silicon based life' is a better option than any other element as an alternative to carbon, but it's really not a very good option, for a huge number of reasons. Lots of aliens might be out there, but the Horta don't seem like it ought'a.
Solar_eclipse#Final_totality. Was thinking about how the moon was a pretty convenient size for GR testing.
