An 'Earth-like' world
The latest research suggests that "Earth-like" planets are more common in our galaxy than we thought.
Since our galaxy is not particularly unique, this would suggest that other galaxies contain lots of Earth-like planets too. However, what exactly do we mean by Earth-like? The honest answer is a world on which we might expect life of a kind we can relate to can exist. We also want evidence of that life to be detectable from here.
A key condition for being Earth-like is for the temperature and air pressure on the surface of the planet to be such that water can exist in all its three forms: vapour, liquid, and solid. The circulation of water and its moving to and fro among its three forms on Earth is key to the existence of life as we know it. On Earth, the "water cycle" keeps our world habitable.
Maintaining such conditions in the long term requires a planet to be massive enough for its gravity to hold down its atmosphere, stopping it diffusing off into space. In addition, it needs a liquid iron core. Flows in this core generate a magnetic field. All stars produce their own versions of the solar wind, a continuous outward flow of particles and magnetic fields, flowing at speeds of hundreds to thousands of kilometres a second. If this wind hits a planet's atmosphere, it gradually scours it away. A strong magnetic field holds this wind away from the atmosphere. This is partly why Earth, protected by its strong magnetic field, still has a thick atmosphere. Mars is a smaller world. Its core has solidified and its magnetic field has disappeared, allowing the solar wind to reach the top of the atmosphere, scraping most of it away. Martian weather is now largely limited to sandstorms, dust devils and occasional dustings of frost on the rocks in the morning.
For life as we know it, just any liquid won't do. Liquid water is essential. Water molecules consist of two atoms of hydrogen and one of oxygen. The electrons in these atoms interact in a way that leaves a bit of a positive charge on one end and a bit of a negative charge on the other, so that they attract each other. In addition, some of the water molecules break up into ions - atoms that have lost an electron or two, or picked up one or two that they should not normally have. The result is a suitable environment for a wider range of different chemical processes than almost any other liquid. We, along with all other life forms on our world, are chemical organisms, with all those complex chemical reactions that give us life taking place in water. If you take a sample of blood and remove all the corpuscles and organic chemicals, what's left is basically seawater. This is a strong hint that life on Earth began in the sea. The ancient oceans provided the environment in which life really got going. We might not see surface oceans on worlds orbiting other stars, but we can search for water vapour in their atmospheres.
Life might have got going a billion years or less after the Earth formed, some 4.5 billion years ago, but for most of that time it was mostly single-celled. It was only 500 million years ago that life proliferated and got more sophisticated, starting on the road to what we see around us today. This suggests the best candidates for supporting life as we know it are yellow and orange stars, which have a stable maturity long enough for life to appear and evolve to advanced forms.
One thing that makes Earth particularly unique among planets we have detected and studied so far is its oxygen atmosphere. This gas is highly reactive and a key element in our metabolism. However, it was a by-product of plant activity. A young, Earthlike planet might not yet have got to the point where its living things have changed its atmosphere to one containing a lot of oxygen.
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• Jupiter and Saturn rise before midnight. Jupiter is the bright one.
• Venus lies low in the sunset glow.
• The moon will be full on the 23rd.
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