Venus may have had life, but it has a runaway greenhouse atmosphere of carbon dioxide and nitric acid, with surface temperatures hot enough to melt lead. Some have proposed to inject blue-green algae into the air, which would metabolize the CO2 into oxygen and water, dropping the temperature and making it rain for the first time. After some period of time, Venus might become habitable for us.
Mars is too small to keep it's atmosphere, which has mostly all escaped into space. All that remains is a thin, sparse covering of carbon dioxide, but water once ran freely on the surface, and may still be there, frozen at the poles and under the Martian soil. Mars may have hosted life at one time, but not intelligent life - there are no canals, and no ruined cities.
Earth is the Goldilocks planet - neither too hot, not too cold. Once it too was covered in a reducing atmosphere, but anaerobic life evolved, and turned the sky oxygen-blue, while comets deposited oceans of water. Oxygen breathing life evolved, became multicellular and eventually what passes for intelligent (the US Congress not withstanding). What is the likelihood this is unique?
Intelligence itself doesn't appear to be unique - chimps, dolphins and elephants seem to have at least some self-awareness, and creatures such as octopi, crows and apes can use tools and solve puzzles. Our sample of one suggests that life eventually gives way to intelligent tool users. So far it looks like planets are common, and life may be too.
So if the universe is full of planets teeming with intelligent tool users - where are they? The Sun (Sol) is a very common type of G class yellow dwarf, a third generation star that has shone for 5 billion years. The universe is about 13 billion years old - while it took some time to make the heavier elements we need, such as iron, silicon and carbon, there has been plenty on time for civilizations to arise before ours. If star travel is possible, why have they not been here? Why are they not here now? We certainly will be out among the stars as soon as we get the technology down - that's our way.
The fundamental problem is that the Universe is too old, and too big. Our galazy, the Milky Way, is one of hundreds of billions or even trillions of galaxies, and holds around 200 billion stars - that's 30 stars for every man, woman and child alive today. In this galaxy alone. Anything that can happen, has happened - somewhere. If star-travelling species can exist, they do exist. And if they do exist, why did they not colonize our solar system already?
There are a few possibilities, none of them very pleasant:
- Life, and especially tool using intelligent life is actually very rare. Maybe we are unique - or civilizations are so spread out as to almost never make contact with each other. What evidence we have so far is rather against this.
- Technology is a fatal disease - all civilizations that develop it die, from pollution, nuclear holocaust, or self made pathogens. None make it as far as communication with other civilizations, or to star travel.
- Star travel isn't possible, and the planet-bound civilizations either don't communicate with each other, or they don't use radio. Perhaps we are too young to have developed sub-space based communications which are instantaneous and efficient, and they are watching our TV signals and shaking their heads (or whatever they shake) over our youthful stupidity. And poor production values.
- Everyone is hunkered down, or dead. Advanced machine civilizations silently cruise the interstellar starways, and when they capture the radio signals from an ignorant and wasteful emerging biological infestation, they send out the clean up crew.
“You are gods; you are all children of The Highest!" - The Bible, Psalm 82 vs 6.
At least that one's hopeful......