Astrobiology, the study of life pertaining to outer space, is the most important and among the least useful fields. It involves the beginning and probable end of life as we know it, but what we find is too large to be used by anyone, or even everyone.
On May 10, 2016, NASA released this image:
The blue circles represent planets that have been previously found (confirmed), mostly by the Kepler satellite in the last few years. The orange shaded circles are those found since NASA's last announcement on the matter.
The size of each circle is proportional to the (estimated) size of the planet, the height is essentially the brightness of the star* that planet orbits. Near the top is our sun, a white** star, and further down are cooler, smaller, redder stars. The further to the right, the less bright that star is from the planet. Notice that Mars is to the right of Earth.
That green band down the middle of the chart, that's the habitable zone. Planets in that range are the possibly the right temperature to support carbon-based life. That doesn't mean these planets can support life, just that the first two criteria, heat and radiation, are in the right zones. Without that, terraforming for long-term carbon-based life is impossible.
Now that we understand the graph, some remarks.
This is amazing! When I graduated from high school, finding other planets meant speculating about a single planet beyond Pluto. Now, NASA confirms the existence of nearly 1300 newly found planets in the last year! Of those, nine new ones are in or near the habitable zone. These are all pointlessly far away, but it's a leap from nothing to something in our lifetimes.
We still don't know how relatively abundant these small, rocky, habitable zone planets are because larger, more massive gas giants are easier to find. It's worth considering that compared to other stars we can observe, the sun is a bit unusual regarding the high amount of metals it has (i.e. anything but hydrogen and helium), compared to other stars like it (i.e. its population). That even this many rocky planets is found is pretty marvelous.
Space is exciting!
Next, do you see how Earth is close to the too-hot edge of habitable zone? It wasn't always that close. This has nothing to do with global warming on a human history scale, main sequence stars get hotter over time.
A few billion years ago when life forms were much simpler, Earth would have shown up more to the right and a little bit down from where it is now. Earth would have been a lot cooler than now were it not for the fact its atmosphere was mostly carbon dioxide and its core had more radioactivity. Not only does Earth support life now, but its conditions have changed to offset the changes in the star it orbits in such a way that life was continually sustainable long enough to develop its current complexity.
The theory that life developed from self-replicating proteins and lipids on Earth is called abiogenesis, as in 'creation from non-life'. However, a growing body of evidence suggests that Terran life is
currently too complex to have developed in time if it started on Earth. A competing theory, called panspermia 'life everywhere', suggests that some very simple life arrived from inside a meteor, after being kicked into space by something Michael Bay dreamed up.
This early life could have come from anywhere, but Mars is a likely source. NASA has also recently found flowing water on Mars, however it tends to boil away quickly without any air pressure. There's substantial evidence to suggest that ancient Mars was much warmer and with a thicker atmosphere, and that the atmosphere slowly boiled away because there wasn't enough gravity and magnetic field to keep it on the planet.
So to get to our current level of life complexity, we needed not one, but two habitable planets in order to buy enough time to develop. We didn't do it with much room for error either.
Remember how Earth is near the too-hot edge of the habitable zone? Well, the sun is still getting hotter, and there's nothing within the bounds of humanity to prevent that. In roughly half a billion years, Earth, assuming its orbit is the same, will have an average temperature of 55 C, and all remaining carbon will be locked away in rocks and out of the atmosphere. Without that carbon, no plant life can exist, and neither can we. Nothing smaller than moving the Earth itself to a wider orbit can prevent that in the long-term.
To put that in perspective, of the time that life can exist on Earth, that span is nearly 90% over, assuming the best-case scenario.
1. We are lucky, insanely lucky, to exist.
2. Regarding the lack of contact from alien life, we could very well be past whatever stops most life from reaching any technological level - otherwise known as the Great Filter.
3. We can't stay home forever.
*assuming main sequence stars, like our sun is.** yes, white. It only looks yellow through our atmosphere.