The questions below were forwarded to me from an inquisitive group of fourth graders. I'm sharing my short replies for other inquisitive readers, both young and old.
Do you believe that there is life on other planets?
Yes. Here on Earth, we see evidence of life arising quickly in the early stages of the solar system. Once the heavy bombardment from asteroids and comets had settled down, life took hold in a few hundred million years. That is "quick" compared to the current age of the solar system at four and a half billion years.
Also, we find life in many extreme conditions. Life can exist in total darkness, frigid cold, and very high salinity (salt content). Given the heartiness life shows here on our planet, it is logical to believe that it has shown similar behavior on other planets. Further, we can now estimate that there are at least a billion other planets in our galaxy alone. It seems highly likely that a percentage of those planets are suitable for life.
If so, where do you think there might be life?
The basic supposition is that life can arise on Earth-like planets in the universe. If life can survive here, it should be able to survive on similar rocky planets. Also, there are several moons around the giant planets in our solar system that are large enough to resemble a rocky planet. Other solar systems with giant planets and large moons could also be hospitable to life.
Would you start looking on Europa for life or somewhere else?
It would be short-sighted to attempt to explore other solar systems without first searching our own solar system thoroughly. Robotic missions to observe and understand all we can about our cosmic backyard are a proper initial path. Europa and several other moons are thought to have sub-surface oceans, which makes them prime candidates to be studied for evidence of life.
Meanwhile, we can do astronomical observations of other solar systems to learn as much about them as possible. We would especially like to be able to isolate the light of a planet, without the light of its host star. By examining that light, we may get measurements of the composition of its atmosphere, oceans, or land masses, and clues to whether biological processes are at work.
If you thought there was life on Europa, how would you get to it or prove it?
Missions to study potential life on Europa have been envisioned, and they are rather difficult. Life would only be found in the sub-surface ocean. Such a mission would need to land on the icy surface and then drill or melt its way down to the ocean layer. If that layer were just a few kilometers under the ice, it could be feasible. However, if the ocean is below a hundred kilometers of ice, then the logistics are extremely difficult. An important prior step would have to be an orbiter around Europa that could help measure the ice thickness and determine if a suitable place to explore can be identified.
What do you think is the best way to find life on other planets?
The only ways we have to find life are the two methods mentioned above: robotic explorations in our solar system and astronomical observations of other solar systems. Your generation is the first to grow up knowing that other solar systems and planets exist. You will be at the forefront of the scientific search for life in the universe. We will move beyond just detecting planets exist to starting to characterize what these planets are like. The coming decades are really exciting.
How would you get a robot to a planet with life?
We can use existing technology to send robotic missions across our solar system. The timescales for such missions are years to decades, well within the human lifespan. No such technology currently exists to get robots to other solar systems. Using current rockets, the timescales to reach even the nearest stars are thousands of years. Efficient interstellar travel is a staple of science fiction, and is also a real problem for future generations to solve if we are to travel to the stars.