Despite my “retirement” from Perkins Observatory, I’m still doing a lot of public programs. During this phase of my astronomical life, I am now at leisure to engage in the activity I have always loved the best — using telescopes large and small to show people what their universe looks like.

Over my long career as a stargazer, people have asked me thousands of questions as the night rolls on, but one of them is still the most common, especially in the hours after midnight. “Do you believe that there’s other life out there in the universe,” folks ask. Alternatively, they ask, “Do you believe in UFOs?”

The questions are complicated by my natural skepticism. As far as the physical universe is concerned, I don’t believe in anything. I often reply, “Believe in your God. Believe in your family. Believe in your country. If you wish, believe in the basic goodness of humanity. I know I do. For all other claims, demand evidence.”

Despite the claims of others to the contrary, the only life we have any solid evidence for is the life we find on our own planet. We use the knowledge we have about it to speculate about the possibility of life on other worlds and what it might look like.

Such assumptions are, of course, a tad specious to start with. “Life” is difficult to define, and it might exist in radically different biochemical forms on other worlds.

Still, we have to go with what we know, so we attempt to determine the circumstances that produced life on Earth, and then we extrapolate those circumstances to the possibility of life on other planets.

Quite a lot of complex requirements had to be met to produce life on planet Earth. The inescapable conclusion: We are durned lucky (or blessed, depending on your religious point of view) that all those circumstances came together here.

The requirement that astrobiologists tend to emphasize most is the need for a mild, universal solvent to get the elements of a living thing interacting with each other. Luckily, such a solvent exists in relative abundance in the universe. It’s called water.

Unfortunately, most of the universe is too cold to produce much water in the required liquid form. The places where it can are very special indeed.

That’s what astronomers are talking about when they refer to the Goldilocks Zone. Every star produces a different amount of energy. As a result, its planets are heated up to varying degrees.

Therefore, every star has a unique band around it where water can remain liquid most of the time. The band around Earth’s sun stretches from the planet Venus on the inside to the planet Mars on the outside. Both of them are barely in the zone. Earth is firmly ensconced inside it. Lucky!

Of course, the energy output of the star must remain stable over a very long time so that the zone does not shift. Many “variable stars,” as they are called, fluctuate wildly in their energy output. Our second bit of luck is that the sun’s energy output is only slightly variable and has been that way for a long time.

We are also lucky that Earth’s orbit is practically a circle, a situation that has remained unchanged over billions of years. Many of the planets orbiting other stars show signs that their orbits have shifted extensively because of close gravitational encounters with other planets.

If a planet had dipped close enough to Earth, its gravitational influence could have driven us out of the Goldilocks Zone entirely. The wandering planet also could have stretched our nearly circular orbit into an ellipse. As a result, Earth would travel in and out of the Goldilocks Zone, and water could not remain liquid long enough to facilitate the long process that creates life.

An impact with another large body like a planet could have had the same effect, and Earth has had at least one of those. Luckily, almost weirdly, such an impact actually helped to make life possible on Earth.

Earth is tilted 23 degrees to its orbit around the sun. Some sort of cataclysmic event like a giant impact must have caused our tilt.

Our seasonal shifts, from summer to winter and back again, are the result of that tilt. The seasons make it possible for water to be liquid at some time or another over a larger part of the planet. Planets without tilts must have a very narrow zone in which liquid water is possible, even if they are in the Goldilocks Zone.

And we are lucky for another truly cataclysmic impact. Early in Earth’s formation, before life was possible on our planet, a Mars-sized object struck Earth with enough force to begin the formation of our much-larger-than-average moon.

Over tens of thousands of years, Earth’s tilt wobbles a bit as it travels around the sun. It would be wobbling a lot more without the stabilizing influence of a large object orbiting around it.

Consequently, we have had a relatively stable and predictable climate for a long time, a situation that will certainly not last forever, as we shall see next week.

A stable climate means that water can remain liquid over much of our planet, but that begs the obvious question, “Where did all the water come in the first place?”

Earth formed from impacts of smaller bodies called planetesimals, which were forced into a violent intersection because of their orbits and their mutually attractive gravity.

The result was a molten ball of rock, made hot by the force of the impacts and made spherical by the tendency of gravity to draw things inward in all directions equally.

Those objects certainly contained frozen water, and that water escaped the rocks in the form of a gas. At some point, Earth became large enough to have enough gravity to support an atmosphere, and thus some of the water did not escape into space.

Increasingly, many astronomers are convinced that the process would not produce enough water to account for Earth’s relative abundance of it. Something must have happened at the very end of Earth’s formation as it began to cool.

That something, and a lucky something it was, was the Late Heavy Bombardment. The LHB began about four billion years ago. As Earth careened around the sun, it was clearing out its orbit of stray material.

In large part, that cosmic detritus consisted of small objects we now call comets and asteroids. Comets in particular are abundant in water. Our oceans, where life originated on Earth, were mostly caused by that lucky late bombardment of comets.

The LHB also added a lot of nitrogen to the atmosphere and a bit of carbon dioxide. We think of carbon dioxide as a gas. Our CO2 came from the solid form that we call dry ice, which comets also have in abundance.

We are left then with a hot ball of rock with a large quantity of water on its surface and a rich chemical mix in its atmosphere and oceans. In that way, the stage was set for life, but life had not yet formed. For life to generate and for humans to develop, a separate set of lucky events had to occur. More on that next time.