Does Life Exist Elsewhere in the Universe?

Words First, Then Distances

A few terms you'll need all week: intelligent life means beings who can build a civilization that shares and stores knowledge — a narrower bar than "aliens." The Goldilocks zone is the band around a star where water can stay liquid. Tidally locked means one face is permanently turned toward the star, the way the Moon always shows Earth the same side.

Then the reason we only ask about our own galaxy. The observable universe is about 90 billion light years across. The nearest galaxy is 2.5 million light years away. Nothing goes faster than light, and space is expanding — so if you left today in a light-speed ship and flew forever, you could never reach 94% of the galaxies out there. The Milky Way isn't a modest question. It's the only one we can ask.

Seven Numbers, Multiplied

Free write first: what's the first thing humans would do if we discovered alien life?

Then the equation. You don't need physics for this — it's a chain of multiplications, and you can follow every step:

• How many stars are in the Milky Way? Somewhere between 100 and 400 billion. We can't count them; we weigh the galaxy by watching it spin and work backwards.
• How many of those have planets? Nearly all of them. We only found the first planet outside our solar system in 1992 — then one telescope confirmed 2,600 more in nine years.
• How many planets could support life? Conservatively, somewhere between 300 million and 2 billion in our galaxy alone.
• How many of those actually grow life? Nobody knows. We have one example, and we don't fully understand how it happened here.
• How often does life get intelligent? Probably rarely. Ten billion species have lived on Earth. One built a civilization.
• How many civilizations send signals we could detect? Signals fade after about 100 light years. Humans have been around 200,000 years and first deliberately transmitted a message in 1974.
• How long do civilizations last? This is the one nobody wants to think about.

Watch what happens as you go down that list: the evidence runs out. That's not a flaw in the equation — it's the honest answer, and knowing where your knowledge ends is a genuine skill.

Open the Discussion

• Is the galaxy full of life, or are we alone? Which part of the equation is really driving your answer?
• If you could learn the true value of one variable, which would you pick and why?
• Does it matter? Would knowing for certain change anything about how you live?

Be ready to point at your reading: what evidence supports the claim that nearly every star has planets? Naming the evidence, not just the conclusion, is the skill this week.

Before you leave, find a resource to bring on Thursday.

So Where Is Everybody?

If the numbers say the galaxy should be full of civilizations, why haven't we heard from any? That's the Fermi paradox, and every answer to it is unsettling in a different way.

• Should we be broadcasting signals announcing that we're here? Does the Fermi paradox make you more or less keen on that idea?
• Would another lifeform even be able to make sense of what we sent?
• Civilizations might not last: weapons that make one mistake final, technology with consequences nobody predicted, resources used up too fast, older civilizations preying on younger ones, or plain bad luck from space. Which worries you most about ours?
• What dangers has our civilization already survived? What's still ahead?

And the UFO question, honestly: a 2021 US government report listed 143 sightings since 2004 that remain unexplained, eighteen of which seemed to show technology the US doesn't have. The report does not say they came from elsewhere. "We can't explain it" and "it's aliens" are different claims — keeping them apart is the whole game.

Score Your Own Week

What went well, what to improve, what you learned, and whether anything you believed changed. Then the participation matrix. Be honest with yourself.