Does Life Exist Elsewhere in the Universe?

Establish the Vocabulary and the Distances

The deck's own teaching notes are blunt: depending on your students' science background, you may need real time up front on basic astronomy before any of the argument works.

• Intelligent life — beings capable of building civilizations that can share and store knowledge. This is a narrower definition than students expect, and the whole equation depends on it.
• Goldilocks zone — the band around a star where liquid water can exist on a planet.
• Tidally locked — an orbiting body rotating at the same rate it orbits, so one face is permanently turned inward. The Moon is tidally locked to Earth.
• The Milky Way — our galaxy: hundreds of billions of stars, and one of roughly a hundred billion galaxies.

Why only the Milky Way? The observable universe is about 90 billion light years across. Andromeda, the nearest galaxy, is 2.5 million light years away. Nothing travels or communicates faster than light, and space is expanding — so a ship leaving today at light speed could never reach 94% of the galaxies in the universe, no matter how long it travelled. Restricting the question to our own galaxy isn't modesty; it's the only version of the question that's answerable.

Assign the readings, due before Tuesday:

• Are We Alone in the Universe? Revisiting the Drake Equation — NASA
• On Average, Every Star Has At Least One Planet — Popular Science
• Half the Milky Way's Sun-Like Stars Could Be Home to Earth-Like Planets — MIT Technology Review

Walk the Equation, Variable by Variable

Mindful moment (5 min): what's the first thing humans will do if they discover alien life? It's a deceptively good opener — the answers reveal what students think the discovery would mean.

Then Carl Sagan's line — "The universe is a pretty big place. If it's just us, seems like an awful waste of space" — and the question of whether the evidence actually supports it.

The equation, term by term. The deck's structure is the lesson: the variables get progressively less knowable.

• R* — stars in the Milky Way: 100–400 billion. We can't count them. The number is deduced by estimating the galaxy's mass from its rotation, estimating what share of that mass is stars, and estimating an average star. Ask what a range that wide does to everything downstream.
• f_p — stars with planets: nearly 100%. This one has genuinely been answered. The first exoplanet was found in 1992; the Kepler Space Telescope confirmed 2,600 planets between 2009 and 2018, and the consensus now is that nearly every star has a system.
• n_e — habitable planets per system: 0.3–2 billion, conservatively. Most stars are small red ones, where a Goldilocks-zone planet would likely be tidally locked and probably inhospitable; stars much larger than the Sun are rare and unstable. Sun-like stars are only 3–4% of the galaxy — about 4 billion — but recent work suggests at least 8% of them host an Earth-like planet in the habitable zone, a floor of 300 million, with a best estimate closer to 50%.
• f_e — planets where life actually starts: cannot be estimated. Earth is our only observation, and we don't fully understand when or how life began here. The strongest case for a high value is how stubbornly life on Earth appears in hostile places.
• f_i — life that becomes intelligent: likely very rare. Neil deGrasse Tyson's framing in Space Chronicles: ten billion species have lived on Earth and exactly one built a civilization.
• f_c — civilizations that emit detectable signals. We can only find civilizations using the electromagnetic spectrum, and leaked signals may degrade beyond recognition after about 100 light years. Humans have existed for 200,000 years and first deliberately transmitted a greeting in 1974.
• L — how long civilizations last. Weapons powerful enough that one mistake ends everything; well-intentioned technologies with fatal consequences; resources consumed before a species can spread; older civilizations preying on younger ones; and ordinary cosmic hazards — meteorites, solar flares, supernovae.

Open the Discussion

Mindful moment (5 min): is the galaxy full of life, or are we alone — and which part of the equation gives you that confidence?

• Do you think our galaxy is full of life? Which variable is doing the most work in your answer?
• If you could learn the true value of any one variable, which would it be, and why?
• Does it actually matter whether other civilizations exist? Would knowing for certain change anything?

Bring the readings in directly: what evidence did the Popular Science piece give for the claim that nearly every star has planets? This is the week's best chance to practise citing a source for a specific numerical claim.

Before students leave: themes, lingering questions, and one new resource for Thursday.

The Fermi Paradox and the Great Filters

Mindful moment (5 min): do you think we have been visited? What are the best arguments against your own position?

Then the question the whole equation sets up — so where is everybody?

• Should we be transmitting signals announcing ourselves? Does the Fermi paradox make you more or less enthusiastic about that?
• Would another lifeform even be able to interpret what we send?
• Of the reasons civilizations might not last — weapons, runaway technology, resource exhaustion, predation, cosmic hazards — which is most threatening to ours? Which is least?
• What great filters has our civilization already passed? Which ones are still ahead?

The UAP question, handled carefully: a 2021 US government report documented 143 unexplained sightings since 2004, eighteen of which appeared to show technology the US doesn't have. The report does not claim extraterrestrial origin, and saying so plainly is part of the lesson.

Score Your Participation

The four reflective questions, then the participation matrix. Students also use the participation rubric to score the week and explain the score.