This document discusses the possibility of extraterrestrial intelligence and estimates the number of advanced civilizations that may exist in the Milky Way galaxy. It begins by acknowledging how little we know and that we only have data from Earth. It then introduces the Drake Equation for estimating the number of civilizations based on factors like star formation rate, the number of habitable planets, and the lifetime of civilizations. The document considers estimates for these factors and concludes that there could be either one civilization (us) or over a million civilizations depending on the assumed lifetime. It ends by discussing the challenges of detecting other civilizations and emphasizes that we should proceed under the assumption that we are alone for now.

1. Extraterrestrial Intelligence:
Are We Alone?
We’ll have to make many (educated) guesses to
evaluate odds of ETs in our huge Galaxy
But for many guesses we have only a SINGLE data
point: our Earth
This makes it almost impossible to be “scientific”

2. You are all ET “experts” now!
• Finally I do not need a tie to symbolize all
my years of specialized scientific training.
• Honestly I don’t know more about this
than you have learned in Astro 3.
• Isn’t college great?
• That is why I cannot put any questions
from this lecture on the Exam!!

3. A Fearless Estimate
Drake “Equation” for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
Simple accounting of our ignorance, which
increases from left term to right

4. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
Star Formation Rate in our galaxy is 5 to 10 per
year on average (higher in past)—the only number
in Drake Equation which is well determined

5. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
Multiple planets, including terrestrials or Moons are
natural byproduct of star formation, BUT Earth is
currently only one known to be “Habitable”?
Liquid Water requires special conditions

6. Continuously Habitable Zone
Later position
of HZ
Stellar Evolution
• Stars get brighter as they age along the main
sequence. This results in an outward migration
of the “habitable zone”. The continuously
habitable zone--the region where liquid water is
always stable—probably only includes 1 planet.
Initial position of HZ

7. Jupiter’s moons Io, and Europa—
way beyond the Habitable Zone

8. Europa: Water ocean, under the ice crust,
melted by internal heat sources—
this could be fairly common
Inferred Ocean

9. Is Europa telling us to expand our
definition of Habitable Planet?
• Recently, we have discovered
that life without the sun is
possible.
• Instead of photosynthesis,
organisms have been
discovered living in a variety of
places via chemosynthesis -
the use of chemical energy.
• Could this happen on other
worlds? A deep-sea hydrothermal vent.

10. A Fearless Estimate
Drake Equation for the number of “technical civilizations”
currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
LAWKI needs complex system of C chains (protein) catalysts
The building blocks (amino acids) are cosmically common.
The “miracle” of Nuclei acids (DNA and RNA)
with the right message
evidently occurred on Earth almost “immediately”

11. Mars had water seas, but “evidence” for fossil
life in Mars rock is unconvincing.
We’ll have to explore Mars further
• Nice shape, but no evolution of shapes
evident. Way too small!

12. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
Although intelligence offers some evolutionary
advantage, there has NOT been any steady march
towards bigger brains (we took >3 Gigayears)
Humans may be a lucky evolutionary accident

13. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
With the lucky breakthrough of language, “culture”
starts Lamarckian evolution
We’ve only had radio communication, space travel
for less than a century; dolphins and chimps might
never get technical

14. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
= 10 x 1 x 0.001 x 0.1 x 1 x L = 0.001 L
Some pure guesses, but it all comes down to L: we
can only project our own natures onto the aliens

15. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = (SFR) x nhabit planets x f life x f intel x f tech x L
Lifetime could be huge (Gigayears) for successes,
or is the power of technology so de-stabilizing that
civilizations cannot survive it for more than
1000 years = L??

16. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = 0.001 x L
= 0.001 x 1000 years
= 1 Technical Civilization in our Galaxy
Pessimistic: this is Us! Probably alone until we
destroy ourselves within several centuries

17. A Fearless Estimate
Drake Equation for the number of “technical
civilizations” currently in Milky Way:
N = 0.001 x L
= 0.001 x 1 Billion years
= 1 Million Technical Civilizations
Optimistic: a Swarming “Galactic Club”, should not
be too hard to find this “super-civilization”
Don’t forget Course Evals! 9

18. ET Question can only be resolved with
more data—who is going to make the
first move, us or them?
• Nearest outpost of the hypothesized
“galactic club” might be about 100 light-
years away
• Within this volume, we already know most
of the suitable stars to search for
habitable planets.
• With luck, we might even discover strong
biosignatures remotely, then send probes

19. We’re already on the verge of identifying
‘habitable planets’; Next: Biosignatures might
be detected in the atmosphere of a planet many
light years away
• Disequilibrium chemistry: our oxygenated
atmosphere is a good example. Without
life, the oxygen would all react with rocks
and be gone from the atmosphere in a
few million years.
• Metabolic by-products: O2 , O3 are
examples, but in other situations, CH4 can
be a good biosignature (not on Mars,
however).

20. Our 25 nearest stellar neighbors.

21. 260,000 stars within 250 light years

22. Pessimists say we have to search almost our entire galaxy to find intelligent company:
200 billion stars offer a lot of possibilities, but typical distances are now tens of
thousands of light years!

23. In that case, why stop with our own Galaxy, since we could reach thousands more,
going another factor of 100 further in distance…

24. What can they do to contact us?
Communication is cheapest:
EM waves such as radio are “easy” for the
persistent (e.g. “Species”) (Arecibo can now
communicate with a copy of itself on other
side of the Milky Way Galaxy!)
• But this is a classic needle-in-a-haystack problem,
with so many frequencies, directions, duty cycles,
signal patterns, to search. We have to make some
assumptions about their transmission, or try
“eavesdropping” (really tough!)

25. Arecibo 1000 foot dish in Puerto Rico, with very powerful radio transmitter

26. HOLD ON!! Are we SURE we WANT
to be Contacted?
• Are you SURE that all advanced ETs are really “nice” and
enlightened?
• What if all of those (bad) Sci-Fi movies were right!?! (as Tim
Ferris reminds us)
• But on the other hand, isn’t it more likely that only a
PEACEFUL Galactic Club could survive billions of years?
• Would you like to bet the fate of the entire human race that
they were wrong? (oops, it’s too late anyway…)
• We must never forget just how ALIEN ETs are going to be!

27. Even George Lucas can’t imagine how strange
ETs will really be, and that is mathematical
certainty
These are all just humans, dressed up in fancy costumes!

28. What can they do to contact us?
That requires speculating about them!
• Odds are overwhelming that they will be thousands to
millions of years ahead of us, since our technical
civilization was just born ---- (how many people in LA
are 10sec old?)
• In thousands of years, one-way voyages of colonization
should be undertaken by some adventurous explorers
(giant space stations are independent of the home star)

29. If Optimists Correct, Where Are They?
1. They’re uninterested. (Want to talk to ants?)
1a. Maybe they’re too far away to have received the RECENT
‘news’ of our development of technology
Remember how LITTLE WE MAY HAVE IN COMMON
(besides basic math and physics)
2. They’re already here, but incompetent or “messing with
us”? No–UFO’s internally inconsistent
3. They are STRICTLY QUARANTINING us new kids on the
block (“Zoo Hypothesis”).
Please credit them with being SERIOUS (unlike USS
Enterprise!).

30. For either possibility 1) or 3),
the only sound position is to assume
we’re alone for now
• Until proven otherwise, this may be the only place
in the entire Universe where consciousness has
developed (we’re only physically insignificant)
• We certainly can’t count on a bunch of
super/enlightened beings to swoop down and ‘save’
us just at the brink of some calamity, cures cancer,
etc—even if ‘they’ are watching, WE’RE ON OUR
OWN
• P.S. MM personally PREFERS it that way!
(my last night on the telescope)

31. (Personal) Overview of some of Homo Sapiens’ Highs and Lows
What snaps would You send to the Galactic Club?
Whatever you say about all these spectacular Highs and Lows,
We HUMANS did them 100% on our own
No matter how many times we mess up, WE HUMANS will never stop trying
I PREFER this to the prospect of some superior aliens doing it all for us