Life in the Universe - unf.edun00006757/astronomylectures/Life in the Univ… · Life in the Universe • The only place we know life exists is here on Earth • One of humanity’s

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Life in the Universe

• The only place we know life exists is here on Earth

• One of humanity’s Big Questions is whether it exists elsewhere

• We can get some clues by considering life’s history here on Earth

• When we do, we can get an idea how likely life “as we know it” is

• We should keep in mind that life as we know it may not be the only kind possible

• But it is the kind that we will be best able to recognize, if it does exist…




When did life arise on Earth?

What do these events tell us about the possibility

that life exists elsewhere in the universe?



• The first evidence of life

appears ~150 million years

after it became possible

• It is not fossil evidence, but

trace chemical evidence

• The evidence is in the ratio of

carbon-12 to carbon-13


Carbon isotope evidence for life

• In >3.8-billion-year-old

rocks like these in

Greenland there is a higher

than normal ratio of 12C:13C

• Living things incorporate 12C more easily than 13C

• So the higher ratio is taken

as indirect evidence for life



• The oldest fossils of living things

date to ~3.5 billion years ago.


Earliest Fossils



• Fossil stromatolite in 3.5-billion-

year-old rock


Earliest Fossils




year-old rock

• This is a living stromatolite

• Stromatolites are layered structures

formed by colonies of bacteria

• They still exist today, typically in

extreme environments like hyper-

salty lakes and lagoons


Earliest Fossils




year-old rock

• This is a living stromatolite

• Stromatolites are layered structures

formed by colonies of bacteria

• They still exist today, typically in

extreme environments like hyper-

salty lakes and lagoons

• Here are some in a lagoon in

Australia



• So fossil evidence shows that life

certainly existed on Earth by 500

million years after conditions

would permit it to survive

• And chemical evidence suggests

it probably existed much earlier

• But how did it come to be?

• We don’t know


How many civilizations are out there?


The Drake Equation

Number of civilizations with whom we could potentially

communicate

= NHP flife fciv fnow

NHP = total number of habitable planets in galaxy

flife = fraction of habitable planets with life

fciv = fraction of life-bearing planets with civilization at

some time

fnow = fraction of civilizations around now


We do not know the following values for the Drake

equation:

NHP : probably billions

flife : ??? Hard to say (near 0 or near 1)

fciv : ??? It took 4 billion years on Earth

fnow : ??? Can civilizations survive long-term?


Where are the aliens?


Fermi’s Paradox

• Plausible arguments suggest that civilizations should be common…

• So why haven’t we detected them?


Possible solutions to the paradox

1. We are alone: life/civilizations much rarer than

we might have guessed

• Our own planet/civilization looks all the more

precious…


2. Civilizations are common, but interstellar travel

is not, perhaps because:

• interstellar travel is more difficult than we think.

• the desire to explore is rare.

• civilizations destroy themselves before achieving

interstellar travel.

These are all possibilities, but they are not very

appealing.



3. There IS a galactic civilization…

… and someday we’ll meet them.

maybe



• If there are other civilizations as advanced as ours….

• What would they be like?

• What would the “people” look like?

• How would they think?

• Presumably they would have similar science to ours…

• What would their art be like?

• What would their philosophy be like?

• What would their religion be like?

Exocivilizations

Documents

Life in the Universe - unf.edun00006757/astronomylectures/Life in the Univ… · Life in the Universe • The only place we know life exists is here on Earth • One of humanity’s