life in the universe - university of north floridan00006757/astronomylectures/ecp4e/last...

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Life in the Universe


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


• We do know that all “life as we know

it” has an inside and an outside

• These are separated by a lipid

membrane (along with a cell wall in

plants)

• Vesicles made of lipids are easy to

make in the laboratory

How did life arise on Earth?


How did life arise on Earth?

• Life as we know it also has a nucleic acid genome

containing instructions for building the organism

• And we know that all of that was accomplished here on

Earth surprisingly quickly

• So we believe that given similar conditions elsewhere, life

will also arise


Necessities for Life As We Know It

• Nutrient source

• Energy (starlight, chemicals, heat)

• Liquid water (hardest to come by)…

• And can only exist if planet is in “habitable zone”


Are habitable planets likely?


Habitable Planets

Definition:

A habitable world contains the basic necessities for life as

we know it, including liquid water.

• It does not necessarily have life.

• There still needs to be sufficient time for life to evolve

• And certain other requirements as well


Constraints on star systems:

1. Old enough to allow time for origin and evolution (rules

out high-mass stars — 1%)

2. Need to have stable orbits (might rule out

binary/multiple star systems — 50%)

3. Size of habitable zone: region in which a planet of the

right size could have liquid water on its surface

Even so… billions of stars in the Milky Way seem

at least to offer the possibility of habitable worlds.


The Bottom Line

• We don’t yet know how important or

negligible these concerns are.

• The general feeling among most scientists is

that microbial life is likely to be common

• But how common intelligent, technological

life like us humans is, is unknown


• 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

life in the universe - university of north floridan00006757/astronomylectures/ecp4e/last...

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