Life in the Universe - University of North Floridan00006757/astronomylectures/ECP4e/18 _LectureOutlin… · Title: Life in the Universe Author: Megan Donahue Created Date: 4/21/2011

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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.


Finding them will be hard

• Looking for an Earth-like planet around a nearby star is

like standing on the East Coast of the United States and

looking for a pinhead on the West Coast—with a VERY

bright grapefruit nearby.

• But new technologies should soon show the way.


• The Kepler Space Telescope

is monitoring 156,000 stars

for transit events for 4 years.







Planned orbiting

interferometers will obtain

spectra and crude images

of Earth-size planets.


Spectral Signatures of Life

Earth

Venus

Mars

oxygen/ozone


Are Earth-like planets rare or

common?


Elements and Habitability

• Some scientists argue that

proportions of heavy

elements need to be just

right for the formation of

habitable planets.

• If so, then Earth-like

planets are restricted to a

galactic habitable zone.


Impacts and Habitability


Jupiter-like planets are

necessary to reduce the

rate of impacts.

• If so, then Earth-like

planets are restricted to

star systems with Jupiter-

like planets.


Climate and Habitability


plate tectonics and/or a

large moon are necessary

to keep the climate of an

Earth-like planet stable

enough for life.


Jovian Moons

• An intriguing possibility is that

life as we know it could exist

on jovian moons

• If, that is, those moons are in

the HZ of their star

• Of the 542 confirmed

exoplanets, 124 of them are in

the HZ

• Only 2 of those are near Earth-

size, and they are close with

highly elliptical orbits

• A number of the jovian planets

are in more Earth-like orbits

• Good for jovian moon life


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


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?


Are we “off-the-chart” smart?

• Humans have

comparatively large

brains.

• Does that mean our

level of intelligence

is improbably

high?


How does SETI work?


SETI experiments look for deliberate signals from E.T.


We’ve even sent a few signals ourselves…

Earth to globular cluster M13: Hoping we’ll hear

back in about 42,000 years!


Your computer can help! SETI @ Home: a screensaver

with a purpose


Where are the aliens?


Fermi’s Paradox

• Plausible arguments suggest that civilizations should be common. For example, even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations!

• 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.



• 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 - University of North Floridan00006757/astronomylectures/ECP4e/18 _LectureOutlin… · Title: Life in the Universe Author: Megan Donahue Created Date: 4/21/2011