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Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
(Please use PHYS214 in e-mail subject)
Lecture 8.
The scale of time and nature of worlds
(Page 64-74)
January 23
Textbook: Pages 64-74
The scale of time - the cosmic calendar
The Observable Universe
The nature of worlds
How do other worlds in the Solar System compare to Earth
Acknowledgment: images NASA, ESA, Hubble, NOAO, AURA, NSF
The cosmic calendar
The Universe is 14 billion years old.
How do we grasp the meaning of this number?
Let’s imagine we compress the entire history of the Universe, from the Big Bang
to present, into a single year –
The cosmic calendar
The Big Bang is on Jan 1st, and the present is the stroke of midnight on Dec 31.
Each month is a little more than one billion years, each day ~ 40 million years,
each second more than 400 years.
Jan 1 – The Big Bang
Feb – The Milky Way
Many generations of stars lived
and died in the subsequent
months, enriching the galaxy
with heavier elements.
Sept – Solar System & Earth
(about 4.5 billion years ago)
Sept 22 – early life on Earth
(more than 3.5 billion years ago)
living organisms remained
microscopic in size until Dec
The cosmic calendar
Dec 17 – Cambrian explosion
(545 million years ago)
Incredible animal diversity
Dec 26 – Rise of dinosaurs
Dec 30 – Dinosaurs extinction
(65 million years ago)
The death of dinosaurs allowed other
species to evolve.
Dec 31, 9 pm – early hominids
(human ancestors) 60 million years
later after dinosaurs extinction…
The cosmic calendar
Dec 31, 11:58 pm – Modern humans evolve
The entire history of human civilization fits into just the last half-minute!
The scale of time
The fact that the Universe is so much older than Earth means that
could be many worlds that had plenty of time for life to arise
and evolve.
These worlds might have had civilizations millions or billions of
years ago.
The scale of time holds sobering lessons for our own future.
Species have come and gone in the months of the cosmic
calendar, and there is no reason to think that our fate should
be different.
How big is the Universe?
The Universe could be infinite, and contain an infinite number of galaxies.
The age of the universe poses some limitations on the portion of the universe that
we can observe with telescopes, due to the limited value of the speed of light.
When we look to great distances, we are also looking far back into the past.
The observable Universe
Andromeda (M31)
Andromeda located 2.5 million light-years away. The photo shows the galaxy
as it was 2.5 million years ago, long before modern humans existed.
It takes 200 million years to rotate once. Probably looks similar today.
However, at much greater distances we begin to see back to a time when the
universe looks different than today.
The observable Universe
FAR: We see a galaxy 7
billions light-years away as
it was 7 billion years ago,
when the Universe was half
its current age of 14 billions
years old.
FARTHER: We see a
galaxy 12 billions lightyears away as it was 12
billion years ago, when
the Universe was about 2
billions years old.
The limit of our observable
universe: Light from nearly
14 billion light-years away
shows the universe as it
looked shortly after the Big
Bang, before galaxies
Beyond the observable universe: We cannot see anything farther away than 14
billions light-years away, because its light has not had enough time to reach us.
The observable Universe
Our observable universe - the portion of the entire universe that we can potentially
observe – lie within 14 billion light-years of Earth.
1) We cannot observe anything more that 14 billion light-years away.
This does not mean that nothing exists beyond that distance or that the universe
might not be infinite.
2) We are the center of our observable universe, since it is defined by a light-travel
distance in all directions from us.
This does not implies we are the center of the Universe.
Every observer in another part of the universe must be at the center of his
observable universe.
Hubble 15 years of discovery
Chapter 9. Looking to the end of time (9 minutes + credits ~15 min)
Number of galaxies in the observable universe
Because the observable
universe has a finite size, it
must contain a finite
number of galaxies.
We do not know how many,
because there are too many
to count and some galaxies
are too faint to be observed.
By counting the galaxies in the
photo, the observable
universe has an estimate of
about 100 billion galaxies.
Hubble Ultra-Deep Field.
More than 10 days of exposure, it shows galaxies some at more than 12 billion light-years away.
Number of stars in the observable universe
The Universe has an estimate of 100 billion galaxies.
Milky Way has an estimate of 100 billion stars.
100 billion x 100 billion =
1022 stars
The number of stars in the observable Universe is comparable to the
number of grains of sand on every beach on Earth.
Due to the incredible size of the universe, our search for extraterrestrial life
will probably be limited to within our Milky Way.
The nature of worlds - solar system
The solar system
(1) All the planets orbit within a few degrees of the equatorial plane
of the Sun
(2) Most of the planets have a rotation axis roughly perpendicular
to the plane of the solar system
Both Pluto and Uranus have rotations that are highly inclined
to the plane of the solar system
(3) Planets are divided into rocky (terrestrial), gas giants
Two major type of planets
Terrestrial planets - small, made mostly of rock & iron
with high densities, near the Sun.
Jovian planets -large, made mostly of gases and liquids
with low densities, far from the Sun
Terrestrial planets
All are similar in overall
composition, however
their surface looks quite
made mostly of rock &
high densities,
close to the Sun (< 5AU),
no rings
Jovian planets - gas giants
Made mostly of gases with a
rocky core
Low densities
Far from the Sun (> 5AU)
Have rings
Jupiter- failed star
-miniature solar system
many moons
Jovian planets - gas giants
The pressure inside is so high that the gases are compressed into liquid and
metallic phases = large magnetic fields
Less likely to be habitable if we assume life needs a solid surface or oceans.
When we are looking for habitable planets in other star systems we should look
for terrestrial planets.
Gas giants - Jupiter
Gas giants - Jupiter
Gas giants - Saturn
Gas giants - Saturn
North pole
South Pole
Saturn dragon eye storm
Gas giants - Uranus and Neptune
Dwarf planets and small bodies orbiting the Sun
True Colours of Pluto
The Solar Systems goes beyond Neptune -the outermost of the official planets.
Pluto counted as a ninth planet until 2006; orbits at a distance about a third farther
from the Sun than Neptune.
There are thousands of icy objects sharing Pluto’s region.
An object larger than Pluto was discovered in 2005 – Eris (Xena).
Dwarf planet - object large enough for their own gravity to make them round.
Small body – object low enough in mass to take any other shape than round.
Small bodies orbiting the Sun - asteroids
Eros (image 12 m across)
Gaspra (Galileo spacecraft 1991)
Asteroids – made mostly of metal and rock.
Asteroids resemble a terrestrial planet in
composition, but are too small to count as a
planet itself.
Small bodies orbiting the Sun - asteroids
Asteroid Itokawa, copyright ISAS, JAXA
Asteroids – made mostly of metal and rock.
Asteroids resemble a terrestrial planet in
composition, but are too small to count as a
planet itself..
Small bodies orbiting the Sun - comets
Comet Neat, 2004
Comets – made mostly of rock and ice.
Comets grow tail when they come close
to the Sun.
Comet Machholz
Small bodies orbiting the Sun - comets
Comet Neat, 2004
Comet Wild 2's Heart
Comet C/2002 T7
Small bodies orbiting the Sun
Most asteroids orbit in the region called the asteroid belt, which
lies between the orbits of Mars and Jupiter.
Comets come from the Kuiper belt and the Oort could.
Small bodies orbiting the Sun
The Kuiper belt is
beyond the orbit of
the planet
The Oort cloud – is ar
from the Sun in a
spherical cloud
surrounding the
solar system.
Small bodies orbiting the Sun
Small bodies orbiting the Sun
Pluto is a member of the
Kuiper belt, along with
many of other moderately
large objects.
These objects are unusually
large comets, having the
same composition with
smaller comets that
occasionally fall inward
towards the Sun.
Small bodies orbiting the Sun
Jupiter role in reducing space debris in the inner solar system - cosmic vacuum
discovered in March 24, 1993 by the Shoemakers and Levy
Unlike all other comets, it was orbiting Jupiter rather than the Sun
Could dwarf planets or small bodies be habitable?
Probably not. They are too far away from
the Sun and are too cold for liquid
water to exist.
They are important to life because they
occasionally crash into planets and
moons and have a profound effect on
the living organisms that inhabit the
Terrestrial planets have
few moons.
Mercury and
Venus have no
moons at all. Mars
has two very small
moons, probably
captured asteroids.
Moons are common for
Jovian planets,
totalling at least
150 moons
Jupiter moon Ganymede and
Saturn moon Titan are
larger than planet Mercury.
Saturn’s moon Titan (2,575 km) Mercury (2,440 km radius)
Jupiter Moons: Ganymede (2,634 km), Callisto (2,403 km), Io (1,821 km), Europa (1,565 km)
Io (1,821 km), Europa (1,565 km), Titan (2,575 km)
Many moons are planetlike in almost every way except their orbits.
Some moons are geologically active, others have water, other atmosphere.
Io is the most volcanically active world in the Solar System.
Europa has occasionally water or ice floating on its surface.
Titan has an atmosphere thicker than the Earth.
Large moons around jovian planets offer a second category (after terrestrial
planets) of potentially habitable worlds.
Next lecture
Nebular theory - the formation of the solar system
+ movie