We especially need imagination in science. It is not all mathematics, nor all logic, but is somewhat beauty and poetry.

Maria Mitchell (1818-1889)Astronomer and first woman elected to theAmerican Academy of Arts and Sciences

• NASA Astrobiology Institute:

• nai.arc.nasa.gov

• Class Web Page

• http://flash.uchicago.edu/~calder/teaching/life05/life_spring05.htm

• Cassini mission to Saturn:

• http://saturn.jpl.nasa.gov

• Textbook

• www.astonomyplace.com

Sites of the Week

Class # 1

• Outline of Course/Class business

• Intro to Life in the Universe/SETI (Cosmos)

• Science, scientific discovery, and the scientific method.

• A Universe of life: Habitable worlds, extra-solar planets, SETI

• History of the Universe

• Tour of Solar System

• Formation of Stars, planets, and Solar System

Outline of Course

• Scientific method, Solar System, Astronomy, the greats- Kepler, Galileo, etc., habitability, space exploration

• Life as we know it (DNA, Organic Chemistry)

• Geologic history of Earth

• Origin and evolution of Life on Earth

• The possibility of Life in the Solar System, Mars, Jovian moons (Titan).

• Nature and evolution of Habitability

• Extra-solar planets

• SETI and the Drake Equation

• Interstellar communication and travel

• Fermi Paradox and implications of Contact.

Class Business

• Syllabus

• astronomyplace.com

• Reading: • Chapt. 1, • Chapt. 2

• Homework (Due Feb 11): • Chapt. 1: Review questions 2, 6, 7, 10, 13,

Problems 3,5,7,14• Chapt. 2: Review questions 1, 2, 4, 5, 9, 12

Class Business

• This is an exciting time to be studying life in the Universe! All are encouraged to bring new items to class for discussion. The instructor will smile!

NASA

ESA, NASA

Aside: Scientific Visualization: an Art

Which best describes coalescing neutron stars?

Scientific Visualization: an Art

More info: books by Tufte

Visualization: Dave Bock, NCSA

Definitions

• Astronomy- The study of the Universe and its contents beyond the bounds of the Earth’s Atmosphere

• Astrophysics- The physical theory of astronomical objects and phenomena

• Cosmology- The branch of astro concerned with the origins, properties, and evolution of the Universe

• Astrobiology- The study of Life in the Universe

• SETI- Search for Extraterrestrial Intelligence

Science and the Scientific Method

• The first thing to note is that there is not agreement on exactly what the scientific method is!

• The scientific method relies on testing and validation. Any assertion is tested (or even attacked), and only after reliable, repeatable experiments demonstrate the assertion is the assertion accepted.

• Most everything presented on issues related to SETI is really informed opinion.

• The key, however, is to base those opinions on scientific fact as much as is possible.

Science and the Scientific Method

• A scientific “fact” begins as an observation. This may be a discovery of some sort such as the recent Huygens data.

• A hypothesis is an educated guess at an explanation for the observation.

• An experiment is a test of a hypothesis. Experiments can easily disprove a hypothesis, but cannot prove a hypothesis.

• Once a hypothesis has been subjected to a battery of experimental tests, none of which reliably disprove it, then the hypothesis becomes accepted. We can then consider the hypothesis a theory.

• Many theories are called laws, but these laws are not always the most accepted. (Newton’s laws of gravity vs. Einstein’s theory of general relativity)

Science v. Pseudoscience

• Can an assertion survive scientific scrutiny?

• ``... only provided further evidence that a technological civilization once existed on Mars and that the artifacts of that civilization are still visibly present. '' -- P. Gersten, Formal Action Committee for Extra-Terrestrial Studies

Hallmarks of Science (Text)

• The Universe is inherently understandable, and we can figure out how it works by observing it and examining the processes that affect it.

• Science progresses through the creation and testing of models (hypotheses), and the best models are the simplest.

• A scientific model should be testable so that we can indeed test its validity and if necessary rethink the model.

Science in a nutshell (Calder)

• The Universe is inherently understandable.

• Scientific knowledge is durable.

• Scientific ideas may change.

• Science explains and predicts.

• Science demands evidence.

• Science may not be able to answer all questions, particularly why.

A Universe of Life: Questions we want to address

• Are there other habitable worlds?

• Is biology universal?

• Should we expect other star systems to be like ours?

• NASA’s three fundamental questions (nai.arc.nasa.gov/):• How does life begin and develop?• Does life exist elsewhere in the Uni?• What is life’s future on Earth and beyond?

• We begin by studying what we know about the Uni and life in the Uni.

The Scientific History of the Universe

Address: Earth

• How would the Universal post office find us?

A large, glowing ball of gas that generates heat and light through nuclear fusion

Star

Planet

A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in

composition.

Moon

An object that orbits a planet.

Asteroid

A relatively small and rocky object which orbits a star.

Comet

A relatively small and icy object which orbits a star.

Solar (Star) System

A star and all the material which orbits it, including its planets and moons

Galaxy

A great island of stars in space, all held together by gravity and orbiting a common center

Universe

The sum total of all matter and energy; that is, everything within and between all galaxies

The Solar System

• The Sun and the objects that orbit it.

Distances not to scale!

The Layout of the Solar System

• Large bodies in the Solar System have orderly motions• planets orbit counterclockwise in same plane• orbits are almost circular• the Sun and most planets rotate counterclockwise• most moons orbit counterclockwise

The Layout of the Solar System

• Planets fall into two main categories Terrestrial (i.e. Earth-like) Jovian (i.e. Jupiter-like or gaseous)

Mars Neptune

Terrestrial Jovian

The Layout of the Solar System

• Swarms of asteroids and comets populate the Solar System

A Few Exceptions to the Rules…

• Both Uranus & Pluto are tilted on their sides.• Venus rotates “backwards” (i.e. clockwise).• Triton orbits Neptune “backwards.” • Earth is the only terrestrial planet with a relatively large

moon.

The Sun – King of the Solar System

• How does the Sun influence the planets?• Its gravity regulates the orbits of the planets.• Its heat is the primary factor which determines the

temperature of the planets.• It provides practically all of the visible light in the

Solar System.• High-energy particles streaming out from the Sun

influence planetary atmospheres and magnetic fields. This streaming of particles is known as the solar wind.

Apply the scientific method to formation of the Solar system!

• Make observations about the solar system

• Hypothesize

• Test or experiment

• Draw conclusions about the validity of our hypothesis

• Repeat

What is density?

density = mass/volume

typical units: [ g/cm3]

Density of water is defined as 1 g/cm3.

Definitions

• meteor- a flash of light caused when a particle from space burns up upon entering our atmosphere

• meteorite- a rock from space that lands on the Earth

• nebula- a cloud of gas in space (usually glowing)

• isotopes- elements with the same number of protons but differing numbers of neutrons

• Asteroid- a small rocky body orbiting the Sun.

• Comet- a small icy body orbiting the Sun.

• Astronomical Unit (AU)- the average Earth-Sun distance, about 15O million km (93 million miles)

Observation: Patterns of Motion

• All planets orbit the Sun in the same direction: counterclockwise when seen from above the Earth’s North Pole.

• All planetary orbits lie in nearly the same plane.

• Almost all the planets travel on nearly circular orbits, with a spacing that increases with distance according to a fairly regular trend.

Patterns of Motion

• Most planets rotate in the same direction they orbit: counterclockwise when viewed from above the Earth’s North Pole.

• Almost all moons orbit their planet in the same direction as the planet’s rotation and near the planet’s equatorial plane.

• The Sun rotates in the same direction in which the planets orbit.

Patterns of Motion

Question #1:Why are the observed motions in the solar system generally so orderly?

• Most planets have fairly small axis tilts, usually less than 25º.

Observation: Categorizing Planets

Can we categorize the planets into groups? How many categories do we need?

Terrestrial Jovian

Smaller size and mass Larger size and mass

Higher density (rocks, metals) Lower density (light gases)

Solid surface No solid surface

Closer to Sun (and closer together) Farther from Sun (and farther apart)

Warmer Cooler

Few if any moons and no rings Many moons and all have rings

Question #2:Why do the inner and outer planets divide so neatly into two classes?

Categorizing Planets

Observation: Asteroids & Comets

• No formation theory would be complete without an explanation of the most numerous objects in the solar system: asteroids and comets.

• Asteroids are small rocky bodies that orbit the Sun between the orbits of Mars and Jupiter, primarily in the asteroid belt.

Observation: Asteroids & Comets

• Their orbits generally lie close to the plane of the planetary orbits, although they are usually tilted a bit more. Some have quite large eccentricities.

• Almost 10,000 asteroids have been identified; these are probably only the largest ones. The largest asteroids have a radius of about 200 km - much less than half of the Moon’s radius.

Asteroids & Comets

• Comets are small, icy bodies residing in one of two regions, the Kuiper Belt and the Oort Cloud.

Question #3: Why are there a large number of asteroids & comets in two different locations?

Asteroids & Comets

Observation: Exceptions to the patterns

• Some objects don’t fit the general patterns:

• Mercury and Pluto have much larger eccentricities and inclinations.

• The rotational axes of Uranus and Pluto are substantially tilted.

Exceptions

• Venus rotates backwards - clockwise, rather than counterclockwise,as viewed from above Earth’s North Pole.

• While most jovian moons orbit with the same orientation as the planet’s rotation, a few orbit in the opposite direction.

• Earth has an exceptionally large moon. Pluto’s moon is almost as big as Pluto.

Exceptions

Question #4: Why are there exceptions to the general patterns?

Four principal characteristics of the Solar System

• Patterns of motion

• Two types of planets

• Asteroids and comets

• Exceptions to the patterns

A theory of the formation of the Solar System must include these.

Solar nebula theory

• In the last 20 years, a lot of evidence has accumulated in support of a model called“solar nebula theory”.

Orion Nebula : an active star-forming region.

• This model holds that our solar system formed from a giant swirling interstellar cloud of gas and dust. “Nebula” is the Latin word for cloud.

• Originally proposed by Kant (1755) and Laplace (~1795)

Solar nebula theory

• Typical star-forming clouds contain enough material to form millions of stars.

• Star systems are born within interstellar clouds where the gas is somewhat denser than 1 atom/sugarcube.

• Triggered by a cataclysmic event such as a nearby supernova?

Collapse

• An individual star system forms from a small part of a giant interstellar cloud. We call the piece of cloud that formed our solar system the solar nebula.

• At its start, the solar nebula was probably a few light-years in diameter.It then began to collapse under its own gravity.

• As it collapsed to a diameter of about 200 AU, about twice the diameter of Pluto’s orbit, three processes gave form to our solar system:

• Heating

• Spinning

• Collisions

Collapse

Heating

• Heating represents energy conservation in action.

• As the cloud shrank, its gravitational energy was converted into the energy of motion of gas particles falling inwards. These particles crashed into one another, converting energy of motion into the random motions of thermal energy.

Heating

• The solar nebula became hottest at the center, where much of the mass collected to form the protosun. The protosun eventually became so hot that fusion ignited at its core - at which point the Sun became a full-fledged star.

Spinning

• Spinning represents conservation of angular momentum.mass x distance x speed = constant

• Like an ice skater pulling in her arms as she spins, the solar nebula rotated faster and faster as it shrank in radius. The spinning helped ensure that not all of the material of the solar nebula collapsed onto the protosun.

Spinning

• The greater the angular momentum of a rotating cloud, the more spread out it will be.

Collisions

• Flattening is a natural consequence of collisions, which is why disks are so common in the universe.

• A cloud may start with any size or shape, and different clumps may be moving in random directions with random speeds.

• As the cloud collapses these clumps collide and merge, giving the new clumps the average of the old speeds.

Collisions

• Collisions also reduce their ellipticities, making the orbits more circular.

• Thus, random motions in the cloud become more orderly as the cloud collapses, changing the cloud’s original lumpy shape into a rotating, flattened disk.

• Flattening of the disk explains why all the planets orbit in nearly the same plane.

Collapse of the Solar nebula

• Collisions explains why most planets have nearly circular orbits.

• Spinning explains why planets orbit in the same direction, and also plays a role in making most of the planets rotate in the same direction.

• Heating, spinning, flattening, and collisions explain the tidy layout of our solar system, thus answering “why the orderliness”, question #1.

Evidence of nebular collapse

Four dusty protoplanetary disks around young stars in the Orion nebula. The red glow in the center of each disk is a newly formed star, roughly a million years old. Each image is a composite of emission lines from ionized oxygen (blue), hydrogen (green), and nitrogen (red).

HST

Sowing the seeds of planets

• The churning and mixing of the gas in the solar nebula ensured that its composition was about the same everywhere: 98% hydrogen and helium,and 2% heavier elements.

• How did the planets end up with such a wide variety of compositions when they came from such uniform material?

• The formation of a solid or liquid particles from a gas is called condensation.

• Pressures in the solar nebula were so low that liquid droplets rarely formed, but solid particles could condense like snowflakes condense from water vapor in our atmosphere. Such solid particles are called condensates.

Sowing the seeds

Category IngredientsCondensation Temperature

Amount in Nebula

MetalsIron, nickel, aluminum

1600 K 0.2%

RocksSilicon-based minerals

500 –1300 K 0.4%

Hydrogen compounds

CH4, NH3, H2O 150 K 1.4%

Light gases Hydrogen, helium Never condensed 98.0%

• The ingredients of the solar nebula condense at different temperatures:

Sowing the seeds

• The temperature differences between the hot inner regions and the cool outer regions determined what kinds of condensates were available to form planets.

• Near Mercury’s orbit, metal started to condense. Moving outwards to Venus and Earth, more rock condensed.

Sowing the seeds

• Only beyond the frost line, which lay between the present orbits of Mars and Jupiter, were temperatures low enough for hydrogen compounds to condense.

• So, the outer solar system contained condensates of all kinds. Since ice was nearly three times more abundant, ice flakes dominated the mixture.

Sowing the seeds

Assembling planetesimals

• Since rocks and metals only made up 0.6% of the material in the solar nebula, the planetesimals in the inner solar system could not grow very large, which explains why the terrestrial planets are relatively small.

Assembling planetesimals

• Beyond the frost line planetesimals could be built from ice flakes. Since ice flakes were more abundant, these planetesimals could grow much larger.

• These large icy planetesimals became the cores of the jovian planets, the composition of which is mostly gas.

Solar nebula theory

• Our ideas of the formation of the solar system answer question #2: the general differences between the terrestrial and jovian planets.

Competing hypothesis: a close encounter

• Early 20th century

• Planets formed from gas pulled from sun in a close encounter with another star

• Lost the competition! Couldn’t explain observed motion of the planets or the two classes of planets, Also, a close encounter with another star is unlikely to have happened.

Leftover planetesimals

• After about 10 million years, there were hundreds of protoplanets in the inner solar system and a few large ones in the outer solar system.

• Young stars tend to blow strong winds - a flow of hot hydrogen and helium gas ejected in all directions.

• About this time our protosun ignited hydrogen in its core to become a real star.

Leftover planetesimals

• That wind is much weaker today, but still with us.

• The strong wind from the young Sun blew away the excess gas, but many planetesimals remained scattered between the newly formed planets.

ulysses.jpl.nasa.gov

Eros

• These leftovers became asteroids and comets. Rock and metal ones are inside the frost line and icy ones outside the frost line.

Leftover planetesimals

Hale-Bopp

Juan Carlos Casado

Solar nebula theory

• The solar nebula theory thus explains why there are numerous asteroids and comets and why they have different compositions, i.e., question #3.

May 1996, Hyakutake

• This era of collisions between planetesimals and protoplanets would have resembled a rain of rock and ice.

Era of bombardment

• After the young Sun cleared out most of the leftover gas from the solar nebula, a period of consolidation took place.

• These three drawings show the results of a super computer simulation of the formation of the inner planets.

a) The simulation begins with 100 planetesimals.

b) After 30 million years, the planetesimals have coalesced into 22 protoplanets.

c) After 150 million years, the inner planets are essentially complete.

Era of bombardment

• Impacts were extremely common in the young solar system. They are a normal part of the accretion process during the late stages of planet formation. This process has slowed down considerable, but still continues today.

Era of bombardment

HST 1994

Solar nebula theory

• Random, giant impacts during the era of bombardment are the most promising explanation for meeting the “why the exceptions?” , question 4.

• Mercury and Pluto have larger eccentricities and inclinations.

• The rotational axes of Uranus and Pluto are substantially tilted.

• Venus rotates backwards.

• Earth has an exceptionally large moon.

• Unfortunately, this idea is hard to prove. But no other idea so effectively explains the oddities we’ve discussed.

Note: extra-solar planets challenge the theory!

• The extra-solar planets observed thus far do not seem to match the pattern of the Solar System. Scientists are trying to explain this.

Matt PetersenJason TranchidaAlison McGeary

Summary of Class #1

• Class Business• Intro to SETI (Cosmos)• Scientific method• “Observed” the Solar System

• Patterns of motion• Two types of planets• Asteroids and comets• Exceptions to the trends

• Solar Nebula theory• Collapse • Types of planets (temperature)• Solar wind• Heavy bomardment

• Extrasolar planets- may lead to some re-thinking!