Eps Sci 3: Lecture 4. Scientific Method and Planet Formation

what can happen to light?

• reflection

• absorption

• transmission

→ provides evidence about distant stars, planets, and the clouds of dust where stars form

scientific process

1. observation

2. hypothesis

3. prediction

4. test

5. revise

confirmation bias

tendency to favor info that supports what we already believe

false consensus effect

occurs when we overestimate how many people agree with us

Why are planets called wandering stars?

• comes from Greek word “planetes” = wandere

Retrogradem otion

the apparent backward movement of a planet across the night sky as seen from Earth

Aristotle (~ 350 BCE)

proposed the geocentric model (Earth is stationary at the center of the universe) → established the Earth-centered view that dominated astronomy for centuries

Aristarchus of Samos (310-230 BCE)

proposed an early heliocentric model (Earth rotates daily and revolves around the Sun annually) → introduced the sun-centered idea long before copernicus, but it was not widely accepted

Ptolemy (2nd century CE)

proposed epicycles (small circular orbits within larger paths) to explain retrograde motion → improved the geocentric’s model ability to predict planetary movements BUT model became increasingly complicated and later more precise observations exposed its inaccuracies

Hypatia (~400 CE)

contributed to the preservation and development of astronomical and mathematical knowledge

Nicolaus Copernicus (1473 - 1543)

proposed a heliocentric model with planets orbiting the sun in circular paths → challenged geocentrism and established the correct order of the known planets

Tycho Brahe (1546 - 1601)

supported an earth-centered planetary system and collected extremely precise astronomical observations → provided the measurements needed to test and improve planetary modelsG

Giordano Bruno (1548 - 1600)

proposed that stars extend throughout infinite space and that other inhabited worlds exist → expanded thinking about the possibility of life beyond earth

Johannes Kepler (1571 - 1630)

proposed that planets move in elliptical orbits, developing 3 laws of planetary motion → successfully explained planetary movements using Brahe’s observations

What evidence tells us how the Solar System formed?

the motion of planets

1. the solar system is relatively flat (most planets orbit with the same plane = ecliptic plane)

2. planetary orbits are mostly circular (most major planets have low orbital eccentricities = circular orbit)

3. all 8 planets orbit the sun in the same direction in counterclockwise (suggests common origin in rotating material)

4. the sun contains 99% of the solar system’s mass

Planetary compositions: inner planets

rocky and dense

planetary composition: outer planets

rich in gases and ices

why do planets have different compositions?

temperature

• early solar system was much hotter near the young sun and colder further away

Asteroids

mostly rocky, sometimes containing ice

between mars and jupiterC

Comets

mostly icy, with some rocky materials

outer solar system and beyond

Meteorites

preserve unprocessed material from early solar system → have common reservoir of material as Sun

Isotopes

atoms of the same element that have different number of neutrons

• some are unstable → can transform into another nucleus through radioactive decay

half-life

amount of time required for half of the radioactive parent atom in a sample to decay

N = N0 (1/2)^t/t(1/2)

Solar Nebula model

our solar system formed from large cloud of interstellar gas and dust

Cloud collapse

1. conservation of energy: as gravity pulls material inward, gravitational potential energy is converted into kinetic and thermal energy

2. conservation of angular momentum: if original cloud is rotating, it leads to spin faster as it contracts

   1. rotation = helps determine final shape of forming solar system

rohow does a spherical cloud form a flat disk?

1. cloud is relatively diffused and roughly spherical

2. gravity pulls materials inward (contraction)

3. because cloud rotates, the collapse does not occur equally in all directions

4. becomes flattened disk surrounding a growing central protostar

protoplanetary disk

rotating disk of gas and dust surroungind a young star

How the solar system was formed causation

1. rotating gas cloud

2. gravitational collapse

3. faster rotation

4. flattened disk

5. planets forming in a shared orbital plane

W

Why would a gas cloud collapse in the first place?

1. gravity pulls the cloud inward

2. thermal gas pressure resists compression

If gravity becomes sufficiently strong relative to internal pressure, the cloud can collapse

Orion Nebula (aka M42)

nearby region of active star formatino

Interstellar Medium (ISM)

gas and dust occupying the space between stars

• 99% gas (primarily hydrogen and helium)

• 1% dust and other solid particles (very small, around 1 µm or less)

Jeans’ Collapse Criterion

helps determine whether a gas cloud can collapse under its own gravity

• the cloud is colder (less thermal pressure resisting compression)

• the cloud is denser (stronger self-gravity)

• has sufficient mass

Jeans mass

approx min mass required for gravitational instability under specified conditions

• increasing temp while density is constant → increased Jeans mass = hotter clouds need more mass before gravity can overcome with thermal pressure

• increase density → lower jeans mass = denser clouds can become gravitationally unstable at smaller masses

Atacama Large Millimeter/submillimeter Array (ALMA)

observes radiation at millimeter and submillimeter wavelengths → study cold material in planet-forming disks

**rotating disks of material around young stars actually exist

HL Tau

a young star surrounded by prominent protoplanetary disk

Porplyd

short for protoplanetary disk

young stellar objects surrounded by disk material

→ observe star formation and disk formation within the same region

• no longer relying on indirect evidence from our solar system

• study planetary systems at early stages of development

The pleiades (aka the seven sisters)

• relatively young star cluster

• most of original parent cloud has dispersed

• stars are clearly white

**star formation is observable, and the stages of stellar formation and cluster dispersal can be studied directly

The Hyades

• older open star cluster

• consists of stars moving together in space

• represents a more mature stellar population

• most of original star-forming gas and dust has dispersed

**star formation is observable, and the stages of stellar formation and cluster dispersal can be studied directly