Accretion and Formation of Planets Animation

Origin of the AsteroidsThe Solar wind cleared the leftover gas, but not the leftover planetesimals.

Those leftover rocky planetesimals which did not accrete into a planet, crash into inner planets, or get ejected from solar system are the present-day asteroids.

Most inhabit the asteroid belt between Mars & Jupiter. Jupiter’s gravity prevented a planet from forming there.

Origin of the Comets

The leftover icy planetesimals are the present-day comets.

Those which were located between the Jovian planets, if not captured, were gravitationally flung in all directions into the Oort cloud.

Those beyond Neptune’s orbit less likely to be destroyed by collision or flung off - remained in the ecliptic plane in the Kuiper belt. Didn’t grow as large as Jovian planets - material density too low - still managed to grow to large sizes- hundreds of kilometers in diameter or more - Pluto probably a Kuiper belt cometThe nebular theory predicted the

existence of the Kuiper belt 40 years before it was discovered!

Exceptions to the Rules

There were many more leftover planetesimals than we see today.

Most of them collided with the newly-formed planets and moons during the first few 100 million years of the Solar System.

This was the heavy bombardment period.

So how does the nebular theory deal with exceptions, i.e. data which do not fit the model’s predictions?

• Why some moons orbit opposite their planet’s rotation– captured moons (e.g. Triton)

• Why rotation axes of some planets are tilted– impacts “knock them over” (extreme example: Uranus)

• Why some planets rotate more quickly than others– impacts “spin them up”

• Why Earth is the only terrestrial planet with a large Moon– giant impact

• Source of water on Earth– impact of icy planetesimals flung into the inner solar system by

gravitational encounters with Jovian planets

Close encounters with and impacts by planetesimals could explain:

Formation of the Moon(Giant Impact Theory)

• The Earth was struck by a Mars-sized planetesimal

• A part of Earth’s mantle was ejected

• This coalesced into the Moon.– it orbits in same direction as

Earth rotates– has lower density than Earth -

formed from Earth’s outer layers– has smaller amounts of easily

vaporized ingredients (e.g., water)

– Earth was “spun up”

Extrasolar Planets

• Planets which orbit other stars are called extrasolar planets.

• Over the past century, we have assumed that extrasolar planets exist, as evidenced from our science fiction.

• We finally obtained direct evidence of the existence of an extrasolar planet in the year 1995.– A planet was discovered in orbit around the star 51 Pegasi.– Over 200 such extrasolar planets are now known to exist.

Detecting Extrasolar Planets

• Can we actually make images of extrasolar planets?– No, this is very difficult to do.

• The distances to the nearest stars are much greater than the distances from a star to its planets. – The angle between a star and its planets, as seen from Earth, is too

small to resolve with our biggest telescopes.

• A star like the Sun would be a billion times brighter than the light reflected off its planets.

• As a matter of contrast, the planet would be lost in the glare of the star.

• Improved techniques of interferometry may solve this problem someday.

Detecting Extrasolar Planets

• We detect the planets indirectly by observing the star.

• Planet gravitationally tugs the star, causing it to wobble.

• This periodic wobble is measured from the Doppler Shift of the star’s spectrum.

Stellar Motion and Planets Animation

Doppler shift allows detection of slight motion of star caused by orbiting planet

Determining Star’s Velocity Animation

A plot of the radial velocity shifts forms a wave.

–Its wavelength tells you the period and size of the planet’s orbit.

–Its amplitude tells you the mass of the planet.

Doppler shift in spectrum of star 51 Pegasi - shows presence of large planet with orbital period of about 4 days.

Determining Planet Mass and Orbit Animation

Remember - Doppler shift only tells us radial motion. If plane of orbit perpendicular to our line of sight - no shift seen. If we view it from edge on, maximum Doppler shift seen. Orbit generally tilted at some angle - star’s full speed not measured. So mass derived from Doppler technique is minimum possible. If changing velocity and varying position in sky measured (as in one case - Gliese 876) orbital tilt can be determined and mass measured accurately. Gliese 876 is only about 15 LY away.

• The Doppler technique yields only planet masses and orbits.

• Planet must eclipse or transit the star in order to measure its radius.

• Size of the planet is estimated from the amount of starlight it blocks.

• We must view along the plane of the planet’s orbit for a transit to occur.– transits are relatively rare

• They allow us to calculate the density of the planet.– extrasolar planets we have detected have Jovian-like densities.

Planetary Transit

Planetary Transit Animation

Orbital distances and approximate masses of first 77 planets discovered. There have been 313 extrasolar planets discovered to date.

Gliese 581 d - the third planet of the red dwarf star Gliese 581 (approximately 20 light years distance from earth)

- appears to be the best example yet discovered of a possible terrestrial exoplanet which orbits close to the habitable zone of space surrounding its star- appears to reside outside of the "Goldilocks" zone, but greenhouse effect may raise the planet's surface temperature to that which would support liquid water.

False-color infrared image of the brown dwarf 2M1207 (blue) and its planetary companion 2M1207b (red), as viewed by the Very Large Telescope.

- only confirmed extrasolar planet to have been directly imaged.

AtmospheresAtmosphere - a layer of gas which surrounds a world

– usually very thin compared to planet radiusPressure is created by atomic and molecular collisions in an atmosphere.

– heating a gas in a confined space increases pressure– number of collisions increase– unit of measure: 1 bar = 14.7 lbs/inch2 = Earth’s atmospheric pressure at sea level

Pressure balances gravity in an atmosphere– atmospheric pressure equal to weight of a column of gas extending upward

Earth’s atmosphere extends several hundred kilometers into space– no official boundary

Low-Earth orbiting satellites (a few hundred kilometers) experience atmospheric drag– slows them down so they spiral downward, eventually burning up as they reenter the dense lower atmosphere– Space Station and Hubble space telescope have to be periodically boosted to higher orbits – larger satellites don’t completely burn up - space debris

AtmospheresAtmosphere - layer of gas surrounding a world

Atmospheric pressure - collisions of individual atoms or molecules in atmosphere

Air molecules in a balloon exert pressure as they collide with the walls pushing outward. Air molecules outside balloon collide with wall and exert pressure inward. Balloon stays inflated when pressures are balanced.

Adding molecules to balloon (blow it up) causes balloon to expand (increases its volume) until pressures are balanced again.

Heating it also increases pressure (increases the speed of the molecules). The balloon expands until pressures are equalized again

Gas in an atmosphere is held down by gravity.

Atmosphere above presses downward, compressing atmosphere below.

At the same time, fast moving molecules exert pressure in all directions, including upward - tends to make atmosphere expand.

Planetary atmospheres exist in balance between downward weight of their gases and upward push of their gas pressure

The higher you go, the less the weight of gas above you, and the less the atmospheric pressure.

1 bar - atmospheric pressure at sea level - equal to weight of a column of gas extending upward from Earth’s surface from sea level

Effects of an Atmosphere on a Planetgreenhouse effect

- makes the planetary surface warmer than it would be otherwiseextreme on Venusjust right for life on Earthweak on Mars

- distributes heat around planetscattering and absorption of light

- absorb high-energy radiation from the Sun- scattering of optical light brightens the daytime sky

creates pressure- can allow water to exist as a liquid (at the right temperature)

creates wind and weatherpromotes erosion of the planetary surface

creates magnetosphere- caused by interaction of atmosphere with the Solar wind when magnetic fields are

present- protects atmosphere from loss of gases- protects surface from high-energy solar particles- leads to aurora

The Greenhouse Effect

Visible Sunlight passes through a planet’s atmosphere.Some of this light is reflected, some is absorbed by the planet’s surface - heats surface upPlanet re-emits this energy (heat) as infrared (IR) light - blackbody (thermal) spectrumIR light is “trapped” by the atmosphere.

- absorbed and re-emitted in random directions by greenhouse gases - H2O, CO2, CH4 (methane)- surrounding air is heated

This causes the overall surface temperature to be higher than if there were no atmosphere at all.

The Greenhouse Effect Animation

Greenhouse Gases

Key to Greenhouse Effect…gases which absorb IR light effectively:

- water [H2O]

- carbon dioxide [CO2]

- methane [CH4]These are molecules which rotate and vibrate easily.

- they re-emit IR light in a random direction

The more greenhouse gases which are present, the greater the amount of surface warming.

Greenhouse Gases Animation

Planetary Energy Balance

Solar energy received by a planet must balance the energy it returns to space

- planet can either reflect or emit the energy as radiation- this is necessary for the planet to have a stable temperature

What Determines a Planet’s Surface Temperature?

Greenhouse Effect cannot change incoming Sunlight, so it cannot change the total energy returned to space.

- it increases the energy (heat) in lower atmosphere

- it works like a blanket - it slows the escape of heat

In the absence of the Greenhouse Effect, what would determine a planet’s surface temperature?

- the planet's distance from the Sun

- the planet’s overall reflectivity

- the higher the albedo (the reflectivity of the surface), the less light absorbed, planet cooler

- Earth’s average temperature would be –17º C (–1º F) without the Greenhouse Effect

The inverse square law for light. At greater distances from the Sun, the same amount of light passes through an area that gets larger with the square of the distance. The amount of light per unit area therefore declines with the square of the distance. The closer a planet it to the Sun, the more light it receives.

The Inverse Square Law

Substance/Object Albedo

Enceladus 0.8

Europa 0.6

Forest 0.05 - 0.10

Granite 0.30 - 0.35

Grass 0.05 - 0.30

Mars 0.25

Moon 0.12

Sand 0.20 - 0.40

Snow 0.6

Soil 0.05 - 0.30

Urban-areas 0.05 - 0.20

Albedo - the fraction of light that is reflected or scattered by a body or surface

Albedo

A = [total scattered light]/[total incident light]

Temperature vs Reflectivity Animation

Greenhouse Effect on the Planets

Greenhouse Effect warms Venus, Earth, and Mars

- on Venus: it is very strong

- on Earth: it is moderate

- on Mars: it is weak

- average temperature on Venus and Earth would be freezing without it

Light Scattering

Atmospheric gases are largely transparent to visible light

Most photons penetrate to the ground, warming it as the light is absorbed

Small portion of light is scattered- why our sky is bright- light is not scattered on Moon, Mercury

- their skies are dark - stars are visible during day- shadows extremely dark

Gas molecules scatter blue light more effectively than red light

Atmospheric gases scatter blue light more than red light. During most of the day, you therefore see blue photons coming from most directions in the sky - sky looks blue. Only red photons reach your eyes at sunrise or sunset - light must travel a longer path through the atmosphere to reach you. Atmosphere on Mars too thin to scatter light effectively - sky is reddish from presence in the atmosphere of reddish dust from surface. On Venus, almost all blue light scattered away - atmosphere dimly lit and appears reddish orange.

Atmospheric Structure

Atmospheric structure determined by interactions of light from the Sun and the atmospheric gases

X rays- ionize atoms & molecules- dissociate molecules- absorbed by almost all gases

Ultraviolet (UV)- dissociate some molecules

- absorbed well by O3 (ozone) and H2O

Visible (V)- passes right through gases- some photons are scattered

Infrared (IR)- absorbed by greenhouse gases

Structure of Earth’s Atmospherepressure and density of atmosphere decrease with altitude

temperature varies “back and forth” with altitude

- these temperature variations define the major atmospheric layers

exosphere

- low density; fades into space

thermosphere

- temp begins to rise at the top

stratosphere

- rise and fall of temp

troposphere

- layer closest to surface

- temp drops with altitude

Reasons for Atmospheric Structure Light interactions are responsible for the structure we see.

Troposphere

- absorbs IR photons from the surface

- temperature drops with altitude

- hot air rises and high gas density causes storms (convection)

Stratosphere

- lies above the greenhouse gases (no IR absorption)

- absorbs heat via Solar UV photons which dissociate ozone (O3)

- UV penetrates only top layer; hotter air is above colder air

- no convection or weather; the atmosphere is stratified

Thermosphere

- absorbs heat via Solar X-rays which ionizes all gases

- contains ionosphere, which reflects back human radio signals

Exosphere

- hottest layer; gas extremely rarified; provides noticeable drag on satellites

Structure of Terrestrial Planet Atmospheres

Mars, Venus, Earth all- have warm tropospheres (and

greenhouse gases)- have warm thermospheres

which absorb Solar X rays

Only Earth has- a stratosphere - because it

contains a UV-absorbing gas (O3)

.All three planets have warmer surface temps due to greenhouse effect

Planets with very little gas like Mercury only have an exosphere

Magnetospheres

The Sun ejects a stream of charged particles, called the solar wind.

- it is mostly electrons, protons, and Helium nuclei

Earth’s magnetic field diverts these charged particles and allows them to enter the atmosphere only near the poles

- the particles spiral along magnetic field lines and impact the atmosphere causing it to fluoresce

- this causes the aurora (aka northern and southern lights)

- this protective “bubble” is called the magnetosphere

Other terrestrial worlds have no strong magnetic fields and thus no magnetosphere

- solar wind particles impact the exospheres of Venus and Mars

- solar wind particles impact the surfaces of Mercury and Moon

The Earth’s Magnetic Field

The rotating molten metallic core of the Earth generates a magnetic field with magnetic field lines like those illustrated by the influence of a bar magnet on iron filings.