The planets. 2© Sierra College Astronomy Department Lecture 5: A Planetary Overview Solar System...

The planets

2© Sierra College Astronomy Department

Lecture 5: A Planetary Overview

Solar System Roll Call

• The Sun the is largest and brightest object in the solar system

• The Sun is hot (5800 K on surface)• The Sun is gaseous and converts matter into

energy in core• The Sun has the greatest influence on the

rest of the solar system (light, solar wind…)

Sun




• Mercury is the smallest planet in the solar system • It rotates every 58.6 days and revolves every 88

days and is tidally locked to the Sun• The produces 88 days of day and 88 days of night,

making temperatures extreme (425°C to -150°C).• One spacecraft has visited Mercury and another is

on its way

Mercury




• Venus is often called Earth’s “twin” because it is nearly the same size as the Earth. But it’s nothing like the Earth…

• It rotates backwards (or upside down) very slowly

• It is covered with an atmosphere of mostly CO2 which allows a runaway greenhouse effect to occur raising the temperature to 470°C (880°F) planetwide

• Its surface pressure in 90 times greater than the Earth and there are clouds of sulfuric acid near the surface of the planet

Venus




• Earth is only world that we know of that has or had life on it

• It is the only world with a significant amount of oxygen in the atmosphere

• It is the only world with significant amounts of liquid water

• It is the closest planet to the Sun to have a moon and our Moon is quite large compared to the Earth

Earth




• Mars may bear the closest resemblance to the Earth

• It has a thin atmosphere of mostly CO2

• It has polar caps made of CO2 and water-ice

• In the past, water very likely flowed on the surface• It has great geological wonders such as a great canyon and

the largest volcano in the solar system• It has two tiny moons• It is the most studied extraterrestrial planet and has several

spacecraft present and proposed to land or orbit Mars.

Mars




• Jupiter is largest planet in the solar system and is made mostly of gas with a Earth sized rocky-ice core in the center

• It has more than 300 times the diameter and 1000 times the volume of the Earth

• Its atmosphere has many storms many of which have lasted for hundreds of years

• Its four largest moons (of 63) have interesting properties too (active volcanoes, subsurface water, magnetic fields)

Jupiter




• Saturn is another gaseous giant planet with a spectacular ring system

• The ring system is made of millions of ice-dust chunks orbiting around the planet

• Saturn has over 50 moons, a few of them midsize moons and one large one, Titan, which has a significant atmosphere.

Saturn




• Uranus (YUR-uh-nus) is a smaller gas giant with a green-blue color due to methane

• It has several dozen moons a few of which are midsize• The entire system (planet, rings, moons) is tilted on their side• It has been visited by only one spacecraft (Voyager 2)• Neptune is just a bit smaller than Uranus and bluer in color• It has a dozen moons, one of which is large (Triton). Triton is the

largest moon to go backward around the planet• It has been visited by only one spacecraft (Voyager 2)

NeptuneUranus




• Pluto (and the other Dwarf Planets) are round object which orbit around the Sun

• Pluto was discovered as a planet in 1930, but was an oddball world. One of its 3 moons is half its size (Charon). It will be visited by spacecraft in 2015.

• Soon in the 1990s other objects out where Pluto lived were being discovered. One of these, Eris, was found to be a little larger than Pluto

• In 2006, the phrase “dwarf planet” was defined for these objects and asteroids (like Ceres) which were round but were found “nearby” other solar system objects

Pluto



Solar System featurs

• Looking at the general characteristics, there are 4 features which stand out:

1. Patterns of motion among large bodies

2. Two major types of planets

3. Asteroids and comets

4. Exceptions to the rules

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Distances In The Solar System

Measuring Distances in the Solar System

• Copernicus used geometry to determine relative distances to the planets.

• Today we measure planetary distances using radar.

• Average distances to the planets from the Sun range from .387 AU for Mercury to 39.53 AU for Pluto.



Feature 1: Patterns of Motion

• All planetary orbits are ellipses, but all are nearly circular.

• Each of the planets revolves around the Sun in the same direction.

• All planets - except Venus, Uranus - rotate in a counterclockwise direction.

• Most of the satellites revolving around planets also move in a counterclockwise direction, though there are some exceptions.

orbits

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Feature 1: Patterns of Motion

• Inclination of a planet’s orbit is the angle between the plane of a planet’s orbit and the ecliptic plane (the plane of the Earth’s orbit).

• The elliptical paths of all the planets are very nearly in the same plane (inclination about 0°), though Mercury’s orbit is inclined at 7° and Pluto’s at 17°.

tilts



Planet Diameters

Diameters of Non-Earth Planets• Diameters are determined from distances (from

the Earth to the planet) and the planet’s angular size via the small angle formula (Cosmic Calculations 2.1)

• Diameter of Sun (1.39 × 106 km) is over 100 times that of Earth (1.3 × 104 km).

• Jupiter’s diameter is 11 times that of Earth.• Pluto’s diameter is 1/5 that of Earth.



Planet MassesMass of the Planets• Kepler’s third law was reformulated by Newton

to include masses (Cosmic Calculations 4.1):

a3/p2 = K (M1 + M2)• Newton’s statement of Kepler’s third law allows

us to calculate the mass of the Sun.• Consider the orbits of planets around the Sun.

Since one of the masses to the Sun (the other being a planet), the sum of the two is essentially equal to the mass of the Sun, and the equation can be rewritten as:

a3/p2 = KM



Planet Masses

• We can do the same sort of calculation for planets as long as they have satellites orbiting them

• The masses of 7 of the 9 known planets can be calculated based on the distances and periods of revolution of these planets’ natural satellites.

• For Mercury and Venus, which do not possess any natural satellites, accurate determinations of their respective masses had to await orbiting or flyby space probes.



Feature 2: Classifying the Planets

• The planets (except Pluto) fit into two groups: the inner terrestrial planets and the outer Jovian planets.

Size, Mass, and Density• The Jovian planets have much bigger

diameters and even larger masses than the terrestrial planets.

• Terrestrial planets are more dense, however.• Earth is the densest planet of them all.

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Insidethe planets

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Classifying the Planets

Satellites and Rings• The Jovian planets have more satellites than

the terrestrials.• 4 Jovian planets: 163 total satellites as of

September 2007 (63 for Jupiter, 60 for Saturn, 27 for Uranus, and 13 for Neptune).

• 4 terrestrial planets: 3 total satellites.• Pluto has 3 satellites.• Each Jovian planet has a ring or ring system.

None of the terrestrial planets do.


Terrestrial JovianNear the Sun Far from the Sun

Small Large

Mostly solid Mostly liquid & gas

Low mass Great mass

Slow rotation Fast rotation

No rings Rings

High density Low density

Thin atmosphere Dense atmosphere

Few moons Many moons

A comparison of planetary characteristics

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Feature 3: Asteroids and Comets

Asteroids• These rocky bodies orbit the Sun, but are

much smaller than planets. Most lie between Mars and Jupiter

Comets• Small icy (water, ammonia, methane) objects

which occasionally visit the inner solar system and become visible

• Comets originate from two regions: the Kuiper Belt and the Öort Cloud

Asteroids

comets



Feature 4: Exceptions to the Rules

• There are objects in the solar system that are unusual or have characteristics which are unusual as compared to the rest of the solar system. Some examples: Venus and Uranus rotate differently (backwards and on its

side, respectively) Small moons of Jupiter and Saturn and the large moon

Triton (around Neptune) revolve in the opposite direction of the rotation of the host planet.

While other terrestrial planets have no moons (Mercury, Venus) or tiny moons (Mars) The Earth’s moon is large compared to the Earth.

Asteroids

comets



The Formation of the Solar System

Evolutionary Theories• All evolutionary theories have their start with

Descartes’s whirlpool or vortex theory proposed in 1644.

• Using Newtonian mechanics, Kant (in 1755) and then Laplace (around 1795) modified Descartes’s vortex to a rotating cloud of gas contracting under gravity into a disk.

• The Solar Nebula Hypothesis is an example of an evolutionary theory.

SolarNebula



The Formation of the Solar SystemCatastrophic Theories• Catastrophic theory is a theory of the formation

of the solar system that involves an unusual incident such as the collision of the Sun with another star.

• The first catastrophic theory - that a comet pulled material from the Sun to form the planets - was proposed by Buffon in 1745.

• Other close encounter hypotheses have been proposed too.

• Catastrophic origins for solar systems would be quite rare (relative to evolutionary origins) due to the unusual nature of the catastrophic incident.



Solar Nebula Hypothesis

Towards a Solar Nebula Hypothesis• The nebular cloud collapsed due the force of gravity

on the cloud. But the cloud does not end up spherical (like the sun) because there are other processes going on: Heating – The cloud increases in temperature, converting

gravitational potential energy to kinetic energy. The sun would form in the center where temperatures and densities were the greatest

Spinning – as the cloud shrunk in size, the rotation of the disk increase (from the conservation of angular momentum).

Flattening – as cloud starting to spin, collisions flattened the shape of the disk in the plane perpendicular to the spin axis

Cloudcollapse2



Testing the Model

• If the theory is correct, then we should see disks around young stars

• Dust disks, such as discovered around beta-Pictoris or AU Microscopii, provide evidence that conditions for planet formation exist around many Sun-like stars.

AU Mircoscopii HD 141569A

Disks around other stars



Solar Nebula HypothesisThe Formation of Planets• As the solar nebula cooled and flattened into a disk some 200 AU

in diameter, materials began to “freeze” out in a process called condensation (changing from a gas to a solid or liquid).

• The ingredients of the solar system consist of 4 categories (with % abundance):

1. Hydrogen and Helium gas (98%)2. Hydrogen compounds, such as water, ammonia, and methane (1.4%)3. Rock (0.4%)4. Metals (0.2%)

Since it is too cool for H and He to condense, a vast majority of the solar nebula did not condense

Hydrogen compounds could only condense into ices beyond the frost line, which lay between the present-day orbits of Mars and Jupiter

Frost line



Solar Nebula HypothesisBuilding the Terrestrial Planets• In the 1940s, Weizsächer showed that eddies would form in a

rotating gas cloud and that the eddies nearer the center would be smaller.

• Eddies condense to form particles that grow over time in a process called accretion. Materials such and rock and metal (categories #3 and #4).

• These accreted materials became planetesimals, which in turn sweep up smaller particles through collision and gravitational attraction.

• These planetesimals suffered gravitational encounters which altered their orbits caused them to both coalesce and fragment. Only the largest planetesimals grew to be full-fledged planets.

• Verification of this models is difficult and comes in the form of theoretical evidence and computer simulations.




Building the Jovian Planets• Planetesimals should have also grown in the

outer solar system, but would have been made of ice as well as metal and rock.

• But Jovian planets are made mostly of H and He gas…

• The gas presumably was captured by these ice/rock/metal planetesimals and grew into the Jovian planets of today.



Solar Nebula Hypothesis• Stellar wind is the flow of nuclear particles

from a star.• Some young stars exhibit strong stellar

winds. If the early Sun went through such a period, the resulting intense solar wind would have swept the inner solar system clear of volatile elements.

• The giant planets of the outer solar system would then have collected these outflowing gases.




Explaining Other Clues

• Over millions of years the remaining planetesimals fell onto the moons and planets causing the cratering we see today. This was the period of heavy bombardment.

• Comets are thought to be material that coalesced in the outer solar system from the remnants of small eddies.




• The formation of Jovian planets and its moons must have resembled the formation of the solar system. Jupiter specifically: Moons close to Jupiter are denser and

contain fewer light elements; Moons farther out decrease in density and

increase in heavier elements.



The Exceptions to the Rule

• Captured Moons – satellites which go the opposite way were likely captured. Most of these moon are small are lie far away from the planet.

• Giant impacts – may have helped form the Moon and explain the high density of Mercury and the Pluto-Charon system. Furthermore, the unusual tilts of Uranus and Venus can also be explained by giant impacts.

Giant impactMoon

Solar Nebula TheorySummary


Lecture 6: A Solar System Overview

RadioactivityRadioactivity• Certain isotopes (elements which contain differing

number of neutrons) are not stable and will decay into two or more lighter elements

• The time it takes for half of a given isotope to decay is called the half-life

• By noting what percentage a rock (or human body) has left of a radioactive element can enable us to estimate the age of that object. This process is called radioactive dating. See Cosmic Calculations 6.1

Half-life


Lecture 6: A Solar System Overview

Radioactivity

Earth rocks, Moon rocks, and meteorites• The oldest Earth rock date back to 4 billion years

and some small grains go back to 4.4 billion years. Moon rock brought back from the Apollo mission date as far back as 4.4 billion years. These tell us when the rock solidified, not when the

planet formed

• The oldest meteorites, which likely come form asteroids, are dated at 4.55 billion years, marking the time of the accretion of the solar system

Half-life



Planetary Systems Around Other Stars?

• Photographing planets around stars directly is very difficult since planet merely reflect (visible) light from the nearby stars. Using the infrared part of the spectrum, we can detect large objects known as brown dwarfs which are neither stars or planets

• Stars exhibiting a discernable wobble from gravitation tugs can be evidence of an unseen companion - such as a large planet or group of planets. One can try to look for positional changes in the sky form this star – the astrometric technique, but this is difficult.

• Since 1995, this Doppler Technique has found evidence of over 170 planets orbiting stars in the near vicinity of the Sun.

• Some of the extrasolar planets can be detected when the transit the star. The star’s brightness dims just a bit during the transit.

• Web link: http://exoplanets.org/

Demo

COMJupiterSun

Doppler

AstrometricJupiterSun

DopplerVelocitycurve

transit

BrownDwarf

http://exoplanets.org/





Planetary Systems Around Other Stars?

• Comparisons to our Solar System Many of these planets are more massive than Jupiter Many of these planets are closer to their star than

Mars is to the Sun• These discoveries are in part due to a selection effect –

these are the easiest to detect Jovian sized planets close to the star is not consistent

with the standard solar nebular model. So how does one form a “hot Jupiter”?

• Planetary migration – the gas giant form in the cooler, outer region of the nebular disk, but due to friction (and a loss of angular momentum) from the nebular disk, the planet in brought to a much closer distance.

Orbits

Mass

Planetarymigration


The End



Planetary Atmospheres & Escape Velocity

The Atmospheres of the Planets

• Ten times the average speed of molecules at a particular temperature provides a good measure of whether a planetary body will retain a gas for billions of years.

• Because of their size (and mass) the Jovian planets have retained almost all of their gases.

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atmosphericspeed



Planetary Atmospheres & Escape Velocity

• Escape velocity is the minimum velocity an object must have in order to escape the gravitational attraction of an object such as a planet.

• Earth’s escape velocity is 11 km/s. The Moon’s escape velocity is only 2.5 km/s. Jupiter’s escape velocity is 59 km/s

• Phobos (a moon of Mars) is so small that its escape velocity is about 50 km/hr (13.9 m/s).

R

GMV

2esc

atmosphericspeed

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A rotating cloud of gas contracts and flattens …

to form a thin disk of gas and dust around the forming sun at the center.

Planets grow from gas and dust in the disk and are left behind when the disk clears.

The Solar Nebula Hypothesis




• An object shrinking under the force of gravity heats up. High temperatures near the newly formed Sun (protosun) will prevent the condensation of more volatile elements. Planets forming there will thus be made of nonvolatile, dense material.

• Farther out, the eddies are larger and the temperatures cooler so large planets can form that are composed of volatile elements (light gases).

PlanetBuilding



Solar Nebula Hypothesis• Problem: The total angular momentum of

the planets is known to be greater than that of the Sun, which should not occur according to conservation laws (i.e. the present Sun is spinning too slowly).

• Solution: As the young Sun heated up, it ionized the gas of the inner solar system. The Sun’s magnetic field then swept through the

ions in the inner solar system, causing ions to speed up.

As per Newton’s third law, this transfer of energy to the ions caused the Sun to slow its rate of rotation.




• A rotating, contracting disk of gas will speed up according to the law of conservation of angular momentum. Angular momentum of an object is the product

of that object’s mass (m), speed of rotation (v), and distance from the center of rotation (r).

Conservation of angular momentum means that (in the absence of an outside force) as the distance to the spin axis decreases (contraction), the speed increases.

A.M. = m×v×rDemo

The planets. 2© Sierra College Astronomy Department Lecture 5: A Planetary Overview Solar System...

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