Solar System

Sun – we’ll talk about this in Part 4

Planets

Moons of the planets

Asteriods

Plutoids

Kuiper Belt objects

Comets

Meteors, meteoroids, meteorites

PlanetsBasic Information

Motions: inclinations to the ecliptic, directions, “inferior” and “superior”

Types: terrestrial and giant

Water-ice clouds, polar ice, polar regions, and geological features can be seen in this full-disk image of Mars.

A true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon Europa casts a shadow on the planet's cloud tops.

Basic Information

How many planets are there?

To answer the first question, we have to ask the question: how do we define a planet?

Basic Info

How do we define a planet?

The first definition was for the objects in the sky that “moved” relative to the background starts. Based on this definition, there were five planets when we were limited to our own vision: Mercury, Venus, Mars, Jupiter, and Saturn.

Basic Info

After the telescope was invented, we realized that earth was a planet (6). We also found two others: Uranus (7) and Neptune (8).

Later, a ninth planet, Pluto, was found.Recently, we have located lots of other things

that move relative to the background star: asteroids, comets, and most recently Kuiper belt objects.

Basic InfoThis large number of objects has called into

question the definition of a planet. The actual definition of a planet is still under dispute since most don’t want asteroids and comets to be included, and most don’t want the Kuiper belt objects to be included also. This brings up the problem of Pluto – it seems to belong to the Kuiper belt group, but it has traditionally been the ninth planet for quite a while.

Basic Info

Because of the disagreements among many astronomers, for the purposes of this class, I will accept either the traditional nine planets:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto;

or I will accept that Pluto does not belong and that there are only eight.

Motions of the Planets:Inclinations to the ecliptic

The orbit of the earth around the sun defines a plane in space called the ecliptic plane. All of the planets orbit close to this plane, but not exactly on it. The furthest off from this plane are the Plutoids Eris (44o off) and Pluto (17o off), with Mercury (7o off) next. The next furthest is Venus at only 3o off.

ecliptic

Mercury at 7o

Pluto at 17o

Motions of the Planets:Direction of orbit, spin, moons

When viewed from a location above the North Pole, all of the planets orbit around the sun in a counterclockwise direction.

When viewed from a location above the North pole, 6 of the 9 planets rotate (spin) in a counterclockwise direction; one rotates almost on its side (Uranus); one rotates clockwise – but very slowly (Venus),; and the other, Pluto, rotates at an angle of 120o.

All of the major moons have orbits that are close to the equators of their planets and go in the same direction as the planet’s rotation except Triton around Neptune.

Basic Information:Order & Distance

Planet radius of orbit in period in size compared rotation length of tilt of 106 miles / A.U days years to earth period “day” axis

Mercury 36 .387 88 d 0.38 58 d 176 d 2o

Venus 67 .723 225 d 0.95 -243 d 117 d 177.3o

Earth 93 1.00 1 y 1.00 24 h 23.5o

Mars 142 1.52 1.88 y 0.53 24 h+39m 25.2o

Jupiter 484 5.20 5.20 y 11.2 9 h+55m 3.1o

Saturn 888 9.54 29.5 y 9.45 10 h+14m 26.7o

Uranus 1,783 19.2 84.0 y 4.01 -16 h+30m 97.8o

Neptune 2,800 30.1 164.8 y 3.88 19 h+6m 29.6o

*Pluto 3,673 39.5 248.6 y 0.18 -6.39 d 118o

*Eris 38-97 560 y 0.19

Motions of the PlanetsSince the planets move around the sun at different

speeds in different orbits, the planets will appear at different places in the sky at different times but always near the ecliptic.

Whenever a planet is a “morning star”, that is, it appears in the morning, it will be “west” of the sun regardless of whether it is in the eastern or western part of the sky.

Whenever a planet is an “evening star”, it will be “east” of the sun regardless of whether it is in the east or west.

“Inferior” PlanetsAn “inferior” planet is one who’s orbit is inside the earth’s

orbit. The inferior planet moves faster around its orbit than the earth since it is closer to the sun and so has to move faster to prevent it from falling into the sun.

The diagram below is from the perspective as being viewed from above the North pole. The orbit of the planet, the earth, and the earth’s spin are all counterclockwise when viewed from above the North pole.

Superior conjunction

Inferior conjuction

Maximum Eastern elongation – planet is “east” of the sun in the evening

noonmidnight

evening

morning

Maximum Western elongation – planet is “west” of the sun in the morning

“Inferior” PlanetsMaximum elongation for Mercury is 28o. This means it rises

or sets at most about two hours before or after the sun.

Maximum elongation for Venus is 48o. This means it rises or sets at most about 3 hours before or after the sun.

28o

48o

Phases for the “inferior” planetsWhen the “inferior” planet is near superior conjunction, most

of the visible side is in daylight. When the planet is near inferior conjunction, most of the visible side in in darkness – leading to a crescent shape when viewed through a telescope.

Superior conjunctionInferior conjunction

earth

Inferior planet

“Superior” planetsA “superior” planet is one who’s orbit is outside the earth’s

orbit. The superior planet moves slower around its orbit than the earth since it is further from the sun and so has to move slower to prevent it from escaping out into space.

The diagram on the next slide is from the perspective as being viewed from above the North pole. The orbit of the planet, the earth, and the earth’s spin are all counterclockwise when viewed from above the North pole.

Note that the planets will always have most of their visible surface in daylight no matter where the planet is in its relative orbit.

“Superior” planets

90o

90o

midnight

noon

eveningmorning

opposition conjunction

Eastern Quadrature – planet is “east” of the sun in the evening by 90o.

Western Quadrature – planet is “west” of the sun in the morning by 90o.

Superior planet

Choose a planetThere is a lot of info about each of the planets in your

text and on the web. You will be responsible for writing a short descriptive paragraph about one terrestrial type planet (Mercury, Venus, or Mars) and one giant type planet (Jupiter, Saturn, Uranus, or Neptune).

Your paragraph might include info about the planet’s: atmosphere (if any), surface features, temperature extremes, magnetic fields, rings (if any), and major moons.

http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys

Terrestrial Planetsbasic information

Planet: Mercury Venus Mars

Temperature range -279 to 801 F ~ 875 F -195 to 70

Ellipse: (million miles) 29 to 43 66.8 to 67.7 128 to 154

Atmosphere: ~ none 90X earth’s 0.7% of earth’s mainly CO2 mainly CO2

Volcanos none active extensive large ones turnover of surface

Moons: none none two small ones Deimon &

Phobos

Moons

This photo illustration shows selected moons of our solar system at their correct relative sizes to each other and to Earth.

Major Moons (r > 1,000 km)

in the Solar SystemMercury: no moons

Venus: no moons

Earth: The Moon (r = .27*Rearth)

Mars: two minor moons, Phobos and Deimos

Jupiter: Io (r = .28*Rearth), Europa (r = .25*Rearth),

Ganymede (r = .41*Rearth), Calliso (r = .38*Rearth)

Saturn: Titan (r = .40*Rearth) Uranus: several minor moons

Neptune: Triton (r = .21*Rearth)

*Pluto: one minor moon, Charon

*Eris: one minor moon, Dysnomia

Major Moons

There is more and more information being collected on the major moons. Read the text or go on-line to find out information about any or all of these major moons.

http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys&Display=Moons

Note: Pluto has a diameter of 2,274 km, or .18*Rearth which means all 7 of the major moons are bigger than Pluto. Mercury has a diameter of 4,880 km, or .38*Rearth, so Ganymede and Titan are both a little bigger than Mercury and Callisto is about the same size as Mercury!

Colonization?

You will be responsible for picking a planet or major moon to colonize. You should be able to write a short paragraph detailing the reasons for your choice. Some criteria you should consider are: closeness to earth, surface (gas, liquid, or rock; stable or volatile), temperature ranges, atmosphere (amount and type), existence of water or other useful materials.

Asteriods

This picture of Eros, the first of an asteroid taken from an orbiting spacecraft, is a mosaic of four images obtained by NASA's NEAR mission immediately after the spacecraft's insertion into orbit.

Asteroids: basic infoAsteroids are “minor planets” that orbit the sun

(rather than objects that orbit planets as moons). They are much smaller than the planets, with the largest being Ceres at about 1,000 km in diameter, (remember that the diameter of the Earth is about 12,800 km) and only another five or so having diameters over 300 km. There are roughly 200 or so with diameters over 100 km. There may be on the order of a million with a diameter of kilometer or more. The total mass of all the asteroids is probably less than the mass of the smallest planet, Pluto.

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids

Asteroids: orbitsThe main group of asteroids, including Ceres, is in the

“asteroid belt” between Mars and Jupiter with orbits roughly between 2.2 and 3.5 AU.

Some of the asteroids have very elliptical orbits, and some - called the Apollo asteroids - come inside earth’s orbit. We think only a few dozen of the earth-crossing asteroids have sizes greater than 1 km.

Another group of asteroids called the Trojan asteroids follow and lead Jupiter by about 60o in its orbit around the sun.

Asteroids

Trojan

Trojan

beltbelt

Apollo

Jupiter

Mars

Earth

Asteroids

On the previous slide, the size of the orbits is more or less to scale. However, the size of the sun, planets and asteroids are NOT to scale.

The sun should be about 1% of the earth-sun distance.

Jupiter should be about 1/10 the diameter of the sun.

Earth should be about 1/10 the diameter of Jupiter.

Mars should be about ½ the diameter of the Earth.

The biggest asteroids should be about 1/10 the size of Mars.

Dwarf Planets

Size Comparisons Date: 11 Dec 2010A size comparison of dwarf planets Eris, Pluto, and Ceres (artist's concepts); Pluto's moon Charon (artist's concept); Earth's Moon; and Earth. Image Credit: NASA

Plutoids and Dwarf Planets

According to the International Astronomical Union (IAU): “Plutoids are celestial bodies in orbit around the sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near spherical) shape, and that they have not cleared the neighborhood around their orbit.”

There are two named Plutoids right now: Pluto and Eris.http://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf

Kuiper BeltThe Kuiper Belt is an area beyond Neptune

from about 30 AU out to about 50 AU that has at least 70,000 small objects with diameters greater than 100 km. (Recall that Pluto has a diameter of about 2,300 km and the earth has a diameter of about 12,800 km.

Note that Pluto and Eris are Kuiper belt objects. Pluto has an orbit with a radius of about 40 AU, but it is very elliptical and it varies between about 30 and 50 AU. Eris has an even more elliptical orbit that varies between 38 and 97 AU.)

http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs

Comets

Comets are objects composed of dust and ice that come from the far reaches of the solar system on extremely elliptical orbits. They appear to come from the Kuiper Belt (for relatively short period

comets) and an area known as the Oort Cloud (for

much longer period comets). The Oort Cloud is hypothesized to be a spherically shaped area (not

confined to the ecliptic plane like most everything else) about 50,000 AU from the sun.

http://www2.jpl.nasa.gov/comet/hyakutake/soho2.html

Comets

In their elliptical orbits, comets move very fast and spend very little time when they come near the sun (due to the strong gravity near the sun), and they move very slowly and spend a very long time when they are far from the sun (due to the very weak gravity far from the sun). Near the sun they can be very bright, but far from the sun they are extremely hard to see even in powerful telescopes.

See the following website http://www.solarviews.com/eng/comet.htm

Comet Kohoutek This color photograph of the comet Kohoutek was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona. They photographed the comet from the

Catalina observatory with a 35mm camera on January 11, 1974. (Courtesy NASA)

Comets – structure

Far from the sun (several AU’s and farther), the comet is just a dirty snowball.

When the comet comes closer to the sun, the heat from the sun evaporates some of the ice and snow which forms a “coma” that surrounds the “nucleus”. The coma and nucleus are called the “head” of the comet. The gas and dust in the coma reflect sunlight and so the comet gets brighter. As the comet gets even closer, the gas absorbs some of the sun’s light and fluoresces (re-emits the light) and becomes brighter still.

Comets – structure (cont.)

As the comet approaches the sun, the gas and dust that were evaporated form a “tail”. This tail can have three parts:

The “dust” tail tends to follow the comet in its orbit.The “hydrogen” envelope tends to be pushed by the

sun’s radiation somewhat behind the head. This hydrogen envelope is not visible from the earth’s surface since our atmosphere absorbs its light – but it has been seen by spacecraft.

The “ion” tail is pushed straight behind the comet by the sun’s radiation.

MeteorsMeteors are grains of dust and small “rocks” that fall

through the atmosphere and glow due to the heat of air resistance as they fall.

Meteorites are the remains that reach the earth’s surface.

Meteroids are the material of meteors in space (before they become meteors). Some of this material comes from dust from comets, some from asteroids.

See the website:http://csep10.phys.utk.edu/astr161/lect/meteors/showers.html

The meteorite found on Mars by Opportunity rover in 2005.