Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

EducatorEntertainer

JU

G G LE

RPLANETARY

Scientist

ScienceExplorer

Chapters 14 & 15 More Solar System Stuff

Pluto Comets

ASTEROIDS

• In 1930, Pluto was discovered and classified as a planet.

• Is Pluto a planet? The debate:A planet must be massive enough:(1) for its gravity to pull it into a roughly

spherical shape, and(2) for it to have cleared out the

neighborhood of its orbit of comparable mass objects.

• This means that the objects lying in both the asteroid and Kuiper belts are not planets.

• Alas, in 2006, Pluto was reclassified as a dwarf planet.

Pluto?

1930

versus

Pluto?

1930

PlutoPluto’s Reclassification: Will the “Real” Pluto please stand up.

Pluto and its largest moon, Charon would fit within the U.S. Charon orbits Pluto at a steep angle to the ecliptic.

Pluto and its Moons

Pluto

Pluto’s moon:Charon

Pluto and its Moons

Two new moons were discoveredin 2005, Nix and in 2006, Hydra.

Pluto is a mix of water ice, rock, methane and frozen nitrogen.

When Pluto is within Neptune’s orbit it has an atmosphere.

As Pluto moves further out into the solar system, the atmospheresnows out onto the surface.

Pluto’s orbit about the Sun is tipped by ~ 17°. This is called Pluto’s orbital inclination.

Pluto

Sun 17°

Trans-Neptunian Worlds• More than 130 have been

discovered, one of them is larger than Pluto: Eris

• Plutinos are bodies that orbit the Sun at more or less the same distance as Pluto.

• Small icy bodies orbiting the Sun past Neptune’s orbit are called Trans-Neptunian Objects or TNOs.

Please insert figure 46.9

Eris

Large Trans-Neptunian Worlds

Name Diameter (km) Mass (kg) Semi-major axis (AU)

Orbital Eccentricity

Eris 2600 1.7 × 1022 67.8 0.44

Pluto 2310 1.3 × 1022 39.4 0.25

2005 FY9 ~1600 ? 45.8 0.16Sedna ~1500 ? 526 0.85

Charon 1210 1.5 × 1021 39.4 ?

Quaoar ~1000 ? 43.6 0.038Orcus ~950 7.5 × 1020 39.4 0.22

The Structure of Comets: Ice and Dust• Comets have two

primary parts, the head and the tail:

• The head consists of the nucleus, a lump of frozen gas mixed with loose rock and dust.

• Only about 10 km across. Dark in color, probably from dust and other materials.

The tail can be hundreds of millions of km long, streaming directly away from the Sun.

The comet’s “coma” is the cloud of evaporated ices and gases streaming from the surface of the nucleus.

Visiting Comets

Comet Halley, visited by Giotto

Comet Wild 2, visited by Stardust

Comet Tempel 1, visited by Deep Impact

5 min

90 sec

20 sec

4 sec

Just after impact

The Origin of Comets

• Comets may originate in either the Oort Cloud or the Kuiper Belt:

• Oort cloud: comet-like planetesimals that are more than 100,000 AU (100 K AU) from the Sun.

• Oort cloud objects may have formed near the giant planets; then tossed outwards by gravity.

• Passing stars or other gravitational influences nudge the comets into the inner Solar System.

Kuiper Belt

Oort Cloud

Comet Orbit

Orbit of Neptune

50 AU

100 K AU

The Kuiper BeltOutside the orbit of

Neptune lies the Kuiper Belt:

Located around 40 AU from the Sun:

Trans-Neptunian Objects (TNOs) such as Pluto are found here.

Many bodies smaller and larger than Pluto are in this region, including Eris and others.

Asteroid BeltKuiper Belt

Asteroid BeltKuiper Belt

~ 3 AU

~ 80 AU

• The Solar System is surrounded by a cloud of comet-like bodies:

• Located around 50,000 AU from the Sun.

• The gravitational forces from passing stars occasionally send comets into the Solar System.

Please insert figure 32.3

The Opik – Oort Cloud:Ernst Julius Öpik (1893 –1985) was an Estonian astronomer.

Jan Hendrik Oort (1900 – 1992) was a Dutch astronomer.

How a comet becomes visibleAs a comet is warmed by

the sun, ice on the surface sublimates and streams away from the comet’s nucleus:

Sublimation is a solid-to-gas phase change

Sublimated gases form the comet’s coma.

Solar photons strike the comet’s dust particles, pushing them away via a process known as radiation pressure.

Solar photons strike the comet’s dust particles

Solar photons : Sunlight

Sun

Dust

The Two Tails of a Comet: Dust tail & Ion tail

Dust tail

Ion tail

Another View of the Comet process

Meteor Showers• As a comet orbits the sun,

it leaves a trail of dust.• The Earth can pass

through the dust trail.• Dust particles enter

Earth’s atmosphere and burn up. We see them as a meteor shower.

• If interplanetary matter survives its descent to Earth and you can pick it up, you are holding?a) meteoroidb) meteorc) meteorite

Meteor Showers

Perseus

TowardPerseus

The Heating of Meteors• When a meteoroid

travels through the atmosphere, it ionizes the air around it. Vaporized material and gas begin to glow. It’s a meteor.

Meteorites• Most meteors burn up in the

atmosphere.• Some meteors survive the

journey to the surface. These are called meteorites.

Three kinds of meteorites:(1) Iron (2) Stony (3) Stony-iron

Stony meteorites (Chondrites) • Carbonaceous chondrite • Ordinary chondrite • Achondrite

Please insert figure 48.3A

A Chondritic Meteorite

Please insert figure 48.3B

The Energy of Impacts• Eventually, a large meteoroid (>10

meter) will strike the Earth.

• Energy released by the impact is:

• With mass m of the meteoroid and V its impact velocity.

• For a 100 kg meteoroid traveling at 30 km/s, the energy released is equal to 10 tons of dynamite.

• This was a small meteoroid.

2K mV2

1E

Giant Meteor Craters

• Giant meteor craters can be found on Earth.

• Barringer Crater: – Meteor was 50 meters

in diameter.– Crater is a mile across.

• Manicouagan Crater:– Meteor was 5 km in

diameter.– Crater is 73 km across.

Mass Extinction Events• About 65 million

years ago, a 10 km- wide meteoroid impacted the Yucatan Peninsula.

• This impact caused massive climate changes, leading to the extinction of the dinosaurs and other forms of life.

• Iridium found in a layer of soil all over the world is the “smoking gun.”

Yucatan

The Asteroid Belt: Most asteroids can be found between the orbits of Mars and Jupiter.

Using Bode’s Rule : The asteroid Ceres was found between the orbits of Jupiter and Mars.

Jupiter

Mars

Jupiter’s Orbit

Mars’ Orbit

Trojan Asteroids: Share an orbit with a planet.

Bode’s Rule for Planets: Also used for asteroids

Bode’s Rule Number Planet True Distance

(0 + 4)/10 = 0.4 Mercury 0.39

(3 + 4)/10 = 0.7 Venus 0.72

(6 + 4)/10 = 1.0 Earth 1.00

(12 + 4)/10 = 1.6 Mars 1. 52

(24 + 4)/10 = 2.8 Ceres (dwarf) 2.78

(48 + 4)/10 = 5.2 Jupiter 5.20

(96 + 4)/10 = 10.0 Saturn 9.58

(192 + 4)/10 = 19.6 Uranus 19.2

(384 + 4)/10 = 38.8 Neptune 30.1

(768 + 4)/10 = 77.2 Pluto (dwarf) 39.5

(1536 + 4)/10 = 154.0 Eris (dwarf) 67.7

• Using Bode’s Rule (a simple mathematical formula) the asteroid Ceres was discovered between the orbits of Jupiter and Mars.

The Shapes and Sizes of Asteroids• Asteroids come in all shapes

and sizes: Big and small.

• Ceres is massive: Large enough to pull itself into a sphere and therefore be classified as a Dwarf planet.

• Most asteroids are small (tens of kilometers across).

• Still large enough to cause tremendous damage if impacting the Earth.

• Spacecraft have only recently visited asteroids.

ErosCeres

Vesta

Asteroid Eros: Potato-shaped

Asteroid Composition

• Asteroids can be grouped into three basic types:– Carbonaceous bodies:• Carbon rich, coal-like substance• Located in the outer part of the asteroid belt

– Silicate bodies:• Composed primarily of silicates (low-density rock)

– Metallic iron-nickel bodies:• Composed mostly of dense metals• Located in the inner part of the asteroid belt

Iridium is the metallic element commonly associated with the doomsday asteroid believed to have wiped out the dinosaurs.

Origin of Asteroids

Asteroids: fragments of planetesimals.The planetesimal being a mixture of

rock and metals, differentiating and thus creating dense metallic cores and lighter, silicate-rich outer shells.

• Collisions with other asteroids shattering the planetesimals:

• Fragments of the inner core form the iron-nickel asteroids and fragments of the outer shell, form silicate asteroids.

Differentiated Asteroid

Asteroid Orbits

• It is likely that the asteroids were unable to form a planet due to the gravitational tidal influence of Jupiter.

• Jupiter “stirs up” the asteroids, keeping them apart.

• Empty regions in the asteroid belt are called Kirkwood Gaps.

• These gaps are present at orbital resonances of Jupiter.

• Asteroids with an orbital resonance get periodic tugs from Jupiter, pulling them out of position.

• Apollo asteroids orbit in the inner Solar system, occasionally crossing Earth’s orbit.

Jupiter revolutions about the Sun

Asteroid revolutions about the Sun

CeresJupiter

Trojan Asteroids

Distance from Sun (semi-major axis) AU

Finding Young Planets• Although watching a planetary system evolve takes too long, it is

possible to find other systems in various stages of development.• In the Orion Nebula, we find many protoplanetary disks of dark,

dusty material orbiting young stars.• The one shown below is a baby of around 10 million years old.

Mark McCaughrean, C. Robert O’Dell and NASA

A popular theory of how solar systems are born concerns giant clouds of molecular dust coalescing to create stars: After which, a gas dust cloud forms a halo (disc) around the new star.

Dust and other particles in the disc collide and stick together and with time form larger and larger masses. Bodies that get sufficiently massive will start to “pull in” surrounding particles and other small objects via gravitational attraction. Some of these bodies will form protoplanets: planetary embryo originating from within this protoplanetary disc (halo).

The Advent of Protoplanets

Location of Star

Disk

Please insert figure 34.3

A planet and its star revolve around a common center of mass. We do not detect the planet directly, rather the resulting “wobble” in the central star. As the star approaches us in its motion around the center of mass, its spectrum will be blue-shifted. As it recedes, the spectrum will be red-shifted.

Detecting Exoplanets

Jupiter-Sized Worlds

Please insert figure 34.4

Most planets we detect are very large: the order of several Jupiter-masses. Planets we detect must be large in order to create a large enough wobble.

A Sample of the Exoplanets

Please insert figure 34.5