Phys 214. Planets and Life

Dr. Cristina Buzea

Department of Physics

Room 259

E-mail: cristi@physics.queensu.ca

(Please use PHYS214 in e-mail subject)

Lecture 9.

The nebular theory + Movie

(Page 74-80)

January 25

Contents

Textbook: Pages 74-80

• The origin of our solar system• Nebular theory• Planetary nebulae• Should habitable worlds be common?

• Movie

• Acknowledgments: NASA, ESA, Hubble

The origin of our Solar System- nebular theory

The origin of our Solar system might give us some insight into findinghabitable worlds in other star systems.

Nebular theory – our solar system was born from the gravitationalcollapse of an interstellar cloud, or nebula, of gas and dust.

Carina nebula

Nebular theory

Nebular theory

The formation of theThe formation of the

solar systemsolar system

according to theaccording to the

nebular theory hasnebular theory has

four steps:four steps:

1. Contraction1. Contraction

2. Condensation2. Condensation

3. Accretion3. Accretion

4. Clearing4. Clearing

Nebular theory

Once the gravitational collapseOnce the gravitational collapse

beginsbegins

•• the solar nebula heats up the solar nebula heats up

•• spin faster spin faster

•• flatten into a disk flatten into a disk

•• shrinks in size shrinks in size

1. Contraction

• The solar nebula began as a large,

diffuse cloud, roughly spherical in

shape.

• The initial cause of collapse is

unknown, perhaps a nearby supernova.

• Similar clouds exist today and they

show they can collapse and give birth

to stars.

Nebular theory

The solar nebula heats up <- law of energy conservation.

Large gravitational potential energy -> kinetic energy & heat as they fall inward andcollide. The cloud becomes hotter near the center, where the star forms.

Spin faster <- conservation of angular momentum.

The total amount of circling motion of an object must be conserved. A shrinking cloudspins faster as it contracts.

Flatten into a disk <- consequence of the spin.

When the particles collide, they tend to add to each other’s motion when they move inthe same direction. However, they cancel each other’s motion in other directions.

Nebular theory

The overall composition of the galaxy and Sun

implies that:

The composition of the solar nebula was:The composition of the solar nebula was:

- 98% hydrogen and helium, &- 98% hydrogen and helium, &

- 2% other elements (essential for planet- 2% other elements (essential for planet

formation) - metal, rock, and hydrogenformation) - metal, rock, and hydrogen

compounds (water, methane,compounds (water, methane, amonia amonia).).

The process of planet formation in the early solarThe process of planet formation in the early solar

system is best described by condensation.system is best described by condensation.

Nebular theory

2. Condensation

The materials present in the solar system withThe materials present in the solar system with

the highest condensation temperaturesthe highest condensation temperatures

were metals.were metals.

Because the temperatures were high in the

inner solar system, only materials with

high condensation temperatures could

become solid (metals and rock)

Nebular theory

2. Condensation

Metal and rock compounds (with high condensation T) could condense within about theMetal and rock compounds (with high condensation T) could condense within about thepresent location of the asteroid belt.present location of the asteroid belt.

Farther out, where temperatures were much lower, in addition to metal and rock, hydrogenFarther out, where temperatures were much lower, in addition to metal and rock, hydrogencompounds could condense to make ice.compounds could condense to make ice.

Nebular theory

3. Accretion and terrestrial planet formation

• Solid particles grew larger = accretion.

• Particles orbit the forming Sun with orderly circular paths, each particle moving at

about the same speed as neighbouring particles.

• Gentle collisions - due to electrostatic forces and not to gravity.

• Particles grew larger in mass -> gravity sticks them together into boulders -

planetesimals (protoplanets or pieces of planets).

Nebular theory

3. Accretion and terrestrial planet formation

• Planetesimals grew to hundreds of km -a few million years (only 1/1000 thepresent age of the solar system).

• Dozens or even hundreds of planetesimals orbiting the Sun between the present dayorbits or Mercury and Mars.

• Continued accreting - sometimes colliding violently.• Computer simulations reproduce the collapse of clouds in spinning disks and the

first stages of of accretion; cannot predict the results of late stages of accretion,especially the balance between shattering collisions and planetesimal growth.

Illustration Credit: T. Pyle (SCSC),JPL-Caltech, NASA

Nebular theory

3. Accretion and terrestrial planet formation

• In the inner solar system, at least 4 objects grew to planetary size, becoming Mercury,

Venus, Earth, and Mars (4.900 –12,000 km).

• At least a few other Moon (3,400 km) -to Mars (6,800 km) size objects might have been

present in early times, but eventually smashed into one of the four planets that survived .

• Moon formed when a Mars-size object collided violently with the young Earth.

Nebular theory

3. Accretion and 3. Accretion and jovianjovian planet formation ( planet formation (scientific debate).scientific debate).

•• The The planetesimalsplanetesimals in the outer solar system contained a in the outer solar system contained a larger amount of ice inlarger amount of ice in

addition to metal and rockaddition to metal and rock..

•• All solids objects that reside in the outer solar system today, such as All solids objects that reside in the outer solar system today, such as comets,comets,

KuiperKuiper belt objects, moons of belt objects, moons of jovian jovian planets planets, all show an ice-rich composition., all show an ice-rich composition.

The The JovianJovian planets most likely formed from planets most likely formed from planetesimals planetesimals of rock and ice attracting of rock and ice attracting

hydrogen and helium gas from the solar nebulahydrogen and helium gas from the solar nebula

Nebular theory

3. Accretion and moons of jovian planet formation

• Similar process to the one that made the disk of the solar nebula (heating, spinning

flattening).

• Each jovian planet - surrounded by its own disk of gas, spinning in the same

direction that the planet rotates.

• Moons - accreted from ice-rick planetesimals within the disks, closed to the

equatorial plane of the planet.

• This model explains why jovian planets have many moons.

Nebular theory

3. Accretion and jovian planet

formation

• The model of accretion

followed by gas capture

explains the observed features

of the jovian planets well.

• A competing model suggests

that disturbances within the

disk of the solar nebula led to

clumps of gas to collapse and

form jovian planets without

the need of forming icy

planetesimals first.

Nebular theory

• 4. Clearing the disk

• As the planets formed, the Sun also

formed and accreted the remaining

gas.

• Young Sun had a strong solar wind,

blowing off particles from its surface

out into space.

• The wind swept away the remaining

gas into the interstellar space, ending

the era of planet formation.

Nebular theory – clearing the disk

Disk of dust

Three trillion mile-long jet from a star

hiding in dust

• Once nuclear ignition is achieved the star releases a massive wind

-sweep out the remaining gas (T Tauri phase)

• The remaining planetesimals close to the Sun will almost all

impact with planets in this region

–creation of the Moon

–About 20,000 of these objects left between Mars & Jupiter

–The rate of impacts was clearly much higher in the past than

it is now

•Planetesimals farther out (mostly icy)

interact with the Jovian planets and

can be thrown out of the solar system!

(comets)

Explaining the worlds – planets rotation

Nebular theory predictions:Nebular theory predictions:Planets rotate in the same direction as theyPlanets rotate in the same direction as they

orbit the Sun and in the same plane.orbit the Sun and in the same plane.

Sun rotates in the same direction (born atSun rotates in the same direction (born atthe centre of the spinning cloud).the centre of the spinning cloud).

The general cloud rotation explains whyThe general cloud rotation explains whymost planets rotate in the same directionmost planets rotate in the same directionand most of the large moons orbit theirand most of the large moons orbit theirplanets in the same direction.planets in the same direction.

Explaining the worlds – planets rotation

Nebular theory:Nebular theory:

The condensation theory does not explainThe condensation theory does not explain

the rotation rate of the planets.the rotation rate of the planets.

Explaining the worlds – almost circular orbits

Planets have nearly circular orbits,

because particles with more

elliptical orbits would have suffered

more collisions.

Most planetesimals ended up in one of

the eight major planets; many

planetesimals shattered into pieces.

Asteroids are the remaining

planetesimals of the inner solar

system. Most reside in the asteroid

belt, others in Kuiper belt.

Explaining the worlds

Oort cloud comets - more difficult to explain.

originated as comets orbiting among jovian

planets.

When they passed near a jovian planet, they

were flung out to a great distance by the

planet gravity (similar to the way the scientists

use Jupiter’s gravity to accelerate spacecraft to

planets beyond).

Explaining the worlds - Exceptions

Uranus is tilted at 98° to the plane of the solar system

• Possibly an off-centre impact, or the fact that the solar

nebular is less dense in the outer parts allowing a higher

probability of being at an angle

Pluto and Mercury lie at 7 and 17 degrees relative to the plane

of the solar system

• Mercury probably suffered an impact during its formation

(it is small and easy to perturb)

• Pluto seems to be a left over planetesimal so probably had

many encounters to knock it into a strange position

Explaining the worlds - Exceptions

• Moons with strange orbits - Triton which

orbits opposite to Neptune’s rotation

Probably a captured planetesimal

• Earth’s moon orbits in the plane of the

solar system, not in the plane of the

Earth’s equator

Impact event occurred in the plane of the

solar system

Should habitable worlds be common?

Formation of the spinning disk -

consequence of physical laws that

operate everywhere

-> most stars surrounded by spinning disks in

which planets may form.

Observations support this idea – many young

stars have such disks!

Hubble telescope photo - flattened spinning

disk around star AU Microscopii (edge on).

Light reflected off dust around the young star

when the star light is blocked.

Hubble Space Telescope near-infrared picture

of a disk around the star HD 141569, located

about 320 light-years away in the constellation

Libra.

Planetary nebulae

Hubble Space Telescope images of four protoplanetary disks around young stars in the Orion nebula, located 1500 light-

years away. Gas and dust disks can be seen in visible light.

Planetary nebulae

Protoplanetary disks in the Orion Nebula as seen by Hubble Edge

on.

Artist impression - HD 98800 system - two pairs of double stars,

with one pair surrounded by a disk of dust. Recent data from

the Earth-trailing Spitzer Space Telescope in infrared light,

indicate that the dust disk has gaps consistent with being

cleared by planets orbiting in the disk. If so, one planet appears

to be orbiting at a distance similar to Mars of our own Solar

System.

Should habitable worlds be common?

We expect to find many planetary systems with

terrestrial and jovian planets laid out the same.

However, most of the extrasolar planetary systems

discovered to date are quite different than our

own solar system:

- jovian planets found close to their parent stars.

- planets orbit closer to their star (closer than

Mercury’s orbit).

Movie. The discovery of most Earth-like planet.

The first image of an extrasolar planet. The planet roughly five

times the mass of Jupiter is orbiting a brown dwarf.

Artificial-colour Hubble Near Infrared Camera and Multi-

Object Spectrometer (NICMOS) infrared-light view of the

brown dwarf star 2M1207 and giant planet companion

candidate - about five times the mass of Jupiter, is the magenta

coloured spot at lower right. The brown dwarf’s location is

within the circle at image centre. The glare of the dwarf, which

is 700 times brighter than the planet candidate has been greatly

reduced through image processing

Should habitable worlds be common?

We cannot say with certainty whether solar

systems like ours should be rare or

common.

However, rare in Milky Way means large

actually. If only 1 in 1 million star has a

system like ours, this is 100,000 systems!

Therefore, it is almost inevitable that our

galaxy contains many worlds that have

liquid water and would be suitable for life.

Movie

• Hubble 15 years of discovery

• Chapter 3. Planetary tales (9 minutes)

Next lecture

• Chapter 4. The habitability of Earth

• Geology