ASTR 330: The Solar System Lecture 5 Review Quiz Dr Conor Nixon Fall 2006 1.Distinguish between remote sensing and in situ sensing, and give examples.

ASTR 330: The Solar System

Lecture 5 Review Quiz

Dr Conor Nixon Fall 2006

1. Distinguish between remote sensing and in situ sensing, and give examples.

2. What is meant by an atmospheric spectral window?

3. What information can we tell about a planet from infrared spectral lines?

4. What are the two main types of telescopes, and name some recent advances in telescope technology.


Announcements


• Yellow forms still required for students: Gilkey, Snyder & Talebi!

• E-mail: anyone not getting e-mail on the class list?

• Homework #1 returned.

Standard was high: mean=42.5. Question 4 problems.

• Materials on-line: HW #1 solutions. Lectures 6 & 7.

• Calculators.


Lecture 5:

Formation of the

Planetary System

Dr Conor Nixon Fall 2006Picture from emuseum@Minnisota State Univ, Mankato


Questions For This Class


1. From what did the solar system form?

2. How did it form?

3. Why are the objects in the solar system all so different?

4. Could it have formed differently, e.g. with a binary star?

5. How long did it take for the planets to accrete most of their mass?

6. How have the planets evolved since the end of their major accretion?

7. Is this evolution continuing today?


Data and Evidence


• In trying to answer these questions we have limited evidence at our disposal today.

• In our own solar system, we have only the end-point of a complex evolutionary process to go on: the observed sizes, orbits, compositions etc of the planets and other bodies.

• It is somewhat like trying to deduce the childhood and experiences of a human, having only a picture of the adult.

• As astronomical technology has progressed, in the last decade we have been able to now view the beginnings of solar systems around other stars: a valuable insight into our own history.

• However, many aspects of solar system formation at this stage are still not certain.


Facts We Can Use: Chemical Composition


1. More than 99% of the material in the solar system is in the Sun.

2. The Sun is composed almost entirely of Hydrogen and Helium.

Hence the initial raw material must have had close to this composition.

3. Jupiter and Saturn have almost the same composition as the Sun.

4. The smaller bodies are depleted in H, He and other light gases.

Hence the inner planets were probably formed without ices or other volatiles.


Facts We Can Use: Orbits and Rotations


1. All the planets have orbits which are approximately circular.

2. These orbits all lie roughly in the same plane.

3. The planets all rotate in the same direction around the Sun.

4. The Sun rotates in the same direction as the planets orbit.

5. The Sun’s equator lies essentially in the same plane of the planet’s orbits.


Early Theories


• The first serious speculations about the formation of the solar and planetary system using the laws of gravity and physics were due to Pierre-Simon, Marquis de Laplace (1796).

• Laplace envisioned a vast rotating interstellar gas ‘cloud’ which collapsed under its own gravity, to form a disk.

• These ideas were improved on by Roche (1854) and are still valid today, though with many changes in the details!

Picture credit:Univ. St. Andrews


Overview Of Formation


Now let’s look at the individual stages in more detail…

Figure: Universities Corp. For Atmospheric Research (UCAR)


In the beginning…


• … was a huge cloud of molecular material, known as the proto-solar or primordial nebula, similar to the Orion Nebula (right).

• This nebula may have only contained only 10-20% more mass than the present solar system.

• Due to some disturbance, perhaps a nearby supernova, the gas was perturbed, causing ripples of increased density.

• The denser material began to collapse under its own gravity…

Picture from stardate.org


Initial Collapse


• The nebula must have possessed some rotation. Due to the spin, the cloud collapsed faster along the ‘poles’ than the equator.

• The result is that the cloud collapsed into a spinning disk.

• The disk material cannot easily fall all the remaining way into the center because of its rotational motion, unless it can somehow lose some energy, e.g. by friction in the disk (collisions).

• The initial collapse takes just a few 100,000s of years.

Picture credit: AnimAlu Productions


Rotation and Angular Momentum


• Angular momentum is a conserved quantity: in the absence of dissipation the total angular momentum of the cloud stays the same.

• Angular momentum is the product of three quantities: mass, size (radius) and rotation speed (or velocity):

L = mrv• If L is constant, then clearly if any one of the other quantities

decreases, another quantity must increase proportionately.

• I.e., if the cloud collapses and becomes smaller (r decreases) and the mass stays the same, then the rotational speed (v) increases: the cloud ‘spins up’.


Rotational Spin-Up


• The spin-up of a shrinking object can be demonstrated by a familiar example:

• An ice skater performing a spin (Michelle Kwan, right) draws in her arms to spin faster without expending any extra effort.

• Now let’s look at some actual protoplanetary disks…


Actual Protoplanetary Disks


• The images (left) show four protoplanetary disks in the Orion Nebula, 1500 light years away, imaged by the Hubble Space Telescope (HST).

• The disks are 99% gas and 1% dust. The dust shows as a dark silhouette against the glowing gas of the nebula.

• Each frame is 270 billion km across: about 1800 AU. The central stars are about 1 million years old – infants!

These image are visible composites from red, green and blue light.


Disk Composition


• The central parts of the nebula were very hot: over 10,000 K.

• Going outwards in the nebula, the temperature drops, and different compounds condense out at different distances from the protostar:

• Calcium, Aluminum oxides first,• then Iron-Nickel alloys (by 0.2 AU, Mercury),• Magnesium silicates and oxides next (by 1.0 AU),• Olivine and Pyroxene (Fe-Si-O compounds),• Feldspars (K-Fe-Si-O compounds),• Hydrous silicates,• Sulfates,• And finally ices (water ice by 5.0 AU).

• This radial variation in composition in the nebula is one cause of the variation in composition of the planets with solar orbit distance.


Planetesimals


• Dust grains and ices were ‘sticky’ (not just chemically, but electrically and magnetically cohesive) and began to clump together (‘accretion’), forming small bodies 0.01 to 10 m across, all orbiting the proto-star in the same direction like Saturn’s rings.

• As their size grew, gravity began to have an effect, and larger bodies around 1 km in size called planetesimals formed.

• The details of planetesimal formation are still uncertain, but km-sized bodies would have appeared by 10,000 years after the disk formed.


The Proto-Sun


• Gravity caused the center of the cloud to collapse into a ball: the proto-sun. The gravitational energy released begins to heat things up.

• When the protosun became hot and dense enough, nuclear fusion was ignited.



T Tauri Phase


• Once the star begins to shine, the stellar wind ‘turns on’, and the star begins to blow material which has not yet accreted outwards.

• T Tauri stars are characterized by vigorous outflows perpendicular to the relatively dense disk.

• After 105 or 106 years, the original gas nebula has been dissipated.

• Young solar-type stars are said to be in the T Tauri phase (named after the first example), and can have wind velocities of 200-300 km/s. This phase lasts about 10 million years.

Picture credit: James Schimbert, U. Oregon, Eugene


Planetesimal Growth


• Gravitational interactions between planetesimals perturbed their orbits into non-circular, collisional trajectories.

• Time passed, and the planetesimals impacted one another. In lower energy collisions or where the sizes are unequal, the planetesimals

merged into a new larger object.

• But in higher-energy collisions, two similarly-sized original bodies were disrupted back into fragments.

• Over time, the larger planetesimals gathered up more and more mass from collisions with smaller impacting bodies.

• In this way, the cores of the inner and outer planets were formed.


Inner Planets


• After about 108 years, the solar wind and accretion of planetesimals had cleared the inner solar system of debris.


• The inner planets had by then accreted almost all their eventual mass.

• A period called the Late Heavy Bombardment, around 3.9 billion years ago is associated with clearing up the last planetesimals on inclined orbits, as inferred from lunar cratering.

• However, the process of collision and accumulations continues to the present day: e.g. meteors, SL-9.


Outer Planets


• The outer planets continued to accrete for longer than the inner planets, and gathered much more ices and volatiles.

• The outer planets are also responsible for the asteroid belt and comets.

Picture: NASA


Differentiation


Below: proposed Ganymede interior: rock core and ice mantle.

Picture: NASA

• As the planets accreted, temperature and pressure rose in the inner regions.

• Heavier substances fell to the core (e.g. metal for the inner planets) and lighter substances floated on top.

• This process, called differentiation, occurred in all the planets but the end result depended on the initial ingredients!


Asteroids


• The major asteroid belt lies between the orbits of Mars and Jupiter, at a distance of around 2.7 AU.

• The asteroids were once thought to be the remains of a planet destroyed by a massive impact.

Picture credit: NASA GSFC


Asteroids


• Current theories hold that the fragmented belt of material is the natural consequence of the presence of the giant planet Jupiter nearby during the planetary accretion phase.

• The massive Jupiter core formed first, and then either gobbled up nearby planetesimals, or, in the case of the asteroids slightly further away; Jupiter was able to disrupt any attempts they made to cling together into a planet! The Asteroids are all less than 1000 km in size.

• Asteroids also exist in groups either preceding or trailing Jupiter in its orbit (Jupiter Trojans) or Mars (Martian Trojans). There are also asteroids which cross the Earth’s orbit, and others.

• Asteroids are important because they are examples of the original planetesimals from 4.6 billion years ago. We will talk more about asteroids in a later lecture.


Edgeworth-Kuiper Belt


• The Edgeworth-Kuiper belt is a band of icy planetesimals outside the orbit of Neptune (40-120 AU), hypothesized in the 1940s.

Picture: Johns Hopkins University

• These objects are relics from the early formation phase of the solar system, which did not manage to form into planets.

• The first EKO detected was found in 1992 (not counting Pluto and Charon!) – now over 800 are known.


EB 313 and Pluto


• The object EB 313, first seen in 2003, caused a major upset to astronomy when its size was announced in mid-2005 to be larger than Pluto! (2400 or 3000 km, according to 2 studies: Pluto is 2300 km)

Graphic: wikipedia

• This animation shows EB 313 moving against the star background in the upper left.

• Astronomers have been grappling ever since with the question of how to define what is a planet!

• A decision in August 2006 has resulted in Pluto being downgraded to a new ‘dwarf planet’ category.


Eris and Dysnomia

Dr Conor Nixon Fall 2006Graphic: wikipedia

• Follow-up observations with the Keck adaptive optics system showed that EB 313 was accompanied by a small moon.

• Originally dubbed ‘Xena’ and ‘Gabrielle’ by the discoverers, they gained official names on Sept 13: Eris and Dysnomia.

• The names mean ‘strife’ or ‘discord’, and ‘lawlessness’ - appropriate to the trouble they are causing!


Dr Conor Nixon Fall 2006Graphic: wikipedia


KBOs and SDOs


• Kuiper belt objects are actually clustered quite closely between 39 and 48 AU - stable orbital zones with respect to Neptune.

• Eris lies at a=68 AU, but its 557-year orbit is highly elliptical, ranging from 38 to 100 AU, and inclined at 45 degrees.

Graphic: wikipedia

• For this reason, Eris is classified as a SDO: or scattered disk object.


Other Kuiper Belts


• We cannot gain a good view of the Kuiper belt as a whole due to our position in the inner solar system … but, we can look elsewhere.

• These HST images show 2 Kuiper Belts around other stars, face on (left) and edge-on (right).

Graphic: wikipedia


Oort Cloud


• A vast reservoir of icy planetesimals at 100s out to 100,000s of AU.

• Named the Oort Cloud, after Jan Oort who guessed its existence in 1950, by noting that long-period comets came from all directions of the sky.

• Ironically, Oort cloud objects formed closer to the Sun the EKOs, but are on extremely eccentric orbits.

Graphic: SWRI


Oort Cloud Formation


• Any planetesimals coming close to mighty Jupiter and Saturn were ejected from the solar system entirely.

• However, icy bodies coming close to Neptune and Uranus were merely flung into very distant and eccentric orbits around the Sun.

• These orbits were no longer confined to the plane of the solar system: and so these icy bodies formed a huge spherical cloud around the Sun, reaching out to 100,000 AU.

• These objects periodically visit the inner reaches of the solar system, and we see their long tails of gas and dust as comets.


Pictorial Summary

Dr Conor Nixon Fall 2006Picture credit: James Schimbert, U. Oregon, eugene


Quiz - Summary


1. Describe the conditions which existed in our part of the Milky Way prior to the birth of the solar system.

2. Why did the gas cloud collapse to a disk and not a point; why did everything not fall into the Sun?

3. Describe how planets formed from the disk.

4. Describe the early history (pre-main sequence) of the Sun.

5. Why are the inner planets volatile-poor while the outer planets are volatile-rich?

ASTR 330: The Solar System Lecture 5 Review Quiz Dr Conor Nixon Fall 2006 1.Distinguish between remote sensing and in situ sensing, and give examples.

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