The Origin of the Solar System

Formation and basic characteristicsof the Solar System

The origin of matter

• All the building material necessary to make our SolarSystem was assembled in the process of nucleosynthesisin the early Universe, in stellar cores and duringsupernova explosions

• Protons, neutrons and electrons were made in the first 3min. of the existence of the Universe

• Hydrogen, deuterium, helium, lithium and beryllium weremade in the first 30 min.

• Heavier elements all the way up to iron were synthesizedby nuclear fusion in stars

• All elements heavier than iron were made in supernovaexplosions

• We are made out of that stuff!!!

The origin of planets• The most likely explanation of the

creation of our Solar System involvesthe familiar process of star formationout of a collapsing molecular cloud ofgas and dust. This is called the solarnebula theory

• A rotating cloud, compressed by apassing shock wave, collapses into aflat, spinning disk with a protostar atits center

• Planets are made out of the diskmaterial in the process ofcondensation and coagulation of dustparticles

• When nuclear fusion begins in thecentral star and it becomes luminousenough, most of the remaining debrisis cleared away (this process is seen inT Tauri stars)

Protoplanetary and debris disks aroundyoung stars

• Properties of the Solar Systemindicate that the solar nebula theoryis the most plausible one

• The solar nebula theory predicts thatplanets should form as a by-productof star formation

• This means that protoplanetary disksshould be common around otherstars and so should be the planets

• Observations show that ~50% ofyoung, solar-mass stars have suchdisks and that some older stars havedusty debris disks around them thatcould hide planetary companions

Planets around other stars

• First extrasolar planets, orbiting a neutronstar, were found in 1992

• The first planet around a sun-like star wasfound in 1995 around 51 Pegasi

• This discovery was made by detectingDoppler shifts of lines in the stellarspectrum due to a wobble of the stararound the center of mass caused byorbiting planets

• As of today, over 150 giant, Jupiter-massplanets have been found around othersolar-type stars. About 5% of such starsappear to have gas giant planets aroundthem. Unlike the Solar System planets,many of them have tight, elliptical orbits

• Current detection methods are notsensitive enough to detect Earth-likeplanets around other stars, but orbitingtelescopes of new generation should beable to find them in the next 10-15 years

Basic properties of the Solar System

• Our planetary system consists of8 planets, and many small bodies(comets, asteroids) left over fromthe time of its formation

• Planets orbit the Sun close to acommon plane. This planeroughly coincides with theequatorial plane of the Sun

• Planets rotate counterclockwisearound the Sun and they spin ontheir axes in the same direction,again in agreement with the solarrotation

• These “clockwork” properties ofthe Solar System point to itsorigin in a rotating disk of matterthat created the Sun

Two kinds of planets

• Planets are clearly divided intotwo types: terrestrial (Earth-like) planets and Jovian planets(gas giants)

• Terrestrial planets, Mercury,Venus, Earth and Mars, lie inthe inner Solar System, aresmall, rocky and dense, andhave less dense atmospheresthan the Jovian planets

• Jovian planets, Jupiter, Saturn,Uranus and Neptune, lie in theouter Solar System, are large,gaseous and low-density objects

• Pluto does not fit either ofthese two categories

Earth-like planets?• The term “Earth-like”

should not be taken tooliterally. It is only valid interms of global propertiesof the terrestrial planets

• Mercury resembles theMoon with its heavilycratered surface and haspractically noatmosphere. Cratering isvery typical in the SolarSystem

• Venus is hot and has verydense atmosphere.Atmospheric clouds hidethe surface of the planetfrom optical observers

The two planetary families compared - I

• Venus can be viewed with the“radar eyes”, which reveal a rocky,volcanic, cratered, and entirelyinhospitable terrain

• Mars, on the other hand, has a thin,transparent atmosphere and again, itis marked by volcanoes and impactcraters

• The terrestrial planets are dense (3-5 g/cm3) and have iron/nickel coressurrounded by rocky mantles

• The Jovian planets have lowdensities (average density < 1.75g/cm3), but their masses are large.For example, Jupiter is 318 times asmassive as Earth

The two planetary families compared - II

• Interiors of Jupiter and Saturnare hot and metallic coresabout 10 times the mass ofEarth. Just above the cores,hydrogen is highly compressedand is believed to conductelectric current like metals liquid metallic hydrogen

• Uranus and Neptune are notmassive enough to form liquidhydrogen. They are believed tohave metallic cores surroundedby oceans of water with deep,dense atmospheres on top ofthat

• Jovian planets, in contrast withthe terrestrial planets haverings and large systems ofsatellites

Comets

• Comets appear as glowingheads with extensive tails,when approaching the Sun

• The dirty snowball modelpostulates that cometary nucleiare balls of dirty ices (mostlywater and carbon dioxide). TheSun’s radiation vaporizes theices to produce a tail of gas anddust that is pushed away by thesolar wind and light pressure

• Comets come close to the Sunfrom the outer parts of the solarsystem and are icy leftoversfrom planet formation. Thismeans that at least parts of thesolar nebula were rich in ices

Comet Hale-Bopp

Asteroids

• Asteroids are small, rockybodies, most of which orbit theSun at the average distance of~2.8 A.U., between Mars andJupiter.

• About a 1000 out of some20,000 identified asteroidshave orbits that intersect withthe orbits of inner planets,which sometimes leads tocollisions

• About 200 asteroids are greaterthan 100 km in size

• Asteroids are believed to beremnants of a failed planetformation at the distance of~2.8 A.U. from the Sun

Eros as seen by an approachingspacecraft

Meteors, meteoroids and meteorites

• Meteors are small chunks ofrock and metal (typically notheavier than ~ 1 g) that fall intothe Earth’s atmosphere and getvaporized by friction at ~80 kmaltitude. Meteoritic dust addsabout 40,000 tons/year toEarth’s mass

• When in space, these objects arecalled meteoroids. Meteors thatare big enough to reach Earth’ssurface are called meteorites

• Meteorites are important in ourstudies of early history and ageof the solar system

The Leonids meteor shower

Age and general properties of the SolarSystem

• The age of the Solar System canbe estimated from radioactivedating of rocks (from Earth, theMoon, Mars and meteorites)

• The initial chemical compositionof rocks changes over time,because radioactive elementscontained in them decay into other(daughter) elements. For example,238U (uranium) decays into 206Pb(lead) with half-life of 4.5 billionyears

• Oldest Earth rocks: 3.9 billionyears old, Moon rocks and Marsmeteorites: 4.5 billion years, SolarSystem meteorites: 4.6 billionyears. All Solar System bodiesformed ~4.6 billion years ago

The Solar System formation:condensation of solids

• Chemical composition of the Sun reflectsthe composition of the solar nebula:mostly H with some He and traces ofheavier elements

• Differences in planet composition reflectthe two different processes that haveformed them

• Jovian planets first formed cores of rockand ice by aggregation. When coresbecame massive enough (~10 Earthmasses) they continued growing bygravitationally capturing H and He fromthe solar nebula

• The terrestrial planets never grewmassive enough to develop H/Heenvelopes they are made of heavierelements

• Planet densities and the condensationsequence tell us that the inner planets(high temperature) formed from highdensity metals and metal oxides, whilethe outer planets (low temperature)formed from low density ices.

Planetesimals and protoplanets• Small dust particles in the solar

protoplanetary disk grew throughcondensation, by adding single atoms

• As grains grew larger, accretion becamemore effective (sticking, attraction bystatic electricity)

• These processes have gradually producesplanetesimals, which were objects ~1 kmin size

• Planetesimals massive enough toconcentrate in the plane of the futureSolar System. This helped to grow themeven larger, by collisions and sticking, tothe stage of protoplanets

• Internal heat helped planetary cores todifferentiate (heavier elements sunkdown) and to create atmospheres byoutgassing of interior matter. This is whythe inner planets do not have originalH/He atmospheres that they could notkeep, because of heat and low gravity

• The Jovian planets formed in about 10million years, before the nebular gas wasblown away by the Sun. The inner planetsformed in < 100 million years

The mature Solar System

• Characteristics of the Solar System supportthe solar nebula hypothesis

• The two types of planets can be understoodwith the condensation sequence caused bydifferent conditions in the inner and theouter parts of the nebula

• The Solar System is different from theother planetary systems found so far: theyfrequently have Jovian planets close toparent stars (after migration caused byinteraction with the nebular gas), andplanetary orbits are significantly ellipticalin most of them

• Jupiter prevented a planet from formingbetween it and the orbit of Mars

• After the planets formed, the system wasalmost cleared from gas and debris byradiation pressure, solar wind, sweep up ofdebris by planets (most craters wereformed ~4 billion years ago era ofheavy bombardment), and gravitationalejection of material from the system