Introduction to Astronomy

• Announcements

– Some notes on your homework:

• USE YOUR OWN WORDS.

• DO YOUR OWN WORK.

Astronomy Image of the Day

“Hanny’s Voorwerp”

Survey of the Solar System

Components

Origin & History

Exoplanets

Components

The Sun

• Hot ball of dense gas– AH = 0.71– AHe = 0.27– Plus other trace elements (basically all of

them)

• Largest, most massive body in the solar system– More on the Sun later

Planets• Massive bodies, but still too small to ignite

nuclear fusion in their cores– “failed stars”…

• Solar system = flattened, spinning pancake– 3 stacked CDs…roughly same relative

thickness-to-diameter ratio

– Exceptions to perfection• Orbital planes not exactly aligned• Tilt of rotation axes• Retrograde rotation (NOT retrograde motion) of

Venus, Uranus & Pluto

Inner vs. Outer Planets• Classified based on size, composition,

location– Inner planets

• Small, rocky bodies with thin or no atmosphere• SiO2 w/ Al, Mg, S, Fe & other heavy metals• Mercury, Venus, Earth, Mars

– Outer planets• Large, gaseous, liquid, or icy bodies with no

crust/atmosphere boundary• Gases “thicken” (get denser) with depth, eventually

liquifying• H2O, CO2, NH3, CH4

• Jupiter, Saturn, Uranus, Neptune

• Compositions– Using observations to infer properties we

cannot directly measure• Kepler’s law for mass• Angular size – distance relation for volume• Then can calculate AVERAGE density

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3

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4RV

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• How do we “know” these compositions?– Have some direct measurements

• Voyager 1 & 2, Pioneer, Cassini, etc…

– Computer simulations– Gravity & Newton’s Laws of Motion

• Caveats– Multiple combinations of substances may give the

same average density– Gravitational compression– Assume initial dust disk was roughly uniform

Satellites (Moons)

• Mini models of the solar system– Recall capture theory, twin formation theory,

fission theory, violent-birth theory

– Mercury & Venus: only planets w/o moons!• Why?• Low mass, proximity to Sun

– Any moon-forming material most likely would’ve been pulled into the Sun

Asteroids & Comets• Asteroids

– Rocky, metallic bodies – Size usually less than 1000 km – Asteroid Belt (between Mars & Jupiter)

• Remnants of another planet that failed to fully form (?), because Jupiter’s enormous gravity interfered

• Comets– Icy bodies usually less than 10 km in size– “dirty snowball”– Vaporized gases create comet “tails”

Halley’s Comet

Next opportunity to see itis ~ 2061

Shoemaker-Levy 9 fragmentsimpacts Jupiter……this was a wake-up call.

• Where do comets come from?– Kuiper Belt

• Disk-like region of comet nuclei (dirty snowballs) starting past Neptune’s orbit, extending out to ~ 60 AU (recall 1 AU = distance from Sun to Earth = 93 million miles)

– Oort Cloud• Huge, spherical shell of comet nuclei that

completely surrounds solar system• 40,000 AU < R < 100,000 AU (huge source of

comet nuclei)

Kuiper Belt & Oort Cloud estimated to contain >1 trillion comet nuclei combined!

Origin & History

• How did the solar system form?

• What physical processes led to the formation of a star surrounded by so much “extra” material?

Huge gas clouds gravitational collapse condensation accretion moon formation atmosphere formation

Interstellar Clouds

• Raw materials– Mostly H, H+, H2, He

– “Interstellar Grains”• Silicates, Iron, Carbon

& diamond (!)

IS cloud

Star cluster

Compose about10% of visiblematter in ourgalaxy

Collapse

• Gravitational attraction pulls outer parts of slowly-rotating gas cloud toward center

• Conservation of Angular Momentum– Like ice-skater– As cloud contracts, rotation speeds up

• Causes cloud to flatten into a thick disk with a bulge at the center

• Happens over few million years

NOT to scale…final collapsed cloud wouldbe nearly 100,000 times smaller than originalsize

Condensation

• A gas, when cooled, starts to form larger and larger “clumps” of its molecules– Gas to solid (flakes)– E.G. if temperature never falls below ~ 500 K,

H2O molecules will never group together (water vapor stays in vapor state)

Condensation

• Heat from newly-forming Sun (at center) prevents H2O molecules from condensing into liquid water or water vapor– All the way out to Jupiter’s orbital distance– But…rocky materials (e.g. Iron & Silicates)

can condense & settle even at high temperatures

– RESULT: could not form solid ice particles in the inner parts of the solar system

• Therefore, collapsing disk separates into rocky interior & icy outer regions

You only see steamoutside the tea kettle,because temperatureoutside is low enoughfor the water vaporto condense to visible-sizedparticles

These are microscopicflakes of Aluminum produced by low-vacuumcondensation of vaporizedAluminum

Area scale (1μm = 10-6 m)

Thickness scale (1nm = 10-9 m)

Accretion

• Building bigger objects out of smaller objects

• Condensed particles attract each other– Initially, electrostatic forces (remember

opposites attract) bind particles together• Atom/molecule-sized

– As composite particles get bigger, collisions take over as binding force

• Pebble/boulder-sized and larger

• “Planetesimals” (building blocks of planets)– Collisions formed outer planets more rapidly,

due to high amounts of ice particles• Large balls of rocky ice and heavy gases sweep up

excess H & He by gravitational attraction

– Collisions in the inner solar system melt the rocky bodies, allow differentiation (heavy Fe & Ni sink to core, lighter silicates remain near surface)

– Few million years

Ultimately, the properties of the planets were determined by large impacts with the material in the early solar system

Moon Formation

• Basically, just a scaled-down version of the formation of the solar system– Moon systems show same types of

regularities as the planets in the solar system• Orbital planes• Compositions

• But recall “violent birth theory” of our moon’s formation…possible to create a moon by different processes than those responsible for forming the solar system

Atmosphere Formation

• Outer planets probably captured their atmospheres by sweeping up H & He– Gravitational attraction + high amounts of

solar nebular material in outer regions

• Inner planets have (or had) volcanic activity to generate their atmospheres– Although recall “atmospheric delivery” by

comets & asteroids

Result of ocean-impact…the aftermath would be circularly-expanding tsunamisstarting at about 1000 ft high. Once nearer shorelines, the crests steepen to1-2 miles!

Introduction to Astronomy

• Announcements

– Some notes on your homework:

• USE YOUR OWN WORDS.

• DO YOUR OWN WORK.

Review

• Solar System Formation– Initial ingredients: Hydrogen, Helium, trace

others– Gravitational collapse– Condensation– Collisional accretion– Moon formation– Atmosphere formation

Cleaning Up

• Heat & solar wind emanating from sun blasts away remaining gases and small particles– Perhaps forming the parts of the Kuiper belt

and the Oort cloud in the process?

Extrasolar Planets(or Exoplanets for short)

• Theories on solar system formation raised important questions

– Do other stars form similar structures of nearby rocky & far-away icy bodies?

– Can we find any?

• Our solar system has formed this way, so is there any reason to expect that other “stellar systems” do not exist?– NO! We have observed (indirectly) many

such systems

Searching for Exoplanets

• Many methods– Direct telescopic observation is still years

away• Exoplanets just too faint• Upcoming Terrestrial Planet Finder (TPF)

– Look for proto-planetary disks (gas clouds in the first stages of solar system formation)

TPF Coronagraph:

Block out direct starlight,so we can see fainter objectsthat may be orbiting

TPF Interferometer:

Operates in the IR to lookfor planetary heat signatures

Funding for TPF was cut by Congress in early 2006. Public outrageensued, and now TPF is back on track!

In Orion Nebula:

Proto-stars and surroundingdisks of dust and gas

Around star β Pictoris:

Disk of dust and gasclearly visible

Dust disk (seen herein IR) around theyoung starHR 4796A

A small disk in thetelescope blots outthe light from thestar itself so thatit’s glare will notobscure the disk…

…this is the sameprinciple seen duringsolar eclipses, andit is how we are ableto see the faint outerlayers of our Sun.

• Observe gravitational effects (indirect)– Star-planet system rotates about its common

center of mass (CoM)– Causes parent star to “wobble” slightly

• Recall Doppler Shift– When star wobbles toward us, see blue-shifted light– When star wobbles away from us, see red-shifted light– Amount of shift tells about speed of parent star’s orbit

about the CoM– Speed of star’s orbit tells us the mass of the planet

– This is two-body example, but still applies to more than one planet (Upsilon Andromedae)

Center of Mass

As unseen planet moves AWAY from observer, parent star moves TOWARDobserver…this causes the starlight to be blue-shifted to shorter wavelengths

Astrometric Stellar WobbleAstrometric Stellar Wobble: observed position actually changes periodically

Practically, this will only work for the nearest stars with the largest exoplanets.

Spectroscopic Stellar WobbleSpectroscopic Stellar Wobble: the positions of spectral lines (both absorption& emission) changes periodically.

This can be translated into a velocity of the star, which then tells us that the star is rotating around it’s system’s center-of-mass

• The Transit Technique– The best way to find exoplanets, but requires

the right circumstances

– An exoplanet passing between us and its star blocks some of the star’s light from reaching our telescopes

• The amount and duration of this dimming tells size and speed of orbiting exoplanet

• Also, can tell if the exoplanet has an atmosphere!– Starlight modified by atmosphere (if it exists) on limb of

planet» New absorption lines not seen in the star itself» Tells chemical composition of any gases that are

present

• Current status of Exoplanetary Hunt– About 300 known exoplanets– Most around Sun-like stars– Statistics

• At most, 1 in 3 Sun-like stars harbor a planetary system

• At least, 1 in 14 Sun-like stars have one• According to this study

– Last year, the most Earth-like planet found so far

• Orbiting Gliese 581 (red dwarf, 21 ly away) at a distance that means liquid H2O could exist on it’s surface!

Properties of Exoplanets

• Most that we’ve seen have masses M ~ MJupiter

• But unlike Jupiter, most exoplanets are VERY close to their star– Imagine replacing Mercury with Jupiter!

• Here’s the kicker: NoNo exoplanetary systems have been discovered that resemble our own solar system!!!!

Wide variety of orbital geometries…

• Does this mean our solar system (and hence Earth) is unique?

• Nope.

• Observational Bias:– High-mass planets close to their stars

produce the largest “wobbles”– Some “wobbles” are too small to detect

• HARPS to the rescue!

– So, most easily-detected “wobbles” come from high-mass exoplanets very close to their stars

What we see depends on how we look for it.

Upsilon Andromedae

• Multi-planet system

• Very elliptical orbits– used to be thought that such elliptical orbits

could not be sustained…too many gravitational perturbations would lead to planetary ejections either out of the stellar system, or into the parent star (planet consumption)

Excerpts from space.com’s Top Ten Most Intriguing Exoplanets

SWEEPS-10 orbits its parent star at only ¾ of a million miles. 1 year on this planet isonly 10 Earth-hours long.

An exoplanet orbiting Coku Tau 4 is less than 1 million years old, makingit the youngest known exoplanet.

An exoplanet orbiting a pulsar (a deadstar that behaves much like a lighthouse)is roughly 12.7 billion years old. This is the oldest known exoplanet, and suggested that planets are very commonin the Universe.

A year on HD209458b is 3½ Earth-dayslong. It is being evaporated by it’s parent star, at an estimated rate of 10,000Earth-tons per second.

Gliese 581 C is the smallest exoplanet ever detected, and is the firstto lie within its parent star’s Habitable Zone. Life could exist here.

NEXT TIME

• The Terrestrial (Inner) Planets– Mercury, Venus & Mars in more detail…