Transcript Survey of the Solar System - USU Department of Physics

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
4 2 a 3
M m
GP2
4 3
V  R
3
M

V
• 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 it
is ~ 2061
Shoemaker-Levy 9 fragments
impacts 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
Compose about
10% of visible
matter in our
galaxy
Star cluster
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 would
be nearly 100,000 times smaller than original
size
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 steam
outside the tea kettle,
because temperature
outside is low enough
for the water vapor
to condense to
visible-sized
particles
These are microscopic
flakes of Aluminum
produced by low-vacuum
condensation of vaporized
Aluminum
Thickness scale (1nm = 10-9 m)
Area scale (1μm = 10-6 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 tsunamis
starting at about 1000 ft high. Once nearer shorelines, the crests steepen to
1-2 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:
TPF Interferometer:
Block out direct starlight,
so we can see fainter objects
that may be orbiting
Operates in the IR to look
for planetary heat signatures
Funding for TPF was cut by Congress in early 2006. Public outrage
ensued, and now TPF is back on track!
In Orion Nebula:
Proto-stars and surrounding
disks of dust and gas
Around star β Pictoris:
Disk of dust and gas
clearly visible
Dust disk (seen here
in IR) around the
young star
HR 4796A
A small disk in the
telescope blots out
the light from the
star itself so that
it’s glare will not
obscure the disk…
…this is the same
principle seen during
solar eclipses, and
it is how we are able
to see the faint outer
layers 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 TOWARD
observer…this causes the starlight to be blue-shifted to shorter wavelengths
Astrometric Stellar Wobble: observed position actually changes periodically
Practically, this will only work for the nearest stars with the largest exoplanets.
Spectroscopic 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: No 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 is
only 10 Earth-hours long.
An exoplanet orbiting Coku Tau 4 is
less than 1 million years old, making
it the youngest known exoplanet.
An exoplanet orbiting a pulsar (a dead
star 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 common
in the Universe.
A year on HD209458b is 3½ Earth-days
long. It is being evaporated by it’s
parent star, at an estimated rate of 10,000
Earth-tons per second.
Gliese 581 C is the smallest exoplanet ever detected, and is the first
to lie within its parent star’s Habitable Zone. Life could exist here.
NEXT TIME
• The Terrestrial (Inner) Planets
– Mercury, Venus & Mars in more detail…