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136 Cards in this Set

  • Front
  • Back
"comparative planetology"
studying similarities / differences amoung planetary bodies

-learn how physical processes that affect planets compare
-origin / history of solar system
-probable nature of solar systems around other stars
our solar system

[composed of, location, formation]
-our sun
-8 planets
-asteroids, comets, dust

-near spiral arm of Milky Way Galaxy

~4.6 bya
-long AFTER big bang [formation of the universe]
orbits / rotation of planets
-most planets and moons orbit CCW
--close to circular orbits (low eccentricity)
---in nearly the same plane

-the sun and most planets rotate CCW
planets' location in relation to the ecliptic plane?
all planets ~ in ecliptic plane BUT PLUTO
order of the planets
terrestrial planets
-rocky (earth-like)

-smaller size / mass

-denser (rocks metals)

-solid surface

-closer to sun (& closer together)


-few moons

-no rings
jovian planets
-gas giants (jupiter-like)

-larger size / mass

-lower density (light gasses...hydrogen compounds)

-NOT solid surface

-further from sun (& farther apart)


-have rings

-many moons
exceptions to the rules

(of our solar system)
-pluto isn't terrestrial or jovian

-uranus & pluto are tilted on their sides

-venus rotates CW (backwards)

-triton orbits neptune CW (backwards)

-earth is the only terrestrial planet w a large moon

-only earth known to have life

-most between jupiter and mars (asteroid belt)

-kuiper belt

-oort cloud
sun = how much of solar system's mass
99.9% of solar systems mass
sun's influence on planets
GRAVITY--planets orbits

HEAT-- planets temp

LIGHT-- solar systems only source of visible light

HIGH ENERGY PARTICLES-- influence atmospheres & magnetic fields
temp extremes
- 800* day, -240* night
-lots of craters
-signs of early shrinkage
-density: 5.4 g/cm3 (IRON RICH... large core)
-geologically dead
-no moons
-size / density like earth's
-thick co2 atmosphere
-extreme GH effect (850* surface temp)
-geologically complex
-no moons
-rocky w. iron core
-VERY geologically active
-n2 & o2 atmosphere
-modest GH effect
-abundant surface water
-large rocky moon
-thin co2 atmosphere
-geologically complex
-lots of near surface ice
-evidence of ancient/recent surface water
-life questionable
-2 tiny moons
-H & He atmosphere
-great red spot
-no solid surface
-faint rings

jupiter's moons
-many active volcanos

-ice + rock
-probable subsurface ocean

-past faulting

-lots of craters
-gas giant
-bright icy mooons

-thick atmosphere
-likely methane seas
-gas giant

-methane ==> bluish color

-axis tilted on side

-many dirty icy moons

-thin dark rings
-gas giant

-methane ==> bluish color

-narrow, clumpy rings

-nitrogen geysers

-other small moons
-largest known kuiper belt object (KBO)
-not terrestrial or jovian planet

-exotic ices (methane, N, co2, co)

-orbit = elliptical & inclined (sometimes closer to sun than neptune)

-large icy moon

-Only Planet unexplored by spacecraft
theory of solar system formation MUST EXPLAIN::
1. orderly motions in solar system

2. two planet types
(-small rocky terrestrial)
(-large H-rich jovian)

3. asteroids / comets exist in certain regions of solar system

4. exceptions to these patterns

(theory of solar system formation)
-solar system formed from giant cloud of gas & dust
**the solar nebula**

-solar nebula cloud collapsed under its own gravity
**(law of gravity, conservation of energy, conservation of ang. momentum)**

-Observational Evidence:::
---stars observed in process of forming today
----this always occurs in intersteller gas clouds
the Gravitational Collapse

of the Solar Nebula (cloud)
-solar nebula initially a blob
-a few light yrs across
-very cold, slightly rotating

-an event gave it a "push"
--(could have been shock from nearby supernova)

--->The nebula SHRANK
---->Gravity Increased
---->caused collapse

-as nebula falls inward Gravitational potential energy converted to heat
**conservation of energy**

-as nebula radius decreases
it rotates faster!!
**conservation of ang. momentum,**
flattening of solar nebula
as nebula collapses

---clumps collide & merge
--collisions cancel out random velocities... remaining motion is general rotating directions

-spinning nebula becomes disk.... hottest @ center
the causes and effects
of solar nebula
GRAVITY--causes nebula to collapse

CONSERVATION OF ENERGY -- causes it to heat up (as it collapses)

CONSERVATION OF ANGULAR MOMENTUM --- causes it to spin faster as radius decreases

COLLISIONS ---- between particals cause it to flatten
sun formed in center of nebula bc...
temp and density high enough here for nuclear fusion

planets formed in rest of disk
the nebular theory explains orderly motions of the solar system...
-all planets lie along same plane
(the plane of the disk)

-all planets orbit in one directions
(the spin direction of the disk)

--the sun rotates in the same direction

-the planets rotate in same direction

--mooons orbit in this direction

-most planetary orbits circular
(survivors of collisions in the disk)
support for nebular theory
from observations
-disks of gas / dust observed around young stars
(probably new planetary systems in formation)
materials in solar nebula
1) light gasses (98%)
-H -He

2) hydrogen compounds (1.4%)
-water -methane -ammonia

3) rocks (.4%)

4) metals (.2%)
-iron -nickel -aluminum
condensation (solidification) of materials in nebula
condensation / solidification
depends on Temperature

so Depends on distance from Sun

-ICES (H-compounds) condense only behind frost line
accretion of planetisimals
small condensed "seeds" stick together then attract via gravity to form
'planetisimals' (building blocks of planets)

-planetisimals collide & combine

--only planets w well behaved orbits survive

near sun

rocky planetisimals collide and combine to form terrestrial planets

behind frost line

-ice-rich planetisimals combine to form large cores

-large icy cores capture gas from outer solar nebula to form jovian planets
formation of jovian planets' moons
each jovian planet formed own mini nebula

-mooons formed in planet's mini-nebula
solar wind
charged particals coming out of sun

-strong solar wind from early active sun
-CLEARED THE NEBULA of leftover gas
nebular theory explaining 2 planet types?::
-cloud collapses forming disk

-solid particals condense from gas (comp. depending on distance!!!!!)

-seeds accrete into larger bodies....eventually planets

-jovian planets capture nebular gasses

-solar wind clears away remaining nebula
when would there be no jovian planets??
if the nebula had been cleared by solar wind before planetismals captured nebular gas
when would there be no terrestrial planets???::
if the whole nebula cooled below condensation temp BEFORE solar wind clearing
origin of asteroids
-solar wind cleared leftover gas... but not leftover planetisimals

-leftover rocky planetisimals that didn't accrete into planets are asteroids!!!!

-most are in asteroid belt between mars & jupiter
(jupiter's strong gravity prevented a planet from forming there)
origin of comets
-leftover icy planetisimals = comets

-comets amoung jovian planets flung out in all directions to form OORT CLOUD

-planetisimals beyond neptune stayed in ecliptic plane to form KUIPER BELT
extrasolar planets
aka ESPs

planets around other stars

first evidence = 1995
(around star "51 pegasi")

>200 ESPs found since 1995
telescopes can't detect ESPs
-planets too close to their stars to resolve them from earth

-planets too dim to image (the star is ~ a bill x brighter than the planets)
ESPs detected by
intersteller wobble

-big planets tug on their star causing it to wobble

-can be detected by detecting DOPPLER SHIFT of light from wobbling star
(blue when moving toward us, red when moving away form us)
doppler shift & stars
-blue moving toward
-red moving away

-the greater the velocity the greater the doppler shift

-shape of doppler shift plot tells us::
----1---- distance of planet from star (plot's WL)
----2---- mass of planet (plot's amplitude)
planetary transits
can be observed when planet's orbit is edge-on relative to us

a TRANSIT is::
-the dimming of a star when planet passes in front of it

Allows us to:
-calc planet's size (& mass / period)
-knowing size can determine density (tells us if terrestrial or jovian)
Exoplanets' properties
-most massive & close to star
(jovian planets migrate toward star by gas drag / gravitational interactions??)
--->could happen before solar wind clearing bc of drag by nebular gasses

-Very Eccentric Orbits

-easiest to detect massive planets close to stars
ppl credited for "nebular hypothesis"
-solar system formed from gaviational collapse of intersteller cloud

40yrs later
-same idea

(intersteller cloud usually called a nebula --> "nebular hypo")
"close encounter hypothesis"
-competed w nebular hypo in 20th century

-planets formed from blobs of gas that had been gravitationally pulled out of sun during near collision

-couldn't account for orbital motions or 2 planet types
-required improbable event (star systems largely seperated)
-forming planets in collisions = rare event = planets should be rare ..... BUT THEY'RE NOT!!!
our solar system was born from...
a cloud of gas that collapsed under its own gravity

(the solar nebula)
When universe was young (beginning of big bang)
only H and He were present!

-heavier elements were produced thru nuclear fusion in stars cores that is released during their death
(and can then be recycled into new generations of stars)
when our solar system formed...

H & He
heavier elements
H & He = 98%
heavier elements = 2%

-THE SUN has this basic comp today
(bc it rep. almost all of the mass in our solar system)
the gas that made up the solar nebula that formed our solar system...
resulted from billions of yrs of recycling that happened before our system was born

-sun & planets born from this gas
-terrestrial planets ended up being made of the heavier elements
solar nebula originally large cold low-density cloud

(how did it collapse??)
it was very spread out originally
-gravity alone couldn't have made it collapse

-collapse prob triggered by impact from explosion of nearby star

-collapse began ... law of gravity ensured it would continue!
when collapse of solar nebula began ... law of gravity ensured it would continue!
strength of gravity follows inverse square law w distance

-mass of cloud remained same as it shrank--- strength of gravity increased as diameter decreased

(when diameter decreased by half, force of gravity inccreased by factor of four)
heating of the solar nebula
temperature increased as it collapsed


-as cloud shrank gravitational potential energy converted to kinetic energy

-particles w kinetic energy crashed into one another converting kinetic ennergy into thermal heat

-Sun formed in center where temp and densities are highest
spinning of nebula
conservation of angular momentum

-radius shrank ---> spins faster

-ensured not all material was in center
..... greater the ang. momentum the more spread out it will be
flattening of solar nebula
it flattened into a disk

-natural consequence of collisions between particles of spinning cloud

-clumps collide and merge ... each clump has same velocity as clumps that formed it
--random motions become more orderly
-collisions also reduce eccentricity (making orbits more circular)
all planets orbit sun in same plane bc..
they formed in the flat disk
why do planets in our system have nearly circular orbits?
collisions tend to make orbits more circular
allows for exceptions in orderly planet motions?
bc the planets are so small compared to the entire disk
solid bits of matter from which gravity could ultimately build planets
general process in which solid particles formed in a gas
H & He

never condense in conditions present in nebula
Hydrogen compounds

(water, methane, ammonia)

solidify into ices @ low temperatures

gaseous @ high temps

condenses at temp between ~500K - 1300K

(iron, nickel, alluminum)

-gaseous @ high temps
-BUT condense into solid at higher temps than rock (1000K - 1600K)
distribution according to temp

(& dist. from sun)
near mercury's orbit tgemp low enough for metals and some rock to condense
while h compounds and other rock remained gaseous

where asteroid belt is... temp low enough for dark carbon rich minerals to condense and contained small amts of water

H-compounds could condense ONLY BEYOND THE FROST LINE (between mars and jupiter)
small size of terrestrial planets?
bc rock and metal made up such small amt of material in solar nebula
planets stayed small in size
process in which small "seeds" grew into planets
boulders large enough to count as peices of planets

(when particals grew in mass and gravity began to aid their process of sticking together)
only largest planetisimals survived..
when diff orbits crossed collisions occured at higher speeds and were more destructive

-collisions produced fragmentation more often than accretion

-only largest planetesimals avoided being shattered and grew into TERRESTRIAL PLANETS!@
jovian planet formation

(& explanation of their moons)
ices were able to condense so more solid material than terrestrial planets

-acquired gas after beginning as large ice-rich planetesimals

-icy planetesimals often grew larger than earth
--gravity strong enough to capture H and He

---gravity grew more and thus they accumulated more gasses

-each planet surrounded by disk of gas spinning in same direction planet rotated in
-moons accreted from ice-rich planetesimals in these disks
---why they orbit in the same directions as planet rotates w orbits close to planet's equatorial plane
what sealed fates of planets?

ended planet formation?

-most of H and He gas never became part of a planet
-swept into space by solar wind of charged particles (protons / electrons) -that are continually blown off from sun

-stronger solar wind in younger stars

-if cleared earlier no jovian
-cleared later all jovian
explaining sun's slow rotation
angular momentum says sun should be rotating faster

-young sun's rapid rotation generate dmagnetic field that helped create solar wind
-high energy radiation ionized gas created charged particals

*ang momentum didn't disappear... it was transferred to another obj.*

----the charged particles gained ang momentum while the sun lost it
"the heavy bombardment"
period in which the majority of collisions of asteroids and cocmets occured ( w/i the first few million years of solar system's history )

water on earth came from the impact of planetesimals that formed farther from the sun
(bc where terrestrial planets formed it was too hot for water & other H-compounds to form)
captured mooons
-explain why some moons orbit backwards
& why some have large inclinations to planet's equator

-obj cannot go from unbound orbit to bound orbit unless some orbital energy is lost

--for jovian planets passing planetesimals prob lost energy to drag in the extended dense gas surrounding the planet
--this friction could reduce orbital energy enough for it to become an orbiting moon
(& since randomly captured --> can account for backwards motions and such)
the existence of our moon
-too large to have been captured by earth(small planet)
-if moon and earth had formed together both should have been made of same comp and density

BUT moon's density is much lower than earths
-and moon's made out of mantle/crust earth material

-planet sized planetesimals collided w earth ("GIANT IMPACT")
-impact blasted away earth's ourter layers and sent material into orbit around planet

-moon made of mantle/crust material
-moon lacks easily vaporized ingredients ---WATER
---heat of the impact would have vaporized these ingredients

*other giant impacts could be responsible for the tilting of planets
entire process of planet formation took...
no more than a few tens of millions of years...

~1% of current age of solar system
radiometric dating
method by which we measure age of a rock
--relies on proportions of differnt atoms and isotopes in the rock

-radioactive isotope can undergo radioactive decay (breaking apart or having a proton turn into a neutron)

*half life = time for half of parent nuclei to decay
radiometric dating tells us
how long its been since rock solidified

... not same age as planet as a whole bc surface generally changing
2 ways to detect planets around other stars
pictures or spectra of the planets themselves

measurements of stellar properties (position / brightness / spectra)

(nearly all discovered indirectly by gravitational tugs)
"gravitational tugs"
how most ESPs have been detected


-planets are discovered by observing stars they orbit

-objects orbit system's center of mass

***star makes small orbital motion bc of gravitational tugs by planets
2 ways to observe gravitational tugs

(or indirectly discover planets)
make precise measurements of a star's position to see if it is moving about in a way that could be caused by tugging of planets

-detects orbital motion by looking for a changing doppler shift in the star's spectrum
(blueshifts = toward us
redshifts = away from us)
"astrometric technique"
makes precise measurements of stellar positions in the sky

1)the farther the star, the smaller the movement appears
*works best for big planets orbiting stars close to us

2)easier to detect larger movements.... planet with larger orbit moves star more...
BUT larger orbits take longer time to complete orbit (kepler's 3rd law)

**astrometric technique has only discovered one ESP and it had already been discovered by the dopplar technique
"doppler technique"
orbiting planet causes star to move toward us then away from us

(alternating blue then red)

-how we discovered 51 pegasi had planet orbiting it

-know the period by measuring crest to crest on dopler shift graph

-distance from star (using keplers 3rd law)
-shape of orbit (planet with circular orbit travels @ constant speed around star -- data curve = perfectly symmetric)
-planet's mass (only when orbit is edge on bc only then do we know the true orbital velocity in full)

**the greater the orbit's tilted toward edge-on the greater the shift we'll see**

-can sometimes tell if there's more than one planet
doppler techniques in determining masses
really minimum masses, not actual masses

-bc we normally view an orbit that is tilted.. not edge on
... so we cannot find the true orbital velocity
fates of the "leftovers"

jupiter's gravity stirred up rocky planetesimals between jupiter and mars
(no planet was able to form)
fates of the "leftovers"

jovian planets stirred up orbits of icy planetesimals in their vicinity flinging them far outward
fates of the "leftovers"

icy planetesimals slowly formed from nebula just beyond neptune

(still orbiting sun)
fates of the "leftovers"

all leftovers -- asteroids / comets
rained down on planets early on
-made impact craters
-bring earth "good stuff" (h-compounds) *WATER!!*
exceptions to planetary rules:
-nearly everything about pluto

-earth's large moon

-mars' moons

-venus' backwards rotation

-uranus' rotation

-neptune's backwards moon (triton)
exceptions to rules explained
many planetesimals collided w planets during "Heavy Bombardment"

-CAPTURED MOONS (why some orbit backwards)

-HIGHLY TILTED ROTATION .. uranus on side (impacts knocked them over)

-FAST ROTATION (impacts can speed up planets rotation)

-EARTH is only terrestrial planet w GIANT MOON (formed from impact --- stuff ejected into space and into orbit)
formation of earth's moon
-earth struck by mars sized object

-portions of earth's outer layers ejected

-easily vaporized materials (water) were lost to space

-debris re-accreted around earth to form moon

-moon is dry
-composition like earth's mantle
-low density (tiny iron core)
the solar system's oldest rocks

~4.5 billion years old
(we conclude the age of the solar system is just over 4.5 byo)
summary of solar system formation

(5 steps - one word)




the SUN
98% Hydrogen / Helium
2% Heavier Elements

-contains > 99.9% of solar system's mass
-surface = sea of hot hydrogen and helium gas
-sunspots = darker bc slightly cooler
-gaseous thruout
-deeper = higher temps

-source of energy = deep in core -- nuclear fusion takes place
-fusion tranforms hydrogen to helium with some lost to energy

-its gravity governs orbits of planets
-its heat determines planet temp
-source of ~all visible light

-charged particles flowing outward (solar wind) create planetary magnetic fields and influence planetary atmosphere
-rocks / metals
-no moons
-hot / cold extremes
-tidal forces from sun = unusual rotation pattern
---rotates 3 times for every 2 orbits
-least studied bc so close to sun
-craters / ancient lave flows

-made mostly of iron
-rocks / metals
-0 moons

-same size as earth
-rotates on axis VERY SLOWLY

(sun rises in west / sets in east)

-dense clouds -- extreme GH effect

-high pressure
-no oxygen
-no water

-mountains / valleys / craters (past and possible present volcanic activity)
-rocks / metals
-1 moon

-just enough carbon dioxide and water vapor to maintain moderate GH effect
-rocks / metals
-2 small moons
(too small to influnce planet)

-smaller than venus & earth
-ancient volcanos / canyon

-polar caps of frozen carbon dioxide & water ice

-dried up riverbeds / minerals

-very low air pressure
-temp below freezing
-little oxygen
-lack of ozone (UV RAYS!)
-hydrogen / helium
->63 moons

-distance from mars to jupiter = double distance from sun to mars

-mass 300x earth
-great red spot = long lived storm

-thin set of rings

-IO = most volcanically active place in solar system

-EUROPA = icry crust (possible subsurface ocean)
-hydrogen / helium
->47 moons

-2nd largest planet
-less dense than jupiter

--- made of tiny particals that orbit it

-TITAIN = thick atmosphere
-same nitrogen content as earth
-erosion even tho temp -180*C
-hydrogen / helium / hydrogen compounds
->27 moons

-twice as far from sun as saturn

-hydrogen compounds = water, ammonia, methane (gives blue color)

-set of rings

--extreme seasons
(on N pole continuous daylight for 1/2 orbit..)

-only voyager 2 visited
- hydrogen / helium / hydrogen compounds
->13 moons

-journey from uranus to neptune LONGEST YET

-higher density than uranus

-only voyager 2 visited

-icy surface
-nitrogen geysers
-orbits BACKWARDS!
(& other dwarf planets)
-sun just bright light
-much smaller / less massive than other planets

-composed of ice / rock
(identical to many comets

-located in Kiuper belt
(not even largest obj in belt... ERIS is)

-"dwarf planet" = any obj that orbits sun and is large enough for its own gravity to have made it round but has not cleared its neighborhood of other bodies
4 major features of solar system:
1) sun / planets / moons generally orbit and rotate in organized way

2) 2 types of planets

3)asteroids and comets locations / orbits / compositions follow patterns

4) exceptions to these rules
terrestrial planets
four inner planets
-small / dense
-rocky surface
-metal in interior
-few moons if any
jovian planets
-four outer
-low in density
-numerous moons
-made mostly of H, He, & H-compounds
-no solid surface
-but intense pressure and temp transform these gasses..
small rocky bodies orbit sun
-much smaller than planets

-most found in asteroid belt between mars and jupiter
orbit sun
-made mostly of ICE
mixed w/ rock

-rare appearence in inner solar system
(appear w/ tails)

-KUIPER BELT = orbits sun in same direction and plane as planets
-occupies region beyond neptune

-other comets with no simple pattern to orbit
-much futher from sun
-uranus rotates nearly on side

-venus rotates backwards

-our moon
determining orbital shape from doppler data curve
perfectly circular orbit
-travels @ constant speed
-data curve perfectly symmetric

eccentric orbit
-varying speed
-asymmetry in doppler curve
doppler technique and planetary masses
doppler shift will tellus orbital velocity
-with velocity we can determine mass

(only gives true velocity if orbit is edge on... therefore we only have minimum possible masses obtained from doppler data)

-but planet's true mass usually no more than dble min mass so still good estimate
doppler technique best suited for
massive planets close to their star

-bc gravity weakens w/ distance
-planet of given size pulls harder when closer to star

-closer planet has shorter orbit so less time to observe shifts

-greater mass yields greater effect on shift of star
when planet moves across face of star

-can determine size of planet

-requires repeated observations bc stars vary in brightness
-need to see dimming repeated in regular period

--->then can calculate planet's orbital distance and mass
(from observing how long orbital period is)

-transits tell us comp of atmosphere
--during transit solid disk blocks light
--gas absorbs only spec wavelengths depending on comp
---determine comp based on absorbtion lines
planets pass behind their star so star blocks light from planet

-dip in brightness smaller in eclipse than in transit bc star so much brighter than planet

-can determine total thermal emission from planet
observing ESPs thru transits & eclipses

-can only work for orbits edge on w earth
-biased for short orbital periods (orbits close to stars)
bc transits more frequently and we need repeated observations to confirm

-with precise measurements of brightness we can detect planets far smaller than possible with astrometric or doppler techniques
-can work in both large & small telescopes
direct detection of planets
still not possible to obtain images or spectra of planets around stars bc of brightness of star themself

-a very large telescope was able to obtain an IR spectrum of planet which showed evidence of H2o molecules (what we would expect for jovian)
Optical Graviational Lensing Experiment (OGLE)
large survey of thousands of distant stars
-already detected several planets by transits

-detected planets by GRAVITATIONAL LENSING
--object's gravity bends/ brightens light of a more distant obj.

(other strategy for detecting planets)

(told by which technique?)
all 3 indirect techniques




(told by which technique?)
calculed when we know

(calc distance by kepler's 3rd law)

(told by which technique?)
need data spanning entire orbit



(transits only give us infor for small part of orbit --- when passes in front of star)

(told by which technique?)
determined by
-orbital period
-mass of star
-speed @ which the planet makes the star orbit its center of mass

*doppler technique tells us part of speed (toward or away from us) - thus tells us min mass
SIZE (radius)

(told by which technique?)

-determined by fraction of light blocked

(told by which technique?)
calculated by size and mass

(since we can only determine size from transits we can only determine density for planets that produce transity AND which we have mass data from astrometric or doppler techniques)

(told by which technique?)
by planet's SPECTRA

--detailed info about comp must be determined by direct detection

--transits / eclipses give limited spectra info
ESPs general
most more massive than jupiter

-least massive found stil 7x more massive than earth

-most seem to be larger and less dense than the jovian in our system
--bc they're closer to the sun the planet's hotter which expands H and He gas... explaining large size / low density

-most close to sun

-highly elliptical orbits

... could have migrated closer.... still follows nebular theory with some additions

... .or could have suffered gravitational interactions w other massive objects (close encounters)
planetary migration
(of ESPs)

*possible explanation for eccentric orbits / close orbits*
planet's gravity can nudge particles in disk
-creating waves
-waves move thru disk
-causes material to bunch up as waves pass
-bunched up matter exerts extra gravitational nudge back on planet

--nudge can cause planet to migrate inward
Encounters / Resonances
(of ESPs)

*possible explanation for eccentric orbits / close orbits*
close gravitational encounters between young jovian planets in outer regions of disk

-can send one planet out of system while other in flung inward into highly elliptical orbit

**OR jovian planet can migrate inward as result of many close encounters with smaller planetesimals

-shared orbital resonances cause orbits to be more elliptical
"kepler mission"
will go into orbit around sun

-stare continuously at same portion of sky for 4 years

-measuring star brightness every 15 min
goal to perform astrometric observations of many stars

-obtaining data from dopller measurements and making position measurements (simultaneously)

--will be able to determine star's motion in all three dimensions.... allowing to determine accurate masses

-also capable of detecting transits
SIM mission
capable of measuring steller positions very precisely

-trying new technology
--"nulling interferometry"
--designed to cancel light from star so can see orbiting planets
"Terrestrial Planet Finder" TPF mission
trying to detect earthlike planets

-huge telescopes w/ masks to block sunlight