Today

● Today: ● – Review of Parts 4 & 5 of the text (Weeks 8-12) – Cover the last of the material Next week – Assignment covering Weeks 8-13 due – Projects also due next week – Class summary – Any student presentations

Review: Weeks 8-12

● History of our Planetary system ● Planets Other than Our Own in our Solar System ● Habitability of places other than Earth ● Finding Planets outside of the Solar System ● Visiting or Communicating?

Our Solar System

● Orbits and Gravity ● Planetary System Formation

Orbits

● Planets are falling towards Sun due to gravitational acceleration ● Moving toward the side fast enough that they miss ● ● Moving too fast – escape entirely, leave Sun Move too slowly – fall into Sun ● Same with satellites circling Earth, or Sun orbiting in our galaxy, or...

Gravity

● Gravity acts between all massive objects ● Gravitational force is equal on both objects ● If orbiting, both objects move, not just one, since both are being acted on by gravity ● Both orbit the center of mass of the system ● Equal mass objects; center of mass is at the center of the two objects

Gravity

● If one body is more massive, then gravitational force is increased ● Center of mass tilts towards more massive body ● ● Forces still equal Equal force on lighter body moves it more than the same force on the heavier body ● Lighter object moves larger distance than heavier object

Gravity

● Force of gravity also increases as objects get nearer ● Inverse Square Law (same as light)

Orbits

● Kepler's Laws: (EMPERICAL) – Planets travel in ellipses, with sun at one focus of ellipse – Area swept out by radius is equal over any equal amount of time – Square of the planet's period (the `year' for that planet) proportional to the distance to the sun cubed.

– P 2 ~ a 3

Planets

● Almost all planets are in same plane ● All planets (except Uranus) rotate more or less in the same plane, as does Sun ● Very suggestive of the idea that planets, Sun formed from a disk, as we discussed before ● Suggested by Laplace in 1600s.

● Disk near star is depleted in Hydrogen, Helium by evaporation

Planet Formation

● As disk cools, gas/dust disk can begin condensing ● Grains form, which themselves agglomerate to larger particles ● Regions where disk is originally dense condense faster, gravitationally attract more material ● Process of continued agglomeration can form planets

Instability

● Some processes are naturally stable ● – Burning in main sequence stars – Core heats up – outer layers puff up – core cools down – Automatically stabilizes itself – Ball in a right-side-up bowl Once there's a region of high density in a gas cloud or disk, increase in gravitational attraction to that region...

● Unstable – Ball on an up-side-down bowl

Planet Formation

● Proto-planetary-core starts sweeping out material and planetesimals at its radius ● Accrete material streams in from just outside or inside its radius ● There is a limit to this process; if there are planets forming on either side, eventually the gaps collide – no more new material ● This process of slowly sweeping up and accreting material can take millions of years

Mystery: `Hot Jupiters'

● A Jupiter couldn't form at 1AU; evaporation would prevent such a gas giant from forming ● Many of the extra-solar planets observed are gas giants at distances ~ 1AU ● What happened?

● Two possibilities: – Migration – Different formation mechanism

Planet Formation

● Migration is possible ● As planets form and accrete material, they experience a drag force ● Drag takes energy from planets motion and they fall inwards

Planet Formation

● Fast formation is also possible ● In sufficiently massive disk, instabilities can occur much faster, and on larger scales ● Can happen quickly enough that perhaps giants can form near star

Our Solar System

● Other Bodies – Mercury – The Moon – Venus – Mars – Gas Giants – Gas Giant Moons

The Moon

● ● No atmosphere No geological activity ● No water ● ● -> no erosion Can provide information about formation of solar system that is absent from Earth

Mercury

● ● Similar to moon Similar size ● Small, empty, simple ● ● Very close to Sun No atmosphere to mediate temperature swings: – +750 o F in sun – -230 o F in shade

Moon's Cratering

● ● Nothing to alter surface Complete history of cratering in Moon's history ● From predicted cratering rate, one expects that crust of moon formed very quickly in solar system history

Possible Moon Formation Scenario

Possible Moon Formation Scenario

● Explains similar Oxygen abundances ● – Very different from meteorites Explains fewer volatiles – If Earth's iron core had already settled, impact would have dislodged crust material – Heat of impact would have vaporized volatiles

Venus

● ● Closest to Earth ¾ as far away from Sun as Earth is ● Very similar to Earth's size, density ● Covered by thick, opaque clouds

Venus

● ● Runaway greenhouse effect Hot: very near sun ● Water begins to evaporate ● Water vapor is a greenhouse gas!

● Surface gets hotter, more water evaporation ● Surface is hundreds of degrees ● No liquid water

Mars

● Red planet between Earth and Asteroid Belt ● Half again as far away from Sun as the Earth is – Expect it to be ~100 o F colder than Earth on average – Average too cool for water – Peak temps ~ 70 o F (but -130 at night!)

Mars

● Near asteroid belt ● – Likely more collisions than Earth Large impacts can blow off significant rocky material ● – Meteorites As well as gases (atmosphere)

Mars

● ● ~1/2 radius of Earth ~1/10 mass ● ~40% surface gravity – Force of a 1 lb weight less than ½ lb on Mars – Less gravity holding the atmosphere in place

Mars

● Too little gravity to be able to hold onto a significant atmosphere ● Atmospheric pressure less than 1% of Earth's

Evaporation

● What causes evaporation of liquid, and what prevents it?

Evaporation

● What causes evaporation of liquid, and what prevents it?

● Fastest moving water (say) molecules can escape into atmosphere ● ● Water molecules in atmosphere can collide into water and become part of the liquid Balance is reached when evaporating water = condensing water

Evaporation

● Can change balance: – Little water in atmosphere, evaporation happens faster ● (Why feel so sticky on a humid day) – If air pressure is very low, evaporated water molecules can move very far away from pool of water ● Fewer around to condense ● Faster evaporation

Evaporation

220 210 200 190 180 170 160 150 140 130 120 110 100 0 Boiling Point at Alt it ude 2000 5000 Alt it ude (ft ) 7500 10000 ● Effect of atmospheric pressure happens on our own planet ● Reason for `high-altitude cooking instructions' on some boxes ● Higher altitude -> lower air pressure -> evaporation is easier > lower boiling point

Evaporation

● Martian atmospheric pressure < 1% of Earth's ● – (Earth's atmosphere at 15 miles / 80,000 ft) Water boiling point is so low that any liquid water evaporates immediately ● No free water possible on surface

Evaporation

● But water ice DOES exist on Mars: – Polar ice caps ● Mostly (on top) dry ice (frozen CO2) ● Underneath, visible when CO2 has sublimated, water ice – Quite likely some trapped under surface: `permafrost'

The Giants

● The Giants are sometimes all called `Jovian' planets after Jupiter ● After more exploration showed their diversity, this term lost favour

The Giants

● The giant planets can be weighed very accurately by measuring the speed of their moons.

● ● Much heavier than Earth, but not so heavy considering their size Densities 600 – 1600 kg/m 3 , compared with Earth's 5700 kg/m 3 ● Mostly made of gas/liquids?

The Birth of Giants

● In outer solar system, cooler ● Less evaporative stripping of volatile gases ● If sufficiently massive cores form, can keep even volatile gases ● These gases will be representative of the very early solar system

The Birth of Giants

● Since early solar system is largely composed of Hydrogen, so will gas giants ● Rocky or Icy or Slushy core ● High-hydrogen atmosphere has some similarities to atmosphere in Miller-Urey experiment ● Can form lots of organics

Jupiter in Infra-red

The Birth of Giants

● Large mass -> high pressure, temperature at centre ● Temperature at centre of Jupiter ~ 4 times surface of Sun!

● Collapse from origin of planet still slowly continuing ● Releases heat energy ● These planets have a source of heat

Jupiter in Infra-red

The Birth of Giants

● Gas giants emit more heat than they absorb from Sun ● At earlier times, would have been much hotter ● Moons, which are nearby, heated by their nearby planet ● Many of these moons are large (planet-sized) ● Moons might be interesting for life?

The Moons of Giants

● Planets large enough that many moons were also formed ● Many of them planet sized in their own right ● Get heat from planet ● Some (Io/Jupiter) effected by planets magnetic field ● Atmosphere? (Titan, Saturn) ● Water? (Europa, Jupiter)

The Moons of Giants

● Formation: like planets around sun ● Rotating body, disk forms ● Moons generally along plane of rotation of planet

Gas Giants

● Convection is a fundamental process ● – Happens everywhere Fluid heated at bottom rises, cools, falls back down ● Gas giants have hot centres ● ● Large-scale motions Mix material

Gas Giants

● Makes it difficult to imagine life forming ● No real surface to live on ● Chemicals constantly being mixed around ● No originally contained environment (`protocell')

Moons

● Gas giants have planet-sized moons ● At least one (Titan) has a significant atmosphere ● Another (Europa) very likely has liquid salty water under a layer of ice

Europa

● Very suggestive it has a liquid underneath ● – No cratering – Many fractures, ridges on surface What would this mean for life?

– If some source of energy on inside (geothermal, chemical), very real possibility of some sort of life

Titan

● ● Very Cold Massive, Cold enough to have an atmosphere (1.5 x as dense as ours!) ● No oxygen ● No liquid water ● Hydrogen rich ● Interesting organic chemistry ● ● Lakes of hydrocarbons?

Huygens probe 2005

How Unique is Earth?

● What is special about Earth?

● How important/rare are those things?

● How many such planets are there likely to be?

Earth

● Atmosphere ● – Large surface gravity Reasonable temperature ● Rocky surface ● Large moon ● Lots of heavy elements

How Important/Rare are these?

● Heavy elements; – Likely ubiquitous in planets around Pop I stars

How Important/Rare are these?

● Rocky Surface – Can happen if there is heavy elements (see above) – Probably true of all planets close enough to have liquid water – (But planet migration)

How Important/Rare are these?

● Atmosphere – Requires not too close to sun – Requires massive enough planet

How Important/Rare are these?

● Reasonable Temperature – `Goldilocks zone’ – Needs to be right distance to star

How Important/Rare are these?

● So we require – Rocky Planet – Of the right mass – At the right distance from the star

Habitable Zone

● Corresponds to further than Venus to about Mars distance for our Sun ● Using inverse-square law, could calculate for other stars ● Main requirement: liquid water in the presence of an atmosphere.

Habitable Zone: Binary Stars

● About half of all stars are in binary systems ● Stars orbit a common centre of mass (more on that next week) ● Can planets have reasonable orbits in such systems?

● ● Yes, but must orbit one star or be far away from both; `Figure 8’ orbits aren’t stable

Finding Other Planets

● Light from planet ● – Reflected visible light – Reflected+generated infrared Dark from planet ● – Transits (shadows from planets) Light bent by planet ● – Gravitational Lensing Star's Motion from planet – Proper Motions – Doppler Shift

Small

brown dwarf (not planet)

companion to a star directly imaged

Light from the planet

● Stars observed by emitting their own light ● Planets don't emit light, but do reflect sunlight ● Problem: reflect a billionth or less of the light from the companion star

Light from the planet

● ● Has yet to be observed What sort of planets/systems does this work best for?

Small

brown dwarf (not planet)

companion to a star directly imaged

Light from the planet

● Would work best for: – Large planets (more reflecting surface) – Reflective planets (ammonia clouds?) – Near enough star to reflect lots of light – Far enough not to be overwhelmed by light from star

Small

brown dwarf (not planet)

companion to a star directly imaged

Light from the planet

● Large planets near star: `Hot Jupiters' ● Gas giants (presumably) very near star

Light from the planet

● How observed?

● Very careful imaging of nearby stars ● Probably with telescopes above atmosphere (Hubble) ● As long as planet isn't in front of/behind star, will be reflecting light towards Earth ● Just a question of being able to observe it

Light from the planet

Small

brown dwarf (not planet)

companion to a star directly imaged ● This is actually an infrared image ● Jupiter-type planets may emit their own infrared light ● Terrestrial planets reflect a lot of infrared ● Star emits most of its light in visible ● Better chance in IR

Time

Planetary Transits/Occultations

● ● ● Light from planet can be blocked by orbiting planet Careful measurement of total light from star can show this Can't see directly; the star is just a point

Time

?

Planetary Transits/Occultations

● If period is measured (multiple transits) and mass estimate for star exists, have: – Planet's distance – Planet's size – Planet's orbital period – Star's size

Planetary Transits/Occultations

● How are these observed?

● Fairly rare events: ● – Has to be exactly along line of sight ● Only planetary systems aligned along line of sight ● Planet directly in front of star only very briefly (Jupiter: ~1 day / 11 yrs) Fairly careful measurements must be made – Jupiter: 1% decrease in Sun's brightness

Planetary Transits/Occultations

● Large survey – Dedicated telescope – Look at large fraction of sky every night (or nearly)

Planetary Transits/Occultations

● Works best for: ● – Large planets (blocks more of star) – Planets near star (shorter period – easier to observe) – Hot Jupiters Has been used to find planets

Gravitational lensing

● A very powerful technique to measure dim objects ● Used in searches for brown dwarfs or other large clumps of `dark matter' ● Requires – distant, bright, source star, – very accurate measurements of the brightness of the source star over time

Gravitational lensing

● At least one planet has been `seen' this way ● Results: ● – Mass of planet, star – Distance to star – Distance planet <-> star Difficult, because only get one chance at measuring system

Gravitational lensing

● Works best for what systems?

– Dim Stars – Massive planets – (relatively) insensitive to distance between star and planet – Jupiters at any radii / temperature

Astrometry: Proper Motions

● Stars motion towards/away from us can be measured very accurately ● – Doppler Shift Motions `side-to-side' on the sky take VERY long time to make noticeable changes

Astrometry: Proper Motions

● If star has a large enough proper motion ● – (probably means very near us) Wobble in the star's motion could indicate that the star is being tugged on by a nearby planet

Astrometry: Proper Motions

● Has been successfully used to detect white-dwarf companions ● Shown below: Sirius ● No successful measurement of planets however

Astrometry: Proper Motions

● Would work best for?

Astrometry: Proper Motions

● Would work best for?

– Nearby stars – Large mass companion – Distant from planet: can pull further distance – Near planet: faster orbit, more visible wobble

Doppler Shifting

● Star has slight motion in orbit ● ● If that motion is largely towards/away from us, might be detected by Doppler shift Motions towards/away can be very accurately measured (few meters/sec)

Doppler Shifting

● Has so far been extremely successful ● ● If can watch for several periods, can get very accurate period measurements Sine wave: circular orbit ● ● `Tilted' sine wave: elliptical orbit Get: period, total velocity induced by planet

Doppler Shifting

● Works best for:

Doppler Shifting

● Works best for: – Large planets – Close in: ● Faster period (easier to detect)

Interstellar Travel, Interstellar Communication

● Interstellar Travel ● – Rockets – Fuel – Speeds – Time Dilation Interstellar Communication – What frequencies do we use?

– Meaningful signals – SETI@home

Gravitational Force

Rockets

Net Force -> acceleration Force exerted by exhaust ● Have to exert force to overcome that of gravity ● Reactions from some sort of fuel ● ● – Chemical – Electrical...

Propel exhaust downwards By Newton's 3 rd law, propel rocket upwards

Grav Force Net exhaust ● Easy to accelerate upwards ● Hard to keep from falling back down!

● Can either: ● – Accelerate very quickly to escape vel (25,000 mph) and coast up ● Gravity will keep decelerating you but never quite pull you back – Or accelerate slowly through ascent Luckily, further up you get, weaker force from Earth's gravity becomes

Rockets: Fuel

● Takes a lot of fuel to move something into Earth's orbit or further ● Would take about as much fuel to launch me into orbit as it takes to heat a Chicago home through an entire winter ● Unlike a car trip, fuel starts weighing a lot, even compared to rocket ● Shuttle launch: – Empty Shuttle: 230,000 lb – Fuel : 2,700,000 lb

Fuel along the way?

● Interstellar medium VERY tenuous ● Sprinkled with hydrogen ● Could it be collected and then burned (nuclear fusion?) ● Hard to see how ● – Drag on ship – Power to magnetic fields But would solve enormous fuel problem

Special Relativity

● Einstein: – Physics is the same in all inertial frames of reference – Speed of light in a vacuum is a fundamental physical constant of the Universe

Special Relativity

● But for higher velocities, can be significant!

● ● Astronaut goes to Alpha Centauri and back at 95% of speed of light Astronaut ages 3 years, people back home 9 ● At closer and closer to speed of light, effect gets bigger and bigger.

Special Relativity

● ● Speed of light becomes moving target Astronaut can put more and more energy into traveling faster ● But because can never pass light (light must always travel at same velocity!) can never pass speed of light ● Takes infinite amount of energy to even get to speed of light

Automated Probes?

● High-tech Voyagers or Pioneers ● Aim towards nearby stars ● Enough fuel to accelerate ● Enough smarts to navigate toward system ● Get solar power once near star ● Send message – To nearby planets – To us

Travel Difficult

● Communication much simpler than Transportation.

Messages

● ● Its a lot easier sending signals than things Messages – Have no mass – Don't require fuel – Don't require food/provisions for long journey – Cheap to produce – Travel at speed of light

What frequencies to use?

● Two choices for long-distance forces: ● – Gravity (difficult) – Electromagnetic But there's an essentially infinite range of frequencies to examine ● Radio waves: – Easy/cheap to generate, focus

SETI@home

● Several different SETI listening experiment ● One is called `Project SERENDIP' ● `Listen in' on other astronomical uses of the Arecibo radio telescope in Puerto Rico ● Can't choose where the observers are looking, but can listen (nearly) 24x7 ● Receiver installed which listens to 168 million narrow channels near 21cm Hydrogen line

SETI@home

● Done as part of screen saver on thousands of volunteer's computers

Results

● Several candidate signals discovered ● 2500 persistent gaussians (longish spikes seen at least twice) ● Need to be checked to make sure not interference/noise ● Also searching data for persistent spikes, pulses, triplets...

Has the Search Happened Already?

● UFO sightings ● What Evidence is Necessary?

● If no UFOs yet, why not?

UFO Sightings

● No shortage of UFO observation stories, photos ● A moment spent with google provides thousands of ernest, probably mostly honest web pages describing – UFO sightings – Abductions

What Evidence is Required?

● Large amount of documentary evidence that the Universe has apparently searched for life here ● Why not accept this as truth?

Extrordinary Claims require Extrordinary Evidence

● Let me make two claims ● – This morning, violence broke out in an up-til-now quiet region of Iraq, in the southern town of Rajaf. Four US soldiers were killed.

– With great effort, I can fly short distances (10-20 ft) using the power of my mind.

Which (if either) do you believe?

Extrordinary Claims require Extrordinary Evidence

● You have exactly the same evidence for both claims: my say-so.

● Clearly, the Iraq claim has more serious immediate consequences (death, future violence) ● Why is the same evidence more likely to be sufficient in one case (the more serious, even) than in the other?

What Evidence is Required?

● Photographs are easily misinterpreted ● Photographs also easily faked ● These: Robert Schaefer

What Evidence is Required?

● Eyewitness evidence notoriously unreliable ● Human brain very good at seeing patterns, filling in blanks ● Too good, in fact, to be good at mundanely reciting uninterpreted observations

Observation Test

● ● ● Quantitative test Count basketball passes by one team (dressed in white) in a complicated, dynamic scene http://viscog.beckman.uiuc.edu/grafs/demos/15.html

Same lab: `change blindness'

● http://viscog.beckman.uiuc.edu/grafs/demos/10.html

Post-event Suggestibility

● Elizabeth Loftus: – Film shown of car accident – Questionaire after film – Followup questionaire afterwards – Leading questions, misinformation in questions could cause people to misremember event afterwards ● Wrong color of car ● `Remembering' stop signs, buildings that weren't there ● ...

What Evidence is Required?

● This doesn't mean that all the evidence is proven wrong/mistaken ● Not enough evidence to be convincing ● What would be convincing evidence?

What Evidence is Required?

● This doesn't mean that all the evidence is proven wrong/mistaken ● Not enough evidence to be convincing ● What would be convincing evidence?

– Chunk of spacecraft material/technology – Cheek swab from alien – ...

Fermi's Paradox

● ● ● No signals from aliens yet.

No visitors yet either, perhaps.

Why not?

Fermi's Paradox

● ● ● ● ● Even if 1,000,000 civilizations in our galaxy today, that's one per ~300,000 stars Would have to explore by chance to find Earth Radio signals identifying Earth are very new: 1960s or so Even if travel speed of light, on has been time for 20ly round trip: Only a handful of stars that close

Next week

● Assignment covering Weeks 8-13 due ● Projects also due next week ● Class summary ● Any student presentations