Planets ● For life on a planet, so far we have three important questions: – How far is it from its Sun? – How massive is it? – What type of planet is it: is it rocky?

Distance from Star ● Must be at distance from star that liquid water can exist with an environment ● Not too close (Venus) ● Not too far (further than Mars)

Period of orbit ● Lenth of time takes to complete one orbit ● Period of orbit and distance are related ● For a given star mass, period squared is proportional to distance cubed ● Large distance – takes more time to orbit ● Closer – orbits faster ● If know (or can estimate) star's mass, period distance

Size of Planet ● Rocky planets in our system: – 0.4 – 1.0 Earth diameters ● Gas Giants – 4.0 – 11.0 Earth diameters ● Size alone gives an idea what sort of planet it is ● Size + mass of planet cinches it (why?)

We can see proto-planetary disks... ● Observed around very young stars ● Obscures new star in visible light ● Glows in infrared (why?) ● Can we see planets? – Much harder – Condensed objects – Lost in glare of star – Until 10 years ago, answer: NO.

Can we see planets? ● Answer today: yes! ● >110 extra solar planets discovered ● More almost every month

Can we see planets? ● Easier to make measurements of nearby stars ● Many of the stars with known planets are easily visible to the eye, even in Chicago ● Gamma Cephei is near the north pole (Polaris)

Can we see planets? ● Easier to make measurements of nearby stars ● Many of the stars with known planets are easily visible to the eye, even in Chicago ● 47 Ursae Majoris is below the big dipper

Finding Extra-solar Planets ● Techniques – Direct(ish) measuring of planet – Indirect measurement – effect on star ● Results of search so far – `Hot Jupiters' ● Implications – Can life be found in these systems? – Are most systems like this? – Migration vs. Direct Formation

Finding Extra-solar Planets ● Direct(ish) Methods – Light from planet ● Visible, Infrared – Dark from planet ● Planet transits – Bending light from other object ● Indirect methods – gravitational effect on star – Pulsar Timing – Astrometry – Doppler Shift

Direct Methods ● Light from planet – Reflected visible light – Reflected+generated infrared ● Dark from planet – Transits (shadows from planets) ● Light bent by planet – Gravitational Lensing

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 Small brown dwarf (not planet) companion to a star directly imaged

● Has yet to be observed ● What sort of planets/systems does this work best for? 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 Small brown dwarf (not planet) companion to a star directly imaged 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

● 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 Small brown dwarf (not planet) companion to a star directly imaged Light from the planet

● Infrared is between visible light and radio ● `Near' infrared most easily detected with telescopes ● Very far infrared can be observed with radio telescopes Light from the planet

● Interferometry ● Allows (with some computation) using several radio telescopes as if it were one large telescope ● Easier to do with radio than with visible light ● Amount of signal proportional to total area ● Resolution increases with size of array ● Infrared interferometry has some promise for observing planets directly Light from the planet

Dark from the planet ● 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 Brightness

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 Brightness

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 Brightness

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 Brightness

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 Brightness

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 Brightness

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 Brightness

Planetary Transits/Occultations ● What sort of planets/systems does this work best for?

Planetary Transits/Occultations ● What information can we get? – If can watch until repeats, can find period of planets orbit – Length of dip: amount of time planet in front of star ● Speed of Planet ● Size of Star – Amount of dip: Size of planet / size of star Time Brightness ?

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 Time Brightness ?

Planetary Transits/Occultations ● How are these observed?

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

● Similar requirements to transit searches ● Lots of careful images of large amount of sky ● Comparison to see any changes ● Lensing searches get transit data `for free' ● Both transit search, lensing data here from same operation (OGLE)

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?

Gravitational lensing ● Works best for what systems? – Dim Stars – Massive planets – (relatively) insensitive to distance between star and planet – Jupiters at any radii / temperature

Indirect Methods ● Gravitational Effect on Star – Pulsar Timing – Astrometry – Doppler Shift

Center of Mass ● `For every action there is an equal and opposite reaction’ ● Gravitational force Earth exerts on Sun the same as the force the Sun exerts on the Earth ● So why does the Earth orbit the Sun, and not vice-versa?

Center of Mass ● Same force, but Sun is much heavier than earth ● Same force moves Sun very little ● But Earth (say) a Lot ● Relative amount of motion = relative masses of objects

Center of Mass ● Same force, but Sun is much heavier than earth ● Same force moves Sun very little ● But Earth (say) a Lot ● Relative amount of motion = relative masses of objects

Center of Mass ● Sun is 300,000 times more massive than Earth ● So Sun moves 1/300,000 as much as Earth ● Both orbit a Center of Mass which is 300,000x closer to center of Sun than Earth – 1/10% of Sun’s radius

Center of Mass ● Sun is 1,000 times more massive than Jupiter ● So Sun moves 1/1,000 as much as Jupiter ● Both orbit a Center of Mass which is 1,000x closer to center of Sun than Jupiter – Sun’s radius

Pulsars ● `Cosmic Lighthouses' ● Send out beam of high-energy radiation ● Rotates ● If we're along line of sight, see very regular bursts of light/energy ● Easy visibility + regularity -> very easy to detect changes

Pulsars ● Two planets have been so far discovered around pulsars ● Significance for life? Probably small. – Pulsar likely the result of a supernova – Neutron star doesn't emit much energy – Column of high-energy radiation every few seconds probably not helpful

Pulsars ● What sort of systems would this work well for?

Pulsars – Need a pulsar – Massive planet (large gravitational effect) – Near the pulsar (large gravitational effect)

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 noticable 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 succesfully 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 strs – 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 succesful ● 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)

Center of Mass ● Sun rotates around a circle 1/10% of Sun’s radius in size every year ● Maximum velocity: 3 inches/sec

Center of Mass ● Sun rotates about a circle its radius in size every 11 years – 10 yards/sec

Hot Jupiters ● How did these planets get so close to their sun? ● Normal planetary formation theory: < 1 AU is too close ● Gasses would have evaporated ● No Juptiers from ● Migration? – High eccentricities – Gas could stay, just not form…

Hot Jupiters ● Jupiters are large enough to disrupt other planets ● Asteroid belt ● Jupiters less than 4 or 5 AU away from sun would probably prevent any Earth-like planets forming within habitable zone.

Hot Jupiters ● Unlikely place for life ● BUT ● If some have ~ Earth sized moons: – Rocky “planets” in habitable zone?

Reading for Next Class (Apr 30) ● Chapter 19, 20: Interstellar spaceflight, communications ● Chapter 19: Interstellar spaceflight – Energy, fuel requirements – Time requirements – Time dilation – Special Relativity – Manned vs. Unmanned probes ● Chapter 20: Interstellar communications – Spectrum, and choice of frequency – Choice of message – Listening vs. Speaking –