Planets

1. 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?

2. 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)

3. 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

4. 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?)

5. 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.

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

7. 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)

8. 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

9. 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

10. 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

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

12. 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

13. Light from the planet • Has yet to be observed • What sort of planets/systems does this work best for?

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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 Brightness Time

21. 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 Brightness Time

22. 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 Brightness Time

23. 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 Brightness Time

24. 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 Brightness Time

25. 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 Brightness Time

26. 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 Brightness Time

27. Planetary Transits/Occultations • What sort of planets/systems does this work best for?

28. 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 ? Brightness Time

29. 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 ? Brightness Time

30. Planetary Transits/Occultations • How are these observed?

31. 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

32. Planetary Transits/Occultations • Large survey • Dedicated telescope • Look at large fraction of sky every night (or nearly)

33. 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

34. 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

35. Gravitational lensing

36. 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)

37. 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

38. Gravitational lensing • Works best for what systems?

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

40. Indirect Methods • Gravitational Effect on Star • Pulsar Timing • Astrometry • Doppler Shift

41. 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?

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. Pulsars • What sort of systems would this work well for?

49. Pulsars • What sort of systems would this work well for? • Need a pulsar • Massive planet (large gravitational effect) • Near the pulsar (large gravitational effect)

50. 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