AST101- Chapter 13.docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Other Planetary Systems [Chapter 13] 13.1 Detecting extrasolar planets  Extrasolar planets: planets around other stars  Brown dwarves: objects with masses greater than 13 times jupiters mass and 0.08 times the suns mass, are like large jovian planets but tiny stars Why is it so difficult to detect planets around other stars?  Too far  A sun like star would be a billion times as bright as the light reflected from any planets and telescopes blur light How do we detect planets around other stars?  Directly: pictures or spectra of the planets themselves constitute direct evidence of their existence  Indirectly: precise measurements of stellar properties (such as position, brightness, or spectra), may indirectly reveal the effects of orbiting planets  Nearly all extrasolar planets have been discovered by indirect techniques Gravitational tugs  Detected by indirectly observing tugs they exert on the stars they orbit (we observe stars without actually seeing the planets themselves- why indirect)  All objects in star system, including sun, orbit the systems “balance point” or center of mass  Jupiters tug on the sun is greater than the rest of the planets combined  Sun and jupiter are always on opposite sides of the center of mass so the sun orbits this point with the same 12 year period as jupiter o Jupiter actually orbits the center of mass every 12 years, but appears to orbit the sun because the center of mass is so close to the sun  Can determine jupiter mass from suns orbital characteristics (a more massive planet at the same distance would pull the center of mass farther from the suns center, giving sun a larger orbit and faster orbital speed around the center of mass)  Saturn exerts second strongest gravitational tug and has a small effect on top of the suns 12 year orbit  We can search for planets in other star systems by carefully watching for the tiny orbital motion of a star around the center of mass of its star system o Two techniques that all us to observe the motion: o Astrometic technique: precise measurements of stellar positions in the sky to look for stellar motion caused by orbiting planets o Doppler technique: can detect orbital motion through changing Doppler shifts in a stars spectrum; stars orbital motion will produce alternating blueshifts as the sun moves towards us in its orbit and redshifts as it moves away The astrometric technique  Complications: the further the star is, the smaller its side to side movement will appear and therefore works best for massive planets around relatively nearby stars  The time required to detect a stars motion; planet with larger orbit has larger effect on its star, but would take many years of observation The Doppler technique  We can learn about the planets orbital characteristics and mass  The mass of the planet is what causes that star to move around the systems center of mass, so for a given orbital distance, a more massive planet will cause faster stellar motion  Doppler data tells us orbital period, so we can calculate orbital distance  Determine orbital shape from the shape of doppler data curve o Figure 13.5  Doppler data is good enough to tell us whether the star has more than one planet  If two or more planets show noticeable gravitational tug, Doppler data will show the combined effect of these tugs  We can observe a Doppler shift only if it has a planet orbiting at some angle other than face on and shift tells us the stars full orbital velocity only if we are viewing orbit precisely edge on  Therefore shift will only be precise in telling us mass of planet if it is in an edge on orbit  All masses found through Doppler shift are minimum masses rather than actual masses  Doppler technique tends to find massive planets in close orbits much more easily than any other type of planet; has limitations Transits and eclipses  Searching for slight changes in a stars brightness caused by a planet passing in front of it or behind it  Transit: one planet appears to move across the face of the star o Block a little of the light  we know the planets existence now, and can calculate size in comparison to that of its star o If repeated dimming occurs, then it is likely telling us orbital period of a transiting planet and we can then calculate the orbital distance and mass of planet  Transits revealed we view the planets orbit edge on so it told us mass from the Doppler technique was the planets true mass and not minimum  The amount of dip in stats brightness allowed us to determine the planets radius and hence its volume and can learn planets density  Transits can also tell us about the composition of a planets upper atmosphere and exosphere  Planets that pass in front of their stars during a transit can also pass behind their stars, and the star blocks the light from the planet – eclipse  Weaknesses of this method: can only work for planetary system whose orbits are oriented edge on to earth  Is biased in favor of planets with short orbital periods- and hence with orbits close to their stars- both because these planets transit more frequently and because we observe repeated transits before we can be confident of a discovery  With precise measurements of stellar brightness, the transit method can reveal planets far smaller than is currently possible with the astrometic or Doppler techniques Direct detection  Indirect techniques just tell us about orbital properties, masses and sometimes radii  To learn more need imagines of planets themselves (surface or spectra of this atmospheres)  One example of a planet around a much fainter star- the star is actually an object known as a brown dwarf (have masses between those of very large planets and many small stars, emit little visible light) o We got pics of this through hubble space telescope Other planet hunting strategies  Gravitational lensing: an effect that occurs when one objects gravity bends or brightens the light of a more distant object directly behind it o But these special alignments never repeat so cant do follow ups  If planet Is present in dust disk- can exert small gravitational tugs on dust particles that produce gaps, waves, or ripples in the disk ** summary of all techniques figure 13.10 13.2 The Nature of Extrasolar Planets  Existence of planets around other stars shows that our solar system is not unique o Nature of planets themselves gives us an opportunity to learn more about the range of possible planets (are there other types of planets?) o Arrangements of other planetary system can tell us whether the layout of our solar system is common or rare (tells us if nebular theory is right) What have we learned about extrasolar planets?  Planetary properties that we can learn with current detection techniques: o Orbital period: using astrometric, Doppler and transits o Orbital distance: using orbital period and Newton’s version of Kepler’s third law o Orbital shape: using astrometric and Doppler techniques but transits cant alone o Mass: using orbital period, mass of its star and the speed at which it makes its star orbit their mutual center of gravity, can learn star’s full orbital speed using astrometric techniques and Doppler techniques only tells us the min. mass o Size(radius): only by observing transits o Density: can calculate a planet’s average density from its size and mass, can determine density only for planets that produce transits and for which we also has mass data from the astrometric/Doppler techniques o Composition: from spectra, transits can provide limited info about the composition of a planet’s upper atmosphere Orbits FIG 13.11  Only handful of extrasolar planets that we have seen have orbits that take them beyond about 5 AU o Most of the planets orbit very close to their host star  None are located as far from their stars as the Jovian planets in our solar system  Many of the orbits are nearly elliptical like the planets in our own solar system o Have large orbital eccentricities  We have found multiple-planet system (20 stars have been found to contain 2 planets)  Found 5 systems with orbital resonances which may have pr
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