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Summary for Final Last week of classes 12/10 to 12/14 First hour (Monday): Second hour (Wednesday): Final, Part 3: SCANTRON Test (Form 882, #2 pencils), 70 questions from Review questions, cumulative, includes question on SGA and Planispheres (70 pts) During your Third Hour Planetarium Sky Quiz (30 pts) Final, Part 1 (Solar System Object Quiz, 20 pts) Final, Part 2: Group effort on questions relating to 3rd hour (20 pts) All extra credit due by 12/14 at NOON Also: HW this week due on Friday, 11:59 PM as usual Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? Until recently, we had no proof that there were other planets around other stars (extrasolar planets). Throughout history the meaning of planets has changed Defining a planet is tricky and was done for our solar system just recently Not officially defined for other solar systems Web site: http://exoplanets.org © Sierra College Astronomy Department 2 Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? The catastrophe theories of solar system formation were prominent until about 50 years ago Many of these ideas required a (near) collision with another star As a consequence, these would predict that other solar systems are very rare Since disks of dust and gas have been seen around other stars, could it be more commonplace to have stars with planets around them? © Sierra College Astronomy Department 3 Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? The difficulty of detecting other planets Giant planets are likely easier to detect because they are more massive and brighter that terrestrial planets Directly observing a Jupiter-like planet around a Sun-like star would be difficult If the Sun were a the size of a grapefruit, Jupiter would be a marble 80 meters away. The star would shine one-billion times brighter than the reflected light from the planet © Sierra College Astronomy Department 4 Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? Successful detection There are 2 basic ways to detect an extrasolar planet: Directly: with pictures or spectra Indirectly: with precise measurements of stellar properties Most of the efforts of the last 10-15 years deal with looking for the gravitational influence of the planet on its parent star. COM While it appears that a planet orbits the sun, it is better to say that both orbit a common center of mass This means the star is slowly moving about this point if at least one planet is present. Multiple planets may make this motion very complicated. There are two techniques: astrometric and Doppler Sun COM 5 © Sierra College Astronomy Department Demo Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? Astrometric Technique This involves looking at the very precise motion of a star over time As an unseen planet tugs on a star, the star will move side to side with a period equal to that of the planet This method is difficult: e.g. if the Sun and Jupiter were 10 light-years away the Sun would appear to move only 0.003 arcsec and it would take 12 years to happen © Sierra College Astronomy Department 6 Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? Doppler Technique This involves looking at the stars motions using the Doppler shift Doppler As the unseen planet tugs on the stars, the star will move alternatively closer and further from us The causes the spectral lines to alternately blue and red shift. This leads to an orbital speed of the star 51 Peg around its center of mass © Sierra College Astronomy Department 7 Lecture 13: Other Planetary Systems Planetary Systems around Other Stars? This was first used successfully on 51 Peg to detect a planet 51 Peg Doppler The star wobbled at 57 m/s with a 4-day period Therefore the planet was going around the star in 4 51 Peg A days! Drawing This planet had to lie close to star and had a surface temp of 1000 K Current techniques allow us to measure velocities to 1 m/s – walking speed This allows to find smaller planets, further away than the one around 51 Peg © Sierra College Astronomy Department 8 other stars Lecture 13: Other Planetary Systems Planets around Other Stars How easy would it be to find a system like ours? For the Jupiter-Sun system the speed of the Sun around the center of mass is only 13 m/s This leads to a Doppler shift of only one part in 20 million The details of this motion allows us to come up with the orbital period of the planet and the average distance the planet is from the other stars star From this info, what method do we use to find the mass of the star (plus the planet)? © Sierra College Astronomy Department 9 Lecture 13: Other Planetary Systems Planets around Other Stars COM Doppler Face-on More scrutiny of the data allows us to get the mass ratio of the system and therefore the mass of the planet COM Caveat: the Doppler shift depends on the Doppler average inclination of the orbit in the plane of the sky If the orbit is edge-on we get the “full” Doppler effect: good and correct estimation of mass If the orbit is tilted by some amount the Doppler shift is reduced because not all the motion is towards and away from us: underestimation of mass If the orbit is face-on, no Doppler shift is measured and the planet remains undetected © Sierra College Astronomy Department 10 Lecture 13: Other Planetary Systems Planets around Other Stars The search begins by looking at stars that are the similar to the sun The newly discovered systems do not look like our own This in part is due to a selection effect – we are more likely to detect large planets close to the star All planets currently detected are giant planets: we don’t yet have the sensitivity to detect Earth-size planets © Sierra College Astronomy Department 11 Lecture 13: Other Planetary Systems Planets around Other Stars Transiting planets In certain extrasolar systems where the planet is edge-on to our line of site, the planet will pass over or transit the star once every orbit. Occasionally, Venus and Mercury do this over the Sun Venus as seen from Earth transit The transit should dim the star slightly The transit should repeat every orbital period The size of the planet may be derived Since the inclination is nearly edge-on an accurate mass may be determined. © Sierra College Astronomy Department 12 Lecture 13: Other Planetary Systems Planets around Other Stars transit The first successful transit occurred with the star HD209458 Star was known to have a planet via Doppler method. The period between transits (3.5 days) exactly matched what was derived with the Doppler method. The mass, radius (and therefore volume) were derived allowing us to calculate a density. This density was consistent with a Jovian planet. Atmospheric information could be derived too. © Sierra College Astronomy Department 13 Lecture 13: Other Planetary Systems Planets around Other Stars The planet was also seen to behind HD209458 (i.e. an eclipse) This resulted in a smaller drop in the total light since the planet is much fainter than the star The temperature of the planet could be derived at about 1100 K © Sierra College Astronomy Department 14 Lecture 13: Other Planetary Systems Planets around Other Stars This transiting method has strengths and weakness Weakness: Only a small fraction of systems have planets which transit their star. Weakness: short orbital period planets are far more likely to be discovered this technique. Strength: smaller planets can be detected this way, including, potentially, Earth-size planets © Sierra College Astronomy Department 15 Lecture 13: Other Planetary Systems Planets around Other Stars Direct detection Getting a direct picture of a planet could more detail about their atmospheres and surfaces. As mentioned before, the glare of the parent star, makes it very difficult to get picture of the planet A planet has been detected around a Brown dwarf failed star-type called a brown dwarf. planet © Sierra College Astronomy Department 16 Lecture 13: Other Planetary Systems Planets around Other Stars Other strategies Gravitational lensing Unseen planet may distort light of background star Looking the influence of planets around the star’s disk of dust Ripples seen in Beta Pictoris indicate planets © Sierra College Astronomy Department 17 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets What do we learn? By studying these extrasolar planets, we learn what types of possible planets there are. We also see whether the layout of our solar system is rare or common. Can the solar nebular theory explain it all or does it have to modified? © Sierra College Astronomy Department 18 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets What have we learned? Here are the extrasolar planetary properties we have derived in the last 15 years: Orbital period Orbital distance Orbital shape Mass Size (radius) Density Composition © Sierra College Astronomy Department 19 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets Orbits masses orbits eccentricity Only a handful of orbits are larger than 5 AU and quite a few are closer than Mercury is from the Sun Most orbits are (very) elliptical Some multiple planet systems have orbital resonances © Sierra College Astronomy Department 20 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets Masses masses orbits eccentricity All of the extrasolar planets are greater than 5.5 Earth masses. Many are more massive than Jupiter. This is clearly a selection effect as more massive object are easier to detect via Doppler, astrometric, and transit methods. © Sierra College Astronomy Department 21 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets Sizes and Densities masses orbits masses Transiting planets allow us to get size and then density. We have found that seven of these worlds are larger but less dense than the Jovian planets. These worlds lie close to the star and puffs up its outer atmosphere making it larger and less dense. There is one planet which has a Saturn-like mass but a Neptune-like density and may contain more rock and ice than the Jovian planets. © Sierra College Astronomy Department 22 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets Composition There is little information on the composition, except through transits and eclipses. We compare spectra during vs. before/after transit We see the Jovian-like planets have hydrogen and water in their atmosphere. © Sierra College Astronomy Department 23 Lecture 13: Other Planetary Systems The Nature of Extrasolar Planets Compare to our Solar System layout masses Most of the extrasolar systems bear little resemblance to our Solar System. The extra-jovian planet should look similar to ones we have. However, many are very close to their star and have very eccentric orbits Labeled as “hot Jupiters” there origin is still under debate Jup vs. Hot Jup © Sierra College Astronomy Department 24 Lecture 13: Other Planetary Systems The Foramation of Extrasolar Systems Challenge to the Solar Nebula theory layout Recall that the solar nebula theory suggested that most of the protoplanetary disk was made of H, He, a little hydrogen compounds, and still less rocks, metals. The rocks and metal were the only thing which condensed close to the Sun while in the outer solar system, everything condensed This lead to the small rocky terrestrial planets in the inner solar system and larger, more gaseous Jovian planets in the outer solar system These extrasolar systems challenges that basic idea © Sierra College Astronomy Department 25 Lecture 13: Other Planetary Systems The Foramation of Extrasolar Systems layout Why are the Jovian planets so close to their star? It does not seem likely that a Jovianlike planet could form so close to their star Astronomer now feel that these “hot Jupiters” formed far away and somehow migrated inward © Sierra College Astronomy Department 26 Lecture 13: Other Planetary Systems The Foramation of Extrasolar Systems migration Planetary Migration In this scenario, a planet forms “quickly” in the protoplanetary disk. As it orbits around a star it starts to bunch up material on either side of it This material starts to interact with the planet causing to migrate inward This did not happen so much with our Solar System, because the solar wind cleared out all this gas and dust © Sierra College Astronomy Department 27 Lecture 13: Other Planetary Systems The Foramation of Extrasolar Systems Encounters and Resonances The eccentric orbits of some of these planets might be due interactions they has with neighboring planets. Resonances also may also forced some of eccentric orbits. © Sierra College Astronomy Department 28 Lecture 13: Other Planetary Systems The Foramation of Extrasolar Systems Change the theory It is too early to tell whether the basic solar nebular theory need to be changed Our solar system may be unique which might suggest that the Earth is unique in this Galaxy © Sierra College Astronomy Department 29 Lecture 13: Other Planetary Systems Finding More Extrasolar Planets Kepler The discovery which will most significant is finding an Earth-like planet Current technology: not quite there Transit detection missions: Kepler (Launch 2008) will be able to monitor 100,000 stars looking transiting planets, down to the size of Mercury! COROT (Lauched 2006) is not as sensitive as Kelpler, but should be able to detect planets down to a few Earth masses © Sierra College Astronomy Department 30 Lecture 13: Other Planetary Systems Finding More Extrasolar Planets Astrometric migration missions: GAIA (launch 2010) will be able to measure star positions down to 10 microarcsecs SIM (Space Interferometer Mission; launch 2011) will be able to measure star positions down to 1 microarcsec That’s good enough to find a Jupiter-like planet 3000 light-year away © Sierra College Astronomy Department 31 Lecture 13: Other Planetary Systems Finding More Extrasolar Planets Direct TPF-I detection missions: TPF (Terrestrial Planet Finder; launch 2014,2020) These two missions will be able to detect planets around by effectively blocking starlight from a photo leaving only the planet(s) Spectra and other analysis can lead to determination of atmospheric properties © Sierra College Astronomy Department 32 The End © Sierra College Astronomy Department 33