<body bgcolor=white text="#000000" link="#0000ff" vlink="#0000ff"> <TITLE>Lecture 6</TITLE> </HEAD> <center> <table border=8 cellpadding=6 bgcolor=magenta> <tr> <td> <img src="http://hendrix2.uoregon.edu/~imamura/121/images/voyager_disc.jpg"> </td> <td> <center> <h2> Topic 4B:<br><p> ARE WE UNIQUE? </center> <P> Planet Hunters:<br> <ul> <a href="http://www.seds.org/billa/tnp/other.html">SEDS</a>, <a href="http://ast.star.rl.ac.uk:80/darwin/welcome.html"> Darwin Home Page</a>, <a href="http://exoplanets.org/">Marcy & Butler</a>. </ul> <P> S.E.T.I.:<br> <ul> <a href="http://www.seti.org">SETI Website</a> </ul> </h2> </td></tr> </table> </center> <p> <h3> The current belief is that the process of <b> Solar System</b> formation is one of many natural outcomes of star formation. That is, it is simply one way in which the collapse of a rotating cloud may turn out. Other outcomes are the formation of binary star systems (or other multiple star system)-- This notion is supported by the fact that over 50 % (perhaps up to 80 %) of all stars are contained in multiple star systems. Given this motivation and the simple desire to know if we are unique has pushed people to look for other planetary systems in our Galaxy. <P> The types of searches can be broken down into <b> direct</b> planetary searches and <b> indirect</b> planetary searches. <ul> <p> <li><b>Indirect</b> searches are based on the fact that the objects in a solar system do not orbit about the star, but rather orbit about the <a href="http://hendrix2.uoregon.edu/~imamura/121/oct13/cofm.gif"> center-of-mass of the system</a>. This means that the star also moves which may be detectable. <p> <li> <b>Direct</b> planetary searches look for eclipses (transits) of stars by planets (although this is very difficult, see below) and searches are geared to locating other <i> Solar Nebulae</i>. <p> </ul> Here, we consider the following strategies: <p> <UL> <LI> astrometric searches (ID) <LI> spectroscopic searches (ID) <LI> occultation searches (D) <LI> gravitational microlensing searches (D) </UL> <p><hr><p> <center> <b> INDIRECT SEARCHES</b></center> </center> <P> In orbital systems, the bodies move around the center-of-mass of the system. The center-of-mass of a system is located a distance D from object 1 (mass = M) and a distance d from object 2 (mass = m). <P> In planetary systems, the star is always much more massive than the planets and the center-of-mass is very close to the star. In our Solar System, the center-of-mass is just outside the surface of the Sun ===> the planets do not orbit about the Sun. The Sun and planets all orbit about the center-of-mass of the Solar System. In our <b> Solar System</b>, the Sun and Jupiter dominate things. ===>The orbital period is roughly 11.9 years and the Sun's orbital speed is roughly 13 meters per second, the speed of a very fast human. <P> <b> Indirect</b> planetary searches look for the small changes in the position of the star (<i> astrometric</i> searches) and for effects due to the fact that the star is undergoing a slow orbital motion (<i> spectroscopic</i> searches and <a href="http://hendrix2.uoregon.edu/~imamura/121/images/Distance_Chart.gif"><i> pulsar-type</i> searches</a>). <P> <center> <i> Astrometric Searches </i> </center> <P> To get a feel for the difficulty of astrometric searches, suppose that we lived on the nearest star to the Earth (excluding the Sun), <b> Proxima Centauri</b>. Could we see our Solar System? <b> Proxima Centauri</b> is a distance of 4.3 light years kilometers from the Earth. This is a long way away -- 1 Astronomical Unit = 8.3 light minutes. <P> Jupiter orbits around the Sun in an orbit of size 5.2 A. U. with a period of 11.9 years. From <b> Proxima Centauri</b> this would correspond to an orbit of angular size = 0.004 arc seconds. Recall 1 degree = 60 arc minutes. 1 arc minute = 60 arc seconds ===> 1 arc second = 1/3,600 of a degree!! <P> Given current technology for ground-based observatories, this is right near the limit of what is <i> do-able</i>. It is possible that Jupiter-sized planets might be detectable in this manner for the nearest stars. <P> There are currently no good candidates based on astrometric searches. In the future, we will develop space-based experiments which will have pointing accuracies of better than 0.00001 arc seconds and may thus be able to see nearby planetary systems. <P> <center> <i> Spectroscopic Searches </i> </center> <P> Spectroscopic searches try to detect evidence of the motion of the star. In order to understand what is done, we need to discuss the phenomenon of <a href="http://hendrix2.uoregon.edu/~imamura/121/images/Doppler_effect.flv"> Doppler Shifts</a> <P> <b> Doppler shifts</b> are part of the phenomenology of waves (here, we treat the light we receive from stars as a wave-like phenomenon). To understand the <b> Doppler</b> phenomenon, consider what happens to a pan-full of water if I drop a rock into it (or, in fact, disturb it in any manner). <p><center> <img src="wave1.jpg"> <p></center> I now define the <i>wavelength</i>. <P><center> <img src="wave2.jpg"> <p></center> Now, consider a slightly different situation. Apply periodic disturbances (that is, tap the water once every second) but instead of tapping the water at the same spot (e.g., at the center of the pan), I move the point at which I tap the water. For example, I start tapping the water at the center of the pan but for each successive tap, I move a short distance out from the center. <P> Schematically, <P><center> <img src="wave3.jpg"> <P></center> Depending upon where I sit, the wavelength of the disturbance will either be smaller or larger than for the nonmoving case. <i> The size of the contraction or stretching depends upon how fast the object moves</i>. <p><font color=greem> Q: What if I sat at the bottom of the picture, what wavelength would I measure?</font> <P> The preceding phenomenon is the <font color=magenta><i>Classical Doppler Effect</i></font>. An analogous effect occurs for the light produced by a star. In this case, the size of the effect (the amount of stretching or contraction) is given by how fast the sending object moves. <P> The orbital speed of the Sun is on the order of 13-14 meters per second. To observe this effect, we need to be able to observe something like a fast person running in the extra-solar system. The rather amazing thing is that with current technology this is possible! We now know of more than 500 planets outside of our Solar System in more than 100 other planetary systems. Note that many of the stars around which planetary systems have been found are similar to the Sun. The way to see this is to go to the <a href="http://exoplanets.org/">Marcy and Butler </a> website and to bring up the Table. In the far left column is the Name of the star about which the planets orbit. Click on the Name of the star (e.g., the first star is HD 142 b) and to find the Spectral Type and Luminosity Class</i></font> of the star. The Sun is of spectral class G2 V. Star with temperatures fairly similar to the Sun will have spectral class F, G, or K. The Luminosity Class is a measure of the power (and diameter) of the star (the Sun is Luminosity class V, a main sequence star). <i><font color="magenta"> There are many dozens of Sun-like stars with planetary companions.</i></font> There is even a planet five times the mass of the Earth in the Gliese 581 system which lies in the <a href=" http://www.planetary.org/news/2007/0425_Most_EarthLike_Planet_Discovered.html"> habitable zone</a> of the star. <P> <table cell border=10 cellpadding=10 bgcolor=green><tr><td> <a href="http://exoplanet.eu/catalog-RV.php?mdAff=stats#tc"> <img width=400 src="http://hendrix2.uoregon.edu/~imamura/121/images/ planet_mass_a_graph_correl.php"></a></td><td><h3><font color=yellow> To the left is the distribution of planet masses with respect to their orbital sizes and to the right is the number of planets with given mass. Note the size of Jupiter's orbit is 5 A.U. and also that most of the disovered planets are much more massive than Jupiter. <font color= pink>Does this mean that our current understanding of Solar System formation is in trouble?</font></font></td> <td><a href="http://exoplanet.eu/catalog-RV.php?mdAff=diag#tc"> <img width=400 src="http://hendrix2.uoregon.edu/~imamura/121/images/ planet_mass_graph_stat.php"></a></td></tr></table> Well, the current lore has it that our basic notions of Solar System formation is probably okay in that we think the general ideas are correct. However, the current results suggest that after planets start to form, their orbits must somehow be modified so that Jupiter-like planets will be found close to their stars, within the zones where water would be in vapor form, not solid form. <font color=magenta>Orbital migration must occur and is a topic of current research.</font> <p><hr><p> <center> DIRECT SEARCHES <p> <i>Transits</i> </center> <P> The obvious problem with direct detections of planets is that stars are very bright and planets are very faint. This means that it is very difficult to <i> see</i> planets in the glare of the stars. Further, because planets are small, eclipses of stars by planets are not likely to lead to large changes in the light output. This makes direct detections of planets difficult. <center><table cell border=10 cellpadding=10><tr><td> <img width=350 src="http://hendrix2.uoregon.edu/~imamura/121/images/kepler_spacecraft.jpeg"></td> <td><h3><center> Kepler </center> <P> The NASA mission <a href="http://kepler.nasa.gov/">Kepler</a>, shown to the left, is observing 156,000 nearby stars to search for <a href="http://hendrix2.uoregon.edu/~imamura/121/images/transit2.jpg"> transits in extra-Solar planetary systems</a> (<a href="http://hendrix2.uoregon.edu/~imamura/121/lecture-6/mercuryc.mpg"> a transit of Mercury</a>). To the right are shown recent planets discovered by Kepler. </td><td> <img src="http://hendrix2.uoregon.edu/~imamura/121/images/kepler-nasa.jpg"> <p><i><h3> Recent discoveries of planets by Kepler. More information can be found <a href="http://kepler.nasa.gov/Mission/discoveries/">Here</a>. </td></tr></table></center> <p><hr><p><center><i> Gravitational Microlensing</i></center><p> <font color="magenta"><i> An exciting recent development was the discovery of an Earth-like planet in the constellation Sagittarius (<a href= "http://www.theaustralian.news.com.au/common/story_page/ 0,5744,17939268%255E601,00.html">news story</a>)</i></font>. <P> The planet was discovered using a new technique that involves <i><font color="magenta">gravitational lensing</font></i>. <p> <center><table cellpadding=16> <tr><td><img src="bh_macho.jpg"></td> <td><img src="bh_hst_image.jpg"></td></tr> <tr><td><h3> If a single star passes in front of a background star then we expect a single increase in the magnification of the light. However, if the lensing star has a planet circulating about it, then we expect to see a secondary magnification in the light from the distant star as the planet passes in front of it.</td></tr> <tr><td><h2>Detection of Earth-like planet: 5x mass of Earth with an orbital pereiod of ~10 years</td> <td><img width=500 src="microlens_planet.gif"></td> </tr></table></center> <p> <p> <center> <p><p> <a href="http://astrobiology.nasa.gov/"> LIFE IN THE UNIVERSE</a> </center> <P> <p> <center> <a href="http://hendrix2.uoregon.edu/~imamura/121/lecture-5/lecture-5.html"> <img width=100 src="http://hendrix2.uoregon.edu/~imamura/122/images/shinkansen-back.jpg"></a> <a href="http://hendrix2.uoregon.edu/~imamura/121/lecture-7/lecture-7.html"> <img width=100 src="http://hendrix2.uoregon.edu/~imamura/122/images/shinkansen-forward.jpg"></a> </center>