Lecture 31 Discovering Other Solar Systems Part I05 04 2014 WHY IS IT SO CHALLENGING TO LEARN ABOUT EXTRASOLAR PLANETS The great challenge stems from great distances from other stars the small sizes of planets in comparison and the vast difference in brightness between stars and planets We can in principle look for planets indirectly or indirectly Two major indirect approaches are Looking for subtle gravitational effects on stars due to orbiting planets Looking for changes in star s brightness as one of its planets passes in front of it Center of mass all objects in a star system including the star itself orbit the system s balance point or Center of Mass Our sun s orbit traces out only a small circle or ellipse with each 12 year period of Jupiter because the Sun s average orbital distance is barely larger than its won radius we generally don t notice the sun s motion By measuring this motion carefully over many years an extraterrestrial astronomer could deduce the existence and masses of the planets orbiting the sun Although smaller planets are more difficult to measure because extremely precise observation is required o Astrometric Method uses very precise measurement of stellar positions in the sky to look for the slight motion caused by orbiting planet o Doppler Method takes advantage of the Doppler effect which allows us to measure changes in a star s velocity toward or away from us that are caused by orbiting planets Techniques for seeing Extra Solar Planets 1 DIRECT DETECTION direct study of images or spectra Limitations currently possible only in rare cases Major Planetary Properties Measured spectra and surface details with future technology Confirmed or Candidate Planets a few 2 ASTROMETRIC METHOD uses very precise measurement of stellar positions in the sky to look for the slight motion caused by orbiting planet side to side Has been used for decades to identify binary star systems because two orbiting stars move periodically around their center of mass Technique works especially well for binary systems in which the two stars are not too close together because the stellar motion tends to be larger in those cases European Space Agency s GAIA mission scheduled to operate 2013 to 2018 goal to perform Astrometric observations of a billion stars in our galaxy to an accuracy that in some cases will be better than 10 microseconds Advantages best for detecting massive planets that orbit far form their stars Limitations o Currently possible only in rare cases o Works best for nearby stars for distant planets require space observatory Major Planetary Properties Measured o Planet mass o Orbital period semimajor axis o Orbital eccentricity o Orbital inclination Confirmed or Candidate Planets a few 3 DOPPLER or WOBBLE METHOD takes advantage of the Doppler effect which allows us to measure changes in a star s velocity toward or away from us that are caused by orbiting planets back and forth Shifts blueshift when a star is moving toward us and redshift when it is moving away from u So alternating blueshifts and redshifts relative to the star s average Doppler shift indicate orbital motion around the center of mass First Discovery 51 Pegasai 1995 by Swiss astronomers and confirmed by American astronomers Planet lies so close to the star that its year lasts 4 Earth days and it surface temperature is over 100K Higher mass causes greater gravitational effect on the star and therefore causes the star to move a t a higher speed around the system s center of mass o Hot Jupiter combination of the planet s high mass and its closeness to the star because it has a Jupiter like mass but a much greater surface temperature Advantages Limitations o Best for detecting massive planets with close in orbits o Detects planets in all orbit orientation except face on o Less sensitive to planets father from their stars o Require large telescopes o Provides only minimum mass when orbital inclination to Earth unknown Major Planetary Properties Measured o Minimum planet mass o Orbital period and distance semimajor axis o Orbital eccentricity Confirmed or Candidate Planets about 1 000 4 TRANSIT METHOD searching for slight changes in star s brightness Planet appears to move across the surface of the star the larger the planet the more dimming it will cause Eclipse many transitioning planets also offer a measurable eclipse as the planet goes behind the star during which we see a dip in the system s brightness Eclipse observations are more easily accomplished in the infrared because planets contribute a greater proportion of a system s infrared brightness than visible light brightness Confirmation At least three repeated transits before concluding that a planet is probably responsible Using the transit method to discover a planet therefore requires monitoring a star s brightness for at least three full orbits of the planet First Discovery HD 209458b Hot Jupiter had already been detected by Doppler method so the transits in this case served as a confirmation rather than a discovery Missions o European Space Agency s COROT mission 2006 o NASA s Spitzer Space Telescope observes infrared light has successfully observed transit even though it was not designed for that purpose o Kepler Mission 2009 2013 monitors about 150 000 stars Advantages o Allows many stars to be observed at once o Can detect very small planets o Feasible with small telescope o Can provide some atmospheric information in cases of measurable eclipses Limitations o Possible only for planets with edge on orbits as viewed from Earth o Small planets requires sensitivity possible only from space observatory Major Planetary Properties Measured o Planet size o Orbital period and distance semimajor axis o Orbital inclination edge on Confirmed or Candidate Planets about 3 000 5 MICROLENSING METHOD example of gravitational lensing in which the light of a star is temporarily magnified as another tar passes in front of it and bends its light Careful study of microlensing event can reveal whether the foreground star has planets Limitations special alignment necessary for microlensing is a onetime event which generally means there is no opportunity for confirmation or follow up observation Lecture 32 Discovery of Other Solar Systems Part II 05 04 2014 Kepler Mission for Transit Observation Launched in 2009 expected to last until 2016 but pointing failure in 2013 Monitored the brightness of 150 000 stars every 30 minutes Capable of detecting an Earth passing in