BU CAS AS 100 - Lecture 31: Discovering Other Solar Systems

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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 planetsLooking for changes in star’s brightness as one of its planets passes in front of itCenter 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 motionBy 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 requiredAstrometric Method – uses very precise measurement of stellar positions in the sky to look for the slight motion caused by orbiting planetDoppler 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 planetsTechniques for “seeing” Extra Solar Planets1. DIRECT DETECTION  direct study of images or spectraLimitations - currently possible only in rare casesMajor Planetary Properties Measured – spectra and surface details (with future technology)Confirmed or Candidate Planets – a few2. 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 casesEuropean 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 microsecondsAdvantages – best for detecting massive planets that orbit far form their starsLimitationsCurrently possible only in rare casesWorks best for nearby stars, for distant planets, require space observatoryMajor Planetary Properties MeasuredPlanet massOrbital period (semimajor axis)Orbital eccentricityOrbital inclinationConfirmed or Candidate Planets – a few3. 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 massFirst 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 massHot 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 temperatureAdvantagesBest for detecting massive planets with close in orbitsDetects planets in all orbit orientation except face-onLimitationsLess sensitive to planets father from their starsRequire large telescopesProvides only minimum mass when orbital inclination to Earth unknownMajor Planetary Properties MeasuredMinimum planet massOrbital period and distance (semimajor axis)Orbital eccentricityConfirmed or Candidate Planets – about 1,0004. 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 brightnessConfirmation - 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 planetFirst 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 discoveryMissionsEuropean Space Agency’s COROT mission (2006)NASA’s Spitzer Space Telescope: observes infrared light has successfully observed transit even though it was not designed for that purposeKepler Mission (2009 – 2013): monitors about 150,000 starsAdvantagesAllows many stars to be observed at onceCan detect very small planetsFeasible with small telescopeCan provide some atmospheric information in cases of measurable eclipsesLimitationsPossible only for planets with edge-on orbits as viewed from EarthSmall planets, requires sensitivity possible only from space observatoryMajor Planetary Properties MeasuredPlanet sizeOrbital period and distance (semimajor axis)Orbital inclination (edge-on)Confirmed or Candidate Planets – about 3,0005. 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 planetsLimitations – special alignment necessary for microlensing is a onetime event, which generally means there is no opportunity for confirmation or follow-up observationKepler Mission for Transit ObservationLaunched in 2009, expected to last until 2016 but pointing failure in 2013Monitored the brightness of 150,000 stars every 30 minutesCapable of detecting an Earth passing in front of the sun (0.008% decrease in


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BU CAS AS 100 - Lecture 31: Discovering Other Solar Systems

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