Key Ideas: The Search for New Planets. Scientific Questions. Are we alone in the Universe? Direct Imaging. Searches for Extrasolar Planets

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1 The Search for New Planets Key Ideas: Search for planets around other stars. Successful Search Techniques: Astrometric Wobble Doppler Wobble major discovery method Planetary Transits planets we can study in detail Gravitational Microlensing the lowest mass planets Orbit timing 3891 planets / 2900 planetary systems / 645 multiple planet systems Are we alone in the Universe? The question of the existence of other planets beyond the Solar System, and whether any can harbor life is an old one in astronomy: Are there solar systems around other stars? Are such solar systems like ours or different? Are any of the planets like the Earth? Has life arisen on other planets? Has intelligent life arisen on other planets? Has intelligent life arisen on Earth? Scientific Questions Speculation is fun, but the scientific problems reduce to searches for: Solar systems in the process of formation. Evolved solar systems around other stars. Evidence of life on other planets. Evidence of technological, intelligent life on other planets (SETI). Searches for Extrasolar Planets There are two basic search strategies: Direct Detection: Take images of planets orbiting other stars Watch planets occulting (eclipsing) their parent star, causing a characteristic drop in brightness. Gravitational Detection: Effects of a planet s gravity on their parent star. Gravitational microlensing by the planet. Planet-planet perturbations Direct Imaging Hard because stars are so much brighter than planets Best for large, young planets very far from star 44 planets around 40 stars (and counting) 102 planets / 84 planetary systems / 3 multiple planet systems 1

2 3 planets, all a few Jupiter masses, innermost about at the orbit of Neptune About 3 times the mass of Jupiter orbit predicted (!) from structure of ring. 870 year orbit. Mostly suppressed image of the star Wobbling Stars Recall Newton s form of Kepler s 1st Law: Planets actually orbit on ellipses with the center-of-mass at one focus. Because the sun is much more massive than any planet, the sun is very close to the center-of-mass but not quite It orbits around the center of mass at a distance of (planetary orbital radius)(m planet /M sun )=R for Jupiter The star will appear to wobble about the center-ofmass of the star-planet system. P S Astrometric Wobble Parent star wobbles back & forth on the sky relative to more distant background stars. Problem: The star is so much more massive than any planets, the motion is small. The motion is best seen when we are looking down on the plane of the orbit. From 18 light years away, the astrometric wobble of the Sun is <0.001 arcseconds! Only one planet detected this way (still true) Astrometric Wobble of the Sun 0.001" " Viewed from 18 light years above the Ecliptic plane 2

3 Doppler Wobble Method Another way is to look for motion along the line of sight using the Doppler Effect Star s spectral absorption lines shift towards the blue when the wobble moves the star towards the Earth. Star s spectrum shifts towards the red when the wobble moves the star away from the Earth. Measuring the orbital motions provides an estimate of the unseen planet s mass. P Doppler Wobble Measurements Star s spectrum Doppler shifts blueward S The greater mass of the star makes its orbital speed about the star-planet center of mass very small again smaller by the mass ratio M planet /M sun To the observer Star s spectrum Doppler shifts redward S P Example: Sun & Jupiter Jupiter: 13 km/sec at 5.2 AU from the center of mass Sun: 13 m/sec at AU from the center of mass Need to be able to measure the Doppler shifts of the lines with extremely high precision. 51 Pegasi Michel Mayor & Didier Queloz at Geneva Observatory observed a periodic wobble in the star 51 Pegasi in Sun-like star about 40 light-years away in the constellation of Pegasus Wobble was 56 meters/second, with a period of only 4.2 days. This implied a planet with a mass of 0.5 Jupiters orbiting at 0.05 AU! 3

4 Mass of planet/mass of Jupiter Doppler Planets As of February planets, 14 multiple planet systems As of March planets around 175 stars, 20 multiple planet systems As of March planets around 270 stars, 33 multiple planet systems As of November planets around 402 stars, 92 multiple planet systems 784 planets / 579 planetary systems / 144 multiple planet systems Some caveats are in order: Doppler Wobble most easily detects large, close planets (bigger wobbles) and has trouble with small distant planets (smaller wobbles) Currently can detect wobbles ~0.5 meters/second (a sprinter doing 100 meters in 10 seconds is moving 10 meters/second) - 1 meter/second is the wobble induced in the Sun by Saturn. The mass estimates assume the orbit is oriented exactly edge on, giving the smallest mass that could make the observed wobble. Also have to watch the star long enough for the planet to make a significant fraction of an orbit Today s Limits Period of planet s orbit Hard to find because the period of the orbit too long Hard to find because velocity of the star is too low Smallest ones are about ~1 Earth mass, but are closer to their stars than Mercury Some with periods of ~years, but they are 1-10 times the mass of Jupiter 4

5 Closest Planet? News! Even Closer Planet! Alpha Centauri (4.3 light years), a triple star system with Beta Centauri and Proxima Centauri Planet has a 3.2 day period, roughly 1.1 Earth masses, surface temperature ~1500K Amplitude of the Doppler wobble of Alpha Centauri? About 1 mile per hour (0.5 m/second) Some skepticism about reality of detection. Planet around Proxima Centauri (4.3 light years). Planetary Transits Planetary Transit If the orbital plane of an extrasolar planet is aligned with the line of sight: The planet will periodically transit across the face of its parent star. As the planet transits there will be a slight (fraction of a %) dimming of the starlight. Requires precision photometry & lots of luck. Combined with Doppler data, it gives the true orbital inclination, and hence the true mass. The Case of HD planetary transit in the star HD Has a Jupiter-sized planet found by the Doppler Wobble method: Sun-like star 158 light years away in Pegasus Planet has a mass of ~0.7 Jupiters Orbits at AU with a period of ~3.5 days Using the Doppler work as a guide, astronomers were able to observe the planet transit its star. 5

6 Why is this important? The planetary transit and Doppler Wobble data combined gives us more information: The orbital tilt: I = ± 0.14 The planet's mass: M=0.69 ± 0.05 M Jup The planet s radius: R=1.347 ± R Jup The planet's mean density: 350 kg/m 3 It is a giant gas planet intermediate in mass between Jupiter & Saturn. Number of Transits 425 confirmed transiting planets, around 321 stars, 71 systems with multiple transiting planets (these numbers are obsolete, see below) >3,5000 candidates from Kepler spacecraft statistical studies say almost all of these planets will be real 2865 planets / 2146 planetary systems / 469 multiple planet systems The Challenges Transit Depth The depth of the transit is set by how much of the star is covered by the planet the fractional change is (radius of planet/radius of star) 2 ~10 4 for the Earth ~10 2 Jupiter To look for the small dips associated with Earth almost certainly requires doing it from space in order to get the necessary stability The Challenges Transit Time The duration of the transit is determined by the orbit the time for the planet to pass in front of the sun transit duration ~ 12(orbital radius/au) 1/2 hours but as a fraction of the orbital period is is only fraction~0.001(au/orbital radius) Distant transits last longer but are a tiny fraction of the orbital period The Challenges Fraction Observable A more distant transiting planet must be closer and closer to having an orbit edge on relative to us the fraction which will transit is very small at large distances Fraction of planets that will transit ~(radius of star/radius of orbit) ~0.005(1 AU/radius of orbit) Kepler Launched in 2009 in theory capable of detecting transiting Earths It stares at one patch of sky and monitors the brightnesses of about 100,000 stars looking for transits. 6

7 1.7 times Jupiter s mass, very hot, 2730K so can see a secondary eclipse Gravitational Microlensing Gravity s Telescope Only ~40 planets so far, but able to find very low mass planets Big projects using this method are coming online 87 planets / 82 planetary systems / 3 multiple planet systems S. Gaudi Gravity bends light rays, and so a foreground star can focus the light of a more distant star onto the observer More complex light curves with a planet Advantage: The amplitude of the signal does not depend on the planet s mass unlike all other methods Disadvantages: It is a one shot observation no follow up observations to study it further or improve the measurements S. Gaudi 7

8 Brightness in weird astronomy units (10 times brighter going from 16.5 to 14) Explaining this one requires two planets orbiting a star with half the mass of the Sun the planets are 0.7 and 0.3 times the mass of Jupiter and are at 2.3 and 4.6 AU from the star Time in days Brightness of background star as a function of time (in days) Orbit Timing Once you have one planet you can look for other planets through their gravitational effects on the orbit of the first planet Can find much lower mass planets than you could find directly, particularly if they are in a resonance with the first planet At it s simplest, the timing of transits changes from the prediction for a simple elliptical orbit 7 systems, 8 planets Total numbers (obsolete) (exoplanet.eu) Confirmed planets (numbers I ve been using so far) 1043 planets around 791 stars, 173 multiple planet systems Then add the ~3500 Kepler candidates 6332 planets / 5149 planetary systems / 801 multiple planet systems (including candidates) Summary Direct imaging favors distant, big and young Astrometric wobble needs spacecraft Doppler wobble very successful, favors close and big Transits most extra information, favors close and big Microlensing anything at ~AU smaller rarer Orbit timing high mass or resonant low mass Gaudi 8