Other Planetary Systems

1. Other Planetary Systems

2. Other Planetary SystemsPlanetary 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://planetquest.jpl.nasa.gov/ © Sierra College Astronomy Department

3. Other Planetary SystemsPlanetary 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

4. Other Planetary SystemsPlanetary 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 the size of a grapefruit, Jupiter would be a marble 80 meters away. • If an observer is 30 light-years away, Jupiter would be no more than 0.5 arcsec away from the Sun. • The star would shine one-billion times brighter than the reflected light from the planet. © Sierra College Astronomy Department

5. Other Planetary SystemsDetecting Planetary Systems 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. • While it appears that a planet orbits the sun, it is better to say that both orbit a commoncenter 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. © Sierra College Astronomy Department

6. Other Planetary SystemsDetecting Planetary Systems 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

7. Other Planetary SystemsDetecting Planetary Systems Doppler Technique • This involves looking at the stars motions using the Doppler shift. • 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 around its center of mass. © Sierra College Astronomy Department

8. Other Planetary SystemsDetecting Planetary Systems • This technique was first used successfully on 51 Peg to detect a planet. • The star wobbled at 57 m/s with a 4-day period. • Therefore the planet was going around the star in 4 days! • This planet had to lie close to the star and had a surface temp of 1000 K. • Current techniques allow us to measure velocities down to 1 m/s – walking speed. • This allows us to find planets that are smaller and/or farther away than the one around 51 Peg. © Sierra College Astronomy Department

9. Other Planetary SystemsDetecting Planetary Systems • 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 star. • From this info, what method do we use to find the mass of the star (plus the planet)? • More scrutiny of the data allows us to get the mass ratio of the system and therefore the mass of the planet. © Sierra College Astronomy Department

10. Other Planetary SystemsDetecting Planetary Systems • Caveat: the Doppler shift depends on the 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

11. Other Planetary SystemsPlanets 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

12. Other Planetary SystemsPlanets around Other Stars Transiting planets • In certain extrasolar systems where the planet’s orbit 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 as seen from Earth. • 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

13. Other Planetary SystemsPlanets around Other Stars • 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

14. Other Planetary SystemsPlanets around Other Stars • The planet also goes 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

15. Other Planetary SystemsPlanets 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 by this technique. • Strength: Smaller planets can be detected this way, including, potentially, Earth-size planets. • A brand new series of telescopes(WASP) has searched for transiting planets © Sierra College Astronomy Department

16. Other Planetary SystemsPlanets around Other Stars Direct detection • Getting a direct picture of a planet could give 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 failed star-type called a brown dwarf. © Sierra College Astronomy Department

17. Other Planetary SystemsPlanets around Other Stars Other Strategies • Gravitational lensing • Unseen planet may distort light of background star. • Looking for the influence of planets around the star’s disk of dust. • Ripples seen in Beta Pictoris indicate planets. © Sierra College Astronomy Department

18. Other Planetary SystemsThe 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 be modified? © Sierra College Astronomy Department

19. Other Planetary SystemsThe Nature of Extrasolar Planets What have we learned? Here are the extrasolar planetary properties we have derived in the last 15-20 years: • Orbital period • Orbital distance • Orbital shape • Mass • Size (radius) • Density • Composition © Sierra College Astronomy Department

20. Other Planetary SystemsThe Nature of Extrasolar Planets Orbits • 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

21. Other Planetary SystemsThe Nature of Extrasolar Planets Masses • Nearly all of the extrasolar planets are greater than 5.5 Earth masses, though recently a 1 to 2 Earth mass planets have been discovered • 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

22. Other Planetary SystemsThe Nature of Extrasolar Planets Sizes and Densities • Transiting planets allow us to get size and then density. • We have found that several 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

23. Other Planetary SystemsThe 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

24. Other Planetary SystemsThe Nature of Extrasolar Planets Compare to our Solar System • Most of the extrasolar systems bear little resemblance to our Solar System. • The extra-Jovian planets should look similar to ones we have. However, many are very close to their star and have very eccentric orbits • Labeled as “hot Jupiters” their origin is still under debate. © Sierra College Astronomy Department

25. Other Planetary SystemsThe Formation of Extrasolar Systems Challenge to the Solar Nebula theory • Recall that the solar nebula theory suggested that most of the protoplanetary disk was made of H, He, a small amount of hydrogen compounds, and still less rocks and 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. • The extrasolar systems challenges this basic idea. © Sierra College Astronomy Department

26. Other Planetary SystemsThe Formation of Extrasolar Systems Why are the Jovian planets so close to their star? • It does not seem likely that a Jovian-like planet could form so close to their star. • Astronomers now feel that these “hot Jupiters” formed far away and somehow migrated inward. © Sierra College Astronomy Department

27. Other Planetary SystemsThe Formation of Extrasolar Systems 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 gravitationally 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

28. Other Planetary SystemsThe Formation of Extrasolar Systems Encounters and Resonances • The eccentric orbits of some of these planets might be due interactions they have with neighboring planets. • Resonances may also force some of the eccentric orbits. © Sierra College Astronomy Department

29. Other Planetary SystemsThe Formation of Extrasolar Systems Change the theory • It is too early to tell whether the basic solar nebular theory needs to be changed. • Our solar system may be unique, which might suggest that the Earth is unique in this Galaxy. © Sierra College Astronomy Department

30. Other Planetary SystemsFinding More Extrasolar Planets • The discovery which will be most significant is finding an Earth-like planet. • We are just finding these now (2011)! • Transit detection missions: Kepler (2009-2013) monitored over 100,000 stars (in the Cygnus Region) looking for transiting planets every 15 minutes, down to the size of Mercury! • Over 2000 have been detected (but only a few dozen confirmed) • Kepler has detected several “super-Earths” • May eventually detect over 50 Earth-size or smaller planets • COROT (Launched 2006) is not as sensitive as Kepler, but should be able to detect planets down to a few Earth masses (useful data has been obtained) • One planet with estimated 1 to 2 Earth masses was detected © Sierra College Astronomy Department

31. Other Planetary SystemsFinding More Extrasolar Planets • Astrometric missions: • GAIA (launch 2013) will be able to measure star positions down to 10 microarcsecs (and therefore detect the star wobbling from gravitional tugs of its planets) • Transits can also be detected from this telescope © Sierra College Astronomy Department

32. Other Planetary SystemsFinding More Extrasolar Planets • Direct detection missions: • TPF (Terrestrial Planet Finder; under consideration); Darwin (under consideration) • Use multiple telescopes and interferometry to produce high resolution images • These two missions will be able to detect planets around by effectively blocking starlight from a photo leaving only the planet(s) – perhaps even by having a dark screen some 1,000 km in front of the scopes • Spectra and other analysis can lead to determination of atmospheric properties • Both have been CANCELLED! © Sierra College Astronomy Department

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