© 2004 Pearson Education Inc., publishing as Addison-Wesley Announcements Second Mid Term Exam Weds Mar 14 On energy, Newtons Laws, light, telescopes, the Solar System, solar system formation, extrasolar planets New homework assignment 6, available today, due Weds Mar 14 2pm

© 2004 Pearson Education Inc., publishing as Addison-Wesley Origin of the Solar System:The Nebular Theory

© 2004 Pearson Education Inc., publishing as Addison-Wesley

And temperature depended on distance from the Sun

© 2004 Pearson Education Inc., publishing as Addison-Wesley Building the Planets So only rocks & metals condensed within 3.5 AU of the Sun… the so-called frost line. Hydrogen compounds (ices) condensed beyond the frost line.

© 2004 Pearson Education Inc., publishing as Addison-Wesley Origin of the Asteroids The Solar wind cleared the leftover gas, but not the leftover planetesimals. Those leftover rocky planetesimals which did not accrete onto a planet are the present-day asteroids. Most inhabit the asteroid belt between Mars & Jupiter. –Jupiter’s gravity prevented a planet from forming there.

© 2004 Pearson Education Inc., publishing as Addison-Wesley Origin of the Comets The leftover icy planetesimals are the present-day comets. Those which were located between the Jovian planets, if not captured, were gravitationally flung in all directions into the Oort cloud. Those beyond Neptune’s orbit remained in the ecliptic plane in what we call the Kuiper belt. The nebular theory predicted the existence of the Kuiper belt 40 years before it was discovered!

© 2004 Pearson Education Inc., publishing as Addison-Wesley Exceptions to the Rules There were many more leftover planetesimals than we see today. Most of them collided with the newly-formed planets & moons during the first few 10 8 years of the Solar System. We call this the heavy bombardment period. So how does the nebular theory deal with exceptions, i.e. data which do not fit the model’s predictions?

© 2004 Pearson Education Inc., publishing as Addison-Wesley Exceptions to the Rules Why some moons orbit opposite their planet’s rotation –captured moons (e.g. Triton) Why rotation axes of some planets are tilted –impacts “knock them over” (extreme example: Uranus) Why some planets rotate more quickly than others –impacts “spin them up” Why Earth is the only terrestrial planet with a large Moon –giant impact Close encounters with and impacts by planetesimals could explain:

© 2004 Pearson Education Inc., publishing as Addison-Wesley Formation of the Moon (Giant Impact Theory) The Earth was struck by a Mars-sized planetesimal A part of Earth’s mantle was ejected This coalesced in the Moon. –it orbits in same direction as Earth rotates –lower density than Earth –Spun up Earth

© 2004 Pearson Education Inc., publishing as Addison-Wesley 9.5 How Old is the Solar System? How do we measure the age of a rock? How old is the Solar System and how do we know? Our goals for learning:

© 2004 Pearson Education Inc., publishing as Addison-Wesley Radiometric Dating Isotopes which are unstable are said to be radioactive. They spontaneously change in to another isotope in a process called radioactive decay. –protons convert to neutrons –neutrons convert to protons The time it takes half the amount of a radioactive isotope to decay is called its half life. By knowing rock chemistry, we chose a stable isotope which does not form with the rock…its presence is due solely to decay. Measuring the relative amounts of the two isotopes and knowing the half life of the radioactive isotope tells us the age of the rock.

© 2004 Pearson Education Inc., publishing as Addison-Wesley The Age of our Solar System Radiometric dating can only measure the age of a rock since it solidified. Geologic processes on Earth cause rock to melt and resolidify.  Earth rocks can’t be used to measure the Solar System’s age. We must find rocks which have not melted or vaporized since they condensed from the Solar nebula. – meteorites imply an age of 4.6 billion years for Solar System Radioactive isotopes are formed in stars & supernovae –suggests that Solar System formation was triggered by supernova –short half lives suggest the supernova was nearby

© 2004 Pearson Education Inc., publishing as Addison-Wesley Extrasolar Planets Since our Sun has a family of planets, shouldn’t other stars have them as well? –Planets which orbit other stars are called extrasolar planets We finally obtained direct evidence of the existence of an extrasolar planet in the year 1995 –A planet was discovered in orbit around the star 51 Pegasi –~160 extrasolar planets are now known to exist

© 2004 Pearson Education Inc., publishing as Addison-Wesley Why is it so difficult to detect planets around other stars?

© 2004 Pearson Education Inc., publishing as Addison-Wesley Brightness Difference A star like the Sun would be a billion times brighter than the light reflected off its planets. As a matter of contrast, the planet gets lost in the glare of the star. Also, planets are relatively close to their stars, need high angular resolution to separate the bodies

© 2004 Pearson Education Inc., publishing as Addison-Wesley Detecting Extrasolar Planets Can we actually make images of extrasolar planets? –this is very difficult to do. The distances to the nearest stars are much greater than the distances from a star to its planets. The angle between a star and its planets, as seen from Earth, is very small

© 2004 Pearson Education Inc., publishing as Addison-Wesley Planet Detection Direct: Pictures of the planets themselves - tricky due to contrast and separation issues

© 2004 Pearson Education Inc., publishing as Addison-Wesley Direct Detection Special techniques can eliminate light from brighter objects These techniques are enabling direct planet detection

© 2004 Pearson Education Inc., publishing as Addison-Wesley Indirect: Measurements of stellar properties revealing the effects of orbiting planets

© 2004 Pearson Education Inc., publishing as Addison-Wesley Gravitational Tugs Sun and Jupiter orbit around their common center of mass Sun therefore wobbles around that center of mass with same period as Jupiter

© 2004 Pearson Education Inc., publishing as Addison-Wesley Astrometric Technique We detect the planets indirectly by observing the star. Planet gravitationally tugs the star, causing it to wobble. We can detect planets by measuring the change in a star’s position on sky However, these tiny motions are very difficult to measure (~0.001 arcsecond)

© 2004 Pearson Education Inc., publishing as Addison-Wesley Easier is the Doppler Technique Measuring a star’s Doppler shift can tell us its motion toward and away from us Current techniques can measure motions as small as 1 m/s (walking speed!)

© 2004 Pearson Education Inc., publishing as Addison-Wesley First Extrasolar Planet detected 1995 using Doppler Technique Planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance

© 2004 Pearson Education Inc., publishing as Addison-Wesley First Extrasolar Planet Doppler shifts of star 51 Pegasi indirectly reveal a planet with 4-day orbital period Short period means small orbital distance First extrasolar planet to be discovered (1995)

© 2004 Pearson Education Inc., publishing as Addison-Wesley Other Extrasolar Planets Large planet mass Doppler data curve tells us about a planet’s mass and the shape of its orbit

© 2004 Pearson Education Inc., publishing as Addison-Wesley Planet Mass and Orbit Tilt We cannot measure an exact mass for a planet without knowing the tilt of its orbit, because Doppler shift tells us only the velocity toward or away from us Doppler data gives us lower limits on masses

© 2004 Pearson Education Inc., publishing as Addison-Wesley Measuring the Properties of Extrasolar Planets A plot of the radial velocity shifts forms a wave. –Its wavelength tells you the period and size of the planet’s orbit. –Its amplitude tells you the mass of the planet.

© 2004 Pearson Education Inc., publishing as Addison-Wesley Transit Method The Doppler technique yields only planet masses and orbits. Planet must eclipse or transit the star in order to measure its radius. Size of the planet is estimated from the amount of starlight it blocks. We must view along the plane of the planet’s orbit for a transit to occur. –transits are relatively rare They allow us to calculate the density of the planet. –extrasolar planets we have detected have Jovian-like mass/density –But these would be easiest to detect

© 2004 Pearson Education Inc., publishing as Addison-Wesley Transits and Eclipses A transit is when a planet crosses in front of a star The resulting eclipse reduces the star’s apparent brightness and tells us planet’s radius No orbital tilt: accurate measurement of planet mass

© 2004 Pearson Education Inc., publishing as Addison-Wesley Spectrum during Transit Change in spectrum during transit tells us about composition of planet’s atmosphere

© 2004 Pearson Education Inc., publishing as Addison-Wesley Transit Result from Spitzer HD b and TrES-1- detected using “transit” method in infra-red light -ie observers

© 2004 Pearson Education Inc., publishing as Addison-Wesley The Nature of Extrasolar Planets What have we learned about extrasolar planets? How do extrasolar planets compare with those in our solar system?

© 2004 Pearson Education Inc., publishing as Addison-Wesley Measurable Properties Orbital Period, Distance, and Shape Planet Mass, Size, and Density Composition

© 2004 Pearson Education Inc., publishing as Addison-Wesley Orbits of Extrasolar Planets Most of the detected planets have orbits smaller than Jupiter’s Planets at greater distances are harder to detect with Doppler technique

© 2004 Pearson Education Inc., publishing as Addison-Wesley Orbits of Extrasolar Planets Orbits of some extrasolar planets are much more elongated (greater eccentricity) than those in our solar system

© 2004 Pearson Education Inc., publishing as Addison-Wesley Multiple-Planet Systems Some stars have more than one detected planet

© 2004 Pearson Education Inc., publishing as Addison-Wesley Orbits of Extrasolar Planets Most of the detected planets have greater mass than Jupiter Planets with smaller masses are harder to detect with Doppler technique

© 2004 Pearson Education Inc., publishing as Addison-Wesley How do extrasolar planets compare with those in our solar system?

© 2004 Pearson Education Inc., publishing as Addison-Wesley Surprising Characteristics Some extrasolar planets have highly elliptical orbits Some massive planets orbit very close to their stars: “Hot Jupiters”

© 2004 Pearson Education Inc., publishing as Addison-Wesley Hot Jupiters

© 2004 Pearson Education Inc., publishing as Addison-Wesley Planets: Common or Rare? One in ten stars examined so far have turned out to have planets The others may still have smaller (Earth- sized) planets that current techniques cannot detect

© 2004 Pearson Education Inc., publishing as Addison-Wesley How will we search for Earth-like planets?

© 2004 Pearson Education Inc., publishing as Addison-Wesley Transit Missions NASA’s Kepler mission is scheduled to begin looking for transiting planets in 2008 It is designed to measure the 0.008% decline in brightness when an Earth- mass planet eclipses a Sun- like star –Transit missions will be capable of finding Earth- like planets that cross in front of their stars (Kepler to launch in 2008)

© 2004 Pearson Education Inc., publishing as Addison-Wesley Astrometric Missions will be capable of measuring the “wobble” of a star caused by an orbiting Earth-like planet GAIA: A European mission planned for 2010 that will use interferometry to measure precise motions of a billion stars SIM: A NASA mission planned for 2011 that will use interferometry to measure star motions even more precisely (to arcseconds)

© 2004 Pearson Education Inc., publishing as Addison-Wesley Direct Detection Determining whether Earth-mass planets are really Earth-like requires direct detection Missions capable of blocking enough starlight to measure the spectrum of an Earth-like planet are being planned Mission concept for NASA’s Terrestrial Planet Finder (TPF)