ASTR100 (Spring 2008) Introduction to Astronomy Other Planetary Systems Prof. D.C. Richardson Sections

But first…

When did the planets form? We cannot find the age of a planet, but we can find the ages of the rocks that make it up. We can determine the age of a rock by analyzing the proportions of various atoms and isotopes within it.

The decay of radioactive elements into other elements is a key tool in finding the ages of rocks.

Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old.

Thought Question Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 3 atoms of argon-40 for every 1 atom of potassium-40. How old is the rock? A.1.25 billion years. B.2.5 billion years. C.5 billion years. D.It is impossible to determine.

Thought Question Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 3 atoms of argon-40 for every 1 atom of potassium-40. How old is the rock? A.1.25 billion years. B.2.5 billion years. C.5 billion years. D.It is impossible to determine.

How do we detect planets around other stars?

Planet Detection Direct: Pictures or spectra of the planets themselves. Indirect: Measurements of stellar properties revealing the effects of orbiting planets.

Gravitational Tugs The Sun and Jupiter orbit around their common center of mass. The Sun therefore wobbles around that center of mass with the same period as Jupiter.

Gravitational Tugs Sun’s motion around solar system center of mass depends on tugs from all the planets. Astronomers who measure this motion around other stars can determine masses and orbits of all the planets. (as seen from 10 ly)

Astrometric Technique We can detect planets by measuring the change in a star’s position in the sky. However, these tiny motions are very difficult to measure (~0.001 arcsec). (as seen from 10 ly)

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!).

First Extrasolar Planet Detected Doppler shifts of star 51 Pegasi indirectly reveal planet with 4- day orbital period. Short period means small orbital distance. First extrasolar planet to be discovered (1995).

First Extrasolar Planet Detected The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.

Thought Question Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? A.It has a planet orbiting inside 1 AU. B.It has a planet orbiting outside 1 AU. C.It has a planet orbiting at 1 AU. D.It has a planet, but we do not have enough information to know its orbital distance.

Thought Question Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? A.It has a planet orbiting inside 1 AU. B.It has a planet orbiting > 1 AU. C.It has a planet orbiting at 1 AU. D.It has a planet, but we do not have enough information to know its orbital distance.

Transits and Eclipses A transit is when a planet passes in front of a star. The resulting eclipse reduces the star’s apparent brightness and tells us the planet’s radius. When there is no orbital tilt, an accurate measurement of planet mass can be obtained.

Direct Detection Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of planets.

How do extrasolar planets compare with those in our own solar system?

Measurable Properties Orbital period, distance, and shape. Planet mass, size, and density. Composition.

Orbits of Extrasolar Planets Most of the detected planets have smaller orbits than Jupiter. Planets at greater distances are harder to detect with the Doppler technique.

Properties of Extrasolar Planets Most of the detected planets have larger mass than Jupiter. Planets with smaller masses are harder to detect with the Doppler technique.

Planets: Common or Rare? One in 10 stars so far have turned out to have planets. The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.

Surprising Characteristics Some extrasolar planets have highly elliptical orbits. Some massive planets orbit very close to their stars: “Hot Jupiters.”

Hot Jupiters

Do we need to modify our theory of solar system formation?

Revisiting the Nebular Theory Nebular theory predicts massive Jupiter-like planets should not form inside the frost line (at << 5 AU). The discovery of “hot Jupiters” has forced a reexamination of the nebular theory. “Planetary migration” or gravitational encounters may explain hot Jupiters.

Planetary Migration A young planet’s motion can create waves in a planet- forming disk. Models show that matter in these waves can tug on a planet, causing it to migrate inward.

Gravitational Encounters Close gravitational encounters between two massive planets can eject one while flinging the other into an elliptical orbit. Multiple close encounters with smaller planetesimals can also cause inward migration.

Modifying the Nebular Theory Observations of extrasolar planets showed that the nebular theory was incomplete. Effects like planet migration and gravitational encounters might be more important than previously thought.

MIDTERM #1 REVIEW Chapters 1–6

Chapter 1: Our Place in the Universe A.Our Modern View of the Universe Planets, stars, galaxies, superclusters. The speed of light: looking back in time. B.The Scale of the Universe Sizes & distances: planets, stars, galaxies. The age of the universe. C.Spaceship Earth Our motion through the universe.

Chapter 2: Discovering the Universe for Yourself A.Patterns in the Night Sky Constellations, celestial sphere, rise & set. B.The Reason for Seasons Axis tilt, equinoxes, precession. C.The Moon, Our Constant Companion Phases, eclipses. D.The Ancient Mystery of the Planets Geocentric vs. heliocentric.

Chapter 3: The Science of Astronomy A.The Ancient Roots of Science Astronomy as the oldest science. B.Ancient Greek Science Birth of modern science. C.The Copernican Revolution Copernicus, Tycho Brahe, Kepler, Galileo. D.The Nature of Science Observe, hypothesize, experiment, predict.

Chapter 4: Making Sense of the Universe: Understanding Motion, Energy, & Gravity A.Describing Motion: Examples from Daily Life Speed, velocity, acceleration, momentum, force, mass, weight. B.Newton’s Laws of Motion Inertia, F = ma, equal and opposite force. C.Conservation Laws in Astronomy Energy and angular momentum. D.The Force of Gravity F = G M 1 M 2 / d 2, tides.

Chapter 5: Light: The Cosmic Messenger A.Basic Properties of Light and Matter EM spectrum, atoms, emission/absorption. B.Learning from Light Spectroscopy, thermal emission, Doppler. C.Collecting Light with Telescopes Bigger is better, space is clearer.

Chapter 6: Our Solar System and its Origin A.A Brief Tour of the Solar System Sun, planets, moons, asteroids, comets. B.Clues to the Formation of our Solar System Orderly motion, planet types, exceptions. C.The Birth of the Solar System The nebular hypothesis. D.The Formation of Planets Accretion, giant impacts, age. E.Other Planetary Systems Detection methods, challenges to theory.

Midterm Information When: Tuesday March 4, 9:30 am Where: here! Bring pencil, student ID No notes, no calculators, no mobiles! Review: Monday March 3, 5-7 pm Where: here! Bring your questions and textbook!

Good Luck!