Planets and ExoPlanets

1. Planets and ExoPlanets Earth, as viewed by the Voyager spacecraft

2. What does the solar system look like?

3. There are eight major planets with nearly circular orbits. • Pluto and Eris are smaller than the major planets and have more elliptical orbits.

4. What are the major features of the Sun and planets? Sun and planets to scale

5. Sun • Over 99.9% of solar system’s mass • Made mostly of H/He gas (plasma) • Converts 4 million tons of mass into energy each second

6. Mercury • Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs • Very hot and very cold: 425C (day)–170C (night)

7. Venus • Nearly identical in size to Earth; surface hidden by clouds • Hellish conditions due to an extreme greenhouse effect • Even hotter than Mercury: 470C, day and night

8. Earth Earth and Moon with sizes shown to scale • An oasis of life • The only surface liquid water in the solar system • A surprisingly large moon

9. Mars • Looks almost Earth-like, but don’t go without a spacesuit! • Giant volcanoes, a huge canyon, polar caps, more • Water flowed in distant past; could there have been life?

10. Jupiter • Much farther from Sun than inner planets • Mostly H/He; no solid surface • 300 times more massive than Earth • Many moons, rings

11. Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons. • Io (shown here): active volcanoes all over • Europa: possible subsurface ocean • Ganymede: largest moon in solar system • Callisto: a large, cratered “ice ball”

12. Saturn • Giant and gaseous like Jupiter • Spectacular rings • Many moons, including cloudy Titan

13. Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. Artist’s conception

14. Cassini probe arrived July 2004 (launched in 1997).

15. Uranus • Smaller than Jupiter/Saturn; much larger than Earth • Made of H/He gas and hydrogen compounds (H2O, NH3, CH4) • Extreme axis tilt • Moons and rings

16. Neptune • Similar to Uranus (except for axis tilt) • Many moons (including Triton)

17. Pluto (and Other Dwarf Planets) • Much smaller than major planets • Icy, comet-like composition • Pluto’s main moon (Charon) is of similar size

18. Motion of Large Bodies • All large bodies in the solar system orbit in the same direction and in nearly the same plane. • Most also rotate in that direction.

19. Two Major Planet Types • Terrestrial planets are rocky, relatively small, and close to the Sun. • Jovian planets are gaseous, larger, and farther from the Sun.

20. Swarms of Smaller Bodies • Many rocky asteroids and icy comets populate the solar system.

21. Notable Exceptions • Several exceptions to the normal patterns need to be explained.

22. Why is it so difficult to detect planets around other stars?

23. Brightness Difference • A Sun-like star is about a billion times brighter than the light reflected from its planets. • This is like being in San Francisco and trying to see a pinhead 15 meters from a grapefruit in Washington, D.C.

24. Planet Detection • Direct: pictures or spectra of the planets themselves • Indirect: measurements of stellar properties revealing the effects of orbiting planets

25. How do we detect planets around other stars?

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

27. Gravitational Tugs • The Sun’s motion around the solar system’s center of mass depends on tugs from all the planets. • Astronomers around other stars that measured this motion could determine the masses and orbits of all the planets.

28. Astrometric Technique • 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).

29. 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!).

30. First Extrasolar Planet • Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period. • This short period means that the planet has a small orbital distance. • This was the first* extrasolar planet to be discovered (1995). Insert TCP 6e Figure 13.4a unannotated

31. First Extrasolar Planet • The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance. Insert TCP 6e Figure 13.4b

32. Other Extrasolar Planets • Doppler shift data tell us about a planet’s mass and the shape of its orbit.

33. 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 give us lower limits on masses.

34. Transits and Eclipses • A transitis 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

35. Spectrum During Transit • Change in spectrum during a transit tells us about the composition of planet’s atmosphere.

36. Surface Temperature Map • Measuring the change in infrared brightness during an eclipse enables us to map a planet’s surface temperature.

37. Direct Detection • Special techniques like adaptive optics are helping to enable direct planet detection.

38. Direct Detection • Techniques that help block the bright light from stars are also helping us to find planets around them.

39. Direct Detection • Techniques that help block the bright light from stars are also helping us to find planets around them.

40. Other Planet-Hunting Strategies • Gravitational Lensing: Mass bends light in a special way when a star with planets passes in front of another star. • Features in Dust Disks: Gaps, waves, or ripples in disks of dusty gas around stars can indicate presence of planets.

41. What have we learned about extrasolar planets?

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

43. Orbits of Extrasolar Planets • Orbits of some extrasolar planets are much more elongated (have a greater eccentricity) than those in our solar system.

44. Multiple-Planet Systems • Some stars have more than one detected planet.

45. 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.

46. Hot Jupiters

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

48. 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 its orbit to migrate inward.

49. Orbital Resonances • Resonances between planets can also cause their orbits to become more elliptical.

50. 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. • Kepler seems to indicate COMMON