PTYS/ASTR 206Comets 4/24/07 Comets and Kuiper Belt Objects

PTYS/ASTR 206Comets 4/24/07 Announcements Reading Assignment -- Chapter 30 quiz today In-class activity and course evaluations on Thursday Public lecture tonight – 7:30PM, this room –Prof. Alfed McEwan, Mars HiRISE Last Lecture – a week from today –Extrasolar planets –Final exam review

PTYS/ASTR 206Comets 4/24/07 Pluto Most distant “planet” –Most of the time, but not all of the time! Its orbit can bring it inside of Neptune’s (as it did from ) Only “planet” not visited by a spacecraft –New Horizons, launched in Jan. 2006, will reach Pluto in Pluto can be seen with an amateur telescope, but it is not easy! –About 14 th magnitude 6 th magnitude is the limit for the naked eye Pluto is about 1600 times dimmer than this

PTYS/ASTR 206Comets 4/24/07 Pluto: Basic Facts Avg. distance to the Sun –39.5 AU Orbital Period –About 250 years Retrograde rotation –Like Venus and Uranus Eccentricity –0.25 –larger than all of the planets

PTYS/ASTR 206Comets 4/24/07 Pluto: Basic Facts Avg. density –About 1900 kg/m 3 –Rock and ice Diameter –0.18 Earth Diameters –Not that well known (+/- 1%) Mass – Earth Masses –Also not well known, although the combined mass of Pluto and Charon is well known Tenuous atmosphere (but not as we have defined in in class) of N 2 that is probably not in equilibrium

PTYS/ASTR 206Comets 4/24/07 Neptune and Pluto are in a 3:2 orbital resonance Pluto’s orbital period is 1.5 times Neptune’s –Neptune is about 164 years –Pluto is about 249 years This is a stable configuration and they will never crash into each other Pluto is like other objects in this sense – comprising a class of objects called “Plutino’s”

PTYS/ASTR 206Comets 4/24/07 Charon: Pluto’s Moon Discovered in 1978 by Jim Christy. –Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon is the largest moon with respect to its primary “planet” in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

PTYS/ASTR 206Comets 4/24/07 Pluto and Charon are in an unusual resonance. Charon has an orbital period equal to both its rotational period AND Pluto’s orbital period From one side of Pluto, Charon just sits in the same place in the sky (never sets, never rises)!

PTYS/ASTR 206Comets 4/24/07 Is Pluto a planet ? Since the discovery of thousands of objects orbiting the Sun, called Edgeworth-Kuiper objects (or Kuiper-belt objects, KBOs), it has been suggested that Pluto is simply one of these and is not a planet Many KBO’s are in orbital resonances with Neptune (3:2 resonance  “Plutino”) This is further complicated by the fact that the recently discovered Eris is larger than Pluto. In 2006, Pluto was downgraded to a new class of objects called “dwarf planets” –But the debate rages on

PTYS/ASTR 206Comets 4/24/07 Kuiper Belt Red: All objects that are in a resonance with Neptune (Neptune Trojans – 1:1 ; Plutinos – 2:3 ; Twotinos 1:2 ) Blue: “Classical” objects

PTYS/ASTR 206Comets 4/24/07 The IAU’s decision in August 2006 The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: (1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

PTYS/ASTR 206Comets 4/24/07 Eris

PTYS/ASTR 206Comets 4/24/07 History of planets Ancients recognized 7 planets –Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn Earth was not a planet! 1543, changed from 7 to 6 planets –Definition changed to an object orbiting the Sun Sun and Moon removed, added Earth 1852, changed from 23 to 8 planets –Asteroids were demoted since they did not have a resolvable disc Uranus and Neptune had been discovered by now 2006, changed from 9 to 8 planets –Pluto demoted

PTYS/ASTR 206Comets 4/24/07 Semantics Asteroids –Small rocky bodies in orbit about the Sun Comets –Small bodies that orbit the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail Meteoroids –Small asteroids Meteorites –the debris collected on Earth Meteors –A brief flash of light (i.e. a shooting star)

PTYS/ASTR 206Comets 4/24/07 Asteroids and Comets Asteroids –Mostly nearly-circular orbits –Mostly confined to the asteroid belt Close to the ecliptic plane –Relatively short orbital periods Comets –Highly elliptical orbits –Random inclinations i.e. not-confined to the ecliptic –Very long orbital periods

PTYS/ASTR 206Comets 4/24/07 Comets: Harbinger’s of Doom Rome: marked the assassination of Julius Ceaser England: blamed for bringing the Black Death Incan Empire: foreshadowed the brutal conquering by Francisco Pizarro

PTYS/ASTR 206Comets 4/24/07

PTYS/ASTR 206Comets 4/24/07 Comet Types (based on orbital characteristics) Short-Period Comets –Comet with an orbital period of less than 200 years –Shortest lived Breakup due to gravitational forces Life expectancy of about 12,000 years Long-Period Comets –Comets with orbital periods greater than 200 years (though typically around millions of years) Other Definitions –Jupiter Family Comets (JFC) Orbital period less than 20 years –Intermediate-Period Comets Orbital period between 20 and 200 years

PTYS/ASTR 206Comets 4/24/07 Comet Origin Kuiper Belt –Jupiter family and intermediate period comets –These comets have low inclinations compared to long- period comets Oort Cloud –Spherical distribution –Frequent observations of long- and intermediate-period comets indicate that the reservoir must be huge –Cannot be seen directly because it is so far away –Perturbed by passing stars or galactic tides

PTYS/ASTR 206Comets 4/24/07 Comet orbits are often highly inclined Long-period comets, originating from the roughly spherically distributed Oort cloud have orbits with random inclinations Intermediate and Jupiter-family comets tend to have orbits more confined to the ecliptic Asteroids seldom have highly inclined orbits

PTYS/ASTR 206Comets 4/24/07 Appearance of Comets When comets are far from the Sun, they are dark and hard to see –Low albedos, far away Because of their large eccentricities, they occasionally come very close to the Sun The body outgases as it heats and releases dust and cometary atoms This produces a long visible tail –Directed away from the Sun