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

PTYS/ASTR 206 Comets4/24/07

Comets and Kuiper Belt Objects


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


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 1979-1999)

• Only “planet” not visited by a spacecraft– New Horizons, launched in

Jan. 2006, will reach Pluto in 2015.

• Pluto can be seen with an amateur telescope, but it is not easy!– About 14th magnitude

• 6th magnitude is the limit for the naked eye

• Pluto is about 1600 times dimmer than this


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


Pluto: Basic Facts• Avg. density

– About 1900 kg/m3

– Rock and ice

• Diameter– 0.18 Earth Diameters– Not that well known (+/-

1%)

• Mass– 0.0021 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 N2 that is probably not in equilibrium


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”


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.


• 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)!


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


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


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".


Eris


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


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)


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


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



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


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


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


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

Documents

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