Trans-Neptunian Objects and Pluto Astronomy 311 Professor Lee Carkner Lecture 21

Trans-Neptunian Region   Beyond Neptune is the region of small, icy, Trans-Neptunian Objects     The region is populated by icy planetesimals that either formed at the edge of the solar system or were ejected out by the planets

Pluto -- God of the Underworld  Pluto is the God of the Dead in Roman mythology  

The Discovery of Pluto  In the late 1800’s it was believed that Neptune’s orbit was being perturbed by a 9th planet    In 1930 a young astronomer named Clyde Tombaugh found a very faint planet near Lowell’s predicted position 

The Discovery of Pluto

Observing Pluto  Through most telescopes Pluto simply appears as a faint star    

Pluto Facts  Size: 2300 km    Orbit: 39.5 AU   Description:

Pluto’s Orbit  Pluto’s orbit is much more eccentric and much more inclined than any planet     Pluto’s orbit carries it inside the orbit of Neptune   Pluto is tipped on its side like Uranus

Composition of Pluto  Pluto has a density of 2000 kg/m 3   Pluto is probably composed of ice and rock  Spectra of Pluto reveal the presence of methane, nitrogen and carbon monoxide  

HST Images Pluto

Features of Pluto    The other bright regions may be areas where impacts have gouged out fresh ice

Pluto and Charon

Charon  Pluto’s moon Charon was discovered as a small bulge in a high resolution image (1978)    Pluto and Charon are in a close, tidally locked orbit  

Where Do Comets Come From?  Comets are small (few km) icy bodies that sometimes come in to the inner solar system on highly elliptical orbits  Short period comets     Long period comets   

The Kuiper Belt  Around 1950 Kuiper and Edgeworth proposed a belt of comets out beyond Neptune   In 1992 the first (besides Pluto) Kuiper belt object was discovered (QB 1 )  

The Kuiper Belt

Discovering Kuiper Belt Objects

The Known Kuiper Belt  There are now hundreds of known Kuiper Belt Objects (KBOs)   Total population of large KBO’s may be (larger than 100 km)  Kuiper belt seems to end at about 50 AU   Larger and larger KBO’s being detected

Eris  The largest KBO currently known is called Eris   Larger than Pluto   Semi-major axis of 68 AU, but is currently at 97 AU due to high eccentricity    Has a small moon, Dysnomia 

Large KBO Size Comparisons

Is Pluto a Planet?  Pro       Con     

What Makes Something a Planet? Planets used to be obvious   Needed new definition when rest of solar system was discovered with telescopes   The International Astronomical Union (which has authority over these things) removed Pluto from the list of planets in 2006  

The Oort Cloud  In 1950 Dutch astronomer Jan Oort postulated a spherical shell of comets surrounding the solar system at about 50,000 AU   

Population of the Oort Cloud  The Oort cloud is the source of the long period comets   They are too far away to see, so we only have indirect methods of studying them 

Diagram of the Oort Cloud

Summary  Past the orbit of Neptune the solar system is made up of many small icy bodies  Kuiper Belt  extends from AU  formed from left over planetesimals at the edge of the solar system  Oort Cloud  extends from ,000 AU  formed from ejected icy planetesimals

Summary: Pluto  Description: small, cold, distant  Pluto resembles a large Kuiper belt object more than a planet  Has a closely orbiting large moon Charon  Properties  Thin atmosphere  Very cold (~50 K)  Bright surface features possibly composed of fresher ice