Physics Photo of the Week

October 15, 2010

Pluto - Photos by Contemporary Astronomy Students: Max Hunt, Zoe MacLellan, Edward Rubin, Qi Shen, and Chelsea Ostiguy
Discussion by Edward Rubin

On October 6-7, Warren Wilson students helped identify Pluto within the constellation Sagittarius.  Using an 8-inch telescope and deep sky CCD camera, photos were taken of star fields at the location predicted by Pluto's ephemeris.  As you can see Pluto is very difficult to see among the brighter stars.  Pluto shows up no brighter than over 1000 stars in the photo. This is because of the low light magnitude Pluto gives off due to its vast distance from the sun.  Pluto is tiny (about 2300 kilometers in diameter - about 2/3 the diameter of Earth's Moon) and far from the sun (about 39 times farther away than Earth); therefore it reflects very little light. 

Even when generating an image of Pluto, it is difficult to distinguish its light from that of other stars.  This is why it took two nights of tracking and careful identification of the star field to find Pluto. By "blinking" a photo of the star field on October 6th with a photo of the same star field on October 7th, we are able to see Pluto’s direct motion on the celestial sphere.  See the animated photo at right. 

All planets in the Solar System follow a direct (eastward) motion around the Sun.  Pluto is so far away that it appears relatively fixed in its position. 

Pluto is a dwarf planet located in the section of the Solar System known as the Kuiper Belt.  It is the second largest dwarf-sized planet (after Eris) in the solar system, and is the tenth largest body observed directly orbiting our sun.  Since its discovery in 1930, its classification as the 9th planet had been debated by scientists until 2006, when the International Astronomical Union formally classified it as a dwarf planet.  Pluto's surface is one of the coldest places in our solar system. Astronomers believe the temperature on Pluto may be about –375 °F (–225 °C). The planet appears to be partly covered with frozen methane gas and contains a thin atmosphere composed mostly of methane. Because Pluto's density is low, astronomers think Pluto is mainly icy.

In 1905, Percival Lowell found that the force of gravity of some unknown object affected the orbits of Neptune and Uranus. In 1915, he predicted the location of a new planet and began searching for it from his observatory in Flagstaff, Arizona. In 1929, Clyde W. Tombaugh used predictions made by Lowell and other astronomers to photograph the sky with a more powerful, wide-angle telescope. Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs (the exact process shown in the Physics Photo). On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement of what is now known as Pluto.

Technology has enabled routine photography of Pluto.  Pluto's orbit is well-studied, permitting an accurate prediction of its coordinates for any date.  The blink comparison is very simple with appropriate software with digital images.  The blink comparator in 1930 was a cumbersome, very expensive, optical-mechanical device.

There will be no Physics Photo of the Week published next week, October 22, 2010, due to Fall Break at Warren Wilson College.  The next Physics Photo will be published on Friday, October 29, 2010.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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