Discovering Pluto
Six of the planets in our solar system were known to the ancients, since they are bright enough to be seen wandering among the stars at night even with the unaided eye. It was therefore a big deal whenever a new planet was discovered. William Herschel's discovery of Uranus in 1781 while he was sweeping the sky with his telescope gave him lasting fame. The discovery of Neptune in 1846 was based on mathematical predictions, based on deviations of Uranus from a standard, elliptical orbit. In the early twentieth century, Percival Lowell, in the United States, was interested in finding Planet X, a ninth planet in our solar system. He set up the Lowell Observatory in Flagstaff, Arizona. After Lowell himself died, a young amateur astronomer, Clyde Tombaugh, was hired by the Lowell Observatory to carry out a search for Planet X. He took thousands of photographic plates, each containing many thousands of star images. He took pairs of plates separated by a brief time, and "blinked" between them using an optical machine that made objects stand out if they weren't the same on both plates. (He usually also took a third plate, to help rule out mere flaws in the photographic emulsions.) In 1930, Tombaugh found the object that was soon named Pluto. (The name has been credited, interestingly, to a young English girl who sent it in, though the name was undoubtedly already under consideration at the Lowell Observatory.)
Weighing Pluto
Even at its discovery, Pluto was not as expected. Its orbit was inclined much more than that of the other planets, and it was far fainter than Planet X had been expected to be, indicating that it was smaller. But for a long while, up through 1968, it was thought to have perhaps 90% the mass of Earth. It is about 2,250 km (about 1400 miles) in diameter.
To find the mass of a celestial body, one finds its gravitational effect on another body. In particular, when a moon was discovered around Pluto in 1978, comparing its orbit with its distance from Pluto gave Pluto's mass accurately. (The ground-based images did not show Pluto and its moon distinctly separated, but the observations were clear enough to allow an approximate distance between them to be measured and so an approximate mass to be found, actually the mass of the two together.) Surprisingly, Pluto's mass was only 1/500th that of Earth, making it an almost negligible object and far less massive than would have been needed to provide the gravitational pull on Uranus that had been thought to lead to Pluto's discovery. It is now thought that it was only Tombaugh's careful search over a wide range of sky that led to Pluto's discovery, rather than the mathematical calculations that had led him to concentrate in certain areas of sky.
When the Hubble Space Telescope was able to observe Pluto and its moon Charon in 1990, it allowed the two objects to be seen distinctly separated for the first time. These observations allowed Pluto's mass to be measured definitively at 1/500th that of Earth.
Pluto's atmosphere
Pluto has an atmosphere, and it can be best detected when Pluto passes (as seen from Earth) in front of a star, which it does rarely. When the starlight passes through Pluto's atmosphere, it is bent and distorted, and careful measurements with telescopes on Earth can invert the situation to discover the temperature, density, and other characteristics of Pluto's atmosphere.
A lone occultation (as such events are known) observation from 1988 wasn't repeated until 2002, when several teams observed an occultation from the Mauna Kea Observatory in Hawaii. If Pluto had no atmosphere, the starlight would be distinguished abruptly when Pluto's advancing disk apparently covered the point image of the star. But since Pluto has an atmosphere, the amount of starlight diminished gradually, and occasional bright spikes showed a focusing effect of layers of the atmosphere. It turned out that Pluto's atmosphere had warmed a bit since the 1988 observation, demonstrating global warming on Pluto.
A light curve (that is, a graph of intensity of starlight on the vertical axis vs. time (shown by sample #) on the horizontal axis. Over an interval of a few minutes, the starlight was seen to dim gradually in this graph of the August 20, 2002, observations from the Mauna Kea Observatory by a Williams College-MIT team of an occultation of a star by Pluto.
It was a bit of a surprise that Pluto's atmosphere had warmed, since Pluto had passed perihelion (the near point to the sun in its orbit) in 1989, but just as daily temperatures on Earth peak about 4 pm instead of at noon, there is a lag in the peak temperature on Pluto.
Observations of a series of occultations of stars by Pluto have monitored Pluto's atmosphere, and its changing appears to have leveled off. Several groups of scientists--one from Williams College and MIT, one from the Southwest Research Institute, and one from the Observatory of Paris--continue to monitor Pluto's atmosphere through occultation studies.
Pluto's atmosphere is largely nitrogen, as is Earth's. Methane has been discovered spectroscopically on its surface, so it must exist in the atmosphere as well. The atmospheric pressure, in any case, is only about 1/100,000 that of Earth.
Pluto's moons
The discovery of Pluto's moon, named Charon, in 1978 provided the breakthrough that allowed Pluto's mass to be assessed accurately. Charon is a substantial object, about half Pluto's diameter, so Pluto-Charon was often called a double-planet system, especially since the center of mass of the system lay between the two objects rather than within Pluto. It wasn't until 2005 that careful observations with the Hubble Space Telescope discovered two more moons around Pluto, though they are much smaller and less massive. Nix and Hydra may be less than 100 km across, compared with Charon's approximately 600 km (about 380 miles) diameter.
Some of the occultation studies of Pluto and Charon have been carefully scanned to see if other moons or if rings around Pluto cause the star's signal to briefly diminish, but no such dropouts have been found.
Charon occulted a star, as observed with several large telescopes in Chile in 2005, when the event was fortuitously visible from some of the world's largest optical instruments. To a high degree of certainty, no atmosphere was found around Charon. Charon was also observed to occult a star in observations from Reunion Island in the Indian Ocean in 2008.
Space Mission to Pluto
NASA launched its New Horizons mission to Pluto but it won't get there until 2015.
NASA's New Horizons mission to Pluto was launched in 2006. It got a gravity assist by Jupiter, testing its cameras successfully on the Jupiter system, in 2007. It is now in hibernation until it is almost at Pluto in 2015.
http://www.nasa.gov/mission_pages/newhorizons/news/index.html
Classifying Pluto
Pluto has long been known to be so different from the longer-known planets--Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune--with a much more elliptical orbit and a much more inclined orbit--that experts in the field have been asking for its reclassification. Since the 1990's, several objects were found in the outer solar system beyond Neptune that, similarly to Pluto, had elliptical and inclined objects. (Advantages in astronomical imaging through, especially, electronic cameras and computer data reduction have led to these discoveries.) These objects are what has become known as the Kuiper belt (with "kui" pronounced to rhyme with "coy"), so they are Kuiper-belt objects or Trans-Neptunian Objects (TNO's).
Michael Brown at Caltech has been a leader recently in finding Trans-Neptunian Objects, a few of which rival Pluto in size (and therefore presumably in mass). Once an object was found that was larger than Pluto, it became difficult to maintain Pluto's full planethood without also promoting the other objects--and perhaps eventually dozens of such objects as they are discovered--to the list of planets.
So the question became whether to stop the list of planets at 8, with Neptune, or to allow new planets to be allocated spaces on the list. The question was sometimes put in terms of whether third-graders should be asked to remember 13 or 20 or 50 planets, or whether it wasn't better for them to have 8 planets and then a bunch of other objects.
The question came to a head with the discovery of Michael Brown of an object larger than Pluto, originally given the catalogue number 2003 UB313. How to name this object?
The International Astronomical Union (IAU) is in charge of naming celestial objects, but different IAU committees would be responsible, depending on whether Pluto was an asteroid (or Kuiper-belt object) or a planet. The IAU's President convened a committee, but they didn't agree definitively. Accordingly, a Planet Naming Committee, under Prof. Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics, was given the charge of reporting to the General Assembly of the International Astronomical Union in Prague, Czech Republic, in August 2006.
In the event, there was much wider public interest in the naming of the new object and thus the definition of a planet than had been anticipated. Though the IAU General Assembly covered many astronomical topics, much of the worldwide publicity became centered on the question of how to classify Pluto.
After discussions and sessions for the membership during the meeting, proposals were made for the final General Assembly. The Chair of the Planet Naming Committee had already left and many others interested in the topic had also left by this final day. Further, the IAU is skewed to astronomers interested in celestial motions and dynamics compared with those in
terested in planetary features like surfaces and atmospheres; many "planetary scientists" do not consider themselves to be astronomers, for example, and don't attend the IAU meetings.
The proposals were amended from the floor, and the meeting was a bit of a runaway. In any case, the result was that a new class of "dwarf planet" was set up, with Pluto as its most prominent member. The criteria for being a dwarf planet included being massive enough so that gravity made it round, and that it "cleared its orbit," whatever that meant. Charon and the asteroid Ceres also met the criteria, so the three objects were the first three dwarf planets. The new object was soon named Eris, after the goddess of discord. Its satellite was named Dysnomia, after the goddess of lawlessness. (This choice was something of a pun based on Mike Brown's use of the name Xena, from a cable television series "Xena, Warrior Goddess," as an informal name for the object pending the IAU's selection of the final name, given that the actress playing Xena was Lucy Lawless.)
The decision met with a worldwide outcry, with some prominent astronomers vowing not to accept the decision. In some sense, such a decision comes to textbook authors rather than decisions of organizations like the IAU. Newspapers around the world printed that Pluto was "demoted," though many astronomers would say that it was "promoted" to being the first and most prominent member of a new class of astronomical object instead of a minor and diminutive member of an older class. Pluto eventually received the asteroid classification of 134340, a number without distinction. (It had previously been offered the round number 10,000, which was turned down in favor at the time of leaving it as only a full-fledged planet. Many more asteroids had been discovered since then, bringing the number above not only 10,000 but even 100,000.)
Among the prominent objectors to the new classification were Alan Stern, the head of the New Horizons Mission, and Mark Sykes, of the Planetary Science Institute. They convened a Great Planet Debate (http://gpd.jhuapl.edu) in August 2008 at the Johns Hopkins University Applied Physics Laboratory, which runs the mission. But the next occasion that any official step could be made to change or reverse the classification of "dwarf planet" is at the General Assembly of the International Astronomical Union to be held in Rio de Janeiro in August 2009. The scientists convened at the Great Planet Debate did not reach a consensus about a definition, nor was there overwhelming sentiment to bring the matter before the next IAU meeting, in part because of the widely expressed sentiment that since the IAU had gotten the definition wrong once, why give them a chance to get it wrong again. But making "dwarf planet" a subcategory of "planet," to contrast with "classical planet" for Mercury through Neptune," was under consideration, in spite of its having been rejected at the Prague IAU General Assembly in 2006.
In 2008, Pluto was declared by the IAU to be the first of a class of dwarf planets known as "plutoids," to distinguish them from Ceres, which is in the asteroid belt. Officially, "Plutoids are celestial bodies in orbit around the sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit." The other officially accepted plutoids are Eris and Makemake.
References
Jay M. Pasachoff and Alex Filippenko, The Cosmos: Astronomy in the New Millennium, 3rd ed., 2007, Cengage Publishing. (www.solarcorona.net)
Jay M. Pasachoff, Peterson Field Guide to the Stars and Planets, 4th ed., 2000, revised 2006, Houghton Mifflin. (www.williams.edu/astronomy/fieldguide)
Jay M. Pasachoff and Michael A. Seeds, Pluto: A Case Study in Science, 2007, Cengage Publishing. ISBN-13: 978-0-495-38405-2. (www.solarcorona.net)
Alan Stern and Jacqueline Mitton, Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, 2nd ed., 2005, John Wiley and Sons.
David A. Weintraub, Is Pluto a Planet? A Historical Journey through the Solar System, 2006; paperback revised 2009. Princeton University Press. (http://press.princeton.edu/titles/8247.html)
Web references
International Astronomical Union resolution
http://www.iau.org/public_press/news/release/iau0603/
Great Planet Debate (2008)
http://gdp.jhuapl.edu
sebastien R.
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