The planetary scientist says that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.
The world's astronomers finally voted today on the highly controversial issue of how to define a planet. The official definition means Pluto is no longer a planet. NPR's David Kestenbaum reports on the pandemonium in the convention halls of Prague, where the astronomers are meeting.
If you have a little model of the solar system at home, you can break off the outermost planet. Astronomers today gave Pluto the heave ho, voting in an official definition for the word planet that leaves Pluto out. The term planet has been around since Ancient Greece, but coming to consensus on a precise definition today proved challenging, even for the world's top scientific minds.
KESTENBAUM: The definition had been changed many times since the meeting began. Last week an official panel recommended keeping Pluto as a planet and adding at least three, perhaps 40 more small objects. But that proposal, which had been drafted in secret, led to an uproar and many, many revisions. Today Astronomer Richard Binzel stood at the podium and read the final proposal so it could be voted on.
KESTENBAUM: Tiny Pluto would not count. It doesn't have the mass to kick its neighbors out. The suburbs of the solar system are filled with small, icy objects, including Pluto. But astronomers continued to debate. Question, what about planets outside our solar system? Answer, this definition is just for our solar system. Question, and what about Neptune? It's not fully alone in it's orbit.
KESTENBAUM: The resolution had a footnote explicitly naming the eight planets. Well, someone said, why not drop the complicated definition and just leave the footnote? Everybody laughed. Then Michael Rowan-Robinson stood up to try to move things along. He's an astronomer and serves as the official representative of the United Kingdom.
KESTENBAUM: And Pluto was out. The resolution does make Pluto one of a number of dwarf planets, which confusingly are not officially planets. So there are eight planets in the solar system, not nine and not 10. Until today, Mike Brown at Cal Tech could claim credit for finding what was arguably the tenth planet. It's been dubbed Xena, and it's bigger than Pluto. It, too, is out.
Mr. BROWN: This is great. This is the definition that we should have had all along. When people think of the word planet, they think of large, special objects, and when you tell people about Pluto and make them realize that Pluto really is quite small, people even start to realize well, okay, maybe Pluto shouldn't be called a planet.
1905: Percival Lowell starts the search for Planet X. The planets, including the newly discovered Neptune, didn't move around the sun in quite the way gravitational laws predicted, and Lowell proposes that an undiscovered planet must be the reason why. He never finds Planet X before his death in 1916.
Feb. 18, 1930: Clyde Tombaugh takes up the search in 1929 at Lowell's observatory and proves that discovering new planets is not glamorous work. For a year, he photographs the same section of sky several nights apart and then searches the images for any objects that move like a planet should. On Feb. 18 he looks at his photographic plates and knows right away that one of the dots is Planet X.
May 1930: A little girl in Britain interested in Greek and Roman mythology tells her grandfather over breakfast that the new planet should be named Pluto. He cables the Lowell Observatory, and they unanimously vote for the name because Pluto is the god of the underworld, which seems appropriate for such a cold and remote planet, and the first two letters of Pluto are Percival Lowell's initials.
Aug. 30, 1992: Pluto's tiny size didn't disqualify it from being a planet, but then David Jewitt, an astronomer at the University of Hawaii, and Jane Luu, a researcher at the University of California, Berkeley, discover Pluto isn't the only chunk of rock and out there in the Kuiper Belt beyond Neptune. Scientists have found hundreds of these objects since 1992, so some astronomers start to think that maybe Pluto isn't so special after all.
Feb. 3, 1999: Pluto's debated status as a planet gets publicity. The International Astronomical Union calms stargazers worried by recent media reports saying the IAU is planning to demote Pluto. Not so, they say in a press statement. They only want to include it in their numbering system for Kuiper Belt objects.
May 11, 2000: Scientists may debate whether Pluto is a planet, but it's place in the classical music canon gets secured. Composer Colin Matthews writes a movement for Pluto into Gustav Holst's The Planets. Although Pluto was discovered in Holst's lifetime, he declined to add it to his suite.
Jan. 5, 2005: Michael Brown, a planetary scientist at the California Institute of Technology, discovers what might be the 10th planet, Xena. He says it's rocky and icy like Pluto. When he announces his discovery on July 29, he forces astronomers to decide what makes a planet.
Oct. 31, 2005: The Hubble Space Telescope Pluto Companion Search Team discovers that Pluto has three moons, not just one. Moons don't qualify an object to be a planet, but having a couple moons doesn't hurt Pluto's case.
April 11, 2006: The Hubble Space Telescope finds that Xena is slightly larger than Pluto. Astronomers now have to make a decision: either Xena and Pluto are both planets or neither is a planet.
August 24, 2006: The International Astronomical Union strips Pluto of its planetary status. The group says a planet must, among other things, have "cleared the neighborhood around its orbit." Because Pluto's orbit overlaps Neptune's, Pluto is out. The celestial body formerly known as the ninth planet will be reclassified as a "dwarf planet."
What few people remember is why the definition of a planet had to be revisited in 2006. The real reason was a recently discovered object called 2003 UB313, at first nicknamed Xena then later re-named as Eris.
But this definition baffled the public and classrooms around the country. For one thing, it only applied to planets in our solar system. What about all those exoplanets orbiting other stars? Are they planets? And Pluto was booted from the planet club and called a dwarf planet. Is a dwarf planet a small planet? Not according to the IAU. Even though a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster.
On Sept. 18, 2014, audience members who attended the Observatory Night talk "What is a Planet?" voted to choose one of three possible definitions for a planet. The result: a planet is "the smallest spherical lump of matter than formed around stars or stellar remnants," and Pluto IS a planet!
"Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Xena," said Mike Brown, planetary scientist at the California Institute of Technology in Pasadena, Calif. Brown's research team discovered Xena, and their results have been accepted for publication in the Astrophysical Journal.
Finding that the largest known KBO is a virtual twin to Pluto may only further complicate the debate about whether to categorize the large icy worlds that dwell in the Kuiper Belt as planets. If Pluto were considered to be the minimum size for a planet, then Xena would fulfill this criterion, too.
On July 14, 2015 at 4:49 a.m. PDT, NASA's New Horizons spacecraft sped past Pluto -- a destination that took nearly nine and a half years to reach -- and collected scientific data along with images of the dwarf planet.
Pluto, famous for once being the ninth planet, was reclassified as a dwarf planet in 2006 after new information emerged about the outer reaches of our solar system. Worlds similar to Pluto were discovered in the region of our solar system known as the Kuiper Belt. The Kuiper Belt --named for astronomer Gerard Kuiper --is a doughnut-shaped area beyond the orbit of Neptune that is home to Pluto, other dwarf planets such as Eris, Makemake, and Haumaea, as well as hundreds of thousands of other large icy bodies, and perhaps trillions of comets orbiting our sun. Over the next several years, the New Horizons spacecraft is expected to visit one to two more Kuiper Belt objects.
The results, positive and negative, aid the handful of observers now hunting for Planet Nine on telescopes. In addition to the groups working on Subaru, Sheppard and Trujillo are leading searches in the high desert of Chile, in case the planet is easier to see from the Southern Hemisphere. There, both the 570-megapixel Dark Energy Camera (DECam) on the 4-meter Blanco telescope and the 6.5-meter Magellan telescopes are contributing to the hunt.
Like Pluto, 2003 UB313 is one of the icy bodies in the so-called Kuiper belt that exists beyond Neptune. It is the most distant object ever seen in the Solar System. Its very elongated orbit takes it up to 97 times farther from the Sun than is the Earth - almost twice as far as the most distant point of Pluto's orbit - so that it takes twice as long as Pluto to orbit the Sun. When it was first seen, UB313 appeared to be at least as big as Pluto. But an accurate estimate of its size was not possible without knowing how reflective it is. A team lead by Prof. Frank Bertoldi from the University of Bonn and the Max Planck Institute for Radio Astronomy (MPIfR) and the MPIfR's Dr. Wilhelm Altenhoff has now resolved this problem by using measurements of the amount of heat UB313 radiates to determine its size, which when combined with the optical observations also allowed them to determine its reflectivity. "Since UB313 is decidedly larger than Pluto," Frank Bertoldi remarks, "it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status." 041b061a72