What Makes a Planet
Three small bodies have been found in orbit around the pulsar PSR 1257+12. They have been designated "PSR1257+12 A, ..B, and ..C". One is about the size of the Moon, the other two are about 2 to 3 times the mass of Earth.
There is so much information on the web we chose to reprint this article. Hope you enjoy the discussion (Dwight 2001)
By the simplest definition, a planet is a large object that orbits a star. But the universe, it seems, is hardly simple, and astronomers have lately been observing things that resemble planets but are drifting freely in space, beyond the gravitational embrace of any star. Last year, two groups of astronomers reported detecting a large number of "free-floating planets" in the Orion Nebula. At an astronomy meeting in Cambridge, England, this month, one of the groups announced that new infrared observations had confirmed the existence of at least 20 such objects in Orion, an active star- forming region more than 1,000 light- years from Earth. Dr. Philip Lucas of the University of Hertfordshire and Dr. Patrick Roche of Oxford University reported the detection of extremely hot water vapor in the objects, which they said confirmed that the objects are young and planetlike.
The faintest of the objects, the astronomers said, appear to be the size of large planets, between 5 and 13 times as massive as Jupiter. They could be detected by the United Kingdom Infrared Telescope on Mauna Kea in Hawaii only because they are young and still warm. Other observations, including those by the Hubble Space Telescope, have produced evidence that such objects, too small to be stars, yet larger than most planets and independent of any stellar host, may be a fairly common phenomenon. But are they planets? That is a question that has astronomers taking sides in often heated debate.
Dr. Mark McCaughrean, of the Astrophysics Institute in Potsdam, Germany, complained of the "unfortunate continued use of the word `planet' where it's unwarranted." It is not just a question of nomenclature, but of formation processes. In an article to be published soon in The Astrophysical Journal (Letters), Dr. Alan P. Boss of the Carnegie Institution in Washington theorized certain conditions under which the same processes of stellar creation could produce small, free-floating objects as small as one Jupiter mass. Jupiter's mass is 318 times as great as Earth's. Such objects with masses below 13 Jupiter masses, Dr. Boss suggested, would not be planets because of the way they formed. They could more appropriately be termed sub-brown dwarfs, neither planet nor star but a new class of in-between objects.
A planet is assumed to form out of the rocky material and gas left in an orbiting disk around a new star. If these objects were formed in that way, scientists must figure out how they were thrown out of their orbits around a star. A star emerges from the collapse of a heavy molecular cloud. When it exceeds a mass 75 times that of Jupiter, it is big enough to be a full- blown star powered by thermonuclear forces. Lesser objects that are formed this way, essentially failed stars, are called brown dwarfs, because their feeble energies give off little light.
So if these newly discovered free- floating objects were indeed formed like a star or a brown dwarf, from a gas cloud in space, then it is misleading to call them planets, Dr. McCaughrean said. He joined a number of astronomers who said they preferred the name sub-brown dwarf. Dr. McCaughrean said he suspected the motives for using the term planet. "Planets are special to us as humans," he said. "We live on one, perhaps there's extraterrestrial life on other planets. By saying `planet,' you can raise enormous public interest." Other scientists cautioned that it was premature to coin new names and more classifications for the objects. Whatever the outcome of the debate, Dr. Roche said, the study of these strange objects "can aid our understanding of the star-formation process, which is one of the major mysteries in astronomy."
The faintest of the objects, the astronomers said, appear to be the size of large planets, between 5 and 13 times as massive as Jupiter. They could be detected by the United Kingdom Infrared Telescope on Mauna Kea in Hawaii only because they are young and still warm. Other observations, including those by the Hubble Space Telescope, have produced evidence that such objects, too small to be stars, yet larger than most planets and independent of any stellar host, may be a fairly common phenomenon. But are they planets? That is a question that has astronomers taking sides in often heated debate.
Dr. Mark McCaughrean, of the Astrophysics Institute in Potsdam, Germany, complained of the "unfortunate continued use of the word `planet' where it's unwarranted." It is not just a question of nomenclature, but of formation processes. In an article to be published soon in The Astrophysical Journal (Letters), Dr. Alan P. Boss of the Carnegie Institution in Washington theorized certain conditions under which the same processes of stellar creation could produce small, free-floating objects as small as one Jupiter mass. Jupiter's mass is 318 times as great as Earth's. Such objects with masses below 13 Jupiter masses, Dr. Boss suggested, would not be planets because of the way they formed. They could more appropriately be termed sub-brown dwarfs, neither planet nor star but a new class of in-between objects.
A planet is assumed to form out of the rocky material and gas left in an orbiting disk around a new star. If these objects were formed in that way, scientists must figure out how they were thrown out of their orbits around a star. A star emerges from the collapse of a heavy molecular cloud. When it exceeds a mass 75 times that of Jupiter, it is big enough to be a full- blown star powered by thermonuclear forces. Lesser objects that are formed this way, essentially failed stars, are called brown dwarfs, because their feeble energies give off little light.
So if these newly discovered free- floating objects were indeed formed like a star or a brown dwarf, from a gas cloud in space, then it is misleading to call them planets, Dr. McCaughrean said. He joined a number of astronomers who said they preferred the name sub-brown dwarf. Dr. McCaughrean said he suspected the motives for using the term planet. "Planets are special to us as humans," he said. "We live on one, perhaps there's extraterrestrial life on other planets. By saying `planet,' you can raise enormous public interest." Other scientists cautioned that it was premature to coin new names and more classifications for the objects. Whatever the outcome of the debate, Dr. Roche said, the study of these strange objects "can aid our understanding of the star-formation process, which is one of the major mysteries in astronomy."