Kennelful of Pluto puppies

Pick of the Pics and Updating Magic Universe

A kennelful of Pluto puppies

Three successive images from the Hubble Space Telescope show two remote objects in the Solar System inching across the sky in front of a distant galaxy (bottom left of each image). The near-vertical streaks are due to the objects moving while Hubble was watching. They are small “trans-Neptunian” objects – comets or asteroids – orbiting the Sun at about 43 times farther out than the Earth. They appear to be companions, at about half the separation of the Earth and Moon. Credits: a negative version of part of Fig. 3 in Fuentes et al., Astrophysical Journal (see references); imagery from HST/ACS/WFC

Soon to be published is the discovery of 14 new members of the Solar System in the so-called Kuiper Belt beyond the most distant “real” planet, Neptune. They are 40-100 km leftover scraps from the building of the Sun’s family of planets. To find them, a team from the Harvard-Smithsonian Center for Astrophysics and Northern Arizona University, led by Carlos Fuentes, trawled through existing images from the Hubble Space Telescope.

As Halley’s Comet and 15 other regular visitors came from the trans-Neptunian Kuiper Belt, I suppose I should be updating my Comets book, but although it mentions “Halley-class” comets orbiting not far beyond Neptune, it doesn’t name the Kuiper Belt. That important feature figures in Magic Universe, which was written two decades later and is more receptive to updating on this point. Here’s the most relevant section in the story called “Comets and asteroids: snowy dirtballs and their rocky cousins”.

Kicking over the boxes

To some experts, the idea of a link between comets and asteroids seemed repugnant. Since the first asteroid, Ceres, was discovered by Giuseppe Piazzi of Palermo in 1801, evidence piled up that asteroids were stony objects, sometimes containing metallic iron. They were mostly confined to the Asteroid Belt beyond Mars, where they went in procession around the Sun in well-behaved, nearly circular orbits.

Two centuries after Piazzi’s discovery the count of known objects in the Asteroid Belt had risen past the 40,000 mark. In 1996-97, Europe’s Infrared Space Observatory picked out objects not seen by visible light. As a result, astronomers calculated that more than a million objects of 1 kilometre in diameter or larger populate the Belt. Close-up pictures from other spacecraft showed the asteroids to be rocky objects, probably quite typical of the material that was assembled in the building of the Earth and Mars.

What could be more different from the icy comets? When they are not confined to distant swarms, comets dash through the inner Solar System in all directions and sometimes, like Halley’s Comet, go the wrong way around the Sun — in the opposite sense to which the planets revolve.

Scientists have a strong urge to place Mother Nature’s objects into neat boxes,’ Donald Yeomans of NASA’s Jet Propulsion Laboratory commented in 2000. ‘Within the past few years, however, Mother Nature has kicked over the boxes entirely, spilling the contents and demanding that scientists recognize crossover objects — asteroids that behave like comets, and comets that behave like asteroids.’

Besides Phaeton, and other asteroidal candidates to be dead comets, Yeomans’ crossover objects included three objects that astronomers had classified both as asteroids and comets. These were Chiron orbiting between Saturn and Uranus, Comet Wilson-Harrington on an eccentric orbit, and Comet Elst-Pizarro within the Asteroid Belt. In 1998 a stony meteorite — supposedly a piece of an asteroid — fell in Monahans, Texas, and was found to contain salt water. Confusion grew with the discovery in 1999 of two asteroids going the wrong way around the Sun, supposedly a prerogative of comets.

Meanwhile the remote planet Pluto turned out to be comet-like. Pluto is smaller than the Earth’s Moon, and has a moon of its own, Charon. When its eccentric orbit brings it a little nearer to the Sun than Neptune, as it did between 1979 and 1999, frozen gases on its surface vaporize in the manner of a comet — albeit with unusual ingredients, mainly nitrogen. For reasons of scientific history, the International Astronomical Union nevertheless decided to go on calling Pluto a major planet.

In 1992, from Mauna Kea, David Jewitt of Hawaii and Jane Luu of UC Berkeley spotted the first of many other bodies in Pluto’s realm. Orbiting farther from the Sun than the most distant large planet, Neptune, these transneptunian objects are members of the Edgeworth-Kuiper Belt, named after astronomers who speculated about their existence around 1950.

An artist's concept of a trans-Neptunian object, far from the Sun. Credit: NASA, ESA, and G. Bacon (STScI)

Some 300 transneptunians were known by the end of the century. There were estimated to be perhaps 100,000 small Pluto-like objects in the belt, and a billion ordinary comets. If so, both in numbers and total mass, the new belt far surpasses what has hitherto been called the main Asteroid Belt between Mars and Jupiter.

These discoveries are something we could barely have guessed at just a decade ago,’ said Alan Stern of the Southwest Research Institute, Colorado. ‘They are so fundamental that basic texts in astronomy will require revision.’ One early inference was that comets now on fairly small orbits around the Sun did not originate from the Oort Cloud, as previously supposed, but from the much closer Edgeworth-Kuiper Belt. These comets may be the products of collisions in the belt, as may Pluto and Charon. A large moon of Neptune, called Triton, could have originated there too. .

Update 2010

In 2003 Eris, 2400 kilometres in diameter, turned up as the first trans-Neptunian object larger than Pluto, which at last became demoted to a “dwarf planet”. The count of known trans-Neptunian grew in 2010, with more than a dozen additions coming from searches of existing images of the Hubble Space Telescope. A kennelful of Pluto puppies, if you will. And that initial study examined only one-third of a square degree of the sky, in a narrow band around the ecliptic – the plane in which major planets orbit.

“We have proven our ability to detect and characterize trans-Neptunian objects even with data intended for completely different purposes,” said Carlos Fuentes of Northern Arizona University, who led the research. He and his team regarded their trawl as only just beginning, with perhaps hundreds more objects waiting to be discovered in the Hubble archive.


Cesar I. Fuentes et al. Astrophysical Journal in press. Preprint available here

Fuentes quoted in Harvard-Smithsonian Center for Astrophysics (CfA) press release

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5 Responses to Kennelful of Pluto puppies

  1. [...] Kennelful of Pluto puppies « Calder's Updates [...]

  2. Neptune is NOT the most distant “real” planet, and Pluto is only marginally comet like. First, I urge you not to accept the controversial demotion of Pluto by four percent of the IAU as fact when it is not. It is only one side of an ongoing debate, and it was opposed by hundreds of professional astronomers in a formal petition led by Dr. Stern. Unlike comets, Pluto is 75 percent rock, is rounded by its own gravity (hydrostatic equilibrium), is geologically differentiated, and has a very stable orbit that never takes it into the inner solar system. Dwarf planet is a term coined by Stern to indicate a third class of planets, objects large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits. It is a misappropriation of the term to say that dwarf planets are not planets at all.

    • Lystrosaurus_science says:

      If you really think that hydrostatic equilibrium things are planets and that you are not just one of those who only want to keep TNO Pluto as a planet because it makes them sad that it is not considered as a planet anymore, I am surprised that you did say nothing when he called Ceres an “asteroid”. If you know something more about this subject you should have known that Ceres is a hydrostatic equilibrium object and geologically differentiated. Vesta too is eologically differentiated and almost round but a bit flattened (This may because of the large impact on Vesta that flattened Vesta and created Rheasilvia crater on it some billion years ago.). And by the way was it so that Jupiter IV (Callisto) is hydrostatic equilibrium but only partially differentiated. And it is a body much more massive than that the too much famous TNO, Pluto. And Vesta has a structure similar to the terrestrial planets and it is less massive than Jupiter IV (Callisto) that is just only partially differentiated. If this more massive body is less differentiated than Vesta then why would you call hydrostatic equilibrium things planets but say nothing about the nicely differentiated Vesta. Or do you think that Vesta is hydrostatic equilibrium? In any case you said nothing about Vesta or Ceres in your post. I have more to say about the hydrostatic equilibrium thing. Saturn I (Mimas) is hydrostatic equilibrium. It is composed mostly ice and only a bit of rock. It IS hydrostatic equilibrium and also mostly ice. It is just 396 km in diameter. Neptune VIII (Proteus) is larger (diameter 420 km) and more massive than Saturn I (Mimas) but it is NOT hydrostatic equilibrium. Both bodies Neptune VIII (Proteus) and Saturn I (Mimas) are objects made of ice and rock. Perhaps Saturn I (Mimas) is hydrostatic equilibrium only because of the the higher temperature near Saturn or tidal heating. This would indicate that the place where the body is DOES affect its physical characteristics. And if I remember right, I’m sorry if I don’t, Dr. Stern was so keen on making a definition for a word “planet” that would not be affected about the place where the object is. In other words he did not succeed to make a definition that he said he would want to make. If you would take Neptune VIII (Proteus) away from the orbit around Neptune and place it orbit around Saturn that would be same that Saturn I (Mimas) has now maybe Neptune VIII (Proteus) higher temperature or tidal heating would make Neptune VIII (Proteus) hydrostatic equilibrium and a planet according to Dr. Stern’s definition for a word “planet”. And the physical characteristics of Neptune VIII (Proteus) other than “roundness” would not change in the process, you would not change it’s mass, you would only change the place where it is.

  3. [...] Kennelful of Pluto puppies « Calder's Updates [...]

  4. Ceres is not an asteroid; it is clearly a small planet. Vesta and Pallas are borderline cases, but Dawn has shown that Vesta is far more planet than asteroid. We need to expand the categories of planets and create some new ones; for example, objects like Vesta should be classed as something like “sub-dwarf planets” or “protoplanets,” as they are clearly much more complex than asteroids.

    The geophysical planet definition is ultimately about an object’s mass, specifically, whether the object meets the mass threshold for being in hydrostatic equilibrium. According to this definition, if an object is massive enough to be in hydrostatic equilibrium, it is a planet, even if it is not geologically differentiated. Callisto is a satellite planet (a planet that orbits another planet), as is Mimas (though a very small one). Proteus’ status is uncertain, partly because its mass is not known, and being so close to Neptune, it is a difficult object to observe. Proteus does not appear to be spherical or to have geological activity. We need to learn more about this object to accurately classify it.

    Preference for a geophysical planet definition has nothing to do with emotion. That is a straw man used to attack supporters of this viewpoint. This isn’t about Pluto; it is about acknowledging the great diversity of planets in our solar system and in others. No one is saying the location of the object should be discounted. That is why we need more categories, such as “satellite planet,” to distinguish classes of planets by many characteristics, including location. Triton is a satellite planet, but if it were in its own orbit, it would be a small primary planet. There will always be borderline objects, which is why we need to keep exploring, as there is no substitute for actually going (robotically) to these objecst and seeing them up close.

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