Tue
Nov 9 2010 4:43pm

Is Pluto the biggest dwarf planet after all?

Pluto’s controversial demotion from planetary status came in 2006 after the rapid discovery of comparably sized bodies—now named Haumea, Makemake and Eris—made Pluto look rather ordinary. In particular, Eris was found to be larger in diameter than Pluto, raising the question of what separated a planet from numerous smaller bodies. The International Astronomical Union decided on a new definition for planets that resulted in a paring down the solar system’s tally of planets to eight, relegating Pluto and its ilk to dwarf planet status.

Pluto lovers of the world may take some small comfort in a new look at Eris that puts Pluto back in the running for the largest dwarf planet, diameter-wise. (Eris seems to retain a lock on the title of most massive dwarf planet for the time being.) Measurements taken as Eris temporarily blotted out the light of a distant star indicate that the dwarf planet’s diameter is on par with, and maybe even smaller than, that of Pluto.

Eris is extremely distant, orbiting much farther from the sun than even Pluto does, and it is difficult to get a good look at the relatively small world. Although initial thermal readings pegged Eris at about 3,000 kilometers (km) in diameter, later infrared observations taken with the Spitzer Space Telescope indicated a diameter of roughly 2,600 kilometers (km), whereas Hubble Space Telescope measurements pointed to a diameter of 2,400 km. Pluto, in comparison, is about 2,300 kilometers across.

On the night of November 5, a fortuitous alignment provided the new data point. As Eris cruised through its orbit, some 14 billion km from Earth, it passed in front of a distant star from Earth’s vantage point, casting a small shadow across our planet in an event known as an occultation. By timing the duration of the occultation at multiple sites, researchers can estimate the size of the shadow and hence the size of the object.

According to Sky & Telescope, three teams witnessed the occultation from sites in Chile. Based on those measurements, astronomer Bruno Sicardy of the Paris Observatory told the magazine that Eris’s diameter is “almost certainly” smaller than 2,340 km.

Mike Brown of the California Institute of Technology, one of the co-discoverers of Eris who seems to relish his role in the Pluto controversy (his Twitter handle is plutokiller), noted on his Web site that the results, though preliminary, are tantalizing. For if Pluto and Eris are approximately the same diameter, yet Eris is substantially more massive, their composition must be fundamentally different. “How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?” Brown wrote. “As of today I have absolutely no idea.”


Scientific American Reprinted from ScientificAmerican.com with permission from Scientific American.
4 comments
Gorbag
1. Gorbag
Hmmm, that's an interesting puzzle. If dwarf planets are presumed to have been conglomerated where their present-day orbits are, of course ...

What is the likeliehood that Eris is instead an planetisimal from a closer-in orbit that Jupiter has flung out of the inner solar system? If so, its chemical composition (and mass / diameter ratio) would necessarily be different to Pluto's.

Alternatively, it's the aliens' starship come to collect its liens and issue torts (no, Virginia, not tortillas) on its long-lost property, the Pithecanthrope Servitors it left on the Moon while heading home to get a damaged radiator fixed ... the Pithecanthrope Servitors obviously threw a line from the Earth to the Moon and climbed down when it was dark ... and proceeded to evolve without permission, thus imperilling the substantial investment the aliens had in their property ... tinfoil hat time ... :) Is your hat made of genuine tin, or are you just pretending - I know the Pentagon tries to sell tinfoil hats made of recycled plutonium .... :)
Gorbag
2. Lady-Lark
The size of Eris relative to Pluto is completely irrelevant to Pluto's planetary status. The new IAU guidelines for what constitutes a planet state:
1) It must be in orbit around a star (not a planet.)
2) It must be massive enough for gravity to have compacted it into a spherical shape.
3) it must have cleared most of the debris (asteroids, etc.) from the neighborhood of its orbit.

Clearly, Pluto meets the first two categories, but fails the third. Regardless of Eris's size, and regardless of whether any other Kuiper Belt objects are discovered which are larger than Pluto (not an impossibility), under the current guidelines Pluto will remain a dwarf planet, not a planet.

Sorry to disappoint all the Plutophiles out there, but that's the way science is. As we learn more , we sometimes learn that older ideas were wrong. Illness really is caused by invisible germs, not bad air as once was believed; the world really does revolve around the sun, not the other way around.

The reclassification of Pluto as a dwarf planet takes nothing away from Pluto as an interesting object to study. It's time to get over the shock and get on with life -- and science.
Gorbag
3. Laurel Kornfeld
Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term "dwarf planet," which was first coined by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, to indicate a third class of planets which are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for "dwarf planets" to be classed as not planets at all. The IAU did not "have" to do anything other than allow Eris's discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems.
Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto's, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets!
Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common?
It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become.
Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don't ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?
My twitter handle is plutosavior, and my upcoming book is titled "The Little Planet That Would Not Die: Pluto's Story."
Rammy Meyerowitz
4. m5rammy
@Lady-Lark, I concur.

People should stop saying "Pluto should be ..." one thing or another.
The question is "What is the definition of 'planet'?" and Pluto just doesn't fit the criteria.

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