Pluto’s Back In First Place … For Now

Volunteers test some of the new LED lighting at the top of the Bentleyville Christmas tree Monday evening. Photo: Bob King

It may seem a little early for Christmas, but here in Duluth the extensive Bentleyville holiday lighting display – including a 120-foot tall metal Christmas tree -  is under construction near the harbor in Bayfront Park. Last night the crescent moon joined the scene as volunteers were testing out the LED lights on the tree. It’s fun to catch the moon in a cityscape, where we witness the meeting of two worlds so different from each other.

The occultation of a faint star by Eris on November 6th, 2010. The observation was made with a robotic telescope from the Andalusian Astrophysics Institute located in Chile. The star blinks out as Eris passes in front and then returns to view as the dwarf planet moves on.

It appear that Pluto, which was demoted to dwarf planet status several years ago, and then further demoted to second place in size among the dwarfs, may soon get its come uppence. Previously, the dwarf planet Eris, located three times farther from the sun than Pluto, was estimated to be slightly larger than the erstwhile planet. Eris and Pluto are both members of a distant swarm of asteroids beyond Neptune called the Kuiper (KY-per) Belt.

Pluto and Eris - with their moons - photographed by the Hubble Space Telescope. Credit: NASA/ESA

On November 6, three observing teams in Chile watched a faint star briefly disappear from view as Eris passed in front of it, an event called an occultation. By carefully timing the star’s disappearance from three different locations, astronomers were able to determine a new estimate of Eris’ diameter. Turns out it’s shy of Pluto’s size by at least a few to a few tens of kilometers. For now, Eris is somewhat smaller than 1,454 miles in diameter while Pluto wears the crown at 1,456 miles. Pluto-as-planet lovers have reason to celebrate even if the orb’s status still remains dwarfish. Read more about the occultation HERE.

The arrowed spot could possibly indicate the return of a darker SEB. The spot is located at CM II = 290. Credit: Christopher Go

There might be news of activity in Jupiter’s South Equatorial Belt (SEB), the stripe that faded away to almost nothing earlier this year. Philippine amateur Christopher Go recorded a small bright spot in the belt early this morning. Sometimes a spot like this one is the beginning of what observers call the SEB revival. It’s the Jovian equivalent of the hootin’ and hollerin’ at an old-fashioned tent revival, except involving multiple spots that appear rapidly in succession. They funnel fresh, darker materials from beneath the cloud tops into view. In short order, these coalesce and a new dark belt takes shape in place of the pale version we’re now seeing. Amateurs will be watching Jupiter closely in the coming days for signs of more activity. Here are a few more of Chris’s photos.

Jupiter and its four bright moons as they'll appear tonight in 7-10x binoculars. Maps created with Stellarium

Tonight you can see at least three of Jupiter’s bright moons through your binoculars. The 4th one, Io, will be very close to the planet’s western edge early in the evening and difficult to make out in Jupiter’s glare. If you wait until later – around 9 or 10 – it will have orbited farther out and be easier to spot. Observers with telescopes can watch Io’s shadow crossing over Jupiter’s cloud tops until about 6:50 p.m. Central time. The best time is between 5:30 and 6 when the sky’s getting dark and the shadow is more centrally located. Use medium to high magnification and look for a small black dot.

Jupiter with its moons and the shadow of Io around 5:30 this evening. South is up as viewed in a typical telescope.

5 Responses

  1. 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?

    1. astrobob

      Hi Laurel, thanks for your considered and thoughtful comments regarding Pluto’s status. Many people feel the same way as you do, which may lead to a change in Pluto’s classification in the future. When I describe Pluto as a dwarf planet, I’m using the current IAU official definition. Pluto does revolve in an elliptical orbit and is spherical, but according to the IAU, because it resides in the Kuiper Belt, there exists the potential for other asteroids to cross into its neighborhood. That means it doesn’t fulfill the 3rd requirement of the new planet definition: it hasn’t cleared its orbit of smaller bodies. That’s the same reason Ceres is defined as a dwarf planet – it resides in the inner asteroid belt. Now if I were defining a planet, I might choose other criteria, but since I’m not a professional astronomer, I feel I have to defer to those who are in this matter.

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