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clyde tombaugh, dwarf planets, planets, Pluto

It Was 82 Years Ago

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The Rise of the Dwarf Planets

A Hubble Space Telescope image of Pluto (central object) and its four largest moons, Hydra (upper left), Charon (lower left), Nix (lower right), and P4 (upper right). Courtesy NASA/ESA/STScI.

February 18th is the 82nd anniversary of the discovery of Pluto, the dwarf planet.  The find was made in 1930 by an observer at Lowell Observatory in Arizona by the name of Clyde Tombaugh.  He had spent months searching through and comparing photographic plates of the sky, looking for a possible new planet. His discovery was confirmed, and the name Pluto was bestowed on March 24, 1930.  I had the pleasure to meet Clyde at a conference some years ago, when he spoke enthusiastically about his work to uncover this distant, frozen world.

Pluto is classified as a dwarf planet — which means it’s a special class of planet, much as white dwarfs are special classes of stars, and some galaxies are termed “dwarfs” based on the characteristics that differentiate them from spiral, elliptical, and irregular galaxies.

One of the fascinating things (among many) about Pluto is that its discovery really opened up a new phase of solar system exploration, resulting the discovery of more dwarf planets  in the outer solar system.

Granted, we’ve done quite a bit of solar system exploration since Clyde’s momentous discovery.  We’ve sent probes to most of the other planets, and studied them with ground- and space-based telescopes.  But, until recently, we didn’t have the technical wherewithal to do more than study Pluto from Earth (or Earth orbit, with Hubble Space Telescope, for example).  That changed when the New Horizons spacecraft was launched in 2006 on an voyage of exploration of the outer solar system.

New Horizons will arrive at Pluto in 2015.  It will study the planet’s atmosphere, surface characteristics, and its nearest moons.  After that, it will continue out to other outer solar system objects — in fact, its larger mission is to study the Kuiper Belt, a region of space that extends out from the orbit of Neptune and in which Pluto orbits .  It’s really the gateway to all the outer solar system worlds, including Pluto.

I mentioned that astronomers have found other icy worlds out in Pluto’s domain, and beyond. Eris is the most massive known dwarf planet (so far), and orbits the Sun out well beyond Pluto.  It’s an icy world roughly the size of Plut0. Then, there are Makemake, Haumea, Charon, Orcus, Quaoar, and Sedna.  They’re all smaller and more distant than Pluto, but there’s no doubt they’re worlds in their own right. Undoubtedly others are out there, making trans-Neptunian space a sort of new frontier.  This is why I see Pluto’s discovery as momentous. So, in celebration of Pluto Discovery Day, I raise a toast to Clyde Tombaugh — whose ashes are aboard the New Horizon spacecraft bound for Pluto space.  Not only did he discover a dwarf planet, but he also opened the gates to discoveries in a sector of the solar system once thought empty and barren.  It’s a bigger solar system than we thought, folks, and we have visionaries like Clyde to thank for helping us figure that out.

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Planet or Not…

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Pluto Has Moons

The Pluto system. Courtesy STScI/NASA/ESA

That distant world called Pluto has surprised astronomers again, yielding up yet another moon.  Pluto’s largest moon is Charon and was discovered in 1978.  Two more — Nix and Hydra — were found in 2005. The new one, called P4 (for now), is quite small, somewhere between 13 to 34 kilometers across, and small enough that it was probably missed in earlier images of the system taken by Hubble Space Telescope. This latest HST image was taken as part of  a search for ring material around the distant dwarf planet, in support of the New Horizons mission, which is en route to Pluto.

So, how would Pluto, itself a small world like many others in the outer solar system, get moons?  The current thinking is that a collision between Pluto and another world early in the history of the solar system would have flung material out into orbit. Eventually, the pieces and parts would have coalesced back together, forming the family of moons we see today.

When I read this story, the first things I wondered were “Why search for rings around Pluto?”  and “Where would the material for Plutonian ringlets come from?”  A long-ago collision would have provided material for rings, but by now, that material would have been cleared away or coalesced into moons, such as Nix, Hydra, P4 (and maybe even Charon?).  To maintain a ring system, you need a constant source of material being tossed out to space.  At Pluto, that source may well be material “chipped away” from the icy surface by the impacts of tiny micrometeoroids.  That would provide chips of ice to form a faint, thin ring. If it exists, it hasn’t yet been detected. But, HST would be the best instrument we have at this time to find the ring.  Once New Horizons gets there, it may well “see” the ring, if it exists.

I like it when HST finds things like this. It’s a continuing reminder that the venerable telescope has a lot of life in it yet; and will keep surprising astronomers with new finds.