They’re Out There

The known solar system continues to expand. While I was on vacation, astronomers gave a name and designation to an outer solar system world that’s roughly 3/4 the size of Pluto.  This plutoid (which is a subclass of dwarf planets), discovered in 2005, is now called Makemake (pronounced mah-kay mah-kay), or (136472) Makemake, if you’re sending a formal invitation for it to join the community of worlds. Mike Brown, the astronomer who discovered Makemake, has a great discussion about the name he selected for this world, which is a Kuiper Belt object. Essentially, he chose the  name of the god of fertility in the mythology of the South Pacific island of Rapa Nui at Easter Island.

We don’t really have a good image of Makemake, but my friend Robert Hurt at IPAC at Caltech, who does double duty as a scientist AND talented space artist, came up with this lovely artist’s conception of what the newest Plutoid might look like. It could have a moon, so Robert put one in.  We won’t know for sure until more detailed imaging and spectra can be done. It’s exciting to see more worlds being discovered “out there” on the frontier of the solar system!

You’re a Plutoid

After two years of what must have been grueling discussion, the International Astronomical Union has decided that things that are like Pluto (which used to be defined as a planet) are now going to be called “Plutoids” as long as they orbit at or beyond the orbit of Neptune. Dwarf planets (as a small-body definition), as you may recall, was an outcome of the 2006 IAU meeting, when it was decided that we needed a new category for worlds that aren’t quite planets, but are bigger than asteroids. The name plutoid for a specific subset of dwarf planets was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive Committee at its recent meeting in Oslo, Norway.

Two Plutoids: Pluto (shown with its companion Charon) and Eris (with Dysnomia)

So, what are the characteristics of Plutoids? They have to be celestial bodies in orbit around the Sun at a distance greater than that of Neptune. They must have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape. And, they haven’t cleared the neighborhood (of debris) around their orbit. So, Pluto and Eris fit this definition, and scientists expect more small worlds like them to be found as astronomers keep finding them out the great beyond.

By this definition, the dwarf planet Ceres is NOT a Plutoid, but it’s still a dwarf planet. This is because its orbit is within the asteroid belt and not transneptunian.

Cold, and then Some

Up until a couple of days ago we were shivering under some pretty cold temps up here in New England. The other night we took the trash out and it was about 10 below zero (F)—cold enough to literally take your breath away. The sky was quite clear that night, and the stars were stunningly beautiful. Mars was like a red beacon…The next day I walked out to get the mail and noticed the ice in the driveway. It reminded me of pictures I’d seen of ice fields on the worlds of the outer solar system. Out there ice doesn’t so much melt off the surfaces of those worlds, but it sublimates—it turns from ice crystals into a gas without going through that pesky liquid phase we see here on Earth.

Oh, there is liquid water out there at the outer worlds. At least, that’s the working hypothesis deduced from various observations. And, how else do you explain what looks like deposits of fresh ice that have somehow oozed up through cracks on the surfaces of places like Enceladus, Europa and Pluto’s moon Charon? It’s only a matter of time before the existence of all those cold oceans are confirmed. And, when I read about them, I can certainly sympathize with the idea of cold—especially after the bitterly cold weather we had last week. However, I am reminded that 10 below zero here on Earth would be a pretty warm day on Mars or Enceladus or Tethys or Pluto or Charon— so we have it pretty good here at home.

Speaking of cold and ice and outer solar system, here’s the latest installment of my ongoing vodcast series. It features an observation made at Gemini Observatory that I wrote about a few months ago, and an image I worked on with the PR folks at Gemini. Come on—let’s go visit some ice worlds!

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Remember a year or so back when the largest Kuiper Belt Object to date, named Sedna, was discovered? It shifted planetary scientists’ attention to the origin and evolution (and existence!) of large, planetoid-sized objects out beyond Pluto. They’ve been working out the fine details of Sedna’s orbit for a while now, using sophisticated models of the early solar system formation. One of the outcomes of this work is the idea that this nearly-Pluto-sized “worldlet” actually formed in place in the frigid deep-freeze of the outermost solar system. Originally scientists thought it was assembled farther in toward the Sun during the early days of the system’s formation, and was somehow ejected out to its current position.
Why does where Sedna formed matter? Astronomers have longed assumed that planetary formation took place in a rather smaller region of the original solar nebula. If Sedna was created from the collisions of smaller bodies out in the “sticks” of the solar system, then the planetary factory is bigger than everybody suspected. It also means that the Kuiper Belt, which hosts countless bodies at what used to be called “the edge of the solar system” is really part of a larger region called the Kuiper disk and played a much more prominent role in the formation of planets and moons.
The modeling that led to these conclusions was done at the Southwest Research Institute in Boulder, Colorado. In the press release they sent out announcing this work, the institute’s Executive Director for Space Studies, Alan Stern (a former colleague of mine from the University of Colorado), talked about some of the assumptions they made in constructing their model: “”The Sedna formation simulations assumed that the primordial solar nebula was a disk about the size of those observed around many nearby middle-aged stars — like the well-known example of the 1,500-AU-wide disk around the star Beta Pictoris.”
It’s interesting work because it gives us a whole lot MORE insight into the infancy of our own solar system, in particular the formation of planets from smaller planetesimals. And, chances are if Sedna formed where the astronomers think it did, then there could well be more large planetoids circling around out there with it — and that what we used to think of as the “emptiness of the outer solar system” isn’t so empty anymore. As astronomers learn more about the Sun’s outermost retinue of planetesimals, they are finding more clues to what conditions were like early in the history of solar system.

You gotta love the Hubble Space Telescope. There it is up there, looking out across the deeps of space and time, spotting shreds of galaxies as they formed some 300,000 to 500,000 years after the Big Bang, and then it turns around and gives us a view of a little shard of a world called Sedna. This place, smaller than the Moon, smaller than Pluto, lies about 90 times the distance between the Earth and Sun, out in a region of the solar system called the Kuiper Belt. It’s a leftover bit of ice (mostly) from the formation of the solar system some 5.5 billion years ago. It’s so dim and small that HST’s image is one pixel across. But, it’s an informative pixel!

For instance, it tells us that (so far) HST hasn’t spotted a companion to this planetoid, although the astronomer who discovered it on March 15, 2004, calculated its spin rate (it’s “day”) and determined that it should have a moon of some kind. The fact that it doesn’t illustrates one of those wonderful “non-results” that tells us valuable information. Science is replete with stuff like this — what looks like a non-result actually helps us put limits on an object’s actions or size or mass or other characteristic. It reminds me of the observation run I did in Hawaii where we studied Comet Hale-Bopp in late 1996. We thought we might be able to spot a plasma tail forming earlier than might be expected. However, when we examined the data, we found no evidence of the plasma tail, which told us that even with a comet of that size, it had to be close enough to the Sun (essentially within about 2 to 2.5 AU of the Sun) before its plasma tail would form. It helped us nail down parts of a descriptive theory we were formulating about how and when and why these glowing tails form when they do as a comet approaches the Sun during its orbit.

Astronomers will keep studying Sedna, and in fact, they have looked at it with other telescopes since its discovery. Expect to see a few more announcements about this frigid world at the frontiers of the solar system!