My Very Exciting Magic Carpet Just Sailed Under Nine Palace Elephants -- it's a SONG now! Memorize the song! Learn your new Planets!

Click image for larger, sleazier.

Okay, Vleeptron is taking a short break from its All-William-Blake format. Staring directly into the face of Eternity, God, Satan, Nature and the Universe was giving us a headache. But Blake's magnificent and disturbing images and poems will very soon be back, by a surprising Popular Demand.

Who'd have guessed that everybody was wild about William Blake?

Now to more Local News, the controversial re-arrangement of the planets of the Solar System. Specifically, the addition of 2 Planets to the traditional 9 we all had to memorize, forget, and get confused about during trivia games.

The very smart, bright, talented 10-year-old girl Maryn Smith won a National Geographic contest to come up with a new mnemonic to memorize the new order of the planets.

Naturally, Postalo Vleeptron, in conjunction with le Societe Astronomique du Vleeptron, issued a postage stamp to honor this achievement and educate everybody about how to remember the new planets.

This is a pretty sleazy, cheesy, cheap-looking stamp.

And it should be. Quite frankly, I spent a lifetime trying to keep the 9 planets straight, and I'm not happy to have to flush all that down the toilet for entirely arbitrary reasons.

We got 2 new planets now: Ceres -- discovered in the 19th century, but only recently found to be considerably bigger than originally thought.

And way out at the far end, some small sphere called Eris, a new discovery.

Ceres is the Goddess of Grain, or Cereal, so to find a suitable image, we have used a Cheerio.

Eris is the Goddess of Discord. She causes Trouble. That's what she does. (She started the Trojan War because she was pissed off that nobody invited her to a big fancy society wedding.) So there she is, looking much like Britney Spears in her famous somewhat bloated and badly rehearsed appearance on an MTV music award show.

Now back to Maryn Smith's remarkable achievement. As National Geo had threatened, they have finally set Maryn's mnemonic to music and made a song out of it. It's sort of ... well ... boring. It's by Lisa Loeb. It sounds like a song from an acoustic set at the Lilith Faire, and that can't be good.

But anyway, start memorizing the song, this will be on many future tests.

Click HERE, scroll down, find the song-player, play the song.

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Wikipedia's wikis about Eris and Ceres:
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Eris (pronounced /ˈɪərɪs/ EER-iss, or /ˈɛrɪs/ ERR-iss as in Greek Έρις),[10] formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2 500 kilometres in diameter and 27% more massive than Pluto.[11][8]

Eris was first spotted in 2003 by a Palomar Observatory-based team led by Mike Brown but not identified until 2005. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets the pair are the most distant known natural objects in the Solar System.[2]

Because Eris is larger than Pluto, its discoverers and NASA called it the Solar system’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris is a "dwarf planet" along with Pluto, Ceres, Haumea and Makemake.[12]

Discovery

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz[2] on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea.[13] The search team had been systematically scanning for large outer solar system bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin reflecting telescope at Mount Palomar Observatory, California, but the image of Eris was not discovered at that point due to its very slow motion across the sky: The team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris' slow motion against the background stars.
Animation showing the movement of Eris on the images used to discover it. Eris is indicated by the arrow. The three frames were taken over a period of three hours.

Follow-up observations were then carried out to make a preliminary determination of its orbit, which allowed its distance to be estimated. The team had planned to delay announcing their discovery until further observations allowed more accurate calculations of Eris' orbit, but brought their announcement forward when the discovery of another large TNO they had been tracking, Haumea, was announced by a different team in Spain.[2]

More observations released in October 2005 revealed that Eris had a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be (1.66 ± 0.02)×1022 kg, 27% greater than Pluto.

Classification

Eris is classified as a dwarf planet and trans-Neptunian object (TNO); the intersection of these categorizations makes it a "plutoid"[14]. Eris' orbital characteristics can be used to more specifically categorize it a scattered disk object (SDO), or a TNO that is believed to have been "scattered" from the Kuiper belt into more distant and unusual orbits following gravitational interactions with Neptune as the Solar system was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt.[15] Inner-belt objects are expected to be generally more massive than outer-belt objects, and so astronomers expect to discover more large objects like Eris in high-inclination orbits, which have traditionally been neglected.
Distribution of trans-Neptunian Objects.

As Eris is larger than Pluto, it was initially described as the "tenth planet" by NASA and in media reports of its discovery.[16] In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet.[17] Brown has since stated his approval of the "dwarf planet" label.[18] The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.[19]

Name
Athenian painting of Eris, circa 550 BC

Eris is named after the goddess Eris (Greek Έρις), a personification of strife and discord.[20] The name was assigned on September 13, 2006 following an unusually long period in which it was known by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets. The regular adjectival form of Eris is Eridian.

Nicknames

Due to uncertainty over whether the object would be classified as a planet or a minor planet, as different nomenclature procedures apply to these different classes of object,[21] the decision on what to name the object had to wait until after the August 24, 2006 IAU ruling.[22] As a result, a number of interim names were used in the media between 2005 and 2006, in particular Xena and Lila.

"Xena" was an informal name used internally by the discovery team. It was inspired by the eponymous heroine of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname "Xena" for the first body they discovered that was larger than Pluto. According to Brown,

We chose it since it started with an X (planet "X"), it sounds mythological (OK, so it’s TV mythology, but Pluto is named after a cartoon, right?),[a] and (this part is actually true) we've been working to get more female deities out there (i.e. Sedna). Also at the time the TV show was still on TV, which shows you how long we've been searching![23]

"We assumed [that] a real name would come out fairly quickly, [but] the process got stalled," Mike Brown said in interview,

One reporter called me up from the New York Times who happened to have been a friend of mine from college, [and] I was a little less guarded with him than I am with the normal press. He asked me, "What's the name you guys proposed?" and I said, "Well, I'm not going to tell." And he said, "Well, what do you guys call it when you're just talking amongst yourselves?"... As far as I remember this was the only time I told anybody this in the press, and then it got everywhere, which I only sorta felt bad about — I kinda like the name.[24]

The nickname "Lila" has also been used by the media. However, this was a misunderstanding of planetlila, the URL path of the discovery web page.[25] The URL path was named after Mike Brown's then-newborn daughter, Lilah.

Choosing an official name

Brown had previously speculated that Persephone, the wife of the god Pluto, would be a good name for the object.[2] The name had been used several times in science fiction,[26] and was popular with the public, having handily won a poll conducted by NewScientist magazine ("Xena", despite only being a nickname, came fourth).[27] However, this was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone.[2] Because IAU regulations require a name from creation mythology for objects with orbital stability beyond Neptune’s orbit, the team had also been considering such possibilities.[25]

With the dispute resolved, the discovery team proposed Eris on September 6, 2006. On September 13, 2006 it was accepted as the official name by the IAU.[28][25] Brown decided that, as the object had been considered a planet for so long, it deserved a name from Greco-Roman mythology, like the other planets. However, the asteroids had taken the vast majority of Graeco-Roman names. Eris, whom Brown described as his favorite goddess, had fortunately escaped inclusion.[24] The name in part reflects the discord in the astronomical community caused by the debate over the object’s (and Pluto’s) nature, while the name of its moon, Dysnomia ("lawlessness"), retains an oblique reference to the dwarf planet’s old informal name Xena, portrayed on TV by Lucy Lawless.

Orbit
The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view while the diagrams on the right are different views from the ecliptic.

Eris has an orbital period of 557 years, and as of 2008 lies at 96.7 astronomical units from the Sun, almost its maximum possible distance. (Its aphelion is 97.5 AU.) Eris came to perihelion between 1698[5]-1699,[29] aphelion around 1977,[29] and will return to perihelion around 2256[29] to 2258.[30] Eris and its moon are currently the most distant known objects in the Solar system apart from long-period comets and space probes.[31] However, approximately forty known TNOs, most notably 2000 OO67 and Sedna, while currently closer to the Sun than Eris, have greater average orbital distances than Eris' semimajor axis of 67.7 AU.[4]

The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8–30.4 AU). Pluto, on the other hand, like other plutinos, follows a less inclined and less eccentric orbit and, protected by orbital resonance, can cross Neptune’s orbit. (It is possible that Eris is in a 17:5 resonance with Neptune, though further observations will be required to know for sure.[32]) Unlike the terrestrial planets and gas giants, whose orbits all lie roughly in the same plane as the Earth's, Eris' orbit is highly inclined: It is tilted at an angle of about 44 degrees to the ecliptic.

Eris currently has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200 mm telescope with a CCD can detect Eris under favorable conditions.[b] The reason it had not been noticed until now is because of its steep orbital inclination; most searches for large outer Solar system objects concentrate on the ecliptic plane, where most bodies are found.

Eris is now in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until 1875. In 2036 it will enter Pisces and stay there until 2065, when it will enter Aries.[29] It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. Because the orbit of Eris is highly inclined, it only passes through a few constellations of the traditional Zodiac.

Size, mass, and density

The diameter of Eris has been measured to be 2397 km, give or take 100 km, using images from the Hubble Space Telescope.[33][34] (The brightness of an object depends both on its size and its albedo, the amount of light it reflects). At a distance of 97 AU, an object with a radius of 3000 km would have an angular size of 40 milliarcseconds,[9] which is directly measurable with the HST; although resolving such small objects is at the very limit of Hubble's capabilities,[c] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[d])
Eris compared to Pluto, Makemake, Haumea, Sedna, Orcus, Quaoar, Varuna, and Earth.

This makes Eris only 0-8% larger than Pluto, which is about 2306 km across. It also indicates an albedo of 0.86, higher than any other large body in the Solar system other than Enceladus. It is speculated that the high albedo is due to the surface ices being replenished due to temperature fluctuations as Eris' eccentric orbit takes it closer and farther from the Sun.[35]

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris's diameter of 2600 (+400; -200) km.[7] The Spitzer and Hubble estimates overlap in the range of 2400-2500 km, 4-8% larger than Pluto.

The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period, 15.774 days,[36][8] Eris is 27 percent more massive than Pluto.

Thermal measurement

Previous observations of the thermal emission of Eris at a wavelength of 1.2 mm, where the object's brightness depends only on temperature and surface area, indicated a diameter of 3000 ± 400 km, about a third larger than Pluto.[37] If the object rotates quickly, resulting in a more even heat distribution and a temperature of 23 to 24 kelvins (-250 to -249 degrees Celsius), a likely diameter would be in the higher portion of the range (best fit 3090 km); if it rotates slowly, the visible surface would be warmer (about 27 K, or -246 degrees Celsius) and a likely diameter would be in the smaller end of the range (best fit 2860 km). The 2860 km figure implies a Pluto-like albedo of 60%, consistent with its Pluto-like spectral signature.

The apparent inconsistency of the HST PSF results (2400 ± 100 km) with the above IRAM results (3000 ± 370 km) will certainly be studied at more length. Brown explains it by a slightly lower absolute magnitude than the one assumed by Bertoldi (−1.12 ± 0.01 versus −1.16 ± 0.1, resulting by itself in almost 100 km difference in diameter). Assuming further the highest diameter (2500 km) and pole-on position of the object[e] the difference between the results would appear consistent with 1.1-σ error margin.

Another possible explanation for the IRAM results is offered by the Max-Planck-Institut für Radioastronomie. The ratio between the bolometric albedo (representing the total reflected energy and used in the thermal method) and the geometric albedo (representing the reflection in some visual wavelength and used to calculate the diameter from HST pictures) is not known with high precision and depends on many factors. By itself, this uncertainty could bridge the gap between the two measures.[37]

Surface and atmosphere

The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on January 25, 2005. Infrared light from the object revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune's moon Triton, which also has methane on its surface.[38]

Due to its distant eccentric orbit, Eridian surface temperatures are estimated to vary between about 30 and 56 kelvins (−243 and −217 degrees Celsius).[2]

Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey.[2] Pluto's reddish colour is believed to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough away from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.[39]

Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublimate. Methane is highly volatile and its presence shows either that Eris has always resided in the distant reaches of the solar system where it is cold enough for methane ice to persist, or that it has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another recently-discovered TNO, Haumea, which reveal the presence of water ice but not methane.[40]

Moon
Artist impression of Eris and Dysnomia. Eris is the main object, Dysnomia the small grey disk just above it. The flaring object top-left is the Sun.

Main article: Dysnomia (moon)

In 2005 the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system.[41] Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle", after the television warrior princess's sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after the Greek demon of lawlessness who was Eris' daughter.

See also

* Astronomical naming conventions
* Cleared the neighbourhood
* Hypothetical trans-Neptunian planets
* International Astronomical Union
* Planet X

Notes

1. ^ Brown is joking on this point. It was in fact the Disney character Pluto that was named after the newly discovered "planet", though Venetia Phair, Pluto's christener, had to counter accusations her whole life that she named the planet after a cartoon dog.[42]
2. ^ For an example of an amateur image of Eris, see Fred Bruenjes' Astronomy
3. ^ The Resolution of the High Resolution Channel of the ACS is 40 marcsec (milliarcseconds) and the size of 1 pixel is ~25 marcsec i.e. ~1875 km at the distance of Eris.
4. ^ The reference to 'direct' measure by HST should not mislead into thinking that this method is as 'direct' and model-independent as measuring say Neptune’s size. Basically, the method consists in finding the statistically best fit to a smeared image of the size of less than 2 pixels by comparing it with smeared images of the background stars, using a given computer model of the optics (PSF). A non technical description of the method is given on Brown’s page, a detailed description of this approach and its limitations are discussed in a paper on Quaoar[43]
5. ^ If the object is in pole-on position the side facing the Sun (and the observer) gets hotter producing stronger emissions thus resulting in overestimation of the diameter using the thermal method.

References

1. ^ Staff (2007-05-01). "Discovery Circumstances: Numbered Minor Planets". IAU: Minor Planet Center. Retrieved on 2007-05-05.
2. ^ a b c d e f g h Mike Brown (2006). "The discovery of 2003 UB313 Eris, the largest known dwarf planet". Retrieved on 2007-05-03.
3. ^ Staff (2004-02-29). "Minor Planet Designations". IAU: Minor Planet Center. Retrieved on 2007-05-05.
4. ^ a b "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center. Retrieved on 2008-09-10.
5. ^ a b Marc W. Buie (2007-11-06). "Orbit Fit and Astrometric record for 136199". Deep Ecliptic Survey. Retrieved on 2007-12-08.
6. ^ Asteroid Observing Services
7. ^ a b John Stansberry, Will Grundy, Mike Brown, John Spencer, David Trilling, Dale Cruikshank, Jean-Luc Margot (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". University of Arizona, Lowell Observatory, California Institute of Technology, NASA Ames Research Center, Southwest Research Institute, Cornell University. Retrieved on 2007-05-18.
8. ^ a b c Michael E. Brown and Emily L. Schaller (2007). "The Mass of Dwarf Planet Eris" (abstract page). Science 316 (5831): 1585. doi:10.1126/science.1139415. PMID 17569855, http://www.sciencemag.org/cgi/content/abstract/316/5831/1585.
9. ^ a b Bertoldi F., Altenhoff W., Weiss A., Menten K. M., Thum C. (2006). "The trans-Neptunian object UB313 is larger than Pluto". Nature 439 (7076): 563–564. doi:10.1038/nature04494.
10. ^ Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/eris (accessed: November 12, 2007). Brown uses /ˈɛrɪs/: "Julia Sweeney and Michael E. Brown". Hammer Conversations: KCET podcast (2007). Retrieved on 2008-10-01.
11. ^ "Dwarf Planet Outweighs Pluto". space.com (2007). Retrieved on 2007-06-14.
12. ^ IAU (2006-08-16). "The IAU draft definition of "planet" and "plutons"". Press release. Retrieved on 2006-08-16.
13. ^ Thomas H. Maugh II and John Johnson Jr. (2005). "His Stellar Discovery Is Eclipsed". Los Angeles Times. Retrieved on 2008-07-14.
14. ^ "Pluto Now Called a Plutoid". Space.com (2008-06-11). Retrieved on 2008-06-11.
15. ^ Gomes R. S., Gallardo T., Fernández J. A., Brunini A. (2005). "On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances". Celestial Mechanics and Dynamical Astronomy 91: 109–129. doi:10.1007/s10569-004-4623-y.
16. ^ "NASA-Funded Scientists Discover Tenth Planet". Jet Propulsion Laboratory (2005). Retrieved on 2007-05-03.
17. ^ "IAU 2006 General Assembly: Resolutions 5 and 6", IAU (2006-08-24).
18. ^ Robert Roy Britt (2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". space.com. Retrieved on 2007-05-03.
19. ^ IAU Circular 8747 — Official publication of the IAU reporting the naming of Eris and Dysnomia
20. ^ Blue, Jennifer (2006-09-14). "2003 UB 313 named Eris". USGS Astrogeology Research Program. Retrieved on 2007-01-05.
21. ^ "International Astronomical Association homepage". Retrieved on 2007-01-05.
22. ^ Green, Daniel W.E. (2006-09-13). "(134340) PLUTO, (136199) ERIS, AND (136199) ERIS I (DYSNOMIA)" (PDF). Central Bureau for Astronomical Telegrams. Retrieved on 2007-01-05.
23. ^ "Xena and Gabrielle" (PDF). Status (January 2006). Retrieved on 2007-05-03.
24. ^ a b Mike Brown (2007). "Lowell Lectures in Astronomy". WGBH. Retrieved on 2008-07-13.
25. ^ a b c "The Discovery of Eris, the Largest Known Dwarf Planet". California Institute of Technology, Department of Geological Sciences. Retrieved on 2007-01-05.
26. ^ "Planet X Marks the Spotwork=TechRepublic" (PDF) (2006). Retrieved on 2008-07-13.
27. ^ Sean O'Neill (2005). "Your top 10 names for the tenth planet". NewScientist. Retrieved on 2008-06-28.
28. ^ "IAU0605: IAU Names Dwarf Planet Eris". International Astronomical Union News (2006-09-14). Retrieved on 2007-01-05.
29. ^ a b c d Yeomans, Donald K.. "Horizons Online Ephemeris System". California Institute of Technology, Jet Propulsion Laboratory. Retrieved on 2007-01-05.
30. ^ Wm. Robert Johnston (2007-08-21). "(136199) Eris and Dysnomia". Johnston's Archive. Retrieved on 2007-07-27.
31. ^ Chris Peat. "Spacecraft escaping the Solar System". Heavens-Above. Retrieved on 2008-01-25.
32. ^ Simulation of Eris (2003 UB313)'s orbit predicting a 17:5 resonance.
33. ^ "Comment on the recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.". Max Planck Institute (2006). Retrieved on 2007-05-03.
34. ^ "Hubble Finds 'Tenth Planet' Slightly Larger Than Pluto". NASA (2006-04-11). Retrieved on 2008-08-29.
35. ^ M. E. Brown, E.L. Schaller, H.G. Roe, D. L. Rabinowitz, C. A. Trujillo (2006). "Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope" (PDF). The Astronomical Journal 643 (2): L61–L63. doi:10.1086/504843, http://www.gps.caltech.edu/~mbrown/papers/ps/xsize.pdf.
36. ^ Mike Brown (2007). "Dysnomia, the moon of Eris". CalTech. Retrieved on 2007-06-14.
37. ^ a b "Comment on the Recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.". Max Planck Institute for Radioastronomy (2006-02-02). Retrieved on 2007-01-05.
38. ^ "Gemini Observatory Shows That "10th Planet" Has a Pluto-Like Surface". Gemini Observatory (2005). Retrieved on 2007-05-03.
39. ^ M. E. Brown, C. A. Trujillo, D. L. Rabinowitz (2005). "Discovery of a Planetary-sized Object in the Scattered Kuiper Belt" (abstract page). The Astrophysical Journal 635 (1): L97–L100. doi:10.1086/499336, http://arxiv.org/abs/astro-ph/0508633.
40. ^ J. Licandro, W. M. Grundy, N. Pinilla-Alonso, P. Leisy (2006). "Visible spectroscopy of 2003 UB313: evidence for N2 ice on the surface of the largest TNO". Astronomy and Astrophysics 458: L5–L8. doi:10.1051/0004-6361:20066028, http://www.aanda.org/articles/aa/pdf/2006/40/aa6028-06.pdf.
41. ^ M. E. Brown, M. A. van Dam, A. H. Bouchez, D. LeMignant, C. A. Trujillo, R. Campbell, J. Chin, Conrad A., S. Hartman, E. Johansson, R. Lafon, D. L. Rabinowitz, P. Stomski, D. Summers, P. L. Wizinowich (2006). "Satellites of the largest Kuiper belt objects" (abstract page). The Astrophysical Journal 639 (1): L43–L46. doi:10.1086/501524, http://arxiv.org/abs/astro-ph/0510029.
42. ^ "The girl who named a planet". BBC News (2006). Retrieved on 2007-06-21.
43. ^ M. E. Brown and C. A. Trujillo (2004). "Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar" (PDF). The Astronomical Journal 127 (7076): 2413 – 2417. doi:10.1086/382513, http://www.gps.caltech.edu/~mbrown/papers/ps/qu.pdf. Describing in detail the method applied to the recent measure of 2003 UB313.

External links

* Michael Brown's webpage about Eris
* Brown's webpage about Dysnomia
* 2007 KCET interview of Mike Brown about Eris and Haumea with Julia Sweeney
* compiled list of data
* MPEC listing for 2003 UB313
* Java 3D orbit visualization
* Spaceflight Now article about 2003 UB313 (Eris), 2003 EL61, and 2005 FY9 (Makemake)
* Slacker Astronomy Interview With Co-Discoverer Trujillo
* Trans-Neptunian Object 2003 UB313 — IAU statement regarding the planetary status of Eris
* Simulation of 2003 UB313's orbit
* Keck observatory page about the discovery of Dysnomia
* Caltech Press Release, 7/29/2005 "Planetary Scientists Discover Tenth Planet".
* Press release on the Spitzer Space Telescope trying to image 2003 UB313 again

Observatories involved

* Keck Observatory, Hawaiʻi, US
* Palomar
* Gemini
* IRAM (Institut de Radio Astronomie Millimétrique (Institute for Millimetric Radio Astronomy)): French-German (Max-Planck Institut fur Radioastronomy, Bonn) and Spanish (where the 30 m telescope is situated) collaborative programme.
* HST

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Ceres (pronounced /ˈsɪəriːz/ SEER-eez,[14] or as Latin: Cerēs), formal designation 1 Ceres, symbolized asin astrology, and occasionally in astronomy, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. It was discovered on January 1, 1801, by Giuseppe Piazzi,[15] and is named after the Roman goddess Ceres — the goddess of growing plants, the harvest, and of motherly love.

With a diameter of about 950 km, Ceres is by far the largest and most massive body in the asteroid belt, and contains a third (32%) of the belt's total mass.[16][17] Recent observations have revealed that it is spherical, unlike the irregular shapes of smaller bodies with lower gravity.[8] The surface of Ceres is probably made of a mixture of water ice and various hydrated minerals like carbonates and clays.[9] Ceres appears to be differentiated into a rocky core and ice mantle.[3] It may harbour an ocean of liquid water underneath its surface, which makes it a potential target of current searches for extraterrestrial life.[17]

Ceres' apparent magnitude ranges from 6.7 to 9.3, hence at its brightest is still too dim to be seen with the naked eye.[10] On September 27, 2007, NASA launched the Dawn Mission space probe to explore Vesta (2011–2012) and Ceres (2015).[18]

Discovery

The idea that an unknown planet could exist between the orbits of Mars and Jupiter was first suggested by Johann Elert Bode in 1768.[15] His considerations were based on the so called Titius-Bode law, a now-abandoned theory which had been proposed by Johann Daniel Titius in 1766.[19][15] According to this law the semi-major axis of the planet should be near 2.8 AU.[19] William Herschel's discovery of Uranus in 1781[15] increased faith in the law of Titius and Bode, and in 1800, twenty-four experienced astronomers combined their efforts and began a methodical search for the proposed planet.[15][19] The group was headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz. While they did not discover Ceres, they later found several large asteroids.[19]
Piazzi's Book "Della scoperta del nuovo pianeta Cerere Ferdinandea" outlining the discovery of Ceres

Ceres was discovered on 1 January 1801, by Giuseppe Piazzi, who was searching for a star listed by Francis Wollaston as Mayer 87 because it was not in Mayer's zodiacal catalogue in the position given.[15] Instead of a star, Piazzi found a moving star-like object, which he first thought was a comet.[20] Piazzi observed Ceres a total of 24 times, the final time on February 11, when illness interrupted his observations. He announced his discovery on January 24, 1801 in letters to fellow astronomers, among them his compatriot Barnaba Oriani of Milan. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet".[15] In April, Piazzi sent his complete observations to Oriani, Johann Elert Bode, and Jérôme Lalande in Paris. The information was published in the September 1801 issue of the Monatliche Correspondenz.[20]

Soon after this, Ceres' apparent position had changed (mostly due to the Earth's orbital motion). It then appeared too close to the Sun's glare, so other astronomers could not confirm the observations of Piazzi until the end of the year. However after such a long time it was difficult to predict its exact position. To recover Ceres Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination.[20] In only a few weeks, he predicted its path, and sent his results to von Zach. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.[20]

In 1802 Herschel estimated Ceres to be 260 km in diameter, and in 1811 Johann Hieronymus Schröter estimated Ceres to be 2613 km in diameter.[21][22]

Name

Piazzi originally suggested the name Ceres Ferdinandea (Ital­ian, Cerere Ferdinan­dea) for this body, after both the mythological figure Ceres (Roman goddess of plants) and King Ferdinand III of Sicily.[15][20] "Ferdinandea" was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany.[23] In Greece, it is called Δήμητρα (Demeter), after the goddess Ceres' Greek equivalent; in English usage, Demeter is the name of an asteroid (1108 Demeter). Due to its uncommon usage, there is no consensus as to the proper adjectival form of the name, although the nonce forms Cerian and Cerean[17] have been used in fiction. Grammatically, the form Cererean would be correct, derived from its Latin genitive, Cereris.[24] Ceres' astronomical symbol is a sickle, (Sickle variant symbol of Ceres), similar to Venus' symbol (Astronomical symbol of Venus) which is the female gender symbol and Venus' hand mirror.[20][25] The element Cerium was named after Ceres.[26] The element Palladium was originally also named after Ceres, but the discoverer changed its name after Cerium was named. Palladium is named after asteroid 2 Pallas.[27]

Status
Ceres (bottom left), the Moon and the Earth.

The classification of Ceres has changed more than once and has been the subject of some disagreement. Johann Elert Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun.[15] Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century until further asteroids were discovered.[15][20][28]

However as other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies.[15] Sir William Herschel coined in 1802 the term asteroid ("star-like") for such bodies,[28] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".[29] As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.[28]

The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet.[30][31] A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet".[32] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.[33] However, it was not accepted, and in its place an alternate definition of "planet" came into effect as of August 24, 2006: A planet is "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit." By this definition, Ceres is not a planet (because it shares its orbit with the thousands of other asteroids in the main belt), and is now classified as a "dwarf planet" (along with Pluto, Makemake, Haumea and Eris). The issue of whether Ceres remains an asteroid was not addressed.[34] Dual classifications such as Main-belt comets do exist, and being a dwarf planet does not preclude having dual designations.[35]

Physical characteristics
Size comparison: the first 10 minor planets profiled against Earth's Moon. Ceres is far left.
Hubble Space Telescope images of Ceres, taken in 2003/4 with a resolution of about 30 km. The nature of the bright spot is uncertain.

Ceres is the largest object in the asteroid belt, which lies between Mars and Jupiter.[9] The Kuiper belt is known to contain larger objects, including Pluto, 50000 Quaoar, and 90482 Orcus, while more distant Eris, in the scattered disc, is the largest of all these bodies.[36]

The mass of Ceres has been determined by analysis of the influence it exerts on small asteroids. Results obtained by different authors are slightly different.[37] The average of the three most precise values as of 2008 is approximately 9.4×1020 kg.[4][37] With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2 ×1021 kg mass of the asteroids in the solar system,[38] together totalling about four percent of the mass of the Moon. Ceres' size and mass are sufficient to give it a nearly spherical shape.[3] That is, it is close to hydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas,[39] 3 Juno,[40] and 4 Vesta[41] are known to be quite irregular.

Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior;[3] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[3] This mantle of thickness from 120 to 60 km could contain 200 million cubic kilometres of water (16–26 percent of Ceres by mass; 30–60 percent by volume), which is more than the amount of fresh water on the Earth.[42] This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling.[4][43] Also, some characteristics of its surface and history (such as the distance from the Sun, which keeps the strong radiation in a low level that possibly let the planet retain some of its primordial elements), point to the presence of Volatile materials in the interior of Ceres.[4]

Surface

The surface composition of Ceres is broadly similar to that of C-type asteroids.[9] However, some differences do exist. The ubiquitous features in the IR spectra of Ceres are that of hydrated materials, which indicates the presence of significant amounts of water in the interior of this body. Other possible surface constituents include iron-rich clays (cronstedtite) and carbonates (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.[9] The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.[9] Sometimes Ceres is classified as G-type asteroid.[44]

The surface of Ceres is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C) on May 5, 1991.[13] Taking into account also the heliocentric distance at the time, this gives an estimated maximum of about 239 K at perihelion.
Diagram showing internal structure of Ceres

Only a few features have been unambiguously detected on the surface of Ceres. High resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres.[44] This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.[45][4] Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, while another was identified as the "Piazzi" feature.[45][4] More recent visible light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the nature of which are currently unknown.[8][46] One of these features corresponds to the "Piazzi" feature observed earlier.[8]

These last observations also determined that Ceres' north pole points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small—about 3°.[3][8]

Atmosphere

Ceres may be surrounded by a tenuous atmosphere containing water vapour.[47] There are some strong indications that Ceres may have a tenuous atmosphere and water frost on the surface.[47] Surface water ice is not stable at distances smaller than 5 AU, so it is spected to sublimate if is exposed directly to the solar radiation, water ice can escape from the deep layers of Ceres to the surface, but will escape in very short time periods, as a result, it is a little hard to detect water vaporization. Until the early 90's, this was observed from the polar region, but it is also easy to see from a fresh impact crater or cracks in the sub-surface layers.[4] Ultraviolet observations by IUE spacecraft detected statistically significant hydroxide water vapour near the Cererean north pole.[47]

Orbit of Ceres

Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 Earth years. The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).[1]

The diagram illustrates the orbits of Ceres (blue) and several planets (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a perspective view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

In the past, Ceres had been considered to be the largest member of an asteroid family.[48] These groupings of asteroids share similar orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres, however, was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the lowest-numbered family member, 1272 Gefion.[48] Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.[49] Self rotation period of Ceres is 9 hours and 4 minutes.[6]

Origin and evolution

The observations imply that Ceres is a remaining protoplanet – planetary embryo, which formed 4.57 billion years ago in the asteroid belt.[50] While the majority of protoplanets (including all lunar- to Mars-sized bodies) were ejected from the Solar System by Jupiter or merged with other protoplanets to form terrestrial planets,[50] Ceres survived relatively intact.[43] Two other possible remaining protoplanets are Pallas and Vesta,[18] but they do not have relaxed shapes, in the case of Vesta perhaps only because it suffered a catastrophic impact after solidifying.[41]

Further evolution of Ceres was relatively simple. Heated by the energy of accretion and by decay of various radionuclides including, possibly, short-lived elements like Al26, Ceres differentiated into a rocky core and icy mantle soon after its formation.[8][43] This event caused resurfacing by the water volcanism and tectonics erasing many geological features.[43] However due to the fast depletion of heat sources Ceres cooled down quickly.[43][51] The ice on the surface gradually sublimated leaving behind various hydrated minerals like clays and carbonates.[9] Now Ceres is a geologically dead body, whose surface is being sculptured only by impacts.[8]

The existence of significant amounts of water ice in Ceres[3] raises a possibility that it has or had a layer of liquid water in the interior.[51][43] This hypothetical layer is often called an ocean.[9] The water layer is (or was) probably located between the rocky core and ice mantle like that in Europa.[43] The existence of the ocean is more likely if ammonia or other antifreeze is dissolved in water.[43] The possible existence of liquid water inside Ceres makes it a target in the search for extraterrestrial life.[17]

Observations

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.[10] This is generally regarded as being just barely too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. The only asteroids that can reach so bright a magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.[52] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.

Some notable observation milestones for Ceres include:

* An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on November 13, 1984.[53]
* Ultraviolet Hubble Space Telescope images with 50 km resolution taken in 1995.[54][44]
* Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.[45]
* Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.[46][8]

Exploration
Artist's conception of Dawn visiting Ceres and Vesta

To date, no space probe has visited Ceres. However, NASA launched the Dawn Mission on 27 September 2007, which will explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.[18]

The mission profile calls for the Dawn Spacecraft to enter orbit around Ceres at an altitude of 5,900 km. After five months of study, the spacecraft will reduce the orbital distance to 1,300 km, then down to 700 km after another five months.[55] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These will be used to examine the dwarf planet's shape and element abundance.[18]

Radio signals from spacecraft in orbit around Mars and on its surface have been used to estimate the mass of Ceres from the perturbations induced by it onto the motion of Mars.[38]

See also
Solar System portal

* Asteroids in astrology
* Ceres in astrology
* Ceres in fiction
* Colonization of Ceres
* Solar System bodies formerly considered to be planets

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Ephemerides

* Hilton, James L., U.S. Naval Observatory Ephemerides of the Largest Asteroids The Astronomical Journal, Vol. 117, p. 1077 (1999).
* Yeomans, Donald K.. "Horizons system". NASA JPL. Retrieved on 2007-03-20. — Horizons can be used to obtain a current ephemeris

External links

* Movie of one Ceres rotation (processed Hubble images)
* How Gauss determined the orbit of Ceres from keplersdiscovery.com
* An up-to-date summary of knowledge about Ceres, plus an Earth-Ceres size comparison (the Planetary Society)
* A simulation of the orbit of Ceres
* A website dedicated entirely to 1 Ceres