* Astronomy

The orbital period of the moon, S/2005 (2003 UB313), is 14 days and the semi major axis 40,000km.
The total mass of the system is 2.6 x 10^22 kg (which is twice the mass of Pluto).

The satellite was found to be 4.43 ±0.05 magnitudes fainter than the primary and to be separated 0".53 ±0".01 from the primary in p.a. 275 ±1 degrees.

The newly discovered Kuiper Belt object, 2003 UB313, has a moon.

The moon is 100 times fainter than Xena and orbits once every couple of weeks. It was spotted on September 10, 2005, with the Laser Guide Star Adaptive Optics system on the 10-meter Keck II telescope at the W.M. Keck Observatory in Hawaii by Michael E. Brown, professor of planetary astronomy, and his colleagues at Caltech, the Keck Observatory, Yale University, and the Gemini Observatory in Hawaii.
A paper about the discovery was submitted on October 3 to Astrophysical Journal Letters.


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The discovery of the moon of the 10th planet from the W.M. Keck Observatory. The planet appears in the centre,
while the moon is the small dot at the 3 o'clock position. Credit: W.M. Keck Observatory


"Since the day we discovered Xena, the big question has been whether or not it has a moon. Having a moon is just inherently cool-and it is something that most self-respecting planets have, so it is good to see that this one does too" - Michael E. Brown.

Brown estimates that the moon, nicknamed "Gabrielle"-after the fictional Xena's fictional sidekick-is at least one-tenth of the size of Xena, which is thought to be about 2700 km in diameter (Pluto is 2274 km), and may be around 250 km across.
To know Gabrielle's size more precisely, the researchers need to know the moon's composition, which has not yet been determined. Most objects in the Kuiper Belt are about half rock and half water ice. Since a half-rock, half-ice surface reflects a fairly predictable amount of sunlight, a general estimate of the size of an object with that composition can be made.
Very icy objects, however, reflect a lot more light, and so will appear brighter-and thus bigger-than similarly sized rocky objects.

Further observations of the moon with the Hubble Space Telescope, planned for November and December, will allow Brown and his colleagues to pin down Gabrielle's exact orbit around Xena. With that data, they will be able to calculate Xena's mass.

"A combination of the distance of the moon from the planet and the speed it goes around the planet tells you very precisely what the mass of the planet is. If the planet is very massive, the moon will go around very fast; if it is less massive, the moon will travel more slowly. It is the only way we could ever measure the mass of Xena-because it has a moon" - Michael E. Brown.

The discovery was made possible by Keck II's recently commissioned Laser Guide Star Adaptive Optics system. Adaptive optics is a technique that removes the blurring of atmospheric turbulence, creating images as sharp as would be obtained from space-based telescopes.
The new laser guide star system allows researchers to create an artificial "star" by bouncing a laser beam off a layer of the atmosphere about 75 miles above the ground. Bright stars located near the object of interest are used as the reference point for the adaptive optics corrections. Since no bright stars are naturally found near Xena, adaptive optics imaging would have been impossible without the laser system.



"With Laser Guide Star Adaptive Optics, observers not only get more resolution, but the light from distant objects is concentrated over a much smaller area of the sky, making faint detections possible" - Marcos van Dam, adaptive optics scientist at the W.M. Keck Observatory, and second author on the new paper.

Title: Discovery of a planetary-sized object in the scattered Kuiper belt
Authors: M.E. Brown, C.A. Trujillo, D.L. Rabinowitz

We present the discovery and initial physical and dynamical characterization of the object 2003 UB313.
The object is sufficiently bright that for all reasonable values of the albedo it is certain to be larger than Pluto.
Pre-discovery observations back to 1989 are used to obtain an orbit with extremely small errors. The object is currently at aphelion in what appears to be a typical orbit for a scattered Kuiper belt object except that it is inclined by about 44 degrees from the ecliptic.
The presence of such a large object at this extreme inclination suggests that high inclination Kuiper belt objects formed preferentially closer to the sun.

Observations from Gemini Observatory show that the infrared spectrum is, like that of Pluto, dominated by the presence of frozen methane, though visible photometry shows that the object is almost neutral in colour compared to Pluto's extremely red colour. 2003 UB313 is likely to undergo substantial seasonal change over the large range of heliocentric distances that it travels; Pluto at its current distance is likely to prove a useful analogy for better understanding the range of seasonal changes on this body.

Since the discovery of the first small objects beyond Neptune astronomers have speculated about the existence of objects larger than Pluto in the Kuiper belt. Extrapolation of the size distribution of smaller Kuiper belt objects (KBOs) has sometimes been used to attempt to estimate the numbers of such larger objects (i.e. Bernstein et al. 2004), but such estimates have proven inconclusive. One of the goals of our ongoing survey for bright KBOs is to find the rare objects at the bright end of the Kuiper belt magnitude distribution. Such bright objects are invaluable as targets for detailed physical study in addition to being potential beacons of previously unknown populations.

The newly discovered KBO 2003 UB313 is currently the fourth brightest object known in the Kuiper belt (after Pluto, 2003 FY9, and 2003 EL61) and is currently the most distant object ever seen in orbit around the sun. As an object notable for its brightness, distance, and size, 2003 UB313 is certain to be the object of intensive study. We present here details on its discovery, preliminary observations about its surface characteristics, and some suggestions about physical processes operating on this object.
2003 UB313 was discovered in data from 21 October 2003 obtained from our ongoing survey at the 48-inch Samuel Oschin telescope at Palomar Observatory. At the time of discovery the object was moving 1.42 arc seconds per hour, slower than the cut-off in our main survey. Our survey obtains three images over a 3 hour period. With typical image quality of from 2 to 3 arc seconds, slower motions are clearly detectable, but we installed a 1.5 arc second per hour lower limit to our analysis to cut down the copious false positives at the slow end.

The discovery of Sedna, with a motion of 1.75 arc seconds per hour, however, suggested a need to efficiently search for distant objects which would be moving at lower rates. We have now reanalyzed all survey data with a second (”slow”) detection scheme in addition to the standard (”fast”) scheme. This slow scheme searches for motions between 1 and 2 arc seconds per hour between the first and third image of a triplet. When a potential object is found it checks for consistency using the second image, but motion need not be detected between either the first and second or second and third images.
Finally, to remove the large number of false positives generated by stationary stars, all potential detections which are within 2 arc seconds of a catalogued USNO star are removed without examination. The slow scheme generates 10 to 20 times more false positives than the fast scheme, leading to approximately 1200 candidates every month.
These candidates are examined by eye and are generally quickly rejected. On occasion we also make use of the Skymorph data base1 to determine that a potentially moving candidate is, in fact, a stationary star. In the two years worth of slow data examined to date we have found only two real objects: Sedna (previously also found in the fast scheme) and 2003 UB313.

The extreme brightness and slow motion of 2003 UB313 made it easy to identify it as a transient in archival data. The object was identified in multiple images from the Skymorph data base and eventually found in a 1989 plate from the UK Schmidt telescope at Siding Springs Observatory.
From this 16-year orbital arc the derived barycentric orbit using the method of Bernstein & Khushalani (2000) gives a semi-major axis, eccentricity, and inclination of a = 67.89±0.01, e = 0.4378±0.0001, and i = 43.993±0.001, respectively. 2003 UB is currently near aphelion at 97.50 ± 0.01 AU from the sun and will not reach perihelion at 38.2 AU until the year 2257.

Based on the semi-major axis and eccentricity, 2003 UB313 would be classified as a typical member of the scattered Kuiper belt. The inclination of 44 degrees is extreme for the scattered belt, however. Only one other otherwise unremarkable scattered object (2004 DG77) has a confirmed orbit with an inclination as high. While initial models of the scattered Kuiper belt were incapable of populating high inclination regions, recent work suggests that a combination of gravitational scattering, mean-motion resonant interaction, planetary migration, and the Kozai mechanism may be able to place objects into orbits such as these.
Additional simulations show that objects that are initially in the inner part of the pre-migration disk (at distances of 20 AU) are scattered into orbits with higher inclinations than those further out (Gomes 2003). We expect that, on average, these inner regions will lead to the formation of larger objects owing to both higher nebular densities and shorter accretion time scales. We might therefore expect to find other large objects at high inclination in the scattered Kuiper belt.

Indeed, the other two recently announced scattered KBOs from our survey, 2005 FY9 and 2003 El61, both have inclinations near 30 degrees and approach the size of Pluto.
3. Spectrum Visible photometry of 2003 UB313 was obtained on 5, 6, and 7 January 2005 using the 1.3-meter SMARTS telescope. Data were obtained and reduced in an identical manner to that described in Rabinowitz et al. (2005). Infrared photometry was obtained on 25 and 26 January from the Gemini North Observatory. No evidence for any photometric variation (http://skys.gsfc.nasa.gov/skymorph/skymorph.html) was seen over the short time scale of observation.

No attempt is made to correct for solar phase effects, which are of order 0.01 magnitudes at Pluto for a 0.5 degree phase. The relative brightness of 2003 UB313 is highest in the R and I filters. We find an absolute magnitude of Hr = -1.48, which corresponds to a diameter of 2250 -1/2 r km, where r is the R albedo. Even if the surface albedo is an unreasonably high 100% at these wavelengths the object has a diameter approximately that of Pluto. Medium resolution near-infrared spectra were obtained on the nights of 25-27 January UT with the Near Infrared Imager and Spectrograph instrument on the Gemini North telescope.
The J, H and K bands were measured using 3 separate grating settings and on-source times of one, one, and two hours, respectively. Relative reflectance was computed by dividing the spectra by a solar analogy G2V star at a similar airmass to 2003 UB313. Each spectra was pair subtracted to remove detector bias, then flattened and rectified. Bad pixels and cosmic rays were masked out in each spectrum prior to extraction.

Extracted spectra were rebinned to a common wavelength scale with regions affected by bright OH lines masked out. Error bars were computed from the reproducibility of spectral data in each wavelength bin. Though 2003 UB313 is relatively bright, the signal-to-noise of the spectrum is only moderate owing to the fact that at the time of discovery the object was quickly setting in the evening sky.



Figure 1 shows the relative reflectance of 2003 UB313, with the individual J, H, and K spectra scaled to match the near-infrared photometry and the relative near-infrared colours of Pluto. Because of uncertainties in spectral slope across the near-infrared, we do not regard the relative scaling between the three separate spectra to be reliable. The near-infrared spectrum is dominated by absorption from CH4 and closely resembles that of Pluto.
At the current signal-to-noise and systematic reproducibility level, no reliable detection is made of any other species on 2003 UB313, including, notably, the 2.15 µm line of N2, the 1.58 µm line of CO, both detected on Pluto, and the 2.01 and 2.07 µm lines of CO2 detected on Triton. In many cases there are potentially detections of these lines, but most are in spectral regions contaminated by bright sky lines or variable sky absorptions and none should be believed without additional observation and confirmation.

One major difference between the spectrum of 2003 UB313 and that of Pluto is that the visible region of 2003 UB313 is considerably less red than that of Pluto. If red visible colours on icy bodies are interpreted as due to irradiated complex organics, the difference between Pluto and 2003 UB313 is surprising given the similarity of the methane spectra of the two bodies. A more subtle difference between the spectra is a slight shift of the positions of the methane absorption lines.
On Pluto methane is a minor component dissolved as a solid solution inside of N2 ice. The isolation of the methane molecules leads to a slight but measurable energy shift in the spectrum. The four best measured methane lines on 2003 UB313, in contrast, appear much closer to the positions measured in the laboratory for pure methane than they do for methane incorporated into N2.

2003 UB313 is the largest known object in orbit beyond Neptune, and, like the second largest object, Pluto, its spectrum is dominated by absorption due to methane. Methane ices subjected to ion and UV radiation irreversibly break down and reassemble into more complex hydrocarbons, leading eventually to the formation of dark red tholins. The continued presence of abundant methane on 2003 UB313 suggests the need, as has been suggested for Pluto, for an interior source to replenish the methane.
The presence of methane on 2003 UB313 as well as Pluto suggests that this process is ubiquitous in the outer solar system and that methane is not retained on smaller objects where escape rates are higher.
The red colours and large spatial albedo variations of Pluto have been suggested to be due to distinct regions covered by these dark red tholins. At Pluto’s current heliocentric distance, dark regions absorb enough sunlight to become too warm for methane condensation, while the bright regions serve as methane cold traps, thus reinforcing any albedo contrast in existence.
At the 97 AU distance of 2003 UB313, however, even dark regions will be sufficiently cold that as methane freezes out of the atmosphere or is replenished from the subsurface it will cover the entire body, lowering albedo contrasts and hiding the red tholins. This model leads to the prediction that 2003 UB313 will have significantly less albedo variation than Pluto and that its albedo will be as high or higher than Pluto.
The lower temperature of 2003 UB313 may also explain the difference in the state of the methane.
Expected subsolar surface temperatures of a 70% albedo body at 97 AU are 30 K. At this temperature the vapour pressure over pure N2 ice is 420 nbar, while the vapour pressure over pure methane ice is below a pbar. Unlike Pluto’s present state, methane on 2003 UB313 is currently essentially involatile and will not be mixed in the atmosphere with nitrogen. As 2003 UB313 moved towards aphelion over the past two centuries nitrogen and methane may have segregated, perhaps vertically. As 2003 UB313 moves back towards perihelion a more Pluto-like mixing may occur.

The discovery of 2003 UB313 provides a new lower temperature laboratory for the study of many of the processes discussed for Pluto, including atmospheric freeze out and escape, ice chemistry, nitrogen phase transitions, and volatile mixing and transport. The temperature variation from perihelion of aphelion of 2003 UB313 is even more extreme than that on Pluto. Higher quality infrared spectra, which should be readily obtainable for this moderately bright object, will be a key component of future studies.

Source (PDF, 142kb)

-- Edited by Blobrana at 21:57, 2005-08-31

UPDATE: on Mr Browns website.

Spitzer Space Telescope observations are often the best way to find the size of objects in the outer solar system.
The Spitzer telescope measures the amount of heat coming from an object. If we wanted to measure the size of a fire, for example, we could do it by measuring the total amount of heat coming from the fire. The temperature of the flames in a match and a bonfire are essentially the same, but a bonfire emits much more heat because it is much bigger.
The same is true of distant planets. Because we know how far away the planet is we have a pretty good idea of the surface temperature (a frosty 405 degrees below zero!), thus when we measure the total heat we can tell how big the object is.

Though we tried earlier to measure the size using Spitzer, those observations failed due to human error which caused the telescope to point in the wrong direction. The Spitzer Space Telescope rarely makes such errors, but these observations were extremely unusual in that they were of a moving object whose position could not be obtained from publicly available web sites at JPL (since JPL didn't yet know of the existence of the object). Instead, a string of human interaction had to occur between our (correct) submission of the orbital elements and the final pointing of the telescope. Somewhere in this string of interactions a mistake was made. Two other Kuiper belt objects (2003 EL61 and 2005 FY9) were observed in the same manner at the same time and the observations proceeded without a glitch, leading us to initially assume that the 2003 UB313 observations were correctly pointed also.
The mistake was caught by one of the many extremely careful members of the Spitzer Science Centre. As soon as the mistake was caught new observations were scheduled and safeguards were put into place to prevent such an occurrence again. Spitzer will again attempt to observe 2003 UB313 at the end of the month.

In the meantime, we are attempting observing from the 30-meter IRAM telescope. This telescope, like Spitzer, measures the heat output. But IRAM measures the heat output in a region of the spectrum where much less heat is output. Nonetheless we have high hopes that these observations will succeed. The combination of Spitzer and IRAM will be especially powerful.

Yet another step to try to measure the size will be to observe the planet with the Hubble Space Telescope and see if we can do some very careful analysis to measure the size in a similar manner as we did for the planetoid Quaoar. These observations are already scheduled and will be taking place shortly, though the observations are optimised for detection of satellite rather than size measurement. We are attempting to secure observations optimized for size measurement.

Source