Note added 31.10.2007: Recent size measurements by Stansberry et al. (2007) of KBOs using Spitzer infrared data show that the size of Eris is consistent with 2600 (-200/+400) km, which is consistent with both the HST and MAMBO measurements.
Comment on the recent Hubble Space Telescope size measurement of 2003 UB313 by Brown et al.
The question naturally arises why our size measurement based on the detection of thermal emission (2600 - 3400 km diameter, a 68% confidence level range typically used, corresponding to one standard deviation in statistical terms) is in apparent conflict with the HST measurement (2300 - 2500 km, 68% confidence). One should realize though that the measurements are consistent at the 1.3 standard deviation (80%) confidence limit of both measurements, i.e. in each measurement there is a 10% chance that the diameter is around 2530 km.
One way to even better reconcile both measurements is to lower the assumed ratio between the bolometric (also called "Bond") albedo, A, and the optical "geometric" albedo, p, from the value we had adopted, q=A/p=0.9, to q=0.7 (q is also called the "phase integral"). In this case, the size derived from the thermal measurement is reduced by 100 km, so that both measurements would agree within the 68% confidence limits at ca. 2500 km.
What does it mean to reduce the value of q? This parameter is not very well constrained,
neither theoretically nor empirically from observations of solar system objects.
It relates two albedos: the bolometric Bond albedo specifies what fraction of the total
incident radiation energy is reflected, whereas the geometric albedo, p (usually measured in the optical
red) specifies what fraction of the red sunlight is reflected toward the observer. Both values
are usually not equal. For many materials, the reflectivity depends on wavelength,
so that the albedo as averaged over all wavelengths, A, is different from that at any particular
wavelength. This is a rather
complicated issue since the back-reflectivity of a tilted material surface
also depends much on the surface roughness. Consider, e.g., a highly reflective surface such as
a metal plate. Placing a smooth metal film on a sphere results in a very low average albedo for this
sphere, because only few rays reflect toward the observer - despite the reflective nature of the
material. A ragged or porous material such as snow on the other hand would reflect back well
even if viewed at some angle.
Besides this ratio of albedos we should point out at least other possible sources of uncertainty in UB313's size measurement. Two important assumptions were made when deriving a size for UB313 from the HST measurements: a specific radial brightness profile on the optical "disk" (the "center-to-limb function"), and second, the implicit assumption that the star observed for comparison with UB313 is a single star.
For the radial brightness profile Brown et al. assumed a reasonable best guess,
i.e. that measured for Triton. Using a different profile, e.g. flat instead of a limb-darkened,
or, in the opposite extreme, a reflective smooth film as mentioned above,
would have a very large effect
on the derived size, an effect much bigger than the statistical measurement uncertainty of 100 km.
Brown and collaborators did use the best reasonable guess they could make (just as we used q=0.9
assuming UB313 is similar to Pluto) - which may be correct, or may not.
Given the inherent uncertainties in both methods, the actual size of 2003 UB313 is not established beyond reasonable doubt by either the MAMBO or HST measurements. Both may be right, in which case we learn something rather interesting about the surface property of this exotic object. Precise measurements of the diameter (and with this the albedo) of UB313 will eventually be possible with powerful ALMA radio interferometer to be installed in the Chilean desert, or with the James Webb Space Telescope .
The most distant object yet known in the
solar system turns out
to be a rich source of puzzling information, and an exciting challenge.
At the bottom of this page you find information in addition to that of the official MPG/Bonn University press release which is repeated first
New "planet" is larger than Pluto:
Claims that the Solar System has a tenth planet are bolstered by the
finding by a group lead by Bonn astrophysicists that this putative
planet, announced last summer and tentatively named 2003 UB313, is
bigger than Pluto. By measuring its thermal emission, the scientists
were able to determine a diameter of about 3000 km, which makes it 700
km larger than Pluto and thereby marks it as the largest solar system
object found since the discovery of Neptune in 1846.
Like Pluto, 2003 UB313 is one of the icy bodies in the so-called Kuiper belt that swarms beyond Neptune. It is the most distant object ever seen in the Solar System. Its very elongated orbit takes it up to 97 times farther from the Sun than is the Earth - almost twice as far as the most distant point of Pluto's orbit – so that it takes twice as long as Pluto to go around the Sun. When it was first seen, UB313 appeared to be at least as big as Pluto. But an accurate estimate of its size was not possible without knowing how reflective it is. A team lead by Prof. Frank Bertoldi from the University of Bonn and the Max-Planck-Institute for Radioastronomy (MPIfR) and the MPIfR's Dr. Wilhelm Altenhoff has now resolved this problem by using measurements of the amount of heat UB313 radiates to determine its size, which when combined with the optical observations also allows them to determine its reflectivity. "Since UB313 is decidedly larger than Pluto," Frank Bertoldi remarks, "it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status."
UB313 was discovered in January 2005 by Prof. Mike Brown and his colleagues from the Californian Institute of Technology in a sky survey using a wide field digital camera that searches for distant minor planets at visible wavelengths. They discovered a slowly moving, spatially unresolved source, the apparent speed of which allowed them to determine its distance and orbital shape. However, they were not able to determine the size of the object, although from its optical brightness it was believed to be larger than Pluto.
Astronomers have found small planetary object beyond the orbits of Neptune and Pluto since 1992, confirming a then 40-year old prediction by astronomers Kenneth Edgeworth (1880-1972) and Gerard P. Kuiper (1905-1973) for the existence of a belt of smaller planetary objects beyond Neptune. The so-called Kuiper Belt contains objects left from the formation of our planetary system some 4.5 billion years ago. In their distant orbits they were able to survive the gravitational clean-up of similar objects by the large planets in the inner solar system. Some Kuiper Belt objects are still occasionally deflected to then enter the inner solar system and may appear as short period comets.
In optically visible light, the solar system objects are visible through the light they reflect from the Sun. Thereby the apparent brightness depends on their size as well as on the surface reflectivity. Latter is known to vary between 4% for most comets to over 50% for Pluto, which makes any accurate size determination from the optical light alone impossible.
The Bonn group therefore used the IRAM 30-meter telescope in Spain, equipped with the sensitive Max-Planck Millimeter Bolometer (MAMBO) detector developed and built at the MPIfR, to measure the heat radiation of UB313 at a wavelength of 1.2 mm, where reflected sunlight is negligible and the object brightness only depends on the surface temperature and the object size. The temperature can be well estimated from the distance to the sun, and thus the observed 1.2 mm brightness allows a good size measurement. One can further conclude that the UB313 surface is such that it reflects about 60% of the incident solar light, which is very similar to the reflectivity of Pluto.
"The discovery of a solar system object larger than Pluto is very exciting," Dr. Altenhoff exclaims, who has researched minor planets and comets for decades. "It tells us that Pluto, who should properly also be counted to the Kuiper Belt, is not such an unusual object. Maybe we can find even other small planets out there, which could teach us more about how the solar system formed and evolved. The Kuiper Belt objects are the debris from its formation, an archeological site containing pristine remnants of the solar nebula, from which the sun and the planets formed." Dr. Altenhoff made the pioneering discovery of heat radiation from Pluto in 1988 with a predecessor of the current detector at the IRAM 30-meter telescope.
The size measurement of 2003 UB313 is published in the 2 February 2006 issue of Nature. The research team includes Prof. Dr. Frank Bertoldi (Bonn University and MPIfR), Dr. Wilhelm Altenhoff (MPIfR), Dr. Axel Weiss (MPIfR), Prof. Dr. Karl M. Menten (MPIfR), and Dr. Clemens Thum (IRAM).