Minor planets are discovered as slowly moving unresolved sources in optical sky photographs taken with astronomical telescopes. Drs. Frank Bertoldi and Wilhelm Altenhoff from the MPIfR recently used the IRAM 30-meter telescope in Spain to measure the thermal radiation of four of the optically brightest minor planets. From the measured intensity they could derive their sizes, which range between 700 and 1200 km (see table below). On October 7 their Caltech colleagues presented their discovery of Quaoar at the annual meeting of the Planetary Sciences Division of the American Astronomical Society, which is held in Birmingham, Alabama. There they also present a direct size measurement of Quaoar from optical images obtained with the Hubble Space Telescope. This unique and first such observation confirms the prior radio size measurement.

The four minor planets are members of a ring of some 100,000 objects in the outskirts of the solar system, beyond Pluto at distances over 4 billion km from the sun, over 30 times the distance between earth and sun. The objects in this "Kuiper belt" circle the sun in stable orbits with periods of about 300 years. In the mid of last century, the existence of a ring of small  planetisemals was first suggested by the astronomers Kenneth Edgeworth (1880-1972) and Gerard P. Kuiper (1905-1973), but the first discovery of an "Edgeworth-Kuiper belt object" (EKO) was not until 1992. By now, over 550 EKOs are known.
 

Figure 1:  The newly discovered EKO Quaoar, compared in size with Pluto, the Moon, and Earth. (image credit: NASA)

A direct size determination of an EKO had not been possible until recently due to the large distance of these small objects. However, using the IRAM 30-m telescope and MAMBO, a very sensitive heat sensor built at the MPIfR in Bonn, the Bonn scientists were able to measure the very weak thermal radiation emerging from the four large EKOs.

"The velocity with which a distant solar system object moves reveals its distance," explains Dr. Frank Bertoldi. "From that we can compute the objects' surface temperature, which is mostly given by the solar irradiation. The intensity of the heat radiation we receive from the EKO depends on its distance, temperature, and size, so knowing the distance and temperature, we find its size. On the other hand, the optical brightness of the object, which is simply reflected sunlight, does not tell us much about its size, because the very low surface reflectivity may vary significantly from object to object." 

"The discovery of two large EKOs by our American colleagues this year is impressive and important," admits Dr. William Altenhoff, who has researched minor planets and comets for decades. "In the coming years I expect the discovery of many more and even larger of such objects. What is interesting to us is to find out the extend of the Kuiper belt, and particularly what the total mass of all the EKOs together might be. This would allow unique insights into the origin of our solar system. The EKOs are the debris from its formation, an archeaological site containing prestine remnants of the solar nebula, from which the sun and the planets formed. A determination of the size and reflectivity of some of the EKOs enables us to estimate also the total mass of the many smaller EKOs, which are too small to measure their individual sizes."

The observations at millimeter wavelengths were made using the IRAM 30-meter telescope at Pico Veleta near Granada in Spain (Fig. 2). The sensitive bolometer detector MAMBO (Fig. 3) used here was developed and built at the MPIfR in Bonn by the group of Dr. Ernst Kreysa. The Institute for Radio Astronomy at Millimeter wavelengths (IRAM) is supported jointly by the German Max-Planck-Society, the French Centre National de Recherche Scientifique (CNRS) and the Spanish Instituto Geografico Nacional. 

[ELN]