Scientists at the Max-Planck-Institute for Radioastronomy (MPIfR) in Bonn were able to determine the diameters of four of the five largest and most distant minor planets in our solar system. The largest of them was discovered last June by planetary scientists of the California Institute of Technology (Caltech), who named their object "Quaoar" after a creation myth of the Californian native Tongva people. The radio observations of the Bonn astronomers and their Californian colleagues show that Quaoar has a diameter of 1250 km, making it the largest object discovered in the solar system since the discovery of Pluto in 1930.
Minor planets are initially discovered as slowly moving unresolved sources in optical sky images taken with small astronomical telescopes. Drs. Frank Bertoldi and Wilhelm Altenhoff from the MPIfR recently used the IRAM 30-meter telescope in Spain to measure the heat radiation of four of the optically brightest such objects. From the measured intensity they could derive the object sizes, which range between 700 and 1200 km (see table below). On October 7 their Caltech colleagues present their discovery of Quaoar at the annual meeting of the Planetary Sciences Division of the American Astronomical Society, which is held in Birmingham, Alabama. Here they also present a size measurement of Quaoar from optical images taken with the Hubble Space Telescope, a unique, first such observation, which confirmed the prior radio size measurement.
The four minor planets are members of a ring of some 100,000 such
objects far 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
the 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" (EKOs) was not until 1992.
By now, over 550 EKOs are known.
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 scicntists were able to measure the very faint heat radiation emerging from four of the five largest 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.
Now, the intensity of the heat radiation we receive from the EKO depends on
its distnace, 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
"The discovery of two large EKOs by our American colleagues this
year is impressive and important," admits Dr. William Altenhoff, who has researched
planets and comets for decades. "In the coming years I expect the discovery
of many more and even larger such objects. What is particularly interesting
to us is to find out the extend of the Kuiper belt, and also what the total
mass of all the EKOs together might be. This would allow unique insights
into the origin of our planetary system, since the EKOs are the rubbish from
its formation, or more politely, an archeaological site containing prestine
remnants of the solar nebula, from which the sun and the planets did forrm.
A determination of the size and reflectivity of some of the EKOs allows us
to estimate also the total mass of the many smaller EKOs, which are too small
for us to measure their sizes."
|Quaoar||42 au||1250 +-50 km
1200 +-200 km
Bertoldi, Brown, Trujillo, Margot
|Ixion||43 au||1055 +-165 km||Altenhoff, Bertoldi||IRAM 30m|
|Varuna||43 au||900 +-140 km||Jewitt, Aussel, Evans||JCMT|
|2002AW197||48 au||890 +-120 km||Margot, Brown, Trujillo, Bertoldi||IRAM 30m|
|1999TC36||31 au||675 +-100 km||Altenhoff, Bertoldi||IRAM 30m|
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. E. 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.
Dr. Frank Bertoldi
Tel.: +49 - 228 - 525-377 or +49 - 179 - 8567872
Fax: +49 - 228 - 525-229
Dr. Wilhelm Altenhoff
Tel.:+49 - 228 - 525-293
Fax:+49 - 228 - 525-229
Dr. Norbert Junkes (MPIfR public outreach)
Fax: +49 - 2257 - 301-105