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
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
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.”

The five largest known Edgeworth-Kuiper Objects
Name Solar distance Diameter measured by  using
Quaoar 42 au 1250 +-50 km
1200 +-200 km		 Brown, Trujillo
Bertoldi, Brown, Trujillo, Margot		 HST
IRAM 30m
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
      Notes:
  • JCMT =  James-Clerk-Maxwell Teleskop, Hawaii
  • HST    =  Hubble Space Telescope
  • au        =  “astronomical unit” = average
    earth-sun distanc, ca. 150 mio. km.
  • Ixion :   Greek mythology, a Thesalian king who was tied to
    a wheel as punishment for advancing Hera

 

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. 

Further information on the internet:

We would be happy to provide you with more information – please
contact us at:

Dr. Frank Bertoldi
Tel.: +49 – 228 – 525-377  or  +49 – 179 – 8567872
Fax:  +49 – 228 – 525-229
e-mail: bertoldi@mpifr-bonn.mpg.de

Dr. Wilhelm Altenhoff
Tel.:+49 – 228 – 525-293
Fax:+49 – 228 – 525-229
e-mail: waltenhoff@mpifr-bonn.mpg.de

Dr. Norbert Junkes (MPIfR  public outreach)
Fax: +49 – 2257 –  301-105
e-mail: njunkes@mpifr-bonn.mpg.de