Beyond Pluto: Max-Planck radioastronomers measure the sizes of distant minor planets

Press Release From: Max Planck Institute
Posted: Monday, October 7, 2002

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
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

  • 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

Dr. Wilhelm Altenhoff
Tel.:+49 - 228 - 525-293
Fax:+49 - 228 - 525-229

Dr. Norbert Junkes (MPIfR  public outreach)
Fax: +49 - 2257 -  301-105

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