Press Release

First Accurate Determination of the Sizes of Pluto and its Moon

5 November 1986

For the first time, an accurate and direct determination of the diameters of the outermost planet Pluto and its moon, Charon, has been made. On the basis of measurements of light changes during eclipses, Pluto was found to have a diameter of 2200±140 km. Charon is approximately half the size; the diameter is 1160±100 km. Charon moves in an almost perfectly circular orbit around Pluto; the orbital period is 6.38 days and the mean distance is 19400 km.

The fundamental observations were made by astronomers Manfred Pakull and Klaus Reinsch from the Institute for Astronomy and Astrophysics of the Technical University, West Berlin, with telescopes at the ESO La Silla observatory. In April 1985, they used a 90 cm telescope to show the absence of eclipses, but one year later, they observed two well-defined eclipses with a 1.5 m and a 2.2 m telescope. On April 2, 1986, Charon passed in front of Pluto (see the figure), and on April 18, Pluto passed in front of Charon. After careful evaluation of the data, the results have now become available. At the time of the observations, Pluto and Charon were about 4300 million km distant from the Earth.

The shapes of the light curves reflect the geometry of the Pluto/Charon system. For instance, with the orbit of Charon accurately known, the total duration of an eclipse is a measure of the sizes of the two bodies. The "solution" of light curves in order to learn the underlying properties of a double system is a well known problem in astronomy, mainly in connection with the study of double stars.

The opportunity to perform these measurements occurs only once every 124 years, when the Earth is near the plane that is defined by Charon's orbit around Pluto. From the presently derived sizes, it is estimated that further observations of eclipses will be possible until 1989. Earlier estimates of Pluto's diameter were significantly larger; soon after its discovery in 1930, a diameter around 6600 km (only a little less than Mars) was surmised. Before the observations of Pakull and Reinsch, the best estimate (from speckle interferometry) was in the 2600 - 4000 km range. Thus, the new determination puts Pluto at a smaller size, but still at least twice as big as the largest known minor planet, but smaller than the Earth's Moon and also than some of the moons of the outer planets.

With the currently accepted mass of Pluto, about 450 times less than that of the Earth, the mean density derived from the measured diameter is 2.1 ± 0.5 g⋅cm-3, similar to some of the icy moons of the outer planets, including the Neptunian moon Triton. Some astronomers think that Pluto may be an escaped Neptunian moon. The “albedo", that is the reflectivity of the surfaces of Pluto and Charon, is now of the order of 0.5 for both objects.

A summary of the results obtained by Drs. Pakull and Reinsch is being published in the Circulars of the International Astronomical Union and a more detailed paper is being submitted to a professional journal.

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Richard West
ESO
Garching, Germany
Tel: +49 89 3200 6276
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