Eris, Pluto’s distant twin

Eris, Pluto’s distant twin
10/27/11

This occultation by Eris was the first TNO to be observed with such a precision for a TNO. It is, moreover, the subject of a publication which has just appeared in the magazine Nature. From la Silla, TRAPPIST observed an occultation of 31 seconds. In San Pedro de Atacama, 600km to the North, the phenomenon occurred with a slight time-delay and it lasted 79 seconds. The very precise recording of the beginning and end of the occultation from the two sites made it possible to measure the exact size of Eris: the measurements are compatible with a sphere of 1163 ± 6 km of radius. “Until then, the best measurements of the diameter of Eris were obtained with the Hubble Space Telescope in the visible, and the Spitzer space telescope in the infrared”, explains our astronomer. “Their estimates were marred by an error of 100 km and 400 km respectively. With these new observations, we have succeeded in estimating the radius of Eris with a precision of 6 km, even though Eris orbits at a distance of 15 billion kilometers from us and we did it with very modestly-sized telescopes.”

On November 6th 2010, at 02h19min18s (UT), TRAPPIST observed the disappearance of a star of the Cetus constellation for 31 seconds. The cause: the passing of the dwarf planet Eris in front of it, visible as a small luminous spot in the same place as the star.

©TRAPPIST/E.Jehin

Eris 4 imagesEN
Finally, and contrary to what astronomers had thought since 2005, Eris is similar in size to Pluto! The values commonly agreed for Pluto vary between 1150km and 1200km, the uncertainty stemming from the presence of an atmosphere of variable thickness which causes confusion. By way of contrast, the reduction in luminosity of the star occulted by Eris is instantaneous, showing that Eris is without an atmosphere.

Eris size The consequences of this measurement are of great importance for our knowledge of the solar system and its formation. As Eris is smaller than what astronomers had originally calculated, and the measurement of its mass itself has not changed, its density is therefore higher. Eris would now appear to be a rocky dwarf planet (up to 85%) covered in a thin layer of ice (15%) rather than a body made up principally of ice like Pluto. Eris is 27% heavier than Pluto, for an equivalent size, its density is 2,5g/cm³ as compared with 3 g/cm³ of the moon (the density of water being 1g/cm³)

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Written by : Elisa Di Pietro Translated by: ISLV Based on the work of:


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