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Solar System

Twice in a blue moon

Nature volume 390, pages 441–443 (1997)Cite this article

The new satellites are ‘irregular’ satellites, in that their orbits are relatively far from the planet, with large eccentricities and inclinations compared with the ‘regular’ satellites. They orbit Uranus at a mean distance of roughly 5.8 million kilometres (227 Uranian radii) — although the orbital parameters are less certain for U1 than for U2 — and are estimated to have diameters of 80 and 160 km, respectively2. News of the discoveries was greeted with relief: until now, Uranus was the only giant planet without irregular satellites of its own (Jupiter currently holds the record with eight irregulars; Neptune has two and Saturn one). Scientists generally do not like deviations from patterns, and are pleased that Uranus has now joined the fold, making irregular satellites a common trait of all giant planets. But however pleased astronomers may be, they cannot help but be reminded that the origin of irregular satellites eludes them.

It is generally accepted that, unlike the regular satellites, the irregular satellites did not form in situ. The regular satellites accreted within circumplanetary disks, but the irregular satellites must have been captured from elsewhere, most likely from orbits around the Sun. So how can a body initially in an unbound orbit (from the planet's perspective) become bound to the planet?

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Figure 1: The discovery of U2.

BRETT GLADMAN

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  1. Jane Luu is in the Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, 02138, Massachusetts, USA

    Jane Luu

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  1. Jane Luu
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Luu, J. Twice in a blue moon. Nature 390, 441–443 (1997). https://doi.org/10.1038/37225

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