* Astronomy

Whether you call it a planet or not, Pluto has officially been overtaken by a more massive planetary object Eris (previously nicknamed Xena).
Pluto, traditionally known as the ninth planet of our Solar System, sits in a giant zone called the Kuiper belt that is filled with asteroids and many other planetary bodies. The discovery of more and more objects in this zone including Eris, in 2005, which was found to be bigger than Pluto led astronomers to try to more strictly define what is and isn't a planet. In 2006, members of the International Astronomical Union (IAU) at a meeting in Prague, the Czech Republic, voted that Pluto be reclassified as a 'dwarf planet'. Eris, by these terms, sits in the same category.

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Title: First Constraints on Rings in the Pluto System
Authors: A.J. Steffl, S.A. Stern
(revised v2)

Simple theoretical calculations have suggested that small body impacts onto Pluto's newly discovered small satellites, Nix and Hydra, are capable of generating time-variable rings or dust sheets in the Pluto system. Using HST/ACS data obtained on 2006 February 15 and 2006 March 2, we find no observational evidence for such a ring system and present the first constraints on the present-day I/F and optical depth of a putative ring system. At the 1500-km radial resolution of our search, we place a 3-sigma upper limit on the azimuthally-averaged normal I/F of ring particles of 5.1x10^-7 at a distance of 42,000 km from the Pluto-Charon barycentre, the minimum distance for a dynamically stable ring (Stern et al., 1994; Nagy et al., 2006); 4.4x10^-7 at the orbit of Nix; and 2.5x10^-7 at the orbit of Hydra. For an assumed ring particle albedo of 0.04 (0.38), these I/F limits translate into 3-sigma upper limits on the normal optical depth of macroscopic ring particles of 1.3x10^-5 (1.4x10^-6), 1.1x10^-5 (1.2x10^-6), 6.4x10^-6 (6.7x10^-7), respectively. Were the New Horizons spacecraft to fly through a ring system with optical depth of 1.3x10^-5, it would collide with a significant number of potentially damaging ring particles. We therefore recommend that unless tighter constraints can be obtained, New Horizons cross the putative ring plane within 42,000 km of the Pluto-Charon barycentre, where rings are dynamically unstable. We derive a crude estimate of the lifetime of putative ring particles of 900 years.



Drizzled HRC F606Wimages of the Pluto system. The image on the left is from the 2006 February 15 visit, while the image on the right is from the 2006 March 2 visit. Pluto, Charon, Nix, and Hydra can be clearly seen. All other features are due to the extended PSF halos around Pluto and Charon or artefacts introduced by the incomplete removal of background star trails or cosmic ray events. The diffraction spikes from Pluto have been used to divide each image into four quadrants. 1500 km width annuli at the orbital distances of Nix (left) and Hydra (right) are shown in red. Four 10° segments (half width), centred on each diffraction spike, are excluded from analysis. A one arcsecond scale bar is shown in the upper left of each image.

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Astronomers have approved the historic new planet definition guidelines today, and downsizing Earth's neighbourhood from nine principal heavenly bodies to eight by demoting Pluto.

On the night of 5/6 August 2006, the newly discovered Pluto satellite Hydra (alias 2005/P1) will occult a star, 2UCAC 26034796, with magnitudes respectively

mK = 12.7 , mH = 12.9, mJ = 13.7, mI =14.8, (rUCAC = 16.2) mV = 16.5.



This rare event might be seen from somewhere in Spain, Portugal (00h04), or Canary Islands (00h05), or Africa and South America (00h13). The faintness of the star requires a large enough telescope and a sensitive camera. The star and Hydra will be about 3 arcsec south of Pluto (mK = 12.9, mV = 13.9) : it is a MUST to resolve the images of Pluto and the star on different pixels.



Depending on Hydra's albedo (m = 22.9), the satellite brightness implies a diameter ranging from 50 to 150 km, i.e. a poorly constrained size. Observing such occultation would provide constraints on the satellite size, an important step in determining its albedo, hence its nature (bright icy body vs dark cometary material) and origin. Size can also constrain Hydra's mass, with important consequences for better estimating its resonant interaction with Nix

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