OUT THERE 



(Continued from page 63) 



another picture of Pluto this past Feb- 

 ruary 15. Sure enough, the two dots 

 were exactly where the calculations had 

 predicted they would be. 



Now directly confirmed as Pluton- 

 ian moons, S/2005 P 1 and S/2005 P 

 2 will receive official names from the 

 International Astronomical Union in 

 due course. Meanwhile, two questions 

 spring to mind: How did they get there? 

 And why were they never seen before? 



It's a lot easier to answer the second 

 question than the first. Frankly, no one 

 looked hard enough — or, more to the 

 point, no one could see the two moons 

 well enough in the dark. Remember 

 how Pluto has shrunk so dramatically 

 over the decades? Of course, Pluto nev- 

 er really changed size; it's held steady 

 for eons at about 1,400 miles across. 

 What has changed is our ability to dis- 

 tinguish relatively faint, little objects 

 billions of miles from Earth. The two 

 new moons are only about one four- 



thousandth as bright as Pluto — rough- 

 ly the difference between a bathroom 

 night-light and all the rest of the light 

 bulbs in your house combined. 



As for the moons' origins, the jury is 

 still out, but Weaver and Stern present 

 a plausible scenario. Computer simu- 

 lations suggest that, like the Earth- 

 Moon system, the Pluto-Charon system 

 formed billions of years ago in a giant 

 collision. An intruder may have crashed 

 into the young Pluto, depositing much 

 of its mass onto the planet and so in- 

 creasing its size. The rest of the mater- 

 ial may then have splashed outward in- 

 to orbit around Pluto, forming Charon. 

 Large shards from the impact may have 

 survived while in orbit and eventually 

 coalesced into the two small moons. 



So do Pluto's new moons make the 

 little ice ball into a big player after 

 all? Surely any object big enough to at- 

 tract not one but three moons deserves 

 to be called a major planet? Well, yes and 

 no. Pluto's trilunar system has certainly 

 increased the planet's reputation beyond 

 what the Pluto-Charon duo once had. 

 But some minor planets, a.k.a. asteroids, 

 have moons too. The first such moon, 

 Dactyl, was discovered orbiting the as- 

 teroid 243 Ida, itself a mere thirty-five 

 miles across, by the Galileo spacecraft 

 more than a decade ago. It may only be 

 a matter of time before my planetary- 

 astronomer colleagues find an asteroid 

 with two, three, or even more moons. 



Here's the bottom line: No matter 

 what we call it or how we classify it, 

 Pluto will remain a fascinating object 

 to study. It will no doubt continue to 

 yield big scientific surprises, and ulti- 

 mately, may provide new and funda- 

 mental insights into the history of our 

 solar system. And when New Horizons 

 approaches Pluto and its moons almost 

 a decade from now, wide-eyed and 

 angling for a better look, we will once 

 again see that, in the grand scheme of 

 the cosmos, it's not how much mass you 

 have — it's how you use it. 



Ch.arles Liu is a professor of astrophysics at the 

 City I Mversity of New York and an associate 

 with the Ai}ierican Museum of Natural History. 



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66 



NATURAL HISTORY May 2006 



UNIVERSITY OF CALIFORNIA PRESS 



