June 17, 2010

From Robbie Hatley:

Re: Off-kilter planetary system surprises astronomers (May 24): That article says: Scientists had generally assumed that when more than one planet orbit a star, the orbits share one plane.

Not so. Almost, but the word “had” makes a falsity of this, because up until recently (the last few months, actually), the word “planet” was defined a little more broadly, and included Pluto (which was “demoted” a few months back from “planet” to “dwarf planet”). Pluto’s orbit is inclined about 17 degrees from Earth’s orbit. Furthermore, Pluto is not the only major body in our solar system with unusual orbital dynamics. Uranus is even stranger: even though its orbit is close to being coplanar with Earth’s orbit, it’s orbit is extremely eccentric (long, narrow ellipse, not even close to being circular), and it’s spin axis is tilted roughly 90 degrees to the ecliptic, so we’re often looking at its poles rather than its equator as it orbits Sol. So when researchers found that the orbits of these two distant planets were atilt 30 degrees relative to each other, while it was surprising I’m sure, it was certainly not unprecedented. I believe your article overstates the “surprise” factor, and would have been more accurate and informative if local precedents (Pluto, Uranus) had been mentioned.

Robbie Hatley
Stanton, CA, USA


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