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From: mcgeer@JI (Rick McGeer)
Newsgroups: net.space
Subject: Re:  Tenth planet
Message-ID: <8512201843.AA13562@ji>
Date: Fri, 20-Dec-85 13:43:40 EST
Article-I.D.: ji.8512201843.AA13562
Posted: Fri Dec 20 13:43:40 1985
Date-Received: Sun, 22-Dec-85 00:13:42 EST
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	As I recall, the existence of a 10th planet is pretty much taken
for granted in the astronomical community.  But it's unlikely we're going to
find it, for two reasons:

(1) It will be dim.  Isn't it true that Pluto has a very high albedo?  Assuming
that the 10th planet is a gas giant, it will have a much lower albedo than
Pluto and will also be further away from the Sun -- hence much dimmer than
Pluto, and Pluto is something like 10th magnitude anyway.

(2) It will have a very low angular velocity, so much that it will be extremely
difficult to pick out from among the fixed stars.  Assume Bode's Law holds
(I know, most people think it's an interesting bit of numerology, nothing more.
But it's the only assumption I have, and, anyway, you can make a quasi-case
for it by talking about the distribution of particles in the gas cloud that
became the solar system).  Anyway, under that assumption, planet X is at 77.2
AU, way out in the boonies.  By Kepler's Harmonic law, that makes its period
77.2^3/2, or:
678 years, which works out to an angular velocity of about 32 minutes of arc
(about half a degree) per year.  Pluto, by contrast, has about a degree and a
half of arc per year, or about three times as much -- and it took years of
Tombaugh's time on a flicker machine to spot Pluto.

There's one other objection, too -- previous planet hunts took place in and
about the ecliptic, in a pretty narrow band of sky.  I've heard speculation
that eccentricity increases with distance from the sun, so you'd have to search
a wider sky band, too.

					-- Rick.