Whoa. Nine? Ten? Keep counting ...

[unixronin]

First, james_nicoll pointed out this New Scientist article, followed shortly by this one from Nature, about 2003EL61, a trans-Neptunian object at first thought to be larger than Pluto, now estimated at a quarter to a third of Pluto's mass, but with a moon of its own accounting for the earlier overestimation of its brightness and hence size (and from the orbit of which the new mass estimate for 2003EL61 was derived).

Now, dmmaus just pointed me at this Sky & Telescope article about 2003UB313, a Kuiper-belt object in a highly inclined (44° from the ecliptic) orbit, which appears to be between Pluto and Luna in size.

"We tried looking at it with the Spitzer Space Telescope and didn't detect it.  So we have an upper limit on the size.  It can't be any more than 3,000 kilometers across^[1]," says Brown.  But the lower limit derived from its brightness — even by assuming its surface is 100 percent reflective^[2] — still makes it larger than Pluto, which is 2,250 km (1,400 miles) across.

So, how many planets DOES this system have?  Ten?  If Pluto's big enough to be a planet, 2003UB313 certainly is.  Eleven, if we count 2003EL61?  Twelve, if 1,600km Sedna qualifies?  Thirteen, counting Quaoar, at 1,250km?

And by all indications, we've barely begun to chart the Kuiper Belt -- especially if there's bodies out there in orbits this highly inclined.  There could be monsters out there, in arcs above and below the ecliptic we've never even thought of looking for planets in.

If we assume the upper limit of 3000km for 2003UB313, and we assume it's a mostly rocky body^[3], a back-of-the-envelope calculation suggests a surface gravity on the order of .12G, just under an eighth of a gee, three quarters of the Moon's gravity.  That's probably enough gravity to establish a permanent base on it someday, if we could get out there.  Even if it's no bigger than Pluto, somewhere around .08G is still a reasonable assumption, just shy of a twelfth of a gee, half a Lunar gravity.

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^[1] Unless, of course, it's a very dark body indeed, which appears not to be the case.

^[2] Emphasis mine.

^[3] I'm not sure how a dirty-snowball structure would affect the numbers.  It's probably too far out and too cold to have much of a gaseous atmosphere.

Comments

[johnkzin.livejournal.com]

I'm betting on 12. It would match a certain symmetry with the writings of Zacharia Sitchen and Sumerian mythology.