Is It or Isn't It

I’m sure you’ve heard by now about last month’s IAU vote officially demoting Pluto. According to the IAU, Pluto is no longer a planet; it’s a “dwarf planet”.

The decision established three categories of bodies orbiting the sun: 1. Planets: objects which are large enough to clear their orbits of any other bodies (aside from moons, presumably). This includes the first eight planets from Mercury to Neptune. 2. Dwarf planets: objects which are large enough that gravitation makes them round, but not large enough to have cleared their orbits. This category includes Pluto, and also some of the larger non-planetary bodies such as Ceres and the recently discovered 2003 UB313. 3. Small Solar System Bodies: anything too small to be a dwarf planet.

The definition has some confusing holes. For example, many of the news articles describing the decision say that Pluto fails the “planet” test because its orbit overlaps with that of Neptune: therefore Pluto hasn’t cleared its orbit and isn’t a planet. But wouldn’t that eliminate Neptune from the planet club as well?

I’m guessing that the news articles got that part wrong, and the non-orbit-clearing that disqualifies Pluto is actually the nearness of other Kuiper belt objects. That makes more sense, but it has holes too. Earth shares its orbit with quite a number of Earth-crossing asteroids — at least 300 have been identified, of course there are a lot more than that: one estimate is that there are around 1500 larger than a kilometer, and 135,000 larger than 100 meters. So by the new definition, Earth can’t be a planet either.

Nor can Mars, since there are plenty of asteroids that veer over into Mars’ orbit. And what about the thousands of Trojan asteroids sharing Jupiter’s orbit? If the new definition doesn’t even include Jupiter as a planet, what good is it?

Not all scientists agree with the IAU’s decision. Dr Alan Stern, leader of the New Horizons mission currently winging its way to Pluto, has been outspoken in his criticism of the decision — not just on scientific grounds, but on political ones (the vote was apparently held on the last day of the meeting, when many attendees were already on their way home; 424 attendees voted, which, Stern notes, represents less than 5% of the world’s astronomers). More than 300 astronomers have signed a petition disagreeing with the IAU’s definition, and vowing not to use it. Some textbook authors and museums are scurrying to make updates, but others are holding off, expecting a backlash.

I don’t think we’ve heard the end of this yet. We may not get Pluto back as a planet; but I hope we at least can look forward to a clearer definition of exactly what constitutes a planet, one on which astronomers can agree. In the meantime, SJAA member Mark Taylor points out one up side: it just got a lot easier for an observer, regardless of location or telescope size, to see all the planets in a single night!

The object of all this controversy is sitting placidly at the foot of Ophiuchus, within reach of medium to large amateur scopes. It’s already past the meridian by the time the sky gets dark, and is fairly far south so it never gets very high. There’s still time to catch it if you start early, but it’ll be a bit more work than it was a few months ago.

Wait — how can it be in Ophiuchus? That’s not a sign of the Zodiac — don’t planets move along the ecliptic and therefore stay within the Zodiacal constellations?

This is an example of the differences between astronomy and astrology . The official boundaries of the astronomical constellations don’t always match the boundaries of the zodiac, which is set up so that each zodiacal sign spans a roughly equal distance along the ecliptic, one sign per month. The astronomical constellation boundaries follow no such restriction. In fact, the constellation Ophiuchus occupies quite a bit more space along the ecliptic than its Zodiacal neighbor Scorpius does, and planets do pass through Ophiuchus’ boundaries. (In fact, last month Pluto was just barely over the border from Ophiuchus into Serpens Cauda, which does not itself reach the ecliptic, but is close enough to it that Pluto’s inclined orbit takes it inside the constellation boundaries.)

That said, an alert reader pointed out that I erred in past months in placing Uranus in Ophiuchus. Oops! It’s actually in Aquarius. This month it’s just south of lambda Aquarii, which should make it a snap to find in binoculars or a telescope. Neptune is in Capricornus, a bubble rising from the top of the “champagne glass”. They’re both well up by the time the sky gets dark, and transit a few hours before midnight, so they’re ideally placed for early evening observers.

Saturn rises about an hour after midnight and will be available to morning observers.

Jupiter is still visible, barely, in the evening twilight, but as the month progresses it will disappear in the sun’s glow. In the latter half of the month Mercury joins it.

Venus and Mars are lost in the sun’s glare and are both in conjunction this month, Mars on the 23rd and Venus (superior conjunction) on the 27th. We won’t see either of them again until December, when Venus will emerge low in the evening sky and Mars in the morning.