Planet X · Grey Nicholson

Planet X-3: Pluto Still Isn't a Planet, Neither Are 2003 UB313 and 2003 EL61

2005-07-30T13:59:00+00:00

They finally found it - again - another tenth planet, this time unnamed but temporarily designated 2003 UB313. It was also announced yesterday that 2003 EL61 may be roughly 70% Pluto's size.

UB313 however, is definitely larger than Pluto, although perhaps only slightly. According to the Washington Post, Mike Brown has labelled the object the tenth planet; they acknowledge that there are astronomers who disagree with this label, but don't mention that Mike is actually one of them. As I wrote about last year, he doesn't consider Pluto to be a planet, and argues that if Pluto is considered a planet, many other objects (including Sedna) must logically also be considered planets.

So UB313 must logically be classified as a planet as long as Pluto is. I'd be willing to bet that by this time next year there'll be another object found to be larger than Pluto, if not several; then, the International Astronomical Union will have to reconsider their definition of what constitutes a planet (or come up with one). For now, if you're asked how many planets there are in the solar system, it's definitely wrong to say “nine”.

Discover Magazine discusses the outer solar system in more depth in their November 2004 issue; you'll have to log in to read past the first page of the article, which usually entails using BugMeNot.

Planet X-2

2004-03-15T19:52:00+00:00

The press release is now online, as is Mike Brown's Sedna page, at http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ and http://www.gps.caltech.edu/~mbrown/sedna/ respectively. Right then - Sedna isn't a planet - it's not even a Kuiper Belt Object. According to Mike Brown, the Kuiper Belt has a fairly sharp edge at 50AU (1 Astronomical Unit is the distance from Earth to the Sun, 150 Gm); Sedna comes no closer than 70AU. The appropriate term for Sedna is “Inner Oort Cloud Object” (and I'd like to take this opportunity to lay claim to the acronym “IOCO”).

The Oort Cloud was theorised by a bloke named Oort, hence the name. It's a cloud (duh) of small icy bodies which inhabits the outer reaches of the solar system; Oort inferred the cloud's existence to explain the origins of dirty snowballs (comets). The Oort cloud is thought to lie much further out than this; it seems there is also an Inner Oort Cloud, apparently created when a passing star... passed.

Mike Brown also gives a very interesting suggestion of how to define a planet. He says a planet is any body whose mass is greater than the total of all other masses in similar orbits. By this definition, Pluto isn't a planet... but we knew that already.

Planet X

2004-03-15T16:20:00+00:00

Well, they finally found it, “Planet X”, a.k.a. Sedna. Of course, if it is decided that it is actually a planet, the name will have to be changed to a Roman god. Sedna is the Inuit goddess of the ocean, and the Roman god of the ocean - Neptune - is already taken.

Interestingly, the Discovery Channel website reports (http://dsc.discovery.com/news/briefs/20040315/planet.html) that the body appears to have a companion (read: “moon”), which is the second reddest object in the solar system, after Mars. Wow, these NASA fellas can even measure redness accurately! Since the rules of moon-naming are less strict than those for planet-naming (or rather there's a justifiably greater set of allowed names), perhaps this moon will end up being called “Sedna”?

Of course, the big question is “is it a planet?” Well, if size is your criterion, most probably yes, given that Pluto is officially a planet and Sedna is (expected to be) not much smaller, and may even be bigger than Pluto. But size isn't the best criterion - Pluto is actually smaller than the Moon, and the Moon's not a planet (although astronomers sometimes describe Earth-Moon as a double-planet because of their relative sizes).

There's some debate over whether Pluto should qualify as a planet - it's usually described as a Kuiper Belt Object (KBO). (The Kuiper Belt is a field of small, rocky bodies orbitting beyond Neptune - essentially a second Asteroid Belt.) Pluto, Sedna, Quaoar et al. are rightly not classified as asteroids - they're too big - but they're basically the same thing... just bigger. The truth is there's really little intrinsic difference between the smallest of space rocks (meteorites and the like) and much larger objects like Mercury and the Moon.

Sedna orbits the Sun at three times the distance of Pluto (taking a whopping 10,500 years to do so). If Sedna is actually a KBO, that means the Kuiper Belt must be absolutely colossal. If it's a KBO, it's as much a planet as Pluto. If it's not part of a belt, then I think it must qualify as a planet.

NASA will give a press conference at 18:00 UTC.

Pluto Isn't a Planet

2004-02-21T02:45:00+00:00

Officially, Pluto is a planet. In reality, it's a member of numerous Kuiper Belt objects which orbit the Sun beyond Neptune. When it was found, 74 years ago, Clyde Tombaugh was searching for a planet and so assumed what he had found was one.

Another large Kuiper Belt object was found a few days ago, as has been reported by the BBC (http://news.bbc.co.uk/1/low/sci/tech/3506329.stm) - 2004 DW. Its diameter is estimated as between 1500km and 2600km; Quaoar, another Kuiper Belt object found in 2002, is about 1200km across; Pluto is 2320km across. The evidence is mounting that Pluto is simply a large example of an ordinary Kuiper Belt object. If 2004 DW turns out to be as large as, or larger than Pluto, will it be called a planet?