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.

Sedna · Grey Nicholson

Planet X-2

Planet X