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SEDNA

The astronomical discovery

of a new breed of planets
Sedna, artistic rendition

Artistic rendition of Sedna, its possible moon, and the Sun




With the discovery of Sedna on November 14, 2003, the boundaries of the Solar System expanded by several billion kilometres (or miles if that's how you learned to count).

Most striking about Sedna is its orbit and its size. It takes approximately 10,500 year for Sedna to orbit the Sun in a highly elliptical orbit. By comparison, Pluto, which for most of the 20th Century was the outermost planet, takes about 248 years to go around the Sun. As for size, Sedna, at about 1,180-2,360 km (730-1,470 miles) in diameter, is larger than Quaoar and almost as big as Pluto.

Sedna, officially known as 2003 VB12 by astronomers, was discovered while its highly elliptical (egg-shaped) orbit brings it closer to the Sun. The closest it gets (known as the perihelion, which will be later this century in the 2070s) is 76 AU, and the farthest away is 990 AU, but that will be another 5,000 years from now, so don't hold your breath waiting. The orbit was pinned down following the discovery of a photograph in the early 1990s which contained Sedna but nobody then had noticed.


Sedna distance from Sun

Note to newbies: AU is astronomer-speak for Astronomical Unit, or the distance between the Earth and the Sun, about 150 million kilometres or about 93 million miles. Do the math, kids. Sedna ranges from approximately 11.5 billion kilometres (7 billion miles) to 148.5 billion kilometres (92 billion miles) from the Sun. By comparison Pluto averages about 39 AU from the Sun.

What the AU measurements tell us is that Sedna is anywhere from 76 to 990 times further away from the Sun than Earth. How far away is that? Well, if you stood on the surface of Sedna and held up the head of a pin at arm's length, it would cover the Sun.

Not that you'd want to do that, because it's so bloody cold. Surface temperatures are estimated to be around -240° C  (-400° F).   That cold temperature inspired the discovering astronomers to name this new planetary body after the Inuit goddess Sedna who mythology says lives at the bottom of the frigid Arctic ocean. They're even recommending to the astronomical stuffed shirts who govern the naming of planets that astronomers name further planetary discoveries (and they think there'll be plenty) in this part of the solar system after arctic deities. We'll see on that one.

Scientists now have a wealth of intriguing factoids to explore with Sedna. Among them: Sedna has a deep red colour, almost as deep as the red colour of Mars, and this colour is at present unexplained. And don't ask what Sedna is made of, except that water ice and methane ice have been ruled out. There's some juicy research grants to be had to figure out this one. But what is known is that Sedna also rotates on its axis once every 40 days, a particularly slow rotation period (only Mercury and Venus rotate more slowly), which for a variety of complex mathematical reasons leads astronomers to conclude that Sedna likely has a moon, as yet undiscovered. The astronomers promise us investigation by the Hubble space telescope at some unnamed future date to check out this potential moon situation.

Given that only 15 per cent of the solar system in the area where Sedna is has been explored by photography, astronomers figure that there will be more large planetary bodies like Sedna (and likely larger) in the remaining unphotographed and unexplored 85 per cent.

So just who are these astronomers? You will not be surprised that the guy who discovered Quaoar, Chad Trujillo, was on the team, along with another Southern California astronomer, Mike Brown, and a Yale astronomer, David Rabinowitz. They were using the Samuel Oschin Telescope at the Palomar Observatory east of San Diego, California. In the past few years cutting-edge sophisticated technological capability has been invented to photograph the deep recesses of the solar system, and these scientists, seeing a good thing as it were, are making a career for themselves with these photographic explorations. Foreshadowing further discoveries, they described the discovery of Sedna as "the tip of the iceberg." Given Sedna's temperature, not a bad double entendre.

Astronomers speculate Sedna and it's yet undiscovered companions in the outer reaches of the solar system were put in place who knows how many hundreds of millions, if not billions, of years ago by vast galactic forces, the likes of which our solar system has not been subject to for eons. They speak of a close pass to our solar system by a travelling star which caused all sorts of planetary phenomenon and orbital shake-ups to occur. Or perhaps the birth of our Sun in a tight knit cluster of other stars which have now migrated elsewhere, Sedna being a left-over from the star-birthing process. So we're talking about the early days of the creation of the solar system here. Remember that the Big Bang which created the Universe was some 13 billion years ago and Earth is some 4 billion years old, so this would have been in the early days of Earth or earlier. (Bet you haven't smoked any stuff this cosmic in a while.)

Sedna is far enough away to be near the postulated Oort cloud of planetary bodies way, way out at the far edge of the solar system, although Sedna isquite a bit closer ( ! ) than where scientists had predicted the Oort objects would be found. It leaves them scrambling to re-arrange some of their theories. But they acknowledge that if the elliptical orbit of Sedna is typical, then likely there are several more planetary bodies at the distant end (aphelion) of their orbits and not visible with current technology. Of course, give the several thousand-year orbits involved, these scientists had better get either a very long life or some even more sophisticated telescopes to discover the other planetary bodies.

Yet it's clear the astronomers have unwittingly stumbled on to a new category of planets with these 10,000-year-plus elliptical orbits and are trying to wrap their scientific minds around what they've found. It does change the definition of our solar system quite significantly. Yet the scientific reaction actually has been predictably amusing. Astronomers are denying Sedna is a planet at all. Well, yes, this planetary body does exist, they harrumph grumpily, but it's not really a planet at all. Then they intone solemnly that Sedna is really a planetoid, and scientists go to huge lengths to assure us that a planetoid is something quite different from a planet.

Uh, yeah, right guys. A planetoid. Like okay, if you say so. Point is, you've just discovered something big out there, so we'll cut you some slack on the nomenclature.

For those of you with a high-speed connection, click here for a 1 MB mpeg video showing the distance of Sedna's orbit from the Sun. For the rest of you on dial-up, check out the image below, which more or less duplicates the video and gives you a sense of how far away Sedna is from the Sun.



Sedna's orbit



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