Trans-Neptunian Objects (TNOs)

 

July 1, 2005 (with diagram)

 

http://cainer.com/ericfrancis/july1.html

 

Hi Eric,

 

You recently wrote, "The TNOs (objects in and beyond Pluto's orbit) talk..." That's one way to say it, but isn't it a bit confusing to define them as using Pluto's orbit when you are talking about trans-NEPTUNIANS? I constantly find astronomy so difficult to understand, but the Minor Planet Center (MPC) put the definition as: "Trans-Neptunian objects have orbits with semi-major axes beyond the orbit of Neptune." At first I couldn't be sure of the meaning of "semi-major axes" as I'm not an astronomer nor do I speak English so well, but I checked that "semi-major axes" is the same as "the mean solar distance" of a planet.

 

I don't know how much the language barrier complicates things for me, but although the name "TNO" should already say it all, it has sometimes been difficult for me to understand in which category a certain object belongs to and why (because of elliptical orbits and all the orbit-crossings, etc.), but now the MPC's definition has made it much more simple for me: I just need to compare the semi-major axes of an object with Neptune's distance from the Sun.

 

Love,

 

K

 

 

Dear K,

 

Thanks for this question. Actually, your English is amazing, just to get that bit out of the way. I think you explain it well, but I want to offer a diagram and a bit of a caption to illustrate the idea of a trans-Neptunian planet. This is the continuation of the asteroid discussion last week, with a link further back in the archives on Sedna and Quaoar provided there.

 

There is this idea floating around that "Pluto is the furthest planet from the Sun." Schools still teach about the "nine planets." This is a little like those old cartoons in MAD magazine where there's a map in front of the classroom that lists Africa as "unexplored." (Some of us actually remember this. Just think -- unexplored. They were very old maps.)

 

A bit of history is in order. In 1781, the first planet outside the traditional seven visible bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn) was discovered -- Uranus.

 

Actually, Galileo got a look at Neptune through his telescope in the winter of 1612-1613 -- but in true Neptune style, he did not think that it was a planet. Rather, because it was close to an exact station, he thought it was a fixed star. So Neptune remained undiscovered until one evening in 1846 when, based on the mathematical calculations of the French astronomer Le Verrier, the enormous, blue world was found by German astronomers after just a half an hour of searching where Le Verrier told them to look. This proves scientifically that the French can be quick and efficient when they really want to be.

 

Bearing in mind that the discovery of asteroids began in 1801 with Ceres, proceeding more or less continuously throughout the period of discovery of the additional major planets (and to the present day), the next big discovery was Pluto. That was in 1930. Unbeknownst to scientists, they had not discovered just a planet, but rather a whole region in space where millions of bodies hang out, some of which get pulled into the inner solar system and become comets, centaurs or asteroids. This was eventually called the Kuiper Belt, after the scientist who theorized its existence.

 

The Kupier Belt, a kind of wide, cloud of relatively small icy planets and asteroids, remained theoretical until the 1992 discovery of (15760) 1992 QB1. This was the first planet discovered beyond Pluto. And I'll bet you a pint you've never heard of it unless you read my more esoteric articles, as I love to mention this interesting bit of space debris.

 

Pluto does have a moon, called Charon (not to be confused with Chiron), which was discovered in 1978. That, technically, is the second Kuiper object discovered (after Pluto). But Charon is so close in size to Pluto as to be considered a binary planet with Pluto. What we call Pluto is really two little planets orbiting each other. Until the actual discovery of the Kuiper Belt in 1992, Charon was just considered a satellite of Pluto. Now it, too, is considered a Kuiper object.

 

Then QB1 was discovered and a whole new discussion began. Yet despite having been discovered 13 years ago, QB1 remains unnamed, which is really interesting, considering that a wide variety of small planets beyond Neptune have since been discovered and named (these include Varuna, Quaoar, Sedna, and a bunch of others).

 

Once QB1 was discovered (the same year as the centaur planet Pholus, much closer to the Sun), the floodgates opened, and astronomers began to classify two categories of things beyond Neptune: Plutinos (little Plutos, things with orbits of around 248 years) and Cubewanos (after QB1, with longer orbits). Both are considered TNOs or trans-Neptunian objects. In both cases, their average distance from the Sun is more than that of Neptune. And both Cubewannos and Plutinos are members of the Kuiper Belt (KBOs). Astronomy loves to name things. That's what the word means -- naming the stars.

 

Astronomers have come up with a naming scheme for the KBOs Plutinos are named for underworld gods. Cubewanos are named for gods of creation and resurrection. Not that astronomers believe in the gods; that's the job of astrology.

 

Let's use Varuna, a Cubewano, as an example. Varuna was discovered in 2000 and was given minor planet catalog number 20,000. It was considered by science to be a vastly important discovery, and in truth it should be equally respected by astrology. It happens that Varuna is in mid-Cancer and next week's Cancer New Moon is exactly conjunct this meaningful little planet.

 

It's named for the supremely important pre-Vedic creation deity, who was demoted to the lord of waters by subsequent kingdoms. Still, there is no way to actually demote a god. He has been described as a force that is "behind everything." My keywords for Varuna include "the great equalizer." One of Varuna's themes, both astrological and mythological, is the punishment of liars.

 

Here is the orbital diagram:

 

The Sun is at the center, but it's not drawn in. The purple circle is the orbit of Jupiter. (The orbits of all the other planets and most of the asteroids are so small that they can be contained way inside the orbit of Jupiter.) The beige or gold one, second orbit out, is that of Saturn. The dark blue one is Uranus. The green one is Neptune.

 

The light blue orbit is that of Pluto. Note that Pluto is an orbit crosser, entering Neptune's orbit in early Virgo and leaving in mid-Sagittarius.

 

The circle that's partly pink and partly gray is Varuna. Notice how long the orbit is; it takes Varuna 283 years to go around the Sun as compared to Pluto, which takes about 248 years. Unlike Pluto, Varuna has a fairly circular orbit, and Pluto crosses the orbit of Varuna as well.

 

The diagram shows how Varuna's aphelion, or furthest point from the Sun, is in Libra, and how its perihelion, or closest point, is in Aries.

 

Like the Moon, all planets have nodes, the point at which they raise north or south of the ecliptic (the apparent path of the Sun around the sky). For Varuna, the planetary nodes are in Cancer and Capricorn.

 

That provides a visual aid to see what some of these orbits look like. Many of the trans-Neptunians have far longer orbits than Varuna, and some are shorter. The diagram is courtesy of Robert von Heeren, my friend, colleague and centaur astrology teacher. You can reach Robert at http://robertvonheeren.de/

 

Catch you next week! Don't forget to tune into PlanetWaves.net for ongoing coverage of the Deep Impact probe and the Cancer New Moon. And remember to subscribe! Your subscription not only gets you the best weekly horoscope in the business, an essay each week and much besides. It helps support this Q & A project and the massive, free Planet Waves web site. Thank you for that.

 

Signing off from Montreal, Quebec, this is Eric Francis.