Characteristics and Origins of the Solar System

Addendum to Lecture 26

April 12, 2004

Comets

Lectures 26 and 27 deal with comets, one of the most important types of solar system object. There is a good chance that at the end of the semester, we will have the opportunity to see one of these objects, as Comet NEAT may become a naked-eye object.

Our best opportunity to see this object will be in mid-May. A finder chart from Sky and Telescope magazine is shown below.

It should be at its brightest, around naked eye visibility, about May 10.

Sedna

A final point in our inventory of the solar system is to discuss a discovery made about one month ago. It is an object called Sedna , and it is of interest because its discovery paper described is as “ the most distant object ever seen in the solar system”.

It is now at a distance of 76 astronomical units, which is the closest it ever gets.

An artist’s conception of Sedna (together with its possible moon) is shown below.

Let’s continue with some of the facts, and then discuss some of the implications.

First of all, Sedna is big. It is larger than the Kuiper Belt object Quaor, and may be comparable to Pluto in size. See the picture below.

The second point, is that it is very far out. Let’s look at its orbit, shown below.

Question for the august assembly: what kind of objects which we have discussed have orbits that look like this?

You can see that right now, even though it is the most distant object ever seen in the solar system, it is at perihelion. The semimajor axis is 480 astronomical units, and the corresponding orbit period is 10,516 years.

Question for the august assembly: How did I know the orbital period?

Here are the problems with our current understanding of Sedna.

(1) It is comparable in size to large Kuiper Belt objects, such as Pluto and Quoar. However, it is beyond the Kuiper Belt, and known Kuiper Belt objects don’t have orbits which are so elliptical.

(2) It might then be thought to be a representative of the Oort Cloud. But there are problems here, too. It is too close for the Oort Cloud, and it is way too big. Comet nuclei don’t get anywhere near this large in size.

The nature of this object should become clearer in time. It is almost certain that we will discover more of them. Perhaps the most probably explanation is that the solar system had a surprise in store for us, and we have discovered a new class of large, icy worlds way out in the solar system.