The Open Source Guide to the Solar System

What is a planet?

In 2002, Chad Trujillo and Mike Brown at Caltech in Pasadena found Quaoar, a large object in the Kuiper Belt, the area of the solar system beyond the planet Pluto. It's the largest object found in the solar system since Pluto's discover in 1930.

The Kuiper belt is assumed to be the origin place of comets, large chunks of ice circling around outer edge of the solar system. Quaoar appears to be at least partially made of ice. It's larger than the largest asteroid but it's smaller than Pluto. It orbits the sun. Perhaps it's too icy to be called a planet.

Quaoar isn't the only large object found on the edge of the solar system. In the case of Varuna, another near-planet-sized object found by a different team, the object is definitely made of rock, not ice. According to Chad Trujillo's web site, there may be things in the Kuiper belt that are as big as other planets in the solar system. So if astronomers find something big like a planet that moves like a planet and consists of material like a planet, is it a planet? Trujillo says no.

On the other hand, some astronomers are using the Hubble Space Telescope to look for wobbly stars. If the stars wobble, they say that the stars must be surrounded by planets. These objects would be completely unlike any planet seen in the solar system, more like failed stars than the planet Earth. And they're basing the conclusion on the study of only one star, the Sun. So, based on the examination of a single case, something that can't be seen, something unlike anything we know of, something that causes a phenomenon unlike anything we've seen, is a planet? These astronomers say yes.

Good science alone would suggest that if one found wobbly stars that the conclusion should be "We found some wobbly stars". The limited data on other stars would not allow one to infer why the stars wobble. Saying "we found a planet" is not science: it's speculation. True, it could be a super-sized planet, but it could also be some other phenomenon that we just haven't encountered before. And, in the case of stars, our limited knowledge makes that a real possibility.

Screenshot: Celestia showing Mars as viewed from Phobos, one of its moons

I started wondering about these questions when I was playing around with Celestia, the open source space exploration program. Celestia runs on Linux, Mac and Microsoft Windows. I had trouble with Celestia under SuSE 10.1--it seemed to lock up shortly after starting unless I was running under root. Using an accelerated 3D driver for your graphics card is highly recommended. If you use an ATI card, ATI provides fast drivers on their web site.

Using OpenGL, Celestia takes a person on a 3D virtual trip through the solar system. What's it like to stand on Phobos and look down on the Martian surface? As this screenshot shows, it's no problem. Wherever possible, Celestia uses actual astronomical data and real photos of the planets. So in the case of Phobos, that's really where Phobos is in relation to the Martian surface and that's what you'd see if you were really there right now.

Screenshot: Use a Celestia plug-in to see all 3 of Pluto's moons

A wide range of plug-ins are available through Celestia Motherlode. These extend Celestia by providing additional astronomical objects (like Pluto's two newly discovered moons), alternate views of planet surfaces (such as a view of Venus' surface beneath its cloudy covering) or even spacecraft (such as the Mars Global Surveyor).

Screenshot: Relive Voyager 2's Flyby of Neptune

Much of what we know about the solar system has been discovered in the past 20 years. There was only one spacecraft to visit Neptune--Voyager 2 on August 25, 1989--and almost everything we know about Neptune comes from this one visit. Voyager revealed new moons that were previously unknown, and Neptune's faint ring system. But as to the source of the planet's deep blue colour, scientists still do not have an answer.

Celestia is a great summary of 20 years of astronomical research. It reminds us of what a vast, complicated place the universe is, how much we've discovered and how much is still unexplored.

Talk back on the Linux Cafe.

August 23, 2006 

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