Jim Baen's Universe

NonFiction articles

The Universal Diagram

Written by Stephen Euin Cobb

Occasionally, important advances in scientific understanding are made not by gathering more or better data, or by developing a new and radical theory, but by simply experimenting with a new way of displaying the data that everyone has already examined to their satisfaction.

One example is the periodic table of the elements. It's a simple chart of all the chemical elements arranged in order of how many protons each has in its nucleus. Yet this modest little arrangement has revealed numerous secrets of the elements because so many of their relationships are interlocking.

Another example is The Hertzsprung-Russell Diagram of stars which is drawn by plotting as many stars as you wish on a sheet of graph paper such that each one's location depends on its luminosity vs. its temperature. (Usually luminosity is vertical and temperature is horizontal, with the lowest values to the lower right and the highest values to the upper left.) Armed with this simple graphical representation, many unexpected secrets of stellar evolution were made plain, and stars which at first seemed completely different were eventually discovered to be of the same kind, differing only in age.

About twenty-five years ago, based on the idea that I might be able to discover unexpected relationships, I devised a method of charting planets which in the childish vanity of my twenties I called "The Cobb Diagram." I drew it several times by hand using all the planets of our solar system, but with so few planets it was mostly empty space and so nothing of significance could be gleaned of their interrelationships. Today, however, that situation is rapidly changing.

Currently, 210 planets have been discovered outside our solar system, with new discoveries being added every few weeks. Finding more planets which orbit stars other than our sun has become a new kind of international space race. The discovery rate has accelerated during the last few years, and within a decade may well reach explosive proportions.

At least eight different organized searches are currently underway for these extra-solar planets, or exoplanets, and more are in the works. A few include:

The DarwinSpace Infrared Interferometry Project.

COROT–A French photometry mission for stellar seismology and planet detection.

ASP–Arizona Search for Planets (a ground based photometric search for giant planets).

MONS–Measuring Oscillations in Nearby Stars (a Danish space mission to study stellar oscillations, but also capable of doing some transit photometry).

MOST–Microvariability and Oscillations in Stars (a Canadian microsat that uses photometry to measure stellar seismology).

STARE–Stellar Astrophysics & Research on Exoplanets at High Altitude Observatory by Tim Brown.

TEP–A search for transits of the eclipsing binary CM Draconis.

University of California Planet Search Project–The work of Geoff Marcy, Paul Butler, Steve Vogt, Debra Fischer and Chris McCarthy (who have detected most of the known extra-solar planets).

Vulcan Camera–A ground based photometric search for short period transiting giant planets by William Borucki.

And launching in October of 2008–the Kepler Mission from NASA. Kepler is specifically designed to "survey our region of the Milky Way galaxy to detect and characterize hundreds of Earth-size and smaller planets in or near the habitable zone. The habitable zone encompasses the distances from a star where liquid water can exist on a planet's surface." Yes, you read that right. Kepler will spend its entire four-year mission looking for earths.

But big money projects are not likely to be the only thing to contribute in a big way to the coming explosion in exoplanet discoveries. One of the newest, and perhaps most innovative, projects is called Systemic. It's a web-based organization which lets amateur astronomers use their home computers to work with the star wobble data collected by professional astronomers. Participants select a star and tweak variables such as a planet's mass and orbital period by moving a slider on their computer screen. The goal being to design a planetary system that best matches the measured wobble for that star. Over 750 amateur planet hunters are involved so far, and you can join them by going to http://oklo.org/?page_id=33

Because of all this activity and enthusiasm and money and fascination, and because of the potential universal access and participation provided by the World Wide Web, I estimate that within three years new planets will be discovered at approximately one per day. What's more, I estimate the complete list of exoplanets to reach a thousand within four years, ten thousand within ten years, and a hundred thousand within twenty years. And I know that last estimate sounds impossible but, in truth, it's not actually going to take that long. I just wanted to make it easier for you to accept. Exponential progressions can be so hard to swallow sometimes.

But even if my own estimates are exceptionally generous it remains clear that we will soon to be faced with an abundance of exoplanets. And because of this I feel it is time we had a means of arranging them in a logical order. Ideally this logical order should be one that allows us to uncover relationships that would otherwise remain hidden. So out of the drawer comes my old method of charting planets.

While it's obvious that every planet is unique based on dozens if not hundreds of variables, I feel that three are most fundamental. These three properties working together may cause as much as 97% of a planet's unique identity. Using this notion, every planet can be reduced to three numerical values, and these values can then be plotted within a three dimensional cube.

Back in 2002, while toying with this system, I surprised myself by realizing that there is no need to limit its use to planets. By shoving the cube's metaphorical walls out a bit farther, the three numerical values could just as easily define stars. And having realized this, I next realized that all celestial objects could be plotted within the cube.

The three numerical values needed for this plotting are: 1–mass (obviously), 2–surface heating (usually received from an outside source, such as the earth's surface being heated by sunlight) and, 3–internal heating. This internal heating can be from thermonuclear fusion, as we see in stars; or it can be from tidal friction, or radioactive decay, or even gravitational collapse. The source doesn't matter, only the amount.

But you must wonder: why these three values?

Mass is clearly the most important and hardly needs any explanation, but you might wonder why I chose energy for both of the other two values instead of, perhaps, chemical composition. Simple: in this universe, with the exception of the short-lived Population Two stars, almost all objects