Image: Vega system
Harvard-Smithsonian Center For Astrophysics
An artist's view of a possible planet being born, seen as yellow clumps of dust and gas at two locations in its orbit around the star Vega.
By Senior Science Writer
updated 12/16/2003 12:36:48 PM ET 2003-12-16T17:36:48

"Solar system" is fast if grudgingly becoming the cosmic Kleenex or Xerox, a phrase used generically to describe other combinations of stars and planets.

A Space.com article last week about possible planets around the star Vega spurred readers to question the use of the term "solar system" to describe the faraway setup.

"In that the planets in question orbit around Vega rather than Sol, isn't it the vegan system, rather than a solar system?" asked Richard Banyard of Wayne, Pa.

The answer, like much having to do with astronomy, is evolving. Interviews with experts and a review of institutional usage show that the issue is far from settled. Astronomers are in fact wrestling to define many terms in the present era of rapid discovery.

For example, the space just beyond Neptune is loaded with small objects packing a dizzying array of labels -- KBOs, EKOs, TNOs -- that astronomers don't agree on. The word moon has a minor identity crisis as researchers realize there is no lower limit to the size of rocks orbiting Jupiter and the other large planets.

Even the word "planet," perhaps the most controversial astronomical term of all, has no official definition.

Extrasolar argument
In writing, some astronomers (or the press officers at their institutions) capitalize "Solar System" when referring to our own.Alan Boss, a researcher at the Carnegie Institution of Washington who theorizes about how planets formed around the sun and other stars, says he's never heard an astronomer complain about the generic application of "solar systems," so long as it's used to describe arrangements that indeed resemble ours.

"When writing, I try to be a bit more precise and refer to 'extrasolar planetary systems,' but I have certainly used 'solar systems' to refer to planetary systems in general," Boss said.

The generic usage is a convenient way to convey an arrangement of planets around a central star. In contrast, there are observational hints of lone, wandering planets not hosted by any star. To say "planetary system" does not make clear whether a star is involved, and to some those words might suggest something else entirely: a planet and its moons.

Alternatives include "extrasolar system," which is not widely used outside scientific papers, or "star system," which could be interpreted to mean simply two gravitationally bound stars.

Meanwhile, examples of "solar system" as a generic expression abound.

A 1998 NASA press release began this way: "NASA astronomers using the new Keck II telescope in Hawaii have discovered what appears to be the clearest evidence yet of a budding solar system around a nearby star." Last year, the European Southern Observatory issued a press release titled "Images of an Infant Solar System." Curiously, the text of that release did not repeat the usage.

Holdouts
The Sol trademark has defenders.

The press release associated with the recent Vega research does not call the setup a solar system (that usage was introduced to the Space.com article by this reporter). The statement refers to it as a "solarlike planetary system."

So I asked Mark Wyatt, leader of the study, his preference.

"Describing another planetary system as a solar system sounds incorrect, since these planetary systems are not bound to the sun," said Wyatt, of Britain's Royal Observatory. Because our solar system "is a concept which is readily understood by the public," however, he said he understands why it is invoked frequently.

"Perhaps using 'solar system' as a noun to describe the material bound to a star will be accepted one day," Wyatt said, "but for now I prefer to use the alternatives."

Wyatt is not alone, but he may be fighting a losing battle.

And then there were none
The term "solar system" derives from Sol, the name given to the star around which we orbit. To the Romans, Sol was a god that personified the bright object that English-speakers now call the sun. Sometime prior to 230 B.C., Greek thinker Aristarchus of Samos was apparently the first to suggest that Earth orbited the Sun. Aristarchus was threatened over the idea and it was forgotten.

For centuries, Earth was considered the center of the universe. In 1543, Polish astronomer Nicolaus Copernicus suggested Earth and the other planets instead revolved around the sun.

Copernicus knew the idea would not be viewed favorably by the church, and he held off releasing a book about it. He died the same year his idea was published, essentially paying the ultimate price to avoid persecution.

Later, Italian philosopher Giordano Bruno extended Copernicus' thinking. Bruno figured that the universe was infinite and that there must be other worlds. He was burned at the stake. Then the early 1600s, Galileo used one of the first telescopes to observationally argue for Earth's utter lack of uniqueness. He was convicted of heresy, forced to deny the things he believed. He died under house arrest.

Eventually, "solar system" and its meaning became agreed upon. All was well until the first world around another sunlike star was discovered in 1995.

More recently, a handful of extrasolar planets have been found that resemble Jupiter, both in their masses and distance from their hosts, sunlike stars. But no one yet knows if there are total systems like ours, with Earthlike planets.

The rise of ambiguity
If anyone has nomenclative prerogative on these extrasolar systems, it would be the legendary planet-hunting team of Geoffrey Marcy at the University of California at Berkeley and Paul Butler at the Carnegie Institution of Washington. They've led the discoveries of more exoplanets than anyone.

In a 2001 press release out of Berkeley, an astronomer on Marcy and Butler's team, Debra Fischer, said, "Most of the 70 planets people have found to date are in bizarre solar systems, with short periods and eccentric orbits close to the star."

In a separate release from the National Science Foundation, discussing the same discovery, the first paragraph went like this: "A team of astronomers has found a Jupiter-size planet in a circular orbit around a faint nearby star, raising intriguing prospects of finding a solar system with characteristics similar to our own."

Yet in both of these statements, Marcy and Butler, when quoted, left Sol to its sole status and instead referred several times to "planetary systems."

Marcy speaks often and charmingly in public, and he moves with ease between the world of science-speak and the rest of us. So I wondered if he ever takes the easy way out to describe his far-out work.

"I sometimes use 'solar system' when I mean 'planetary system' in communicating with the lay public, simply to emphasize the essence of the object: a central star and planets going around it," he said.

The planet hunter realizes there is some ambiguity surrounding the usage.

"However, I find that the ambiguity is often minimal, taken in context," he said. "The sentence, 'The solar system was found by detecting the dust around the host star' is not ambiguous, as it clearly refers to another planetary system, not ours."

Not a vegan
So what should we call Vega and its presumed companions? There are several terms to choose from, but the "Vegan System"is not among them. Besides sounding like a fringe diet, it’s a term astronomers aren't using.

Vikki Meadows thinks about places like Vega a lot.

Meadows leads a NASA effort to create an advanced computer program that will model synthetic worlds around other stars in an effort to determine what scientists ought to look for when searching for extraterrestrial life -- the ultimate goal of all planet searches.

"I don't mind it when we talk about 'other solar systems' as a group, since I hear 'other planetary systems like our solar system' as implicit in there," Meadows said. "The alternative is 'extrasolar planetary systems,' which is a bit of a mouthful. However, I would prefer not to hear things like 'the solar system around Vega,' or the 'Vega solar system.' 'Planetary system' would be more appropriate there, since Vega is not Sol."

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