What is a planet?

[vakkotaur]

The original definition of planet was a wanderer in the sky. Something that, unlike the apparently fixed stars, moved. This included the Sun and Moon as well as Mercury, Venus, Mars, Jupiter, and Saturn. The definition certainly changed when the heliocentric theory put the Sun at the center of things and the planets around it. That excluded the Sun and Moon. The Sun as it couldn't be orbiting itself, and Moon was orbiting the Earth, not the Sun. Further, the Earth, which moved around the Sun, was itself a planet. The definition might not have changed exactly I just said, but I'm willing to call it a reasonable guess.

Eventually astronomer William Herschel found something unusual - a planet that hadn't been noted before. This was Uranus, as it was eventually named. It was considered a planet right off as it orbited the Sun in the same plane as other planets and was, when one looked at a model of the solar system, in what seemed to be the right place. It fit a pattern, whether the pattern was a guide or not.

The discovery of Uranus lead to the discovery of Neptune. Uranus didn't move in its orbit quite as expected. Something had to be affecting it. That something was Neptune, which was confirmed by telescope, but its location predicted mathematically.

The pattern of planet orbits seemed to have an odd gap. Between Mars and Jupiter there was no obvious planet, but the pattern looked like there ought to be one there. When astronomers went looking for this missing planet they found something new: Minor planets, or planetoids, or as they are more commonly called now, asteroids.

There were other problems. Mercury's orbit moved in a way that Newtonian mechanics didn't predict. And Neptune seemed to be slightly out of place. Astronomers searched for a planet, dubbed "Vulcan," closer to the Sun than Mercury. Also, there was search for a "Planet X" beyond Neptune. Einstein's Relativity accounted for the motion of Mercury's orbit, and the search Planet X resulted in the discovery of Pluto.

There were still problems. Pluto wasn't quite in the same plane as the other planets. Its orbit was very eccentric (non-circular), and it didn't have enough mass to account for Uranus' and Neptune's positions being slightly off of prediction. There might still be a Planet X out there, some thought.

The problem was solved by the Voyager spacecraft. The predictions were wrong because the masses of the planets weren't known well enough. With the right information, everything fits. But this, and recent discoveries of various small objects in the outer solar system brings up a question...

Just what is a planet? There is some disagreement about Pluto. Does it qualify as a planet, or is it too small, its orbit too eccentric, or have some other disqualification? There is no set answer as what a planet is has never been formally defined.

I don't have an answer, but I do have a preference for one proposal. Someone suggested that a planet has sufficient mass that its gravity forms it into a nearly spherical shape, but isn't as massive as a star. I like this as it depends on the body itself, and not some arbitrary view of things from Earth.

It would change things. Pluto would be a planet, but so would some of the other objects in the outer solar system. And perhaps some of the larger asteroids would qualify as planets rather than "minor planets." There would be more than just the traditional planets - and what those are has changed over time anyway, so I don't see it as a big deal. Perhaps the degree of sphericity will be arbitrary. How close to a sphere is close enough? What about things not quite there, but not too oblong? Would these be the new minor planets, and the truly oblong things be asteroids?

Comments

[kiwihunter8.livejournal.com]

Coincidentally, I just started listening to 'Don't know Much About the Universe' and they just mentioned that this was a question :)

[wordslinger.livejournal.com]

I like the "enough gravity that it compresses it into a sphere."

Of course, this then redefines a gazillion moons as planets, too... so perhaps the caveat needs to be added "and orbiting a sun and not another body."

[vakkotaur.livejournal.com]

Oh yeah. I guess I didn't make that explicit, but should have.

[jdm314.livejournal.com]

Funny you should mention that, a friend of mine recently posted a very similar entry on his blog.

[vakkotaur.livejournal.com]

His final paragraph reminds me of how someone once suggested that an alien visitor to our solar system might describe it as "a system of one star, four planets, and assorted debris" with only the gas giants being counted as planets.

[chakawolf.livejournal.com]

a planit is sumthing that monsteres liv on

[vakkotaur.livejournal.com]

Considering that the classic definition of monster would include centaurs as monsters, I have no problem with that. But I haven't heard of any centaurs from other solar system objects than Earth... does that make Earth the only planet?

[chakawolf.livejournal.com]

Well, theres the martians what liv on Martia, and the venuseians what liv on venusia, and sum others i dont know of

(I gotta get some sleep!)

[vakkotaur.livejournal.com]

Oh, the sleeps came from anesthesia.

[chakawolf.livejournal.com]

He he he he he . . . (Falls out of chair and crawls off to bed.)