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The new solar system

Last week, the International Astronomical Union began work on a rather important counting-angels-on-the-head-of-a-pin exercise: defining precisely which bodies orbiting our sun qualify for the appellation of "planet". Naturally, this has provoked considerable controversy, and I've been watching the ongoing arguments with amusement (especially the debate between SF writers John Scalzi and Scott Westerfield, who are both jolly excellent chaps, if a trifle over-excited right now — I blame the brain-eater).

One of the causes of controversy is the dubious status of Pluto. Pluto is way smaller than Mercury — indeed, it's smaller than Luna — and orbits the sun in a rather eccentric catch-me-if-you-can way. Back in 1930 a case was made for it being a planet before its size was established, but these days it's clearly just one of the easiest to see among a whole pack of also-rans, including Ceres (in the asteroid belt, over there), Charon (which it co-orbits with), Xena, Quaoar, and a bunch of other large Kuiper belt bodies. Rather than demote Pluto to the status of a mere chunk of rock like any other KBO, the IAU has compromised by defining a new class of planet, called a "Plutonoid".

This is bullshit. We all know that the only real planets — the big ones that accreted from the solar disk right at the beginning — are Jupiter, Saturn, Nepture and Uranus. They're self-accreting bodies that aren't massive enough to undergo fusion and that formed in orbit around a star. OK? That's a planet.

Naturally you're biased: you live on Earth after all. But I have to tell you, these days we have this theory called the heliocentric model that holds that Earth isn't the centre of the universe. Guess what? Earth isn't a real planet, either. It's just a ball of rocky left-overs that didn't get its fair share of gas when the accretion disk was still swirling. Indeed, the same goes for Mars, Venus, and Mercury. These tiny rocks (Earth, the largest, is barely a thousandth the mass of Jupiter) orbit in the wrong damn place, far to close to their primary star to have any hope of hanging onto a volatile envelope of hydrogen and a bit of helium. In fact, I think it's about time the IAU bit the bullet and admitted that these dwarfish rocky cores are just that, and introduced a new category, "failed planetary nuclei", to define the rocky Earthlike bodies of the inner solar system.

Given that the "Plutonoids" are believed to be mostly condensed gassy stuff, we can (subject to confirmation) then redesignate them as "failed planetary atmosphere fodder". The asteroids and small KBOs can then be allocated to one group or the other, or a fourth, catchment category: "irritating little shit". And the rationalization of the solar system is done.

It'll be so much easier to teach kids the names of the planets when we've pruned them back to four!

30 Comments

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1:

I say we base the definition on Mercury. Anything that would loose most of it's mass if it had Mercury's orbit isn't a planet. :)

2:

After we've gotten rid of the anthropocentric definition of planet, let's do something about the number system. Base 10 is idiosyncratic, inefficient and makes us look backward.

3:

These so-called planets, these hell-holes, with their bizarre oxygen chemistries, searing heat, and other life threatening characteristics (just what can you build a spacecraft from if your structural dihydrogen monoxide becomes a liquid?), are clearly freaks, unworthy of the label, and in no way should be used as a classification template.

While I applaud the requirement for a shape controlled by the equilibrium between gravity and the solidity of matter, it can only be a partial and incomplete definition. You humble correspondent will devote some of his unworthy time to the task of completing it.

Yours modestly,

Nadreck, Palain VII.

4:

All bases self-represent as Base 10, once transliterated.

For instance, one might use any arbitrary symbol string, such as S, U, P, E, R, I, N, K, L, A, D, Y.

And all your bases are belong to US.

5:

But, but... only in the heliocentric model is earth a planet. In the geocentric model, the Sun is a planet, and the Moon is a planet, but Tellus isn't.

6:

Bah. I don't buy that definition of planet in the least... if you are not happy with the current trends and want to get radical, I'd rather lean towards the 1 planet solar system. Define a planet as a orbitting mass that can retain a life sustaining atmosphere.

7:

Orbiting what? In the Tychonian system, the Sun orbits the Earth (as does the Moon), but all the other planets orbit the Sun...

8:

To a first approximation, Jupiter is the Sun's only planet. The others are just second-order perturbations, the largest being Saturn.

The clever part of the new semiofficial semidefinition is not the 480 kilometer round-thingie part, which lets Ceres in and slams the club door in the face of the entourage. No, it's the "orbiting a star" part. That excludes Luna, which doesn't orbit the Sun (but does have an orbit concave to the sun), but, rather, orbits a point beneath the surface of the Earth. Whereas Charon orbits the Sun, because it's centroid with Pluto is above the surface of Pluto (where methane skiing is the main sport). Unresolved is what to call round thingies orbiting brown dwarfs, and the objects formerly known as planets which have, by 3-body trajectories, been kicked out of solar systems, and wander the interstellar depths in a Pail of Airy way. Or that feverish Twilight Zone episode. Or (for Luna) Space 1999.

9:

Wow. And I thought Scalzi and Westerfeld were hardcore. This is impressive.

10:

For an encore, I now need to come up with a definition of "star" which excludes Sol. Hmm ...

11:

Star: any self-gravitating nuclear fusion mixture of hydrogen, helium, metals, and dark matter, which is not visible from the Earth's surface by naked eye during daylight.

12:

To inhabitants of solar systems around Red and Brown Dwarfs, Sol would be one of the rare short lived giant stars.

13:

FREE-FLOATING PLANETS: THEIR ORIGIN AND DISTRIBUTION.
M.J. Fogg - users.globalnet.co.uk

... We rule some of the bubbles. Of the waters we know nothing...��?
Larry Niven & Jerry Pournelle, The Mote in God’s Eye, 1975. Page 3.

ABSTRACT

Evidence for the existence of planetary mass objects, unattached to any star and free-floating in interstellar space, has recently emerged. In this paper, this evidence and the history of the concept of free-floating planets is reviewed and a classification is proposed, based on mode of origin. It is suggested that free-floating planets can originate in two settings:
1) interstellar space, where the object forms in the manner of a star;
2) circumstellar space, where the object forms in the manner of a conventional planet and is subsequently lost to interstellar space. We designate the former type of object a planetar and the latter an unbound planet. Three possible scenarios of planetar formation and four scenarios of unbound planet origin are explored and discussed. Estimates of the abundance of these objects suggest that planetars in the mass range of 1 – 13 M♃ may be about as common as stars and brown dwarfs. The number of unbound planets however may exceed the number of stars by two orders of magnitude, although most of them should be low-mass rock/ice planetary embryos ejected from planetary systems in formation. It seems likely therefore that advances in observational techniques, such as infrared astronomy and microlensing, will lead to the discovery of many more free-floating planets in the future, securing their recognition as genuine astrophysical objects.

14:

An excellent theory. We could go further, and rename the four planets of the Solar System 1,2,3 and 4, thus avoiding the need for pointless memorisation.

I'm afraid I'm going to be a little bit cranky about something which seems endemic in the SF community, and which drives me quite unreasonably up the wall. There are official astronomical names for various well-known cosmic bodies. The planet we live on is called Earth, not Terra. Its satellite is called the Moon (capital M), not Luna. The star it orbits is called the Sun (capital S), not Sol. And, just for completeness, the galaxy they all inhabit is called the Galaxy (capital G). I don't know where this tendency arose in the SF world to use pretentious Latin names rather than the plainer, scientifically understood names, but it really grates with me for some reason. It's like calling napkins serviettes.

15:

[keanu]There is no planet[/keanu].

16:

Just on the off chance that a "Plutonoid" doesn't count as a 'celestial body', I hereby claim Pluto for me.

Frist, bitches!

17:

Remember, folks, to an astronomer the periodic table has three elements: Hydrogen, Helium, and Metal.

18:

12 Planets?
9 Planets?
4 Planets?
It won't matter a bit postSingularity, because Ray Kurzweil, or his avatar, is going to take them all apart and use them for computronium.

19:

Four planets? I recall Isaac Asimov once said "the solar system consists of Jupiter plus debris". He was right.

20:

Mr. Coleman is quite right.

Furthermore, we should stop using pretentious adjectives to describes things associated with various celestial bodies. There are perfectly good English words for almost any object. For example - sunny, mercurial, venereal, earthy, martial, cereal, jovial, saturnine. Uranus is a problem. 'Uranic" is a word, but not one ever used. I suggest "urinal" instead, and "plutescent" for Pluto.

Can't quite figure out what to with Neptune. "Nepotistic"?

21:

I am constantly amazed at what people spend their time mulling over and debating about. This debate reminds me of a silly movie I once saw called, "The Man who wnet Up a Hill and Came Down a Mountain." A true story about some small town who got so upset when their precious mountain was reclassified a hill that they spent literally days carting dirt up the side of the hill and piling it on top to make the knoll tall enough to make the grade. Of all the stupid things to do. This current topic falls into that catagory. When it is all said and done, who the crap cares if Earth is a planet or not? Obviously, it all depends on one's point of view, but Geez people! Why don't we spend our time on more lucrative endeavors like wondering if the same Earth turns clockwise or counter-clockwise. Or better yet... how do they get the chewy center inside of a Tootsie Pop?

22:

Tony: because it's funny.

23:

Right... so who's going to volunteer to take the first load of dirt to Pluto?

24:

Well it's all moot now. The final decision is that Pluto has been demoted, there are now only eight Planets and a pile of "Dwarf Planets".

http://www.space.com/scienceastronomy/060824_planet_definition.html

25:

>Nepture

Neptune?

26:

The final decision is that Pluto has been demoted, there are now only eight Planets and a pile of "Dwarf Planets".

You know, this may come back to bite us in 400 years when the Plutonic colonists start a war in order to get full planetary voting rights in the Solar Parliament.

27:

John Varley dealt in several venues with Pluto being the odd planet out, in a solar system economy. Network Topology (and bandwidth) is politics.

28:

> Earth, the largest, is barely a thousandth the mass of Jupiter

Where by "barely a thousandth" I think you mean 1/317.8

29:
John Varley dealt in several venues with Pluto being the odd planet out, in a solar system economy. Network Topology (and bandwidth) is politics.
Communications is not the only consideration in politics. Energy and time to deliver physical objects, whether cargo or warships also matter. A simple measure would be deltaV (or, if you use constant boost engines, deltaE, since the total change in V for a payload is much greater than that between the endpoints of your trajectory). Oh, and if you use solar sails for transporting non-perishable cargo, clearly the outer system is at a disadvantage.
30:

The deeper problem with the revised definition is that is fails to apply to other solar systems, where (one hopes) the human race will spend more of its time than it did in this one. For example:

Planet Or Failed Star? One Of Smallest Stellar Companions Seen By Hubble

Source: European Space Agency
Date: September 8, 2006

Astronomers using the NASA/ESA Hubble Space Telescope have photographed one of the smallest objects ever seen around a normal star beyond our Sun. Weighing in at 12 times the mass of Jupiter, the object is small enough to be a planet. The riddle is that it is also large enough to be a brown dwarf, a failed star.

[This NASA/ESA Hubble Space Telescope image shows one of the smallest objects ever seen around a normal star. Astronomers believe the object is a brown dwarf because it is 12 times more massive than Jupiter. The brown dwarf candidate, called CHXR 73 B, is the bright spot at lower right. (Credits: NASA, ESA, and K. Luhman (Penn State University, USA))]Ads by Google Advertise on this site

The Hubble observation of the diminutive companion to the low-mass red dwarf star CHRX 73 is a dramatic reminder that astronomers do not have a consensus in deciding which objects orbiting other stars are truly planets - even though they have recently provided the definition of 'planet' for objects inside our Solar System.

Kevin Luhman of Penn State University, USA, leader of the international team that found the object (called CHRX 73 'B') is casting his vote for a brown dwarf. "New, more sensitive telescopes are finding smaller and smaller objects of planetary-mass size," said Luhman. "These discoveries have prompted astronomers to ask the question, are planetary-mass companions always planets?"

Some astronomers suggest that an extra-solar object's mass determines whether it is a planet. Luhman and others advocate that an object is only a planet if it formed from the disk of gas and dust that commonly encircles a newborn star. Our Solar System planets formed 4.6 thousand million years ago out of a dust disk around our Sun.

Brown dwarfs, by contrast, form just like stars: from the gravitational collapse of large, diffuse clouds of hydrogen gas. Unlike stars, brown dwarfs do not have quite enough mass to ignite hydrogen fusion reactions in their cores, which power stars such as our Sun.

CHXR 73 B is 31.2 thousand million kilometres from its red dwarf sun. This is roughly 200 times farther than Earth is from our Sun. Being about two million years old, the star is very young when compared with our middle-aged 4.6-thousand-million-year-old Sun.

"The object is so far away from its star that it is unlikely to have formed in a circumstellar disk," Luhman explained. Disks around low-mass stars are about 8 to 16 thousand million kilometres in diameter. There isn't enough material at that distance from the red dwarf to create a planet. Theoretical models show that giant planets like Jupiter form no more than about 5 thousand million kilometres from their stars.

Hubble's Advanced Camera for Surveys discovered the object while conducting a survey of free-floating brown dwarfs. Astronomers have found hundreds of brown dwarfs in our galaxy since the first brown dwarfs were spied about a decade ago. Most of them are floating through space and not orbiting stars.

"The study of sub-stellar objects in orbit around a star allows us to determine the age, and over time also the mass of the companion. Such studies help us to improve our understanding of the formation and inner structure of brown dwarfs and planets," says Wolfgang Brandner of the Max-Planck-Institute for Astronomy in Heidelberg, Germany.

One way to further settle the uncertainty would be if a disk of dust could be observed around CHRX 73's companion. Like stars, brown dwarfs have circumstellar disks, too. They would be no more than about 4 thousand million kilometres in diameter.

NASA's Spitzer Space Telescope has detected disks around several free-floating brown dwarfs. But CHRX 73 B is too close to its star for Spitzer to detect the disk. So astronomers will have to wait for the launch of the NASA/ESA/CSA James Webb Space Telescope in 2013 to determine if this companion has a disk. The Webb telescope will combine Hubble's sharpness, which is needed for detecting close companions, and Spitzer's infrared sensitivity, which is necessary for seeing cool, dusty disks.

There is also a roughly 0.1 percent probability that CHRX 73 B is a background object that by chance happens to align with CHRX 73.
The team's results will appear in the 20 September 2006 issue of the Astrophysical Journal.

Members of the research team are K. L. Luhman, Penn State University, USA; J. C. Wilson, M. F. Skrutskie, M. J. Nelson, and D. E. Peterson, University of Virginia, USA; W. Brandner, Max-Planck Institute for Astronomy, Germany; and J. D. Smith, M. C. Cushing, and E. Young, University of Arizona, USA.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.


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This page contains a single entry by Charlie Stross published on August 17, 2006 10:09 AM.

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