Pluto Perspectives, Part 1- Intro and Size

[MeteorWayne]

Reanalysis:<br /><br />Let's begin with the history of planets.<br /><br />The original word refers to the wanderers, those objects that move against the background stars. In the geocentric world of the time that included the Sun and Moon.<br />Once the heliocentric world came about, and the sun and moon's place in the firmament became clear, it became Mercury, Venus, Earth, Mars, Jupiter and Saturn.<br />Everyone can easily see them with a little effort. They clearly wander. And are six.<br /><br />Next to join the club was Hershel (Uranus) in 1781. It too is a naked eye object, even though it was discovered by telescope, but it takes effort and dark skies.<br />Even in light polluted NJ, on a clear night, and knowing where to look, I can spot it. That's seven, that I therefore say that are indisputable planets because they fit the original definition. The wanderers. All visible to humans without optical aid, moving against the background stars.<br /><br />Interesting fact I never knew. Ceres, the first and largest main belt asteroid (containing 1/3 of the mass of the belt) was discovered on the first day of the 19th century: 1801, January 1. It is a very borderline naked eye object, as are several other asteroids, all dimmer than Ceres, though.<br /><br />By 1850 things were getting complicated as Kevin Heider posted this great link :<br /><br />1. Go to http://spaceweather.com/ <br />2. In the upper right under "View archives:" go to Sept 14th, 2006, and click view.<br /><br />Neptune had been discovered as well as 10 asteroids.<br />Now there were 18, but by 1920 the futility of adding hundreds of teeny weeny planets had been realized, and we were back to 8. That included Neptune. Herschel had been renamed Uranus. The asteroids were the asteroids.<br /><br />In 1930, Pluto was discovered serendipitously and telescopically. It was thought to be a massive world, much larger than the asteroids, which had been relegated to their own c <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[kheider]

Mike Brown vs Alan Stern<br /><br /><br /><b>Stern</b>: it creates a situation—untenable, in my view—that a given object can be a planet in some circumstances and not in others. For example, the Earth, by this definition, would count as a planet at its current distance from the Sun. But if you moved the Earth out into the distant reaches of the solar system and discovered it there, it would not be a planet [because it wouldn’t have cleared its orbit]. And the same is true if you put Jupiter in the Oort cloud.<br /><br />If it were orbiting another planet, I would call it a satellite that’s a planetary body.<br /><br /><br /><b>Brown</b>: Earth further away, Jupiter further away—I would agree you wouldn’t classify those as planets by this definition. But that actually makes it very interesting. Why would you not classify a Jupiter in the Oort cloud as a planet? Because it behaves very differently. It has had a very different history than these other eight planets.<br /><br />But if you put it in orbit around another planet, it suddenly becomes a satellite. If you really go with the physical criteria, you don’t care where it’s located.<br /><br /><br />Heider: Mars in orbit around Jupiter. Titan in orbit around Saturn. Huya a Planet. Equal rights for equal mass? <img src="/images/icons/smile.gif" /> Perhaps the definition either needs to be ALL inclusive or dynamically exclusive!<br /><br />-- Kevin Heider

 

[kheider]

Mass plotted against Density (not created by me)<br /><br />Vesta has a rather high density for a non-spheroid.<br /><br />-- Kevin Heider

 

[ittiz]

The clearest dividing line I see is between Neptune and Earth at 3.9. The actual number of bodies doesn't really matter. It's also very bad to use it when we have only know the number and distribution of small bodies in one solar system. I'm sure that in the grand scheme that the size of planets and number probably follows the inverse square law like gravity. So to say that non of the bodies of Pluto size exceed 1.4 times the next ten smaller bodies is just as arbitrary as using Pluto as the dividing line between planets and small bodies.

 

[MeteorWayne]

Look at the revised list here which uses mass rather than diameter.<br /><br />Saturn is 6.6 times the mass of Uranus.<br />Neptune is 17.2 times the mass of Earth<br />Venus is 7.6 times the mass of Mars.<br />Mercury is 13.8 times the mass of Eris, which is more massive than Pluto. (Mercury is 22 times the mass of Pluto)<br /><br />Pluto is only 3.6 times the mass of 2005 XY9. It is much more likely that the gap between them will be filled, as we discover more objects, rather than more larger objects between Mars and Eris.<br /><br />Everybody also, get over the arbitrary thing.<br />If you are going to categorize things, you must pick dividing lines, which are by definition arbitrary.<br /><br /><br />To me since Mercury has always been a planet, the fact that it's nearly 14 times the mass of the next smaller object is a pretty good place to draw that dividing line, since below that nothing is more than 4 times the mass of the next on the list.<br /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[kheider]

HD206267 + 2MASSJ21385699+5730455 in IC1396<br />2,450 light-years away<br /><br />The potential planet-forming disk (or "protoplanetary disk") of a sun-like star is being violently ripped away by the powerful winds of a nearby hot O-type star in this image from NASA's Spitzer Space Telescope. At up to 100 times the mass of sun-like stars, O stars are the most massive and energetic stars in the universe.<br /><br />Links:<br />Planets Prefer Safe Neighborhoods<br />Picture<br /><br />-- Kevin Heider