Archive for March 5th, 2019

New Definition Of A Planet Makes Pluto A Planet

March 5, 2019

In the past, one distinguished among three sorts of bodies in the Solar System: planets, satellites, asteroids. However, it turned out that Pluto, once thought to be Earth sized, is much smaller… Although its atmosphere extends way out, it’s nearly extended as far as the diameter of Earth! Yet, some smart ass astronomer, greedy for fame as academic types tend to be, their feeding depends upon their renown, then made his claim to fame by demoting Pluto. He even sold T-shirts, I have one.

Planet means “wanderer”, in greek. Because the planets wander across the stars without reason which prehistoric men, or even the Greeks, could discern. Upon closer inspection (astronomer Tycho and his pupil Kepler), planets turned out to follow slowly changing ellipses.

The modern scientific sense of planet as “world that orbits a star” was prominent in Giordano Bruno’s cosmology, giving the sadists in the Vatican a reason to torture him to death for seven years (Catholic sadistic abusers are persistent, they have been around for nearly two millennia).

Very round, and very blue. Don’t tell me that’s not a planet. Real picture of Pluto from nuclear energized New Horizon Spacecraft, as it left the planet behind….

Pluto was loudly demoted, officially speaking, because it was argued that “Pluto didn’t clear its orbit”. The statement sounds superficially impressively scientific. But, as always in science, the devil lurks in a more careful examination of the situation. Actually, Pluto, as far as I can see, does clear its orbit. So this was an example of the “Big Lie Technique” dear to Hitler: the bigger the lie, the more it convinces people… Just like Obama fixing healthcare with “Romneycare“…

Pluto clears it so well it is accompanied by an entire cortege of satellites, including the relatively enormous Charon, a companion so large Pluto and Charon orbit around a common point exterior to Pluto.

Pluto is usually farthest from the Sun. However, its orbit is closer than Neptune’s orbit for 20 years out of every 248 years. Pluto got closer than Neptune on February 7, 1979, temporarily becoming the 8th planet from the Sun. Pluto crossed back over Neptune’s orbit again on February 11, 1999, resuming its place as the 9th planet from the Sun for the next 228 years. 

In truth (see Note), Pluto overflies Neptune orbit, when at closest point to the Sun. So the two orbits never intersect. One could introduce the notion of orbit disk (the part of the planetary plane of a planet inside the orbit). The intersection of the planetary disks of Pluto and Neptune have planar measure zero! (First Objection to the “clearing” notion).

Anyway, one may as well say that Neptune didn’t clear its own orbit (as Pluto occupies it sometimes, according to those who aren’t smart enough to understand the First Objection, namely that the orbits don’t cross).

Pluto could be more colorful than expected… Charon hanging through the blue haze, which is bue for the same exact reason as Earth atmosphere (Rayleigh scattering….) Mountains are made of water ice with very different properties than terrestrial ice (it’s much harder).

To satisfy all, it was decreed that Pluto was a “Dwarf Planet”… Other Pluto-sized objects have been found since, further out. But some have weird shapes… Some expected Pluto’s atmosphere to freeze down… But the New Horizon robot found it alive and well. The Pluto system has turned out to be very complex.

As one finds exoplanets, the possibility exists that Earth-sized satellites will be found in orbit around giant planets. As in the movie Avatar. Actually one may have been discovered (there is a controversy, as with all new scientific discoveries).

One will want to call Earth sized moons orbiting giant planets “planets”.

So what would be a planet? One can still use the official criterion brandished by the International Astronomical Union. And then I would add the following NEW criterion:

Worlds around giant planets do exist, and one may have already been found. (from variation of the light of the local sun, with slow downs and accelerations similar to those observed in Jupiter’s satellites in the 17C…)

An object large enough, and round enough, to hold an atmosphere all around its entire surface should be called a planet. That would make the giant asteroid Ceres NOT a planet: it doesn’t hold an atmosphere, and didn’t clear its orbit (it’s part of the asteroid belt). According to that definition Ganymede, which has an oxygen atmosphere, and a diameter of 5262 kms, is a planet, so is Titan (very thick atmosphere; 5150 kms diameter). Mercury, clearly a planet, has a diameter of 4880 kms, a tiny bit larger than Callisto, Jupiter second largest satellite… which also has a tenuous atmosphere.

Whether one wants to call large moons equipped with atmospheres planets is a matter of taste.

But, “Pandora” like world would be planets (Ganymede is 4.5 billion years old, at least as old as Earth… Plenty of time to evolve life…)

So what’s a planet? As Giordano Bruno said, a world. And certainly an atmosphere all around, especially if propitious to life, makes it a world. Pandora is a world, that is, a planet. It doesn’t matter that, as it turns around its sun, Pandora also turns around a hyper giant planet.

And certainly, Pluto is a world, too. A much smaller one, but still a world.

Worlds, here we come!

Patrice Ayme



Note 1;

Pluto has cleared its orbit. That’s why it’s so full of satellites….

Diagram of Pluto’s and Neptune’s orbit, on a distance scale in AUs.

Pluto and Neptune will never collide. You can see this in the image below, which shows a view as seen from the side as the planets orbit around the Sun.

Neptune is cleared of Pluto’s orbit, and reciprocally

Most planets only make small excursions in the vertical and radial directions, but Pluto makes large ones. Pluto at times will move closer to the Sun than Neptune, but it is always well “below” the orbit of Neptune when this happens. The orbits never actually cross the same point in space. Simulations have shown this is stable for the next 4 billion years.


Note 2:

Could one have an irregular shaped body with an atmosphere only in some basins? Probably, but unlikely to be naturally sustainable (one would need to make computer simulations taking into account the Roche limit, and Quantum effects on geological stress… Atmospheric pressure varies significantly on parts of Mars… which has giant high mountains, but also deep basins…)