Has Earth Lost Its Status as a Planet?

[DaveC426913]

TL;DR Summary

But at least you can pal around with Pluto now...

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System,[1] a planet is a celestial body that:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighbourhood" around its orbit.

Yeah, well.... Not anymore...
https://www.earth.com/news/earths-new-quasi-moon-will-stick-around-for-hundreds-of-years/

 

[Vanadium 50]

There goes the neighborhood.

Anyway, it can hoin 3753 Cruithne,

The stupidest thing about the Antiplutonic Resolution is that exoplanets are not planets.

 

[Baluncore]

How, without departing from Earth, could we detect or know if there was another planet behind the Sun, in a synchronous orbit with the Earth?

We could remove it with Occam's razor, but would it really be gone?

I would name that planet "Pantomime" because "He's behind you".
https://en.wikipedia.org/wiki/Pantomime

Audience participation, including calls of "He's behind you!" (or "Look behind you!"), and "Oh, yes it is!" and "Oh, no it isn't!"

 

[Vanadium 50]

I would prefer Mondas. Other would prefer Gor.

The best evidence against it is that we don't see it from Mars or various space probes: But we do see Earth and the Moon.

The second best reason is the precession of the perihelion of Mercury. The Earth-moon system induces a precession about twice the GR effect.

 

[Vanadium 50]

As a more serious PS, one could turn this around and ask "how large a planet could be hiding behind the sun?"

It looks to me that anything Ceres-sized or larger would have been discovered by now. Smaller is possible, but we would need a good reason to look in the right place and right time.

 

[Baluncore]

It looks to me that anything Ceres-sized or larger would have been discovered by now.

Now we can get off the Earth and go take a look, but before the 1950s that was not possible.
An Earth sized planet on the other side of the Sun would appear to be a small point. It would be hidden behind the Sun, that has a diameter of about 0.5°.

The differential effect of Jupiter on Earth and Pantomime must not exceed 0.25°, which is about an 8 hour advance in orbital position. We would then need to be looking through the solar corona during a total eclipse, to see a slightly out-of-place planet behind the Sun.

If the hidden planet was the size of our Earth and had a similar moon, then we might see that moon. It is quite possible that there is a small body parked there, something that is always too close to the Sun to be seen from Earth.

How stable would that opposite point be. I suspect there is little coupling between the two, so it might be unstable, so wander out along the shared orbit, to a point where it can be synchronised by the Earth.

 

[Bandersnatch]

^That's an unstable configuration, no? If we treat the other body as having negligible mass it drifts away from the hiding spot on a time scale on the order of a hundred years. I presume having it being Earth-sized would only magnify the instability.

has "cleared the neighbourhood" around its orbit.

A 15m-across rock is not 'of comparable size' with the Earth, so the criterion remains passed despite what you cheekily insinuate in the OP. The link helpfully provides descriptions of more stringent ways of quantifying the admittedly vague term, where it talks about the three types of 'determinants' in use.

 

[Vanadium 50]

An Earth-sized, or more accurately, an Earth-massed Mondas would be immediately visible in the precession of Mercury.

Ceres-sized is probably at the edge of measurability. Mars-sized would be a dirty trick of Nature, because that would exactly counter the GR term and would have us confused until the mess will have been sorted out. (Most likely through the precession of Venus, which is a tough nut to crack since its orbit is so circular)

As far as the stability of such an orbit, I don't see where there would be any restoring force (or torque) if a perturbation caused its orbit to advance. This is the L3 Lagrange point, so I am sure someone somewhere worked it out.

I suspect without doing the calculation that this counter-earth would cause the moon's apogee to precess, and the non-observation would cause us to put similar limits on what could be there.

 

[DaveC426913]

A 15m-across rock is not 'of comparable size' with the Earth, so the criterion remains passed

Never let facts stand in the way of a good larf.

 

[Vanadium 50]

"Clearing the neighborhood" is wooly - even Jupiter has its Trojansy

 

[Baluncore]

I agree that an L3 pantomime is in an untenable position. A planetary pantomime text will clearly need to remain part of the fiction collection.

The presence of the JWST at L2 cannot preclude an object at L3, because it is also in line. We can always say about the JWST, (which is looking the other way), "It's behind you".

I still believe that an L3 pantomime is more credible than is a flat or a hollow Earth, "Oh, no it isn't!" I hear you shout, "Oh, yes it is!" I reply.

 

[DaveC426913]

... woole...

No idea what you were even trying to type here.

 

[Bandersnatch]

"Clearing the neighborhood" is woole - even Jupiter has its Trojans.

Are they comparable to Jupiter in size?

 

[Baluncore]

"Clearing the neighborhood" is woole - even Jupiter has its Trojans.

Maybe "woole" was a typo, hit O twice by mistake, then K in place of L.
"Clearing the neighborhood" is woke - even Jupiter has its Trojans.

 

[Vanadium 50]

No idea what you were even trying to type here.

Wasn't sure if it was wooly or wooley so I split the difference.

 

[Vanadium 50]

I still believe that an L3 pantomime is more credible than is a flat or a hollow Earth,

How can a flat earth be hollow?

Are they comparable to Jupiter in size?

No. But that doesn't solve the problem. If Pluto isn't a planet because it hasn't cleared Neptune, but Neptune is so heavy it doesn't have to clear Pluto, does that mean Pluto would be a planet if it were a little farther out, a modest amount closer in, or even a more circular orbit? Worse, does it mean that Pluto used to be a planet moved into its present position (but isn't any more)?

 

[Bandersnatch]

IIRC they have a provision in the definition for resonance orbits.
Come on. Stop hating on the IAU. The definition is alright. Nobody would care about Pluto if it weren't named after the cartoon dog, riiight?

 

[Baluncore]

How can a flat earth be hollow?

I don't know. How can a flat Earth be hollow?

 

[Vanadium 50]

Stop hating on the IAU.

Not going to happen. I think they did a rush job, and it shows.

The problem they were trying to solve wasn't Pluto. Pluto was just collateral damage. The problem was Sedna. There are probably fifty other Sednas (Sednae?) out there, but their orbits are so eccentric that we can't see them and are unlikely to see them for hundreds or thousands of years.

This situation was viewed as intolerable by some people (e.g. Michael Brown) and they decided to act. Why did the IAU act so quickly rather than come up with a better definition? Because that faction had the votes and were not sure they would have them in the future.

The definition is problematic for many reasons:

1. Exoplanets are not planets.
2. Dwarf galaxies are still galaxies, and dwarf stars are still stars, but dwarf planets are not planets.
3. "Clearing the neighborhood" (this subject of this thread) is not well defined, with at least two definitions in common use.
4. Whether or not an object is a planet depends not only on what it is, but also were it is and when it is. I'm not just talking about Pluto: in this view, Uranus and Neptune started out as planets, and are planets today, but for part of the intervening time (a few hundred million years into their history), they weren't. But then "they got better".

In my view, they took a definition with one undesirable feature and exchanged it for one with many undesirable features.

 

[Ken G]

Fortunately, the only scientists that matter are "planetary astronomers", and to them, anything they study is a planet. So when we take future missions to the Moon, planetary astronomers will use language like "we are now landing on the planet's surface," with no apologies to the IAU! If it exhibits the kind of physics that planetary astronomers are interested in, then it's a planet, it's just that simple. (And yes, anything that looks like a prefix attached to the word planet is for sure got to be a planet itself, or it's just ridiculous.) For me, let them debate what is a "major planet" vs. a "minor planet", but they're all planets, obviously.

 

[Vanadium 50]

what is a "major planet" vs. a "minor planet"

But that is easy to fix. A major planet has a radius greater than X, or if you like a mass greater than X. I like the mass one - a major planet has a mass 10x greater than needed to bring it into hydrostatic equilibrium. That would include the 8 present planets as major planets, and if they were no moons, Earth's moon, Titan, Io. Callisto, Ganymede, Europa but not Triton.

This is somewhat arbitrary, but no worse than what was adopted. Further, the line ends p in a sensible place (even if it de-planetizes Pluto) in that Triton, and for that matter, Pluto, are much more Rhea-like than Earth;s Moon-like.

And you don't have nonsense like deplaneting Uranus and Neptune.

 

[DaveC426913]

Wasn't sure if it was wooly or wooley so I split the difference.

Still don't know. Is that a thing where you're from?

 

[renormalize]

Still don't know. Is that a thing where you're from?

Definition of:
woolly (UK/AU/NZ)
wooly (US)
...4. ADJECTIVE
If you describe a person or their ideas as woolly, you are criticizing them for being confused or vague.
[disapproval]
"...a weak and woolly Government."
"It is no good setting vague and woolly goals."
Synonyms: vague, confused, clouded, blurred
(https://www.collinsdictionary.com/dictionary/english/woolly)

 

[DaveC426913]

Ohhhh! Now it all makes sense.

 

[Ken G]

But that is easy to fix. A major planet has a radius greater than X, or if you like a mass greater than X. I like the mass one - a major planet has a mass 10x greater than needed to bring it into hydrostatic equilibrium. That would include the 8 present planets as major planets, and if they were no moons, Earth's moon, Titan, Io. Callisto, Ganymede, Europa but not Triton.

This is somewhat arbitrary, but no worse than what was adopted. Further, the line ends p in a sensible place (even if it de-planetizes Pluto) in that Triton, and for that matter, Pluto, are much more Rhea-like than Earth;s Moon-like.

And you don't have nonsense like deplaneting Uranus and Neptune.

Right, that's what I mean, it's fine to have some arbitrary distinction when one is only trying to decide what is "major" or "minor", then people can try to decide if Pluto is one or the other. But when you are saying what is or is not a planet, that's when it gets kind of silly, because Pluto is obviously a planet, based on how many papers there are by planetary astronomers on that topic. And I agree that to say neither a "minor planet," nor an "exoplanet", are planets, is really beyond reason. I really don't think anyone pays any attention to the IAU decision on that, least of all the members of the IAU-- maybe children's books on the solar system, that's about it!