Starship Asterisk*

johnnydeep wrote: ↑Wed Sep 14, 2022 6:32 pm

Chris Peterson wrote: ↑Wed Sep 14, 2022 4:03 pm

johnnydeep wrote: ↑Wed Sep 14, 2022 3:58 pm

Reading the wikipedia article on dwarf planet made my head hurt. I'd like to see a definitive hierarchical classification chart showing all the different types of solar system objects and how they relate.

"Dwarf planet" as officially (IAU) described is an absurdity. How can a dwarf planet not be a planet? Logically, "dwarf" should be seen as a qualifier like "terrestrial" or "gas giant". Really silly.

Rationally, "planet" should define any body that is sufficiently massive to form a spherical shape under its own gravity and which orbits a star (or originated in orbit around a star). From there we can further classify them. That whole "clears its orbit of debris" qualification is pretty useless and pointless.

Pleased to hear you say that. I always hated the "clearing its orbit" attribute. And, yeah, a "baby human" is clearly still a human! Now, let's hear it for all the dwarf terrestrial rogue exoplanets out there!

Contrary to what some of the other answers claim, it is possible for two bodies of approximately equal size to (nearly) share an orbit around a larger third body. There are no known planets (or dwarf planets) with such orbits, but a well known example exists among the moons of Saturn: the co-orbital moons Janus and Epimetheus.

What happens in such a system is that the co-orbiting satellites don't stay at a fixed distance from each other: whichever one happens to be closer to the central body will orbit it slightly faster, and will therefore gradually catch up to the other one. However, once the satellites get close enough, their gravity will start to pull them together, so that the trailing satellite gains angular momentum and the leading satellite loses it.

Somewhat paradoxically, this momentum transfer increases the orbital radius of the trailing satellite, thus slowing it down, while the leading satellite's orbital radius is reduced, causing it to speed up. Thus, the satellites end up effectively "swapping orbits", so that the leading satellite becomes the faster one and starts gaining distance on the trailing one again, until it eventually catches up to the other satellite from behind and the process repeats.

In a frame rotating around the central body at the satellites' average orbital velocity, the net trajectories of the satellites end up looking like horseshoes. Here's such a schematic of the movement of Janus and Epimetheus with respect to each other as they orbit around Saturn (not to scale, obviously), taken from Wikipedia:

Epimetheus-Janus Orbit.png Cartoon of the horseshoe orbits of Janus and Epimetheus around Saturn, by Wikipedia user Jrkenti, used under the Creative Commons Attribution-ShareAlike 3.0 license.

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As for whether two objects in such an orbit around the Sun would be considered planets, assuming that they satisfied the other requirements, I'd say your guess is as good as mine: since there are no such objects known in the solar system, and since it's not expected that any more would be found, the IAU had no need to consider this particular issue.

As far as speculation goes, one could argue that, if such a planetary pair did exist in the solar system, the IAU would've found some way to shoehorn them in. One possible way to do so might've been to consider the smaller object in the pair to be an unusual kind of satellite of the larger one, or at least to be "otherwise under its gravitational influence", allowing at least the larger body to be counted as a planet, much like the Earth is considered a planet despite the presence of the Moon in essentially the same orbit.

pferkul wrote: ↑Thu Sep 15, 2022 7:50 pm We know what Earth's north star is. What are the approximate north stars of each planet? And what is the sun's north star?

Do all moons have the same tilt as their planet?

Code: Select all

Planet	Pole Star
Sun*	There is no bright star close to the NCP of the sun
Mercury	Omicron Draconis
Venus	42 Draconis
Earth	Polaris
Mars	Sadr and Deneb
Jupiter	Zeta Draconis
Saturn	Saturn's north celestial pole (NCP) is not directed toward any prominent star
Uranus	Inclined almost 90 degrees, has a pole oriented toward Orion's "head" region
Neptune	Delta Cygni
Pluto*	Delta Draconis

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Moon's Axial tilt:	
1.5424° 	to ecliptic[8]
6.687° 		to orbit plane[2]
24° 		to Earth's equator [9][/quote]

Earth's Axial tilt: 	
23.4392811°	to ecliptic[4]

And here's a nice diagram from https://en.wikipedia.org/wiki/Orbit_of_the_Moon

Click to view full size image

johnnydeep wrote: ↑Thu Sep 15, 2022 8:06 pm
Interesting questions! The first is answered here (though not sure how accurate it is) - https://astronomy.stackexchange.com/que ... tem-planet:

pferkul wrote: ↑Thu Sep 15, 2022 7:50 pm We know what Earth's north star is. What are the approximate north stars of each planet? And what is the sun's north star?

Do all moons have the same tilt as their planet?

XgeoX wrote: ↑Sat Sep 17, 2022 1:44 pm “Rationally, "planet" should define any body that is sufficiently massive to form a spherical shape under its own gravity and which orbits a star (or originated in orbit around a star). From there we can further classify them. That whole "clears its orbit of debris" qualification is pretty useless and pointless.
Chris“

Spot on Chris, thank you for putting it into words better than I could!

XgeoX wrote: ↑Sat Sep 17, 2022 1:57 pm

pferkul wrote: ↑Thu Sep 15, 2022 7:50 pm We know what Earth's north star is. What are the approximate north stars of each planet? And what is the sun's north star?

Do all moons have the same tilt as their planet?

All moons don’t but at Saturn alone four of it’s major share the same inclination to within a third of a degree.
That’s a great question BTW, something I’ve never thought about!

Eric

VictorBorun wrote: ↑Sun Sep 18, 2022 5:40 am

XgeoX wrote: ↑Sat Sep 17, 2022 1:57 pm

pferkul wrote: ↑Thu Sep 15, 2022 7:50 pm We know what Earth's north star is. What are the approximate north stars of each planet? And what is the sun's north star?

Do all moons have the same tilt as their planet?

All moons don’t but at Saturn alone four of it’s major share the same inclination to within a third of a degree.
That’s a great question BTW, something I’ve never thought about!

Eric

They say when Saturn's spin does precession (once in 1.83 million years) so do its satellites' orbital spins;
when Saturn's spin was changing its tilt (when Saturn's spin precession was once in 1.87 million years and resonated with the precession of Neptune's orbital spin) so were the satellites' orbital spins.

johnnydeep wrote: ↑Sat Sep 17, 2022 2:48 pm

XgeoX wrote: ↑Sat Sep 17, 2022 1:44 pm “Rationally, "planet" should define any body that is sufficiently massive to form a spherical shape under its own gravity and which orbits a star (or originated in orbit around a star). From there we can further classify them. That whole "clears its orbit of debris" qualification is pretty useless and pointless.
Chris“

Spot on Chris, thank you for putting it into words better than I could!

So, your reposting of Chris' definition naturally makes me ask: are there any planetary systems around a central object that didn't end up accumulating enough mass during formation to initiate nuclear fusion? That is, a "substellar object", or a brown dwarf or even a "sub brown dwarf". And if so, what do we call any spherical "planetary mass objects" that might be orbiting it? Perhaps any such systems are simply failed stellar systems, or "rogue planets" with satellites, but that never actually went rogue from any star.

johnnydeep wrote: ↑Sat Sep 17, 2022 2:48 pm

XgeoX wrote: ↑Sat Sep 17, 2022 1:44 pm “Rationally, "planet" should define any body that is sufficiently massive to form a spherical shape under its own gravity and which orbits a star (or originated in orbit around a star). From there we can further classify them. That whole "clears its orbit of debris" qualification is pretty useless and pointless.
Chris“

Spot on Chris, thank you for putting it into words better than I could!

So, your reposting of Chris' definition naturally makes me ask: are there any planetary systems around a central object that didn't end up accumulating enough mass during formation to initiate nuclear fusion? That is, a "substellar object", or a brown dwarf or even a "sub brown dwarf". And if so, what do we call any spherical "planetary mass objects" that might be orbiting it? Perhaps any such systems are simply failed stellar systems, or "rogue planets" with satellites, but that never actually went rogue from any star.

VictorBorun wrote: ↑Wed Sep 21, 2022 5:22 pm

johnnydeep wrote: ↑Sat Sep 17, 2022 2:48 pm

XgeoX wrote: ↑Sat Sep 17, 2022 1:44 pm “Rationally, "planet" should define any body that is sufficiently massive to form a spherical shape under its own gravity and which orbits a star (or originated in orbit around a star). From there we can further classify them. That whole "clears its orbit of debris" qualification is pretty useless and pointless.
Chris“

Spot on Chris, thank you for putting it into words better than I could!

So, your reposting of Chris' definition naturally makes me ask: are there any planetary systems around a central object that didn't end up accumulating enough mass during formation to initiate nuclear fusion? That is, a "substellar object", or a brown dwarf or even a "sub brown dwarf". And if so, what do we call any spherical "planetary mass objects" that might be orbiting it? Perhaps any such systems are simply failed stellar systems, or "rogue planets" with satellites, but that never actually went rogue from any star.

rogue-born planets with their satellites?
They may be a lot like Kuiper Belt bodies. But those can later choose slavery like Triton did

johnnydeep wrote: ↑Wed Sep 21, 2022 7:30 pm It wouldn't technically be a "rogue" since it didn't wander out of the gravity well of a star. It just failed to become a star, yet still had its "planets" (now merely satellites) as a retinue. I guess "failed star" with satellites is a much better description.

VictorBorun wrote: ↑Wed Sep 21, 2022 11:07 pm

johnnydeep wrote: ↑Wed Sep 21, 2022 7:30 pm It wouldn't technically be a "rogue" since it didn't wander out of the gravity well of a star. It just failed to become a star, yet still had its "planets" (now merely satellites) as a retinue. I guess "failed star" with satellites is a much better description.

ok, rogue-born is oxymoron, let it be free-born

Offtopic. A failed star like Neptune is black and a captured Kuiper like Triton has spikes in this pic by Webb

Chris Peterson wrote: ↑Thu Sep 22, 2022 12:08 am

VictorBorun wrote: ↑Wed Sep 21, 2022 11:07 pm ok, rogue-born is oxymoron, let it be free-born
Offtopic. A failed star like Neptune is black and a captured Kuiper like Triton has spikes in this pic by Webb

Because this is in the near-IR, we're basically seeing reflected sunlight, and in the near IR Triton's albedo is much higher than Neptune's. And Neptune has diffraction spikes just like Triton does, they're just a bit less apparent being spread out over a much greater width and less bright because Neptune is less bright.

VictorBorun wrote: ↑Thu Sep 22, 2022 12:02 pm

Chris Peterson wrote: ↑Thu Sep 22, 2022 12:08 am

VictorBorun wrote: ↑Wed Sep 21, 2022 11:07 pm ok, rogue-born is oxymoron, let it be free-born
Offtopic. A failed star like Neptune is black and a captured Kuiper like Triton has spikes in this pic by Webb

Because this is in the near-IR, we're basically seeing reflected sunlight, and in the near IR Triton's albedo is much higher than Neptune's. And Neptune has diffraction spikes just like Triton does, they're just a bit less apparent being spread out over a much greater width and less bright because Neptune is less bright.

and I thought it was just me :)
So, Neptune has broad and pale pink spikes while Triton has narrow and bright cyan ones.
Both bodies look white (disk or rim) because of saturation, but show their true IR-mapped colours with their spikes.

But, confusingly, the main disk of Neptune is rather gray than white. Why not pink? Is it just the rim that is pink?

Chris Peterson wrote: ↑Thu Sep 22, 2022 2:02 pm
Well, blue and green are mapped to shorter IR that is mainly reflected sunlight, and red and green are mapped to longer IR that contains a significant thermal component. I don't see gray; to me, the body of Neptune looks the same pinkish as the scatter or inner diffraction spikes around it. That suggests to me that the atmospheric methane is absorbing enough of the shorter wavelengths that the longer ones dominate, so we see a reddish tint over most of the planet. Perhaps the rim reflects a lot more sunlight, resulting in the direct view being saturated (so, white) but the diffraction spikes catching the actual wavelength, that of the shortest wavelength signal, which is just reflected sunlight alone.