Mystery orbits in outermost reaches of solar system not caused by 'Planet Nine'

January 21, 2019, University of Cambridge
Mystery orbits in outermost reaches of solar system not caused by 'Planet Nine'
Kuiper Belt's ice cores. Credit: ESO/M. Kornmesser

The strange orbits of some objects in the farthest reaches of our solar system, hypothesised by some astronomers to be shaped by an unknown ninth planet, can instead be explained by the combined gravitational force of small objects orbiting the Sun beyond Neptune, say researchers.

The alternative explanation to the so-called 'Planet Nine' hypothesis, put forward by researchers at the University of Cambridge and the American University of Beirut, proposes a disc made up of small icy bodies with a combined mass as much as ten times that of Earth. When combined with a simplified model of the solar system, the gravitational forces of the hypothesised disc can account for the unusual orbital architecture exhibited by some objects at the outer reaches of the solar system.

While the new theory is not the first to propose that the gravitational forces of a massive disc made of small objects could avoid the need for a ninth planet, it is the first such theory which is able to explain the significant features of the observed orbits while accounting for the mass and gravity of the other eight in our solar system. The results are reported in the Astronomical Journal.

Beyond the orbit of Neptune lies the Kuiper Belt, which is made up of small bodies left over from the formation of the solar system. Neptune and the other gravitationally influence the objects in the Kuiper Belt and beyond, collectively known as trans-Neptunian Objects (TNOs), which encircle the Sun on nearly-circular paths from almost all directions.

However, astronomers have discovered some mysterious outliers. Since 2003, around 30 TNOs on highly have been spotted: they stand out from the rest of the TNOs by sharing, on average, the same spatial orientation. This type of clustering cannot be explained by our existing eight-planet solar system architecture and has led to some astronomers hypothesising that the unusual orbits could be influenced by the existence of an as-yet-unknown ninth planet.

The 'Planet Nine' hypothesis suggests that to account for the unusual orbits of these TNOs, there would have to be another planet, believed to be about ten times more massive than Earth, lurking in the distant reaches of the solar system and 'shepherding' the TNOs in the same direction through the combined effect of its gravity and that of the rest of the solar system.

"The Planet Nine hypothesis is a fascinating one, but if the hypothesised ninth planet exists, it has so far avoided detection," said co-author Antranik Sefilian, a Ph.D. student in Cambridge's Department of Applied Mathematics and Theoretical Physics. "We wanted to see whether there could be another, less dramatic and perhaps more natural, cause for the unusual orbits we see in some TNOs. We thought, rather than allowing for a ninth planet, and then worry about its formation and unusual orbit, why not simply account for the gravity of small objects constituting a disc beyond the orbit of Neptune and see what it does for us?"

Professor Jihad Touma, from the American University of Beirut, and his former student Sefilian modelled the full spatial dynamics of TNOs with the combined action of the giant outer planets and a massive, extended disc beyond Neptune. The duo's calculations, which grew out of a seminar at the American University of Beirut, revealed that such a model can explain the perplexing spatially clustered orbits of some TNOs. In the process, they were able to identify ranges in the disc's mass, its 'roundness' (or eccentricity), and forced gradual shifts in its orientations (or precession rate), which faithfully reproduced the outlier TNO orbits.

"If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs," said Sefilian, who is a Gates Cambridge Scholar and a member of Darwin College.

Earlier attempts to estimate the total mass of objects beyond Neptune have only added up to around one-tenth the mass of the Earth. However, in order for the TNOs to have the observed orbits and for there to be no Planet Nine, the model put forward by Sefilian and Touma requires the combined mass of the Kuiper Belt to be between a few to ten times the mass of the Earth.

"When observing other systems, we often study the disc surrounding the host star to infer the properties of any planets in around it," said Sefilian. "The problem is when you're observing the disc from inside the system, it's almost impossible to see the whole thing at once. While we don't have direct observational evidence for the disc, neither do we have it for Planet Nine, which is why we're investigating other possibilities. Nevertheless, it is interesting to note that observations of Kuiper belt analogues around other stars, as well as planet formation models, reveal massive remnant populations of debris.

"It's also possible that both things could be true—there could be a massive disc and a ninth planet. With the discovery of each new TNO, we gather more evidence that might help explain their behaviour."

Explore further: In search of the ninth planet

More information: Antranik A. Sefilian and Jihad R. Touma. 'Shepherding in a self-gravitating disk of trans-Neptunian objects.' Astronomical Journal (2019). arxiv.org/abs/1804.06859

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Osiris1
1.3 / 5 (6) Jan 21, 2019
Rather imagine that the article ...could...be right. But then again so could those that posit a 'planet nine'. We NEED to find which possible solution of this is true. I know from engineering that problems can often have many exact solutions, some even imaginary or irrational. However, the answer to this can affect us in other ways. Say we send out a probe to some star or to the Oort cloud and this craft does not take the large body solution into account when the large body solution IS, say, the right one. splat on spacecraft worth many millions. If a manned spacecraft and this is in its path, then the cost would be irreplaceable. I know that skeptiks want to destroy discoveries rhetorically by supposing mundane 'solutions', but the math behind planet nine is there too, and we should know for sure...just in case its orbit comes close to us when it gets closer to the sun as it possibly would in an elliptical orbit.
granville583762
2.5 / 5 (8) Jan 21, 2019
We have lost our ninth planet

Surrounding our ninth planet, which is a highly active planet, with moons
are millions of similar and smaller planets
as the redesignated ninth planet
Until we redesignate our ninth planet to its original status
there are no planets that are larger than our ninth planet
all of these planets surrounding our ninth planet and beyond fit the designation given to our ninth planet
This why we cannot find our ninth planet
So as we send satellites to investigate and photograph our ninth planet as we know its coordinates
Officially it is lost, and with it any other planets in this Kuiper Belt
granville583762
2.5 / 5 (8) Jan 21, 2019
Our tenth planet Eros redesignated to Pluto's status

1992 Pluto is part of objects called the Kuiper belt
similar to Pluto were discovered so Pluto was reclassified as a Kuiper belt object
July 29 2005, astronomers at Caltech announced the discovery of a Newtonian object Eris substantially more massive than Pluto

If proof were needed
we found our tenth planet, Eros; a planet larger than our ninth planet Pluto
even though Eros, under Pluto's original designation would have given us our tenth planet
that is larger than our ninth planet Pluto
even our tenth planet being larger, is redesignated
so as we search this Kuiper Belt
containing billions of icy fragmented fragments
as we are lucky to find our ninth and tenth planets, Pluto and Eros
as we have designated this Kuiper belt the same designation as Pluto and Eros
as this reality dawns
No planets larger than Pluto and Eros, exist in this Kuiper Belt
Shootist
4 / 5 (4) Jan 21, 2019
Rather imagine that the article ...could...be right. But then again so could those that posit a 'planet nine'..


Ultima Thule has a nearly circular orbit and has for billions of years been unaffected by supposed "planet 9s".
Mark Thomas
4 / 5 (4) Jan 21, 2019
if the hypothesized ninth planet exists, it has so far avoided detection


A nice way of saying that it should have been found by now, so we need to find another way of creating all those lopsided orbits detected out there.

total mass of objects beyond Neptune have only added up to around one-tenth the mass of the Earth. However, in order for the TNOs to have the observed orbits and for there to be no Planet Nine, the model put forward by Sefilian and Touma requires the combined mass of the Kuiper Belt to be between a few to ten times the mass of the Earth.


So we are barking up the wrong tree again. Instead try considering the thousands or tens of thousands of stellar objects that have passed by the solar system over the last 4.6 billion years. Consider the close passes of Scholz's Star and Gliese 710, and realize there must have been much closer encounters than these.

https://en.wikipe...27s_Star
Gigel
3 / 5 (2) Jan 21, 2019
Stars passing by shouldn't have a systematic influence on several TNOs. Their orbits shouldn't appear grouped if they were disturbed by a passing star. A star rotating around the Sun at a large distance could do that. But my first guesses go with a large TN disc or a ninth planet.
antigoracle
4.4 / 5 (7) Jan 21, 2019
Planet Nein.
Nik_2213
3 / 5 (2) Jan 21, 2019
IMHO, the long-term effects of Scholz' Star's 70ky pass have yet to show. IIRC, most perturbed Oort objects, potential comets, are unlikely to reach us for many millennia yet.
---
At least this report's 'just crowd perturbation' hypothesis is simply falsifiable, by the systematic or serendipitous discovery of a neptune-mass planet out in that deep, deep dark...

As I understand it, most of the remaining search zones are 'heavily cluttered' by back-ground objects. IMHO, a #9 would be too dark and cold to appear on the gorgeous Gaia scans. But, those may serve to eliminate some of the clutter...

Who knows ? Perhaps RECONS' on-going study of our solar neighbours may find a 'Golden Ticket' in the 'out-field'...
rrwillsj
1.5 / 5 (4) Jan 21, 2019
Yet another blow against the inviolability of planetary objects being required to always assume repetitious orbits. Just because it is almost true of the major planets in the Solar System.

Humans put a lot of effort to impose orderly rules upon natural phenomena & try to limit categories of random objects.

The Universe ain't cooperating.
Frosty the Blushing Snowman is laughing at your indignant wroth.

Myself? U think a Chaotic Cosmos it is just terrific!
The similarities you worship are boring!

It is the uniqueness of every planet & star & galaxy. Of every evolutionary lineage driven by genetic crapshoot. That each planet & moon & asteroid will have some unexpected idiosyncrasy.
granville583762
4 / 5 (4) Jan 21, 2019
Nik_2213
Nik_2213> IMHO, the long-term effects of Scholz' Star's 70ky pass have yet to show. IIRC, most perturbed Oort objects, potential comets, are unlikely to reach us for many millennia yet.
At least this report's 'just crowd perturbation' hypothesis is simply falsifiable, by the systematic or serendipitous discovery of a neptune-mass planet out in that deep, deep dark...
As I understand it, most of the remaining search zones are 'heavily cluttered' by back-ground objects. IMHO, a #9 would be too dark and cold to appear on the gorgeous Gaia scans. But, those may serve to eliminate some of the clutter...
Who knows ? Perhaps RECONS' on-going study of our solar neighbours may find a 'Golden Ticket' in the 'out-field'...

Nik_2213, due to Pluto is now longer in the planet club
as all the Kuiper Belt fragments of ice
are no larger than Pluto
what is your suggestion
In finding a planet in these fragmentary fragments in this Kuiper Belt
Mark Thomas
3.5 / 5 (4) Jan 21, 2019
Stars passing by shouldn't have a systematic influence on several TNOs.


Depends on how close they passed. The TNOs that happened to be closest would have been attracted by gravity towards the passing star, thus leaving some of them in elongated orbits for us to ponder millions of years later. Nik_2213's point is that there is clearly an effect, but it takes a while to fully play out. Regarding comets, I tend to agree. In addition to passing stars, how about passing planets orbiting those stars and that pass by even closer to the TNOs? How about rogue planets? How about black holes, neutron stars, etc? How about interactions from much closer stars in the cluster where our sun was presumed to be born? Astronomers have already mentioned this as a possibility in articles I have read.

Sure it is calm right now in the outer solar system, but a lot has happened in the last 4.6 billion years. Scholz's Star and Gliese 710 are big hints at that dynamism.

granville583762
5 / 5 (3) Jan 21, 2019
This Starry Club of Planets

Where in days of yore
planet Pluto used to frequent
as having that unique status
none other planets held
as the furthest
remotest
planet, in our solar system
in these now distant days
work was afoot
to find an even more distant planet than Pluto
little did we know
there was dastardly work afoot
so devious
it would eliminate
any chance
of
finding an even more distant planet than Pluto
because
as Pluto was refused entry in this starry club of planets
as having his membership revoked
as when Pluto was thrown lock stock and barrel out on the street
Pluto got the last laugh
Pluto lives in this Kuiper Belt
there are no planets larger than Pluto
so as Planet Pluto
sits in orbit
around our sun
is now one of the largest planets
In this Kuiper Belt
Mark Thomas
3.3 / 5 (3) Jan 21, 2019
So what good is an hypothesis if it can't make a prediction you might ask?

I predict the outer solar system will not be the pristine graveyard for original solar system materials we think it is. We may find less original objects than expected the further we push towards the Oort Cloud and a smattering of objects formed in multiple other solar systems captured by our sun. I would also expect to find objects that have been exposed to unexpectedly high temperatures inconsistent with the scant warmth provided only by our sun at these extreme distances, but consistent with exchange of objects with other solar systems. If I am right, the extreme outer solar system is going to be far more interesting than we think it will be and it wouldn't be the first time we have been surprised in this way.
granville583762
5 / 5 (3) Jan 21, 2019
Planet Pluto sends kindest regards

To all those
who worked so hard
in removing Pluto's designation
from this Starry Club of Planets
Pluto thanks one and all
in being give the privileged membership
of
This Starry Club of Kuiper Belt Planets
Old_C_Code
not rated yet Jan 21, 2019
I predict the outer solar system will not be the pristine graveyard for original solar system materials we think it is. We may find less original objects than expected the further we push towards the Oort Cloud and a smattering of objects formed in multiple other solar systems captured by our sun.


Yes, other Star's Oort clouds hit our oort cloud and solar system generally. Because their are other stars that pass by us at close range (10,000 AU), and their oort clouds hit us, and ours hits theirs. The next star to pass by us is in only 1.2 million years at a distance of 9k to 13k AU.
We have the best chance of discovering alien life on stars that pass by us ( in only 1.2 million years...lol, ugh)..
granville583762
5 / 5 (3) Jan 21, 2019
And to think, Nik_2213
we found the tenth planet
and it was thrown away
Eros is our tenth planet
and with it, Nik_2213
as there are larger planets than Eros
we have thrown untold planets away
as now, Nik_2213
we have thrown any chance of finding any further planets
as apart from Eros and Pluto
no other planets come closer to this starry club of planets
Entry requirements
Mark Thomas
3.8 / 5 (4) Jan 21, 2019
We have the best chance of discovering alien life on stars that pass by us ( in only 1.2 million years...lol, ugh).


To your point, sure, 1.2 million years sounds like forever, but to my point, if a star like Gliese 710 passed by every 1.2 million years, that would be 38,333 similar close encounters in 4.6 billion years since the solar system formed. Astronomers believe other stars were very close in the cluster where our sun formed, but there also must have been a lot of close encounters since then as well.

The answers are in the outer solar system beyond Neptune. We need much more powerful telescopes (and as I often end my comments), to know for sure we need to boldly go where no one has gone before.
granville583762
3.7 / 5 (6) Jan 21, 2019
In 5billion years 4000 Suns graze the Ort cloud
Old_C_Code> Yes, other Star's Oort clouds hit our oort cloud and solar system generally. Because their are other stars that pass by us at close range (10,000 AU), and their oort clouds hit us, and ours hits theirs. The next star to pass by us is in only 1.2 million years at a distance of 9k to 13k AU.
We have the best chance of discovering alien life on stars that pass by us ( in only 1.2 million years...lol, ugh)..

1.2 million years in 4.5billion years
by this down to earth calculation
3750 Stars have grazed our Ort cloud
and
come within 10,000AU
if these produce neptunium sized planets
amongst the fragmentary fragments of ice
when we build our telescopes
these 4000 neptunium sized planets
are
Going to be pretty hard to miss
jonesdave
3.3 / 5 (7) Jan 21, 2019
these 4000 neptunium sized planet are....


Neptune-sized. Neptunium is a f***ing element. Jesus H. Christ.
Mark Thomas
3.7 / 5 (3) Jan 21, 2019
granville, you are right, I should have had about 3 thousand not 30 thousand, my bad.

I used 4.6 billion years because the solar system formed a little before Earth. I divided by 1.2 million years and should have got about 3,833 close passes. However, I don't want to put too fine a point on this because I have no idea how close this truly is to being average. I am only trying to get into the ballpark, i.e., thousands of close approaches may be the right ball park, maybe.

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud.


https://en.wikipe...r_System
granville583762
5 / 5 (3) Jan 21, 2019
13,000 light years Planet Is one of the Farthest ever Detected

A NASA telescope has co-discovered one of the most distant planets ever identified: a gas giant about 13,000 light-years away from Earth.
the technique used by the Spitzer Space Telescope, called microlensing, is so new that it has only yielded about 30 planet discoveries so far. But the telescope's potential for finding far-away worlds is vast
as we search the Kuiper belt
are telescopes come up blank
At 13,000Lyrs are telescopes can see
https://www.space...own.html
jonesdave
3.9 / 5 (8) Jan 21, 2019
.....the technique used by the Spitzer Space Telescope, called microlensing, is so new that it has only yielded about 30 planet discoveries so far


I did a project for a BSc degree on 'Microlensing as a means of detecting exoplanets.' The big players back then (~2010) were OGLE in Poland and MOA in New Zealand. Fascinating stuff.

granville583762
5 / 5 (3) Jan 21, 2019
No star has grazed our Sun
Mark Thomas> granville, you are right, I should have had about 3 thousand not 30 thousand, my bad.

I used 4.6 billion years because the solar system formed a little before Earth. I divided by 1.2 million years and should have got about 3,833 close passes. However, I don't want to put too fine a point on this because I have no idea how close this truly is to being average. I am only trying to get into the ballpark, i.e., thousands of close approaches may be the right ball park, maybe.

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud.


https://en.wikipe...r_System

I've found with calc98
the hard way, you cannot believe the answer its calculated
but
there is one certainty
No star has grazed our Sun
as
We would not be here to tell the tale
granville583762
5 / 5 (3) Jan 21, 2019
We spoke to soon -January 2006 a planet at 22,000Lys
.....the technique used by the Spitzer Space Telescope, called microlensing, is so new that it has only yielded about 30 planet discoveries so far


jonesdave> I did a project for a BSc degree on 'Microlensing as a means of detecting exoplanets.' The big players back then (~2010) were OGLE in Poland and MOA in New Zealand. Fascinating stuff.

Microlensing is capable of finding the furthest and the smallest planets of any currently available method for detecting extrasolar planets. In January of 2006 scientists announced the discovery through microlensing of a planet of only five Earth masses, orbiting a star near the center of our galaxy, 22,000 light-years away! It was the lowest mass planet detected up to that time, and also the furthest from the Earth.
http://www.planet...ing.html

jonesdave
3.9 / 5 (7) Jan 21, 2019
Microlensing is capable of finding the furthest and the smallest planets of any currently available method for detecting extrasolar planets. In January of 2006 scientists announced the discovery through microlensing of a planet of only five Earth masses, orbiting a star near the center of our galaxy, 22,000 light-years away! It was the lowest mass planet detected up to that time, and also the furthest from the Earth.


Yep, pretty sure I referenced that paper in the project. The disadvantage of microlensing is the low detection rate. It requires two stars to pass pretty close to each other in line of sight. The benefits of it is that it is not biased on mass, as is the radial velocity method. And it is not biased by the length of observation, as is the transit method. As such it should give a more realistic view of planetary masses and distances from the primary.

Mark Thomas
4 / 5 (2) Jan 21, 2019
No star has grazed our Sun


I would be shocked if one had. I was thinking more along the lines of a passing star grazing our solar system. Scholz's Star may have passed through the Oort Cloud at "only" 52,000 AU or 0.82 light years from the sun about 70,000 years ago.

https://en.wikipe...27s_Star
peabody3000
2 / 5 (1) Jan 21, 2019
how about the culprit being a planet that got ejected from the solar system after doing a dance with one of the gas giants and spending a couple decades swinging through the outer realm?
yaridanjo
1 / 5 (4) Jan 22, 2019
Most astronomers agree that planet Nine has not been found. Maybe those who govern us don't want it to be found because it is indirectly threatening Earth. Certainly, Planet Nine won't be flying thru our inner solar system. But comet swarms (formed from Kuiper Belt Objects that fall towards and round our Sun when their normal orbits are interrupted by Planet Nine's gravity) could threaten Earth. Moreover, debris from these comet swarms could be responsible for past Ice Ages. If such a swarm may soon be near our inner solar system, mum's the word. And that could be the real reason Planet Nine 'has not been found':
http://barry.warm...led.html
VULCAN REVEALED
A Dangerous New Jovian Sized Body In Our Solar System
granville583762
5 / 5 (3) Jan 22, 2019
A theoretically sound argument

The strange orbits
of some objects
in the farthest reaches
of our solar system
hypothesised
by some astronomers
to be shaped
by
an unknown ninth planet
can instead
be explained
by the combined gravitational force
of
small objects orbiting the Sun beyond Neptune

As the fragmented fragmentary fragments
orbit our Sun
as the fragmented fragmentary fragments coalesce
under gravitational attraction
as these fragmented fragmentary fragments
grow into larger fragments
their gravitational influence abounds
these larger fragments
increased gravitational influence
draws together increased numbers of fragments
in regions of this Kuiper belt
over millions of miles
that from our distant view point from this Kuiper belt
appear as large collection of mass
with a large gravitational attraction
as though it is a planet
Even though it is fragmentary fragments coalescing
granville583762
5 / 5 (3) Jan 22, 2019
A theoretically sound argument

Which answers the question
are there planets in this kuiper belt
YES
Planets have by this process grown
as
they are as large as Eros and Pluto
because
this process takes time
billions of years of time
where
as we found Eros and Pluto
there are
countless planets
of this size of Eros and Pluto
but
Do not hold your breath for any planet larger
hat1208
5 / 5 (3) Jan 22, 2019
@yandanjo

No
Mark Thomas
3.7 / 5 (3) Jan 22, 2019
granville, minor spelling correction, but you are thinking of Eris, not Eros (an asteroid).

https://en.wikipe..._planet)

https://en.wikipe...433_Eros

As we discussed once on another thread, by my personal definition of "planet" there are ten known planets in the solar system including Pluto and Eris. I believe Alan Stern, head of NASA's New Horizon mission would agree.
torbjorn_b_g_larsson
3 / 5 (3) Jan 22, 2019
This is a better model than the first attempt at looking at Kuiper Belt Objects (KBOs) as responsible for various observations, but it is still much less predictive than the Planet Nine model. The former hypothesis does explain neither all the TNO groups nor the planet system tilt which the latter hypothesis does predict.

The naturalness of either hypothesis is arguable, but we expect to see superEarths and distant planets, while massive KBO debris disks are not. I don't think we know of any example of a massive protoplanetary disk that has been converted to a massive debris disk.
torbjorn_b_g_larsson
3 / 5 (3) Jan 22, 2019
Ultima Thule has a nearly circular orbit and has for billions of years been unaffected by supposed "planet 9s".


Is that a naive attempt of "gotcha"? Of course it has been predicted it will not do so or the hypothesis would not work among astronomers. The major influence PN would have on the KBOs is that it would sculpt the Kuiper Belt "cliff" - the observed abrupt outer edge - which no other hypothesis has explained.

comet swarms (formed from Kuiper Belt Objects that fall towards and round our Sun when their normal orbits are interrupted by Planet Nine's gravity) could threaten Earth.


Yikes. No. We have 4 billion years of data that says that does not happen - is that not enough for you?

But more generally KBOs are not the main supply of comets, the Oort cloud is. But PN has cleared the KBOs (see the "cliff" observation above) and the Oort cloud inasmuch as it ever was so far in. And that is one reason why it is seen as a planet (has cleared its orbit).

yaridanjo
1 / 5 (1) Jan 22, 2019
@hat1208
Because?
granville583762
5 / 5 (3) Jan 22, 2019
Planets come in a range of sizes
Mark Thomas> granville, minor spelling correction, but you are thinking of Eris, not Eros (an asteroid).
https://en.wikipe..._planet)
As we discussed once on another thread, by my personal definition of "planet" there are ten known planets in the solar system including Pluto and Eris. I believe Alan Stern, head of NASA's New Horizon mission would agree.

Thanks for correcting the spelling mistake
due to a planets status redefined
as dwarfs and planets do not mix
as Pluto has shown there are small planets
where Jupiter has shown there are large planets
as dwarf is no proper description
as planets come in a range of sizes
there are smaller and larger planets
dwarfs are fictional characters that do not exist
planet Pluto is not a dwarf planet
planet Pluto is a small planet
Planets come in a range of sizes
maholmes1
1 / 5 (2) Jan 22, 2019
The ninth planet is already known. It's called Neptune.
maholmes1
1 / 5 (1) Jan 22, 2019
granville, minor spelling correction, but you are thinking of Eris, not Eros (an asteroid).

https://en.wikipe..._planet)

As we discussed once on another thread, by my personal definition of "planet" there are ten known planets in the solar system including Pluto and Eris. I believe Alan Stern, head of NASA's New Horizon mission would agree.


You mean the geophysical definition of planet with a 2000-kilometer minimum diameter that was proposed by a committee of the IAU in 2005, right?
granville583762
5 / 5 (3) Jan 23, 2019
Planet Pluto is 2326 km diameter
granville, minor spelling correction, but you are thinking of Eris, not Eros (an asteroid).

https://en.wikipe..._planet)

As we discussed once on another thread, by my personal definition of "planet" there are ten known planets in the solar system including Pluto and Eris. I believe Alan Stern, head of NASA's New Horizon mission would agree.

maholmes1> You mean the geophysical definition of planet with a 2000-kilometer minimum diameter that was proposed by a committee of the IAU in 2005, right?

As Planet Pluto has shown it is a planet of 2326 km diameter
Planets come in a range of sizes
Planet Pluto is a small planet
granville583762
5 / 5 (3) Jan 23, 2019
Maholmes1, what is this minimum 2000km diameter of which you speak
Is not Planet Pluto exceeding this minimum diameter by 16.3%
even as this highly active planet, Planet Pluto, is a small Planet
Planet Pluto exceeds definitions by 16.3%
so
even under any designation Planet Pluto is not a dwarf
dwarfs are fictional characters
Planet Pluto is not fictional
Planet Pluto is real
Planet Pluto is a small Planet
hat1208
not rated yet Jan 23, 2019
@hat1208
Because?


Because I said so.
Mark Thomas
3 / 5 (2) Jan 23, 2019
You mean the geophysical definition of planet with a 2000-kilometer minimum diameter that was proposed by a committee of the IAU in 2005, right?


Fantastic! I came up with this independently, but I can see the IAU beat me to it by many years to our mutual credit. Unfortunately, although the 2,000 km limit got the most votes of three presented definitions, they dropped it and went to the stupid definition they currently use.

https://en.wikipe...f_planet

Here is my definition:

A planet is a sub-stellar, celestial body at least 2,000 kilometers in diameter.


Although it is not perfect, I would put my personal planet definition up against the IAU's any day of the week. Orbiting the sun, or anything else for that matter, is not required, neither is "clearing the neighborhood."

Mark Thomas
3 / 5 (2) Jan 23, 2019
A planet is a sub-stellar, celestial body at least 2,000 kilometers in diameter.


This short definition covers our solar system and leaves us with 10 planets including Pluto and Eris. It covers exoplanets and rogue planets because it is not necessary to orbit OUR sun (how stupid) or any other star. Orbit and neighborhood should be irrelevant. The "sub-stellar" term is flexible enough to allow for continuing research to help separate planets at the high end from objects like brown dwarfs. Yes 2,000 km is arbitrary, but it is easy to remember and precise enough to quickly identify most objects. Anything that big should be rounded to a very significant degree like one would expect a planet to be.
Mark Thomas
3 / 5 (2) Jan 23, 2019
A planet is a sub-stellar, celestial body at least 2,000 kilometers in diameter.


I covered some of my thinking on this in greater detail here:

https://phys.org/...net.html

and here:

https://phys.org/...net.html
granville583762
5 / 5 (4) Jan 23, 2019
A planet, is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces (nearly round) shape, has cleared the neighbourhood around its orbit.
A dwarf planet, is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces (nearly round), has not cleared the neighbourhood around its orbit, is not a satellite

Planet Earth
has not cleared the neighbourhood around its orbit
whereas the moon
is in the process of clearing its orbit
as in time it will be independently orbiting the Sun as a lone planet
although in years to come
when future astronomers observe planet earth they will have to take into account it did not clear its orbit
Just as planet Pluto is 16.3% larger than the minimum diameter
planet Earth is slightly more than 16.3% larger than the minimum diameter
Diameters are obviously not the deciding factor
granville583762
5 / 5 (4) Jan 23, 2019
As we have kicked our planets out this starry club of planets

Before we kicked these planets out on the street, lock stock and barrel
Eris is 27 percent more massive than Pluto. Eris has a density of 2.52g/cm3
substantially denser than Pluto,
composed largely of rocky materials
Eris, is potentially a larger planet than planet Pluto
Eris, being composed of rocky material could have been sling shot into a larger orbit
as potentially
Eris could be our ninth planet as Pluto could be our tenth
so how ever we look at it
we have thrown our planets out on the street
before
we could get satellite observational, magnetic, mass and a whole host of planetary data
as now
there is a possibility Eris is actual before Pluto in our queue of planets
so how ever we look at it
we have thrown our planets out on the street
Ninth, tenth, eleventh, twelfth, thirteenth, infinitum
Mark Thomas
3 / 5 (2) Jan 23, 2019
The later rejected IAU definition with the 2,000 km limit:

"A planet is any object in orbit around the Sun with a diameter greater than 2000 km." (eleven votes in favour)

My definition:

A planet is a sub-stellar, celestial body at least 2,000 kilometers in diameter.

My definition drops the orbiting the sun requirement and adds "sub-stellar, celestial body" to cover the high end and better define the object.

Here is the IAU current definition:

(1) A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

"Celestial body" makes sense, but (a) and (c) are wrong; and (b) is inherent in the 2,000 km diameter minimum. Ceres appears to be in hydrostatic equilibrium and it is less than half (946 km) of the minimum diameter (2,000 km) necessary to be a planet.
granville583762
5 / 5 (4) Jan 23, 2019
Pluto's Unusual Orbit

It takes 248 Earth years for Pluto to complete one orbit around the Sun
Its orbital path doesn't lie in the same plane as the eight planets
but is inclined at an angle of 17°
its orbit is also more oval-shaped, or elliptical, than those of the planets
that means that sometimes Pluto is a lot nearer to the Sun than at other times
at times Pluto's orbit brings it closer to the Sun than Neptune
the last time this happened was from 1979 to 1999. It won't happen again until 2227

As if we need proof of orbits
planet Pluto's orbit is eccentric
Pluto's orbit is highly susceptible
to planetary gravitational influence as it is almost Cometary in its orbit because planet Pluto is so distant from the sun
so as we look for are ninth planet
Eris could be our ninth and Pluto our tenth
This needs thorough investigation
https://airandspa...rbit.cfm
granville583762
5 / 5 (4) Jan 23, 2019
This is why, Mark Thomas
I bypass this worry
by describing planets
as larger and smaller planets
as I sit on the river bank
watching Eris and Pluto sick with worry
Contemplating their fin rot under their bridge
rrwillsj
3 / 5 (2) Jan 23, 2019
There may have been, may still be, regularly scheduled fly-byes of other star systems.

However if any got close enough to intermingle Oort Clouds? Wouldn't the orbits of the visitor star & the Sun alter each other? Maybe just a little course change, adding up after several passages?

But I would think most visitations were indeterminately, irregular, randomly one-time occurrences.

The moon Rhea might be a captured visitor?

Some people would claim that a planet / planetoid is determined by it's shape. That any body that has a gravity deep enough to form a sphere? Would qualify, such as Ceres.

Personally. I find the subject boring.
How about we flip a coin?
granville583762
5 / 5 (4) Jan 23, 2019
Asteroids are minor planets
especially of the inner Solar System
larger asteroids also called planetoids.
in 1801, while making a star map, Giuseppe Piazzi accidentally discovered a small object 1000km in diameter between the orbits of Mars and Jupiter
Piazzi named the object Ceres
It was the first asteroid to be discovered.
Eris is too large to be a planetoid
Eris is a small planet
it has an active radioactive volcanic core
the more we dig
The more we find there already exists an adequate definition laid down naturally over the centuries
Mark Thomas
3.7 / 5 (3) Jan 23, 2019
Eris is too large to be a planetoid . . . Eris is a small planet


I agree. One reason to define it as a planet is that it is the ONLY planet not visited by a probe, and as such, one can argue that this a serious problem to be rectified. I am confident that once we get a probe to Eris and high quality photos returned that some people will find it fascinating there is a whole world in the solar system they never even knew about.

Maybe it is time for the human race to grow up a little and realize all these distant worlds are real and incredibly fascinating. This ability will come in handy as we try to imagine what the trillion plus planets in the Milky Way are like. It is electrifying to imagine what is and what might be out there.
yaridanjo
1 / 5 (1) Jan 24, 2019
Our Vulcan, that appears to be somewhat like Planet Nine, is thought to be an ultra tiny brown dwarf star. Like the Sun has planets (Venus thru Neptune) it has planets, or more precisely dwarf planets:
http://barry.warm...s.html#d
A NEW GIANT PLANET IN OUR SOLAR SYSTEM?
The dwarf planets are:
Pluto
2004 XR190 ? Maybe
Eros
2015 RR245 ? maybe
Biden
BTW, the Earth/Moon system and the Neptune/Triton system are double planets.
torbjorn_b_g_larsson
1 / 5 (1) Jan 24, 2019
Batygin's - a Planet Nine [PN] researcher - criticism of the paper was interesting, it was along the line of accumulated low likelihood features, with few Kuiper belt objects observed outside the so called "Kuiper cliff". [ https://gizmodo.c...31961723 ]

But even without this problem the prior is on the PN side since it predicts several features (such as the Kuiper cliff, several of the eccentric TNO families, and the tilt of the planet orbits in relation to Sun equator). The new hypothesis predicts only one feature.
torbjorn_b_g_larsson
1 / 5 (1) Jan 24, 2019
they ,,, went to the stupid definition they currently use.

https://en.wikipe...f_planet



It is useful rather than "stupid" so of course they use it. The original astronomical classification was to avoid having to name myriad debris objects. But it also happens to correspond with observed exoplanets of mature systems (not so much in ongoing accretion, obviously) since it was sufficiently physical motivated to be general.

The astrophysical definition you describe has its uses too, but they are different.

Can we stop criticizing experts for their good work? There are cases where they do less useful classifications, the 20+ variants of biological species definitions comes to mind where only 3 or so would suffice, but this is no such case. Same as humanity mistook Ceres for a planet long since (and no one gripes about that) we misplaced Pluto.
Mark Thomas
3 / 5 (2) Jan 24, 2019
It is useful rather than "stupid" so of course they use it.


It is useful in excluding Pluto and Eris, but it makes very little sense otherwise.

No need to be in orbit around the sun, that IS stupid. This isn't 1991, here in 2019 we know the vast majority of stars have multiple "planets." Time to acknowledge that our solar system's planets and the other stars' exoplanets are all just planets. There is no principle of science to separate them.

Then, even worse, "has cleared the neighbourhood around its orbit." What does that even mean? So Earth is a planet, but if were moved to the outer solar system it wouldn't be? Object permanence means location is irrelevant, it is a planet regardless.

Finally, "rounded" is good, but not good enough because many of those objects are just too small. Again, this does not help us to draw a clear line like the 2,000 km minimum 11 astronomers voted for.
maholmes1
5 / 5 (1) Jan 27, 2019
A planet is a sub-stellar, celestial body at least 2,000 kilometers in diameter.
.


Around 2000 km is when a planet becomes massive enough to hold on to an atmosphere, which most people associate with planethood. Moons, also; I keep hearing "Pluto has moons, so it's a planet," but asteroids can and do have moons. But atmosphere is a different story; no irregularly shaped little asteroid has an atmosphere, but planets do. However, Ceres, which geophysically is a planet, is too small to hold on to an atmosphere, unlike Pluto. Eris, according to one report I dug up on Google Scholar, may only have an atmosphere in places where it's warm enough to where the gases don't freeze onto the planet's surface. It would probably have an atmosphere when it's closer to the Sun--because it's massive enough to have one.

Or one could just go with mass and say that hydrostatic equilibrium is enough.This is a matter of scientific debate, and it's being debated in spite of the IAU.
Mark Thomas
not rated yet Jan 28, 2019
Or one could just go with mass and say that hydrostatic equilibrium is enough.


A range in mass might be good enough to cover both ends of the spectrum, maybe.

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