Planet Nine: A world that shouldn't exist

May 3, 2016, Harvard-Smithsonian Center for Astrophysics
An artist's conception of Planet Nine. Credit: Caltech/R. Hurt (IPAC)

Earlier this year scientists presented evidence for Planet Nine, a Neptune-mass planet in an elliptical orbit 10 times farther from our Sun than Pluto. Since then theorists have puzzled over how this planet could end up in such a distant orbit.

New research by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) examines a number of scenarios and finds that most of them have low probabilities. Therefore, the presence of Planet Nine remains a bit of a mystery.

"The evidence points to Planet Nine existing, but we can't explain for certain how it was produced," says CfA astronomer Gongjie Li, lead author on a paper accepted for publication in the Astrophysical Journal Letters.

Planet Nine circles our Sun at a distance of about 40 billion to 140 billion miles, or 400 - 1500 astronomical units. (An astronomical unit or A.U. is the average distance of the Earth from the Sun, or 93 million miles.) This places it far beyond all the other in our . The question becomes: did it form there, or did it form elsewhere and land in its unusual orbit later?

Li and her co-author Fred Adams (University of Michigan) conducted millions of computer simulations in order to consider three possibilities. The first and most likely involves a passing star that tugs Planet Nine outward. Such an interaction would not only nudge the planet into a wider orbit but also make that orbit more elliptical. And since the Sun formed in a star cluster with several thousand neighbors, such stellar encounters were more common in the early history of our solar system.

However, an interloping star is more likely to pull Planet Nine away completely and eject it from the solar system. Li and Adams find only a 10 percent probability, at best, of Planet Nine landing in its current orbit. Moreover, the planet would have had to start at an improbably large distance to begin with.

CfA astronomer Scott Kenyon believes he may have the solution to that difficulty. In two papers submitted to the Astrophysical Journal, Kenyon and his co-author Benjamin Bromley (University of Utah) use computer simulations to construct plausible scenarios for the formation of Planet Nine in a wide orbit.

"The simplest solution is for the solar system to make an extra ," says Kenyon.

They propose that Planet Nine formed much closer to the Sun and then interacted with the other gas giants, particularly Jupiter and Saturn. A series of gravitational kicks then could have boosted the planet into a larger and more over time.

"Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they'll go higher and higher," explains Kenyon. "Then the challenge becomes not shoving the planet so much that you eject it from the solar system."

That could be avoided by interactions with the solar system's gaseous disk, he suggests.

Kenyon and Bromley also examine the possibility that Planet Nine actually formed at a great distance to begin with. They find that the right combination of initial disk mass and disk lifetime could potentially create Planet Nine in time for it to be nudged by Li's passing star.

"The nice thing about these scenarios is that they're observationally testable," Kenyon points out. "A scattered gas giant will look like a cold Neptune, while a planet that formed in place will resemble a giant Pluto with no gas."

Li's work also helps constrain the timing for Planet Nine's formation or migration. The Sun was born in a cluster where encounters with other stars were more frequent. Planet Nine's wide orbit would leave it vulnerable to ejection during such encounters. Therefore, Planet Nine is likely to be a latecomer that arrived in its current after the Sun left its birth cluster.

Finally, Li and Adams looked at two wilder possibilities: that Planet Nine is an exoplanet that was captured from a passing star system, or a free-floating planet that was captured when it drifted close by our solar system. However, they conclude that the chances of either scenario are less than 2 percent.

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17 comments

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ViperSRT3g
4.1 / 5 (10) May 03, 2016
In the related stories, this article is listed first and foremost: http://phys.org/n...lar.html

Is this not a possible source of this theoretical 9th planet?
krundoloss
2.9 / 5 (10) May 03, 2016
Planet Nine circles our Sun at a distance of about 40 billion to 140 billion miles, or 400 - 1500 astronomical units. (An astronomical unit or A.U. is the average distance of the Earth from the Sun, or 93 million miles.)


Oh, it does huh? Quite the span of orbit possibilities there LOL. What I dont get, is how we can see the effects of Planet Nine on Keiper Belt Objects, why then cannot we 'see' this hypothetical Planet? Would it not at least pass in between us and some distant object, like a star or galaxy, or anything detectable?
Azrael
4.7 / 5 (13) May 03, 2016
Oh, it does huh? Quite the span of orbit possibilities there LOL.


That's not a range of orbit possibilities, as if it's a perfectly circular orbit somewhere between those distance values... It's the minimum and maximum distance of the planet from the sun, given it's theoretically odd, elliptical orbit.

why then cannot we 'see' this hypothetical Planet? Would it not at least pass in between us and some distant object, like a star or galaxy, or anything detectable?


We're currently trying to pin down it's most probable position in it's very large orbit, based on models of orbital dynamics. We've already constrained it's likely position considerably, but there's still a lot of sky to search.

Stellar occultation just might be the best way to spot the thing. I have no doubt that, right now, people are searching past and current images of the star field for occultations related to this hypothetical planet as we discuss.
krundoloss
5 / 5 (11) May 03, 2016
That makes sense with the elliptical orbit, I did not think of it like that. Thank you for your response. I love it when people reply with information and do not include insults with it, LOL. Good Luck with your search!
ThomasQuinn
1 / 5 (11) May 04, 2016
Planet Nine circles our Sun at a distance of about 40 billion to 140 billion miles, or 400 - 1500 astronomical units. (An astronomical unit or A.U. is the average distance of the Earth from the Sun, or 93 million miles.)


Oh, it does huh? Quite the span of orbit possibilities there LOL. What I dont get, is how we can see the effects of Planet Nine on Keiper Belt Objects, why then cannot we 'see' this hypothetical Planet? Would it not at least pass in between us and some distant object, like a star or galaxy, or anything detectable?


It may just be me, but I get really annoyed when people can't even COPY a name from an article correctly - Kuyper Belt. Not Keiper Belt. If you're this cavalier with details, it makes me wary of trusting the rest of what you write.
Enthusiastic Fool
5 / 5 (8) May 04, 2016
Kui·per belt*
ˈkīpər ˌbelt
(credit to google)
@TQ
There's no mention of Kuiper Belt in the above article. If you're this cavalier with reading comprehension and pretentious enough to incorrectly correct someone it makes me wary of trusting the rest of what you write.

@Krundo
It'd have to emit some light and it's likely very cold so it won't be easy to find on infrared wavelengths. The other option is that it has to have a chance transit with a star as we're watching it. It's suspected to have an orbital period of 10k years at the fastest. I don't know the math for the ellipse but that'd be 28 years to pass through a degree of sky if the orbit was circular. (10k/360=27.777777...) The ellipse and our relative position inside the orbit makes this time relatively useless but It does help to illustrate the kinds of time we have to invest in finding it in a given place in the sky without more info. Hope this helps a little.
krundoloss
4.4 / 5 (7) May 04, 2016
Kuyper Belt. Not Keiper Belt. If you're this cavalier with details, it makes me wary of trusting the rest of what you write.


What a douchebag. Anyway I will try to be more careful with my spelling, I originally wrote KBO, and decided to spell it out. I make these comments quickly at work, so its not thorough scientific stuff. Besides, they are Comments, not updates, not scientific rebuttals.

The sad thing is that while scientists do the difficult work of searching for unknown celestial bodies, meanwhile Trolls search for typos in the comments section of websites, just so they can feel a little better about themselves, while anonymously trashing someone else.
Mark Thomas
5 / 5 (3) May 04, 2016
"However, an interloping star is more likely to pull Planet Nine away completely and eject it from the solar system."

Maybe Planet 9 existed long enough to have the effect on certain Kuiper Belt objects Astronomer Mike Brown observed, but was subsequently ejected from the solar system.

In any case, that ol' gut feeling that Planet 9 is a wild goose chase keeps getting stronger and stronger.
torbjorn_b_g_larsson
5 / 5 (2) May 04, 2016
"The Sun was born in a cluster where encounters with other stars were more frequent. Planet Nine's wide orbit would leave it vulnerable to ejection during such encounters."

I like orbit expert and PN 'discoverer' Batygin's own theory best (though I would like to see simulations). E.g. PN was ejected in the first 10 Myrs or so and braked by gas and dust interactions with the remaining protoplanetary disk. Precisely because the Sun was still in its birth cluster the event had to be confied to where it's at now.

The 6th ejected gas giant is less likely. since that would put unusually many planets in our system originally. With PN the system is "just right", more Neptunes than Jupiters.

[In fact, with unbiased classification Saturn is most likely a giant neptune, and Earth is the superEarth we miss, making our system having planets from all classes and most of them - 4 each - neptunes and terrestrials.]
Whydening Gyre
5 / 5 (3) May 04, 2016
What I dont get, is how we can see the effects of Planet Nine on Keiper Belt Objects, why then cannot we 'see' this hypothetical Planet?


Because it is difficult ? It is not like going to the fridge to get another beer.

Well, it kinda is - if ya don't remember which fridge the beer is in...:-)
TechnoCreed
5 / 5 (5) May 04, 2016
@ThomasQuinn
It may just be me, but I get really annoyed when people can't even COPY a name from an article correctly - Kuyper Belt. Not Keiper Belt. If you're this cavalier with details, it makes me wary of trusting the rest of what you write.

Thanks for the laugh!

I don't know if you if you know much about astronomy, but I know that you know your gin and might had absorbed a wee bit too much of it ;-)
Mark Thomas
4.5 / 5 (8) May 04, 2016
I suspect Comet Lovejoy (the one leaking alcohol) was from the De Kuyper Belt. :-)
Rosser
3.7 / 5 (3) May 04, 2016
So, we're reasonably certain it's out there somewhere. And we have narrowed the possible places to look. Looking forward, has NASA given any consideration to launching a probe to take pictures and measurements? I know it may seem a bit premature to be doing this planning, but given the lead time necessary to get something off the ground, they might want to at least be looking at the potential to do so. Comments?
BongThePuffin
May 04, 2016
This comment has been removed by a moderator.
Whydening Gyre
5 / 5 (5) May 04, 2016
So, we're reasonably certain it's out there somewhere. And we have narrowed the possible places to look. Looking forward, has NASA given any consideration to launching a probe to take pictures and measurements? I know it may seem a bit premature to be doing this planning, but given the lead time necessary to get something off the ground, they might want to at least be looking at the potential to do so. Comments?

Would take an awful long time to get out there...:-)
rgw
3 / 5 (1) May 05, 2016
"Because it is difficult ? It is not like going to the fridge to get another beer."

Might be, if your fridge is in Antarctica.
Shootist
3 / 5 (2) May 08, 2016
not a comment

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