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Planet Nine is a hypothetical large planet in the far outer Solar System which would explain the unusual orbital configuration of a group of trans-Neptunian objects (TNOs) whose orbits lie mostly beyond the Kuiper belt.

Following the initial 2014 work of Chad Trujillo and Scott Sheppard, researchers Konstantin Batygin and Michael E. Brown at Caltech announced on 20 January 2016 calculation-based evidence of a massive ninth planet in the Solar System. The predicted planet would be a super-Earth with an estimated mass of about 10 times that of Earth (5000 times the mass of Pluto), have a diameter two to four times that of Earth, a thick atmosphere of hydrogen and helium, and a highly elliptical orbit so far away that it could take it 15,000 years to orbit the Sun.

In their discussion, the authors considered models of planet formation that might include planetary migration from the inner Solar System, such as the fifth giant planet hypothesis.

Characteristics

Orbit

Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun, with an orbital period of 10,000–20,000 years. The planet's orbit would have a semi-major axis of roughly 700 astronomical units (AU), or about 20 times the distance of Neptune from the Sun, though it might come as close as 200 AU (30 billion km), and its inclination estimated as 30 (± 20) degrees. The high eccentricity of Planet Nine's orbit could take it as far away as 1200 AU at its aphelion. The aphelion, or farthest point of the estimated orbit, would be in the general vicinity of the constellations of Orion and Taurus, while the perihelion, or nearest point in the orbit to the Sun, would be in the vicinity of the southerly areas of Serpens (Caput), Ophiuchus, and Libra.

Size

The planet is estimated to have 10 times the mass and two to four times the diameter of Earth. An infrared survey by the Wide-field Infrared Survey Explorer (WISE) in 2009 does not exclude such an object because its results allow for a Neptune-sized object beyond 700 AU. A similar study in 2014 focused on possible higher-mass bodies in the outer Solar System and ruled out Jupiter-mass objects out to 26,000 AU. Brown estimates that Planet Nine's mass is higher than the mass required to clear its feeding zone in 4.6 billion years, and that it is hence indeed a planet.

Composition

Brown speculates that the predicted planet is most likely an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas.

Informal names

Brown and Batygin have used the names "Jehoshaphat" and "George" for Planet Nine. Brown has stated: "We actually call it "Fatty" when we're just talking to each other." In August 2014 Lorenzo Iorio proposed the name "Thelisto" in Monthly Notices of the Royal Astronomical Society for the hypothetical planet causing the unusual orbits of the sednoids and detached objects.

Indirect detection

Case for a new planet

The existence of Planet Nine would explain a peculiar cluster of six extreme trans-Neptunian objects in a stable configuration of orbits mostly outside the Kuiper belt (namely Sedna, 2012 VP113, 2007 TG422, 2004 VN112, 2013 RF98, 2010 GB174), and the perpendicular tilt (orbit with a right angle, compared to the other objects) of other objects such as 2008 KV42, 2012 DR30.

A closer look at the data shows that these six objects trace out elliptical orbits that point into approximately the same direction in physical space, and lie in approximately the same plane. These, according to simulation, would only occur with 0.007% probability by chance alone. The predicted planet would have an orbit oriented approximately opposite to those of the six TNOs (would have an longitude of perihelion offset by about 180°). All known minor planets with perihelion greater than 30 AU and a semi-major axis greater than 250 AU currently support the Planet Nine hypothesis.

Early speculation

The discovery of Sedna and its peculiar orbit in 2004 led to the conclusion that sometime in the past something beyond the known eight planets perturbed Sedna away from the Kuiper belt. That could have been another planet; it could have been a star that came close to the Sun; or it could have been a lot of stars if the Sun formed in a cluster.

After analysing the orbits of a group of trans-Neptunian objects with highly elongated orbits, Rodney Gomes of the National Observatory of Brazil created models that demonstrated the possible existence of an as yet undetected planet (of unknown size and undetermined orbit) that could be too far away to influence the motions of Earth and the other inner planets, yet close enough to the scattered disc objects to sway them into the elongated orbits.

The announcement of the discovery of 2012 VP₁₁₃ in March 2014, which shared a few odd orbital characteristics with Sedna and other extreme trans-Neptunian objects further raised the possibility of an unseen super-Earth in a large outer orbit.

Planet Nine hypothesis

The first strong argument for the existence of Planet Nine was published in 2014, by astronomers Scott Sheppard of the Carnegie Institution of Science and Chad Trujillo of Hawaii's Gemini Observatory, who suggested the similar orbits of certain objects such as sednoids might be influenced by a massive unknown planet at the edge of the Solar System. Their findings demonstrated that a Super-Earth of about 2 to 15 Earth masses beyond 200 AU with a possible highly inclined orbit at 1500 AU could shepherd the extreme Kuiper belt objects into similar type orbits.

Computer simulations by Caltech's Michael E. Brown and Konstantin Batygin, originally developed to refute the 2014 paper, instead provided further evidence that Planet Nine may exist. Their theoretical model ties together three elusive aspects of the Kuiper belt (namely, the physical alignment of the distant orbits, the generation of detached objects such as Sedna and the existence of a population tracing out perpendicular orbital trajectories) into a single, unifying picture.

Brown later described the hypothesized planet as a perturber of extreme KBOs, and speculated that, if current findings prove correct, Planet Nine could have developed into the core of a gas giant had it not been flung into the Solar System's farthest reaches.

Brown thinks that if the new object exists and is confirmed to have the effects observed, it needs to be even more massive if it is farther away. He thinks that no matter where it is speculated to be, if it exists, then it dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions.

Simulation

The capture of KBOs into long-lived apsidally anti-aligned orbital configurations occurs, with variable success, across a significant range of companion parameters (semi-major axis a ~ 400–1500 AU, eccentricity e ~ 0.5–0.8). For their best-fit nominal simulation, they selected a = 700 AU, e = 0.6, M = 10 M_E (meaning a body with ten times the mass of Earth), orbital inclination i = 30°, and initial argument of perihelion ω = 150° (compared to roughly 310° the average for the six analyzed TNOs).

The simulations showed that planetesimal swarms could be sculpted into collinear groups of spatially confined orbits by Planet Nine if it is substantially more massive than Earth and is on a highly eccentric orbit. Furthermore, the confined orbits would cluster in a configuration where the long axes of their orbits are anti-aligned with respect to Planet Nine, signalling that the dynamical mechanism at play is resonant in nature. This mechanism, known as mean-motion resonance, prevents trapped trans-Neptunian objects from colliding with Planet Nine, and keeps them aligned.

Simulations have shown that objects that have a semi-major axis less than 150 AU are largely unaffected by the presence of Planet Nine as they have very low chance of coming in its vicinity.

Inference

Batygin is cautious in interpreting the results, saying "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo."

Brown puts the odds for the existence of Planet Nine at about 90%. Greg Laughlin, one of the few researchers who knew in advance about this paper, gives an estimate of 68.3%. Other skeptical scientists demand more data in terms of additional objects to be analysed or final evidence through photographic confirmation. Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a serious search for a new planet and assures everyone that it will not be a wild goose chase.

Brown is supported by Jim Green, director of NASA's Planetary Science Division who says that "the evidence is stronger now than it's ever been before".

Tom Levenson concluded that, for now, Planet Nine seems the only satisfactory explanation for everything now known about the outer regions of the Solar System.

Direct detection

Radiation

A distant planet such as this would reflect little light, but—because it is estimated to be a large body—its radiation signature is more likely to be detected by Earth-based infrared telescopes (such as ALMA). However, this still would need to be confirmed with visual corroboration as the ALMA cannot readily distinguish between a small, nearby body and a large, distant one.

Visibility

Telescopes are searching for the object, which, due to its extreme distance from the Sun, would reflect little sunlight and potentially evade telescope sightings. It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto. A preliminary search of the archival data from the Catalina Sky Survey, Pan-STARRS and WISE has not identified Planet Nine. The remaining areas to search are near aphelion, which is located close to the plane of the Milky Way. The primary search is being conducted with the Subaru Telescope located in Hawaii, as it is predicted to be visible in the Northern Hemisphere, and is expected to take up to five years.

Location

If the planet exists and is close to its perihelion, astronomers could identify it based on existing images. For its aphelion, the largest telescopes are required. However, if the planet is currently located in between, many observatories could spot Planet Nine. Statistically, the planet is more likely to be closer to its aphelion at a distance more than 500 AU. This is because objects move more slowly when near their aphelion, in accordance with Kepler's Second Law. The search in databases of stellar objects performed by Brown and Batygin has already excluded much of the sky the predicted planet could be in, save the direction of its aphelion, or in the difficult to spot backgrounds where the orbit crosses the background Milky Way, which is near the directions of aphelion or to the side of its perihelion in the general direction of Scorpius and Sagittarius. This aphelion direction is where the predicted planet would be faintest and has a complicated field of view to spot it in.

Exploration

Brown thinks that if Planet Nine is confirmed to exist, a probe could reach it in as few as 20 years with a gravity assist around the Sun.

More predicted objects

Batygin and Brown also predict a yet-to-be-discovered population of distant objects. These objects would have semi-major axes greater than 250 AU, but they would have lower eccentricities and orbits that would be aligned with Planet Nine. The larger perihelia of these objects would make them fainter and more difficult to detect than the anti-aligned objects.

Finding more such objects would allow astronomers to make more accurate predictions about the orbit of the predicted planet. The Large Synoptic Survey Telescope, when it is completed in 2023, will be able to map the entire sky in just a few nights, providing more data on distant Kuiper belt objects that could both bolster evidence for Planet Nine and help pinpoint its current location.

Origin

According to Batygin and Brown, the solar nebula would have needed to be "exceptionally expansive to be compatible with in situ formation of a planet on such a distant and eccentric orbit," and so they speculate that Planet Nine, if it exists, likely formed nearer to the Sun but was eventually knocked farther away from the Sun by Jupiter or Saturn during the nebular epoch, flinging it into the outer extremes of the Solar System, via a mechanism reminiscent of the ejection of a hypothetical fifth giant planet in recent variations of the Nice model. However, gravitational interactions with the Sun's birth cluster, and possibly gaseous remnants of the Solar Nebula, could have influenced Planet Nine as it was ejected, putting it in a very wide but consistent orbit well outside the Kuiper belt, but also well within the inner Oort cloud.

According to Batygin's current estimates, for the ejection theory to be a feasible explanation, the timeline for ejection would have been between three million and ten million years after the formation of the Solar System. This timing suggests that Planet Nine is not the planet ejected in the Nice model instability, unless that occurred too early to be the cause of the Late Heavy Bombardment, which would then require another explanation. Batygin also agrees that these ejections must have been two separate events.

Ethan Siegel, who is deeply skeptical of the existence of an undiscovered new planet in the Solar System, nevertheless speculates that at least one super-Earth type planet, which have been commonly discovered in other planetary systems but have not been discovered in the Solar System, might have been ejected from the inner orbits of the Solar System due to the inward migration of Jupiter during the early Solar System. Hal Levinson thinks that the chance of an ejected object ending up in the inner Oort cloud is only about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit.

Astronomers expect that the discovery of Planet Nine would aid in understanding the processes behind the formation of the Solar System and other planetary systems, as well as how unique the Solar System is compared to other planetary systems.

See also

• History of Solar System formation and evolution hypotheses
• Hypothetical planets of the Solar System

Notes

1. The New Yorker put the average orbital distance of Planet Nine into perspective with an apparent allusion to one of the magazine's most famous cartoons, View of the World from 9th Avenue: "If the Sun were on Fifth Avenue and the Earth were one block west, Jupiter would be on the West Side Highway, Pluto would be in Montclair, New Jersey, and Planet Nine would be somewhere near Cleveland."

References

1. ^ "Where is Planet Nine?". findplanetnine.com.
2. ^ Witze, Alexandra (20 January 2016). "Evidence grows for giant planet on fringes of Solar System". Nature. 529 (7586): 266–7. doi:10.1038/529266a. PMID 26791699.
3. ^ Batygin, Konstantin; Brown, Michael E. (20 January 2016). "Evidence for a distant giant planet in the Solar system". The Astronomical Journal. 151 (2): 22. doi:10.3847/0004-6256/151/2/22.{{cite journal}}: CS1 maint: unflagged free DOI (link)
4. ^ Trujillo, Chadwick A.; Sheppard, Scott S. (27 March 2014). "A Sedna-like body with a perihelion of 80 astronomical units" (PDF). Nature. 507 (7493): 471. Bibcode:2014Natur.507..471T. doi:10.1038/nature13156. PMID 24670765.
5. ^ Burdick, Alan (20 January 2016). "Discovering Planet Nine". The New Yorker. Retrieved 20 January 2016.
6. ^ Achenbach, Joel; Feltman, Rachel (20 January 2016). "New evidence suggests a ninth planet lurking at the edge of the solar system". The Washington Post. ISSN 0190-8286. Retrieved 20 January 2016.
7. "Time To Welcome Planet Nine? Scientists Obtain Substantive Proof".
8. ^ Fesenmaier, Kimm (20 January 2016). "Caltech Researchers Find Evidence of a Real Ninth Planet". California Institute of Technology. Retrieved 20 January 2016.
9. Drake, Nadia (20 January 2016). "Scientists Find Evidence for Ninth Planet in Solar System". National Geographic.
10. Plait, Phil (21 January 2016). "More (and Best Yet) Evidence That Another Planet Lurks in the Dark Depths of Our Solar System". Slate. Retrieved 22 January 2016. The best fit was for a planet that never got closer than 200 AU from the Sun (Neptune is roughly 30 AU out from the Sun). The orbit of this planet would be highly elliptical, but it's not clear how elliptical; that's less well constrained. At its most distant, it could be between 500 and 1,200 AU out. The most likely mass of the planet is about 10 times Earth's mass
11. ^ See RA/Dec chart at Konstantin Batygin; Mike Brown (20 January 2016). "Where is Planet Nine?". The Search for Planet Nine (http://www.findplanetnine.com). Retrieved 24 January 2016. {{cite web}}: External link in |website= (help)
12. See embedded video simulation at Michael D. Lemonick; (Worldwide Telescope, Caltech/R. Hurt (IPAC)) (20 January 2016). "Strong Evidence Suggests a Super Earth Lies beyond Pluto". Scientific American. Retrieved 22 January 2015.
13. ^ Hand, Eric (20 January 2016). "Astronomers say a Neptune-sized planet lurks beyond Pluto". Science. Retrieved 20 January 2016.
14. Watson, Traci (20 January 2016). "Researchers find evidence of ninth planet in solar system". USA Today.
15. Lakdawalla, Emily (27 August 2009). "The Planetary Society Blog: "WISE Guys"". The Planetary Society. Retrieved 26 December 2009.
16. Luhman, K. L. (20 January 2014). "A search for a distant companion to the Sun with the Wide-Field Infrared Survey Explorer". The Astrophysical Journal. 781 (4): 4. Bibcode:2014ApJ...781....4L. doi:10.1088/0004-637X/781/1/4. Retrieved 21 January 2015.
17. ^ "Is Planet Nine a "planet"?". 19 January 2016.
18. "Phattie".
19. Iorio, Lorenzo (16 August 2014). "Planet X revamped after the discovery of the Sedna-like object 2012 VP₁₁₃?". Monthly Notices of the Royal Astronomical Society. 444 (1): L78 – L79. arXiv:1404.0258. doi:10.1093/mnrasl/slu116.{{cite journal}}: CS1 maint: unflagged free DOI (link)
20. Hruska, Joel (20 January 2016). "Our solar system may contain a ninth planet, far beyond Pluto". Extreme Tech.
21. ^ Seigel, Ethan. "Not So Fast: Why There Likely Isn't A Large Planet Beyond Pluto". Starts with a Bang. Forbes. Retrieved 22 January 2016.
22. ^ "Search for Planet 9 – Premonition". 19 January 2016.
23. "How did we miss Planet 9? — Bob McDonald". It's like seeing a disturbance on the surface of water but not knowing what caused it. Perhaps it was a jumping fish, a whale or a seal. Even though you didn't actually see it, you could make an informed guess about the size of the object and its location by the nature of the ripples in the water.
24. Lakdawalla, Emily (20 January 2016). "Theoretical evidence for an undiscovered super-Earth at the edge of our solar system". The Planetary Society.
25. "MPC list of "q>30 and a>250"". IAU Minor Planet Center.
26. "A Conversation With Pluto's Killer: Q & A With Astronomer Mike Brown".
27. "Planet X? New Evidence of an Unseen Planet at Solar System's Edge". More work is needed to determine whether Sedna and the other scattered disc objects were sent on their circuitous trips round the sun by a star that passed by long ago, or by an unseen planet that exists in the solar system right now. Finding and observing the orbits of other distant objects similar to Sedna will add more data points to astronomers' computer models.
28. "Dwarf planet discovery hints at a hidden Super Earth in solar system".
29. ^ "A New Planet or a Red Herring?". We plotted the real data on top of the model" Batyagin recalls, and they fell "exactly where they were supposed to be." That was, he says, the epiphany. "It was a dramatic moment. This thing I thought could disprove it turned out to be the strongest evidence for Planet Nine.
30. ^ "Is a real ninth planet out there beyond Pluto?".
31. "Our solar system may have a ninth planet after all — but not all evidence is in (We still haven't seen it yet)". The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. But for a study to be a slam dunk, the odds of failure must be higher such as 1 in 1,744,278. "When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have - But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team" - Sara Seager, a planetary scientist at MIT. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion — by researching observed anomalies in the movement of Uranus. "The idea of a large planet at such a distance from the Sun isn't actually that unlikely" - Bruce Macintosh, a planetary scientist at Stanford University
32. "We can't see this possible 9th planet, but we feel its presence". "Right now, any good scientist is going to be skeptical, there is always that chance that it's not real. But I think it's time to mount this search. I mean, we like to think of it as, we have provided the treasure map of where this ninth planet is, and we have done the starting gun, and now it's a race to actually point your telescope at the right spot in the sky and make that discovery of planet nine." - Mike Brown
33. "Can there really be a planet in our solar system that we don't know about?".
34. "A Little Perspective on the New "9th Planet" (and the 10th, and the 11th)".
35. Wall, Mike (21 January 2016). "How Astronomers Could Actually See 'Planet Nine'". Space.com. Retrieved 24 January 2016.
36. Adam Becker, Lisa Grossman, Jacob Aron (2016). "How Planet Nine may have been exiled to solar system's edge". New Scientist. Retrieved 2016-01-25. {{cite web}}: Italic or bold markup not allowed in: |publisher= (help)CS1 maint: multiple names: authors list (link)
37. "Planet Nine: A New Addition to the Solar System?".
38. Totten, Sanden (20 January 2016). "Caltech researchers answer skeptics' questions about Planet 9". Southern California Public Radio.
39. Chang, Kenneth (20 January 2016). "Ninth Planet May Exist Beyond Pluto, Scientists Report". The New York Times.
40. Gomes, R.; Levison, H. F.; Tsiganis, K.; Morbidelli, A. (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets". Nature. 435 (7041): 466–469. Bibcode:2005Natur.435..466G. doi:10.1038/nature03676. PMID 15917802.
41. Drake, Nadia (22 January 2016). "How Can We Find Planet Nine? (And Other Burning Questions)". National Geographic Phenomena. National Geographic. Retrieved 23 January 2016. 'These two events are both ejections, but they are well separated in epoch,' Batygin says. 'The planet that was ejected during the giant instability of the solar system—which, by the way, coincided with the formation of the Kuiper Belt—that planet, if it was there, it was just ejected. It doesn't get to stick around.'
42. Siegel, Ethan (3 November 2015). "Jupiter May Have Ejected A Planet From Our Solar System". Forbes. Retrieved 22 January 2016.
43. "New Object Offers Hint of "Planet X" By Kelly Beatty".
44. "Has 'Planet X' finally been found? A cautionary tale".

External links

• A New Planet in our Solar System? NASA Takes a Look (NASA video, 21 January 2016)
• A new 9th planet for the solar system? (Science Magazine video, 20 January 2016)
• The Search for Planet Nine – blog by study authors
• A summary of the history behind the search & claims for a ninth planet
• Could You Live on Planet Nine? - Science article by Rhett Allain
• 'Planet Nine': Facts About the Mysterious Solar System World (space.com infographic)

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