Planet Nine
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This article is about the hypothetical planet first suggested in 2014. For other uses, seePlanet Nine (disambiguation).
An artist's impression of Planet Nine
| |
| Orbital characteristics | |
|---|---|
| Aphelion | 1200 AU (est.)[1] |
| Perihelion | 200 AU (est.)[2] |
| 700 AU (est.)[3] | |
| Eccentricity | 0.6 (est.)[2] |
| 10,000 to 20,000 Earth years[2] | |
| Inclination | 30° to ecliptic (est.)[2] |
| 150 | |
| Physical characteristics | |
Mean radius
| 13,000–26,000 km (8,100–16,000 mi) 2–4 R⊕ (est.)[2] |
| Mass | 6×1025 kg (est.)[2] ≥10 Earth masses (est.) |
| >22 (est.)[1] | |
Planet Nine is a hypothetical large planet in the far outer Solar System, whose presence would explain the unusual orbital configuration of a group of trans-Neptunian objects(TNOs)[3][4][5] whose orbits lie mostly beyond the Kuiper belt.
Following the initial 2014 work of Chad Trujilloand Scott Sheppard, researchers Konstantin Batygin and Michael E. Brown at Caltechannounced 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 (approximately 5,000 times the mass of Pluto), have a diameter two to four times larger than that of Earth, a thick atmosphere of hydrogen andhelium, and a highly elliptical orbit so far away that it could take it 15,000 years to orbit the Sun.[6][7]
In their discussion, the authors considered models of planet formation that might includeplanetary migration from the inner Solar System, such as the fifth giant planet hypothesis.
Contents
[hide]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 approximately 700 AU, or about 20 times the distance from Neptune to the Sun, though it might come as close as 200 AU (30 billion km), and its inclination estimated to be around 30 (± 20) degrees.[1][2][8][A] The high eccentricity of Planet Nine's orbit could take it as far away as 1200 AU at its aphelion.[9][10] The aphelion would be in the general vicinity of the constellations of Orion and Taurus, while the perihelion would be in the vicinity of the southerly areas of Serpens (Caput), Ophiuchus and Libra.[11][12]
Size
The planet is estimated to have 10 times the mass[8][13] and two to four times the diameter of Earth.[6][14] 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.[15] 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.[16] 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.[17]
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.[1][6]
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."[5] In August 2014 Lorenzo Iorio proposed the name "Thelisto" in Monthly Notices of the Royal Astronomical Society for the hypothetical planet responsible for the unusual orbits of thesednoids and detached objects.[18]
Indirect detection
Case for a new planet
Trujillo and Shepherd analyzed the orbits of trans-Neptunian objects (TNOs) with perihelion greater than 30 AU and a semi-major axis greater than 150 AU, and found they had a clustering of orbital characteristics, particularly in terms of the argument of perihelion, which describes the orientation of elliptical orbits within their orbital planes.[4][3] They proposed a "single body of 2–15 Earth masses in a circular low inclination orbit between 200 AU and 300 AU" to explain the pattern. It was not the only way to create the clustering of the orbital orientations. Brown and Batygin then analyzed 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),[3] A closer look at the data showed that these six objects trace out elliptical orbits that are aligned in approximately the same direction in physical space, and lie in approximately the same plane.[19][20] They found that this would only occur with 0.007% probability by chance alone.[21] These six are the only known minor planets with perihelia greater than 30 AU and a semi-major axis greater than 250 AU as of January 2016.[22]
| Object | Orbit | Body | |||||
|---|---|---|---|---|---|---|---|
| Period (Earth years) | Semi-major axis (AU) | Perihelion (AU) | Eccentricity | Argument of perihelion (ω) | Magnitude | Diameter (km) | |
| 2012 VP113 | 4,300 | 263 | 80 | 0.696 | 294° | 23.4 | 600 |
| 2013 RF98 | 5,600 | 317 | 36 | 0.88 | 316° | 24.4 | 80 |
| 2004 VN112 | 5,850 | 327 | 47 | 0.854 | 327° | 23.3 | 200 |
| 2010 GB174 | 6,600 | 351 | 48 | 0.869 | 347° | 25.2 | 200 |
| 2007 TG422 | 11,200 | 501 | 36 | 0.928 | 286° | 21.9 | 200 |
| 90377 Sedna | 11,400 | 506 | 76 | 0.855 | 311° | 21.0 | 1,000 |
They then simulated the effect of a planet of mass 10 M⊕ with a semi-major axis ranging from 200 to 2000 AU and an eccentricity varying from 0.1 to 0.9 on these extreme TNOs and inner Oort cloud objects and found that orbital parameters centered around aphelion ~1200 AU, perihelion ~ 200 AU, semi-major axis ~700 AU, eccentricity ~0.6, orbital period 10,000 to 20,000 Earth years, an inclination ~30° to the ecliptic, and an argument of perihelion of 150°, produced the kind of distribution of orbits observed. The object's orbital orientation was approximately opposite to those of the six TNOs (with an longitude of perihelion offset by about 180°).[3] The results of their simulations also predicted there should be a population of objects with a perpendicular orbital inclination (relative to the first set of TNOs considered and the Solar System in general) and they realized that objects such as 2008 KV42 and2012 DR30 fit this prediction of the model.[3][23][24]
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.[25]
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.[26]
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.[27]
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 ofHawaii's Gemini Observatory, who suggested the similar orbits of certain objects such assednoids might be influenced by a massive unknown planet at the edge of the Solar System.[4] Their findings suggest that a super-Earth of about two to 15 Earth masses beyond 200 AU with a highly inclined orbit at 1500 AU could shepherd the extreme Kuiper belt objects (KBOs) 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 explains 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.[19]
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.[6]
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.[17]
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⊕ (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).[3]
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.[19] This mechanism, known as mean-motion resonance, prevents trapped trans-Neptunian objects from colliding with Planet Nine, and keeps them aligned.[8]
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.[3]
Inference
Batygin was 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."[28]
Brown put the odds for the existence of Planet Nine at about 90%.[6] Greg Laughlin, one of the few researchers who knew in advance about this paper, gives an estimate of 68.3%.[5]Other skeptical scientists demand more data in terms of additional KBOs to be analysed or final evidence through photographic confirmation.[29][30] 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.[31]
Brown is supported by Jim Green, director of NASA's Planetary Science Division, who said that "the evidence is stronger now than it's ever been before".[32]
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.[28]
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.[33]
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.[6] It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto.[1] A preliminary search of the archival data from the Catalina Sky Survey, Pan-STARRS andWISE has not identified Planet Nine. The remaining areas to search are near aphelion, which is located close to the plane of the Milky Way.[1] 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 the search expected to take up to five years.[13][34]
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.[8] Statistically, the planet is more likely to be closer to its aphelion at a distance more than 500 AU.[1] 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.[11] This aphelion direction is where the predicted planet would be faintest and has a complicated field of view to spot it in.[11]
Exploration
Brown thinks that if Planet Nine is confirmed to exist, a probe could reach it in as few as 20 years with a powered slingshot around the Sun.[35]
