Following the discovery of the planet
Neptune in 1846, there was
considerable speculation that another planet might exist beyond its
orbit. The search began in the mid-19th century and culminated at the
start of the 20th with
Percival Lowell 's quest for
Planet X. Lowell
Planet X hypothesis to explain apparent discrepancies in
the orbits of the giant planets, particularly
Uranus and Neptune,
speculating that the gravity of a large unseen ninth planet could have
Uranus enough to account for the irregularities.
Clyde Tombaugh 's discovery of
Pluto in 1930 appeared to validate
Lowell's hypothesis, and
Pluto was officially named the ninth planet.
Pluto was conclusively determined to be too small for its
gravity to affect the giant planets, resulting in a brief search for a
tenth planet. The search was largely abandoned in the early 1990s,
when a study of measurements made by the
Voyager 2 spacecraft found
that the irregularities observed in Uranus's orbit were due to a
slight overestimation of Neptune's mass. After 1992, the discovery of
numerous small icy objects with similar or even wider orbits than
Pluto led to a debate over whether
Pluto should remain a planet, or
whether it and its neighbours should, like the asteroids , be given
their own separate classification. Although a number of the larger
members of this group were initially described as planets, in 2006 the
International Astronomical Union
International Astronomical Union reclassified
Pluto and its largest
neighbours as dwarf planets , leaving
Neptune the farthest known
planet in the Solar System.
While today the astronomical community widely agrees that
as originally envisioned, does not exist, the concept of an
as-yet-unobserved planet has been revived by a number of astronomers
to explain other anomalies observed in the outer
Solar System . As of
March 2014, observations with the WISE telescope have ruled out the
possibility of a
Saturn -sized object (95
Earth mass) out to 10,000 AU
, and a
Jupiter -sized (≈318
Earth mass) or larger object out to
In 2014, based on similarities of the orbits of a group of recently
discovered extreme trans-Neptunian objects , astronomers hypothesized
the existence of a super-
Earth planet, 2 to 15 times the mass of the
Earth and beyond 200 AU with possibly a high inclined orbit at some
1500 AU. In 2016 further work showed this unknown distant planet is
likely on an inclined, eccentric orbit that goes no closer than about
200 AU and no further than about 1200 AU from the Sun. The orbit is
predicted to be anti-aligned to the clustered extreme trans-Neptunian
Pluto is no longer considered a planet by the
International Astronomical Union
International Astronomical Union , this new hypothetical object has
become known as
Planet Nine .
* 1 Early speculation
* 2.1 Discovery of
Planet X title
* 2.3 Further searches for
Planet X disproved
* 3 Discovery of further trans-Neptunian objects
* 4 Subsequently proposed trans-Neptunian planets
* 4.1 Orbits of distant objects
* 4.2 Probability
* 4.3 Kuiper cliff
* 4.4 Other proposed planets
* 5 Constraints on additional planets
* 6 See also
* 6.1 Survey telescopes
* 7 References
* 8 Bibliography
* 9 Further reading
See also: Discovery of
Jacques Babinet , an early
proponent of a trans-Neptunian planet
In the 1840s, the French mathematician
Urbain Le Verrier
Urbain Le Verrier used
Newtonian mechanics to analyse perturbations in the orbit of Uranus,
and hypothesised that they were caused by the gravitational pull of a
yet-undiscovered planet. Le Verrier predicted the position of this new
planet and sent his calculations to German astronomer Johann Gottfried
Galle . On 23 September 1846, the night following his receipt of the
letter, Galle and his student Heinrich d\'Arrest discovered Neptune,
exactly where Le Verrier had predicted. There remained some slight
discrepancies in the giant planets ' orbits. These were taken to
indicate the existence of yet another planet orbiting beyond Neptune.
Even before Neptune's discovery, some speculated that one planet
alone was not enough to explain the discrepancy. On 17 November 1834,
the British amateur astronomer the Reverend Thomas John Hussey
reported a conversation he had had with French astronomer Alexis
George Biddell Airy
George Biddell Airy , the British Astronomer Royal. Hussey
reported that when he suggested to Bouvard that the unusual motion of
Uranus might be due to the gravitational influence of an undiscovered
planet, Bouvard replied that the idea had occurred to him, and that he
had corresponded with
Peter Andreas Hansen , director of the Seeberg
Gotha , about the subject. Hansen's opinion was that a
single body could not adequately explain the motion of Uranus, and
postulated that two planets lay beyond Uranus.
Jacques Babinet raised an objection to Le Verrier's
calculations, claiming that Neptune's observed mass was smaller and
its orbit larger than Le Verrier had initially predicted. He
postulated, based largely on simple subtraction from Le Verrier's
calculations, that another planet of roughly 12
Earth masses, which he
named "Hyperion", must exist beyond Neptune. Le Verrier denounced
Babinet's hypothesis, saying, " absolutely nothing by which one could
determine the position of another planet, barring hypotheses in which
imagination played too large a part."
In 1850 James Ferguson , Assistant Astronomer at the United States
Naval Observatory , noted that he had "lost" a star he had observed,
GR1719k, which Lt. Matthew Maury, the superintendent of the
Observatory, claimed was evidence that it must be a new planet.
Subsequent searches failed to recover the "planet" in a different
position, and in 1878,
CHF Peters , director of the Hamilton College
Observatory in New York , showed that the star had not in fact
vanished, and that the previous results had been due to human error.
Camille Flammarion noted that the comets 1862 III and 1889
III had aphelia of 47 and 49 AU , respectively, suggesting that they
might mark the orbital radius of an unknown planet that had dragged
them into an elliptical orbit. Astronomer George Forbes concluded on
the basis of this evidence that two planets must exist beyond Neptune.
He calculated, based on the fact that four comets possessed aphelia at
around 100 AU and a further six with aphelia clustered at around 300
AU, the orbital elements of a pair of hypothetical trans-Neptunian
planets. These elements concorded suggestively with those made
independently by another astronomer named
David Peck Todd , suggesting
to many that they might be valid. However, sceptics argued that the
orbits of the comets involved were still too uncertain to produce
In 1900 and 1901,
Harvard College Observatory director William Henry
Pickering led two searches for trans-Neptunian planets. The first was
begun by Danish astronomer Hans Emil Lau who, after studying the data
on the orbit of
Uranus from 1690 to 1895, concluded that one
trans-Neptunian planet alone could not account for the discrepancies
in its orbit, and postulated the position of two planets he believed
were responsible. The second was launched when Gabriel Dallet
suggested that a single trans-Neptunian planet lying at 47 AU could
account for the motion of Uranus. Pickering agreed to examine plates
for any suspected planets. In neither case were any found.
Thomas Jefferson Jackson See
Thomas Jefferson Jackson See , an astronomer with a
reputation as an egocentric contrarian, opined "that there is
certainly one, most likely two and possibly three planets beyond
Neptune". Tentatively naming the first planet "Oceanus", he placed
their respective distances at 42, 56 and 72 AU from the Sun. He gave
no indication as to how he determined their existence, and no known
searches were mounted to locate them.
In 1911, Indian astronomer Venkatesh P. Ketakar suggested the
existence of two trans-Neptunian planets, which he named
Vishnu , by reworking the patterns observed by
Pierre-Simon Laplace in
the planetary satellites of
Jupiter and applying them to the outer
planets. The three inner
Galilean moons of Jupiter, Io , Europa and
Ganymede , are locked in a complicated 1:2:4 resonance called a
Laplace resonance . Ketakar suggested that Uranus,
Neptune and his
hypothetical trans-Neptunian planets were locked in Laplace-like
resonances. His calculations predicted a mean distance for
38.95 AU and an orbital period of 242.28
Earth years (3:4 resonance
with Neptune). When
Pluto was discovered 19 years later, its mean
distance of 39.48 AU and orbital period of 248
Earth years were close
to Ketakar's prediction (
Pluto in fact has a 2:3 resonance with
Neptune). Ketakar made no predictions for the orbital elements other
than mean distance and period. It is not clear how Ketakar arrived at
these figures, and his second planet, Vishnu, was never located.
In 1894, with the help of William Pickering,
Percival Lowell , a
wealthy Bostonian, founded the
Lowell Observatory in Flagstaff,
Arizona . In 1906, convinced he could resolve the conundrum of
Uranus's orbit, he began an extensive project to search for a
trans-Neptunian planet, which he named
Planet X, a name previously
used by Gabriel Dallet. The X in the name represents an unknown and
is pronounced as the letter, as opposed to the
Roman numeral for 10
(at the time,
Planet X would have been the ninth planet). Lowell's
hope in tracking down
Planet X was to establish his scientific
credibility, which had eluded him due to his widely derided belief
that channel-like features visible on the surface of
Mars were canals
constructed by an intelligent civilization .
Lowell's first search focused on the ecliptic , the plane encompassed
by the zodiac where the other planets in the
Solar System lie. Using a
5-inch photographic camera, he manually examined over 200 three-hour
exposures with a magnifying glass, and found no planets. At that time
Pluto was too far above the ecliptic to be imaged by the survey.
After revising his predicted possible locations, Lowell conducted a
second search from 1914 to 1916. In 1915, he published his Memoir of
a Trans-Neptunian Planet, in which he concluded that
Planet X had a
mass roughly seven times that of Earth—about half that of
Neptune—and a mean distance from the
Sun of 43 AU. He assumed Planet
X would be a large, low-density object with a high albedo , like the
giant planets. As a result, it would show a disc with diameter of
about one arcsecond and an apparent magnitude of between 12 and
13—bright enough to be spotted.
Separately, in 1908, Pickering announced that, by analysing
irregularities in Uranus's orbit, he had found evidence for a ninth
planet. His hypothetical planet, which he termed "
Planet O" (because
it came after "N", i.e. Neptune), possessed a mean orbital radius of
51.9 AU and an orbital period of 373.5 years. Plates taken at his
Peru , showed no evidence for the predicted
planet, and British astronomer P. H. Cowell showed that the
irregularities observed in Uranus's orbit virtually disappeared once
the planet's displacement of longitude was taken into account. Lowell
himself, despite his close association with Pickering, dismissed
Planet O out of hand, saying, "This planet is very properly designated
"O", is nothing at all." Unbeknownst to Pickering, four of the
photographic plates taken in the search for "
Planet O" by astronomers
at the Mount Wilson Observatory in 1919 captured images of Pluto,
though this was only recognised years later. Pickering went on to
suggest many other possible trans-Neptunian planets up to the year
1932, which he named P, Q, R, S, T and U; none were ever detected.
DISCOVERY OF PLUTO
Pluto Clyde William Tombaugh
Lowell's sudden death in 1916 temporarily halted the search for
Planet X. Failing to find the planet, according to one friend,
"virtually killed him". Lowell's widow, Constance, engaged in a legal
battle with the observatory over Lowell's legacy which halted the
Planet X for several years. In 1925, the observatory
obtained glass discs for a new 13 in (33 cm) wide-field telescope to
continue the search, constructed with funds from Abbott Lawrence
Lowell , Percival's brother. In 1929 the observatory's director,
Vesto Melvin Slipher , summarily handed the job of locating the planet
Clyde Tombaugh , a 22-year-old Kansas farm boy who had only just
arrived at the
Lowell Observatory after Slipher had been impressed by
a sample of his astronomical drawings.
Tombaugh's task was to systematically capture sections of the night
sky in pairs of images. Each image in a pair was taken two weeks
apart. He then placed both images of each section in a machine called
a blink comparator , which by exchanging images quickly created a time
lapse illusion of the movement of any planetary body. To reduce the
chances that a faster-moving (and thus closer) object be mistaken for
the new planet, Tombaugh imaged each region near its opposition point,
180 degrees from the Sun, where the apparent retrograde motion for
objects beyond Earth's orbit is at its strongest. He also took a third
image as a control to eliminate any false results caused by defects in
an individual plate. Tombaugh decided to image the entire zodiac,
rather than focus on those regions suggested by Lowell.
Discovery photographs of
By the beginning of 1930, Tombaugh's search had reached the
constellation of Gemini. On 18 February 1930, after searching for
nearly a year and examining nearly 2 million stars, Tombaugh
discovered a moving object on photographic plates taken on 23 January
and 29 January of that year. A lesser-quality photograph taken on
January 21 confirmed the movement. Upon confirmation, Tombaugh walked
into Slipher's office and declared, "Doctor Slipher, I have found your
Planet X." The object lay just six degrees from one of two locations
Planet X Lowell had suggested; thus it seemed he had at last been
vindicated. After the observatory obtained further confirmatory
photographs, news of the discovery was telegraphed to the Harvard
College Observatory on March 13, 1930. The new object was later
precovered on photographs dating back to 19 March 1915. The decision
to name the object
Pluto was intended in part to honour Percival
Lowell, as his initials made up the word's first two letters. After
discovering Pluto, Tombaugh continued to search the ecliptic for other
distant objects. He found hundreds of variable stars and asteroids ,
as well as two comets , but no further planets.
PLUTO LOSES PLANET X TITLE
Discovery image of Charon
To the observatory's disappointment and surprise,
Pluto showed no
visible disc; it appeared as a point, no different from a star, and,
at only 15th magnitude, was six times dimmer than Lowell had
predicted, which meant it was either very small, or very dark.
Because Lowell astronomers thought
Pluto was massive enough to perturb
planets, they assumed that its albedo could be no less than 0.07
(meaning that it reflected only 7% of the light that hit it); about as
dark as asphalt and similar to that of Mercury , the least reflective
planet known. This would give
Pluto an estimated mass of no more than
70% that of Earth. Observations also revealed that Pluto's orbit was
very elliptical, far more than that of any other planet.
Almost immediately, some astronomers questioned Pluto's status as a
planet. Barely a month after its discovery was announced, on 14 April
1930, in an article in the
New York Times
New York Times , Armin O. Leuschner
suggested that Pluto's dimness and high orbital eccentricity made it
more similar to an asteroid or comet: "The Lowell result confirms the
possible high eccentricity announced by us on April 5. Among the
possibilities are a large asteroid greatly disturbed in its orbit by
close approach to a major planet such as Jupiter, or it may be one of
many long-period planetary objects yet to be discovered, or a bright
cometary object." In that same article,
Harvard Observatory director
Harlow Shapley wrote that
Pluto was a "member of the
Solar System not
comparable with known asteroids and comets, and perhaps of greater
importance to cosmogony than would be another major planet beyond
Neptune." In 1931, using a mathematical formula, Ernest W. Brown
asserted (in agreement with E. C. Bower ), that the presumed
irregularities in the orbit of
Uranus could not be due to the
gravitational effect of a more distant planet, and thus that Lowell's
supposed prediction was "purely accidental".
Throughout the mid-20th century, estimates of Pluto's mass were
revised downward. In 1931, Nicholson and Mayall calculated its mass,
based on its supposed effect on the giant planets, as roughly that of
Earth; a value somewhat in accord with the 0.91
Earth mass calculated
in 1942 by Lloyd R. Wylie at the
US Naval Observatory , using the same
assumptions. In 1949,
Gerard Kuiper 's measurements of Pluto's
diameter with the 200 inch telescope at
Mount Palomar Observatory
Mount Palomar Observatory led
him to the conclusion that it was midway in size between Mercury and
Mars and that its mass was most probably about 0.1
In 1973, Dennis Rawlins conjectured, based on the similarities in the
periodicity and amplitude of brightness variation between
Neptune's moon Triton , that Pluto's mass must be similar to Triton's.
This is, in fact, true, and had been argued by astronomers Walter
Baade and E. C. Bower as early as 1934. However, because Triton's
mass was then believed to be roughly 2.5% that of the Earth–Moon
system (more than ten times its actual value), Rawlins's determination
for Pluto's mass was similarly incorrect. It was nonetheless a meagre
enough value for him to conclude that
Pluto was not
Planet X. In
1976, Dale Cruikshank, Carl Pilcher and David Morrison of the
University of Hawaii
University of Hawaii analysed spectra from Pluto's surface and
determined that it must contain methane ice , which is highly
reflective. This meant that Pluto, far from being dark, was in fact
exceptionally bright, and thus was probably no more than 0.01 Earth
Mass estimates for Pluto:
Nicholson & Mayall
0.1 (1/10 Earth)
0.025 (1/40 Earth)
0.01 (1/100 Earth)
Cruikshank, Pilcher, & Morrison
0.002 (1/500 Earth)
Christy roughly one five-hundredth that of
Earth or one-sixth that
of the Moon, and far too small to account for the observed
discrepancies in the orbits of the outer planets. Lowell's
"prediction" had been a coincidence: If there was a
Planet X, it was
FURTHER SEARCHES FOR PLANET X
After 1978, a number of astronomers kept up the search for Lowell's
Planet X, convinced that, because
Pluto was no longer a viable
candidate, an unseen tenth planet must have been perturbing the outer
In the 1980s and 1990s, Robert Harrington led a search to determine
the real cause of the apparent irregularities. He calculated that any
Planet X would be at roughly three times the distance of
the Sun; its orbit would be highly eccentric , and strongly inclined
to the ecliptic—the planet's orbit would be at roughly a 32-degree
angle from the orbital plane of the other known planets. This
hypothesis was met with a mixed reception. Noted
Planet X sceptic
Brian G. Marsden of the Minor
Planet Center pointed out that these
discrepancies were a hundredth the size of those noticed by Le
Verrier, and could easily be due to observational error.
In 1972, Joseph Brady of the Lawrence Livermore National Laboratory
studied irregularities in the motion of Halley\'s
Comet . Brady
claimed that they could have been caused by a Jupiter-sized planet
Neptune at 59 AU that is in a retrograde orbit around the Sun.
However, both Marsden and
Planet X proponent P. Kenneth Seidelmann
attacked the hypothesis, showing that Halley's
Comet randomly and
irregularly ejects jets of material, causing changes to its own
orbital trajectory, and that such a massive object as Brady's
would have severely affected the orbits of known outer planets.
Although its mission did not involve a search for
Planet X, the IRAS
space observatory made headlines briefly in 1983 due to an "unknown
object" that was at first described as "possibly as large as the giant
Jupiter and possibly so close to
Earth that it would be part of
this Solar System". Further analysis revealed that of several
unidentified objects, nine were distant galaxies and the tenth was
"interstellar cirrus "; none were found to be
Solar System bodies.
In 1988, A. A. Jackson and R. M. Killen studied the stability of
Pluto's resonance with
Neptune by placing test "
Planet X-es" with
various masses and at various distances from Pluto.
Neptune's orbits are in a 3:2 resonance, which prevents their
collision or even any close approaches, regardless of their separation
in the z axis . It was found that the hypothetical object's mass had
to exceed 5
Earth masses to break the resonance, and the parameter
space is quite large and a large variety of objects could have existed
Pluto without disturbing the resonance. Four test orbits of a
trans-Plutonian planet have been integrated forward for four million
years in order to determine the effects of such a body on the
stability of the Neptune–
Pluto 3:2 resonance. Planets beyond Pluto
with masses of 0.1 and 1.0
Earth masses in orbits at 48.3 and 75.5 AU,
respectively, do not disturb the 3:2 resonance. Test planets of 5
Earth masses with semi-major axes of 52.5 and 62.5 AU disrupt the
four-million-year libration of Pluto's argument of perihelion.
PLANET X DISPROVED
Harrington died in January 1993, without having found
Planet X. Six
E. Myles Standish had used data from Voyager 2's 1989
flyby of Neptune, which had revised the planet's total mass downward
by 0.5%—an amount comparable to the mass of
Mars —to recalculate
its gravitational effect on Uranus. When Neptune's newly determined
mass was used in the
Jet Propulsion Laboratory
Jet Propulsion Laboratory Developmental Ephemeris
(JPL DE), the supposed discrepancies in the Uranian orbit, and with
them the need for a
Planet X, vanished. There are no discrepancies in
the trajectories of any space probes such as
Pioneer 10 ,
Pioneer 11 ,
Voyager 1 , and
Voyager 2 that can be attributed to the gravitational
pull of a large undiscovered object in the outer Solar System. Today,
most astronomers agree that
Planet X, as Lowell defined it, does not
DISCOVERY OF FURTHER TRANS-NEPTUNIAN OBJECTS
Artistic comparison of
Pluto , Eris ,
Haumea , Sedna , 2007 OR10 , Quaoar , Orcus
Earth along with the
Moon . See also: History of the Kuiper
After the discovery of
Pluto and Charon, no more trans-Neptunian
objects (TNOs) were found until
(15760) 1992 QB1 in 1992. Since
then, thousands of such objects have been discovered. Most are now
recognized as part of the
Kuiper belt , a swarm of icy bodies left
over from the Solar System's formation that orbit near the ecliptic
plane just beyond Neptune. Though none were as large as Pluto, some of
these distant trans-Neptunian objects, such as Sedna , were initially
described in the media as "new planets".
In 2005, astronomer Mike Brown and his team announced the discovery
of 2003 UB313 (later named Eris after the Greek goddess of discord and
strife), a trans-Neptunian object then thought to be just barely
larger than Pluto. Soon afterwards, a
NASA Jet Propulsion Laboratory
press release described the object as the "tenth planet".
Eris was never officially classified as a planet, and the 2006
definition of planet defined both Eris and
Pluto not as planets but as
dwarf planets because they have not cleared their neighbourhoods .
They do not orbit the
Sun alone, but as part of a population of
similarly sized objects.
Pluto itself is now recognized as being a
member of the
Kuiper belt and the largest dwarf planet, larger than
the more-massive Eris.
A number of astronomers, most notably
Alan Stern , the head of NASA's
New Horizons mission to Pluto, contend that the IAU's definition is
flawed, and that
Pluto and Eris, and all large trans-Neptunian
objects, such as
Makemake , Sedna , Quaoar , Varuna and
should be considered planets in their own right. However, the
discovery of Eris did not rehabilitate the
Planet X theory because it
is far too small to have significant effects on the outer planets'
SUBSEQUENTLY PROPOSED TRANS-NEPTUNIAN PLANETS
Although most astronomers accept that Lowell's
Planet X does not
exist, a number have revived the idea that a large unseen planet could
create observable gravitational effects in the outer Solar System.
These hypothetical objects are often referred to as "
although the conception of these objects may differ considerably from
that proposed by Lowell.
ORBITS OF DISTANT OBJECTS
Planet Nine The orbit of Sedna (red) set against the
Uranus (green), Neptune
Pluto (purple) Prediction of hypothetical Planet
Nine's orbit based on unique clustering Sedna's orbit
When Sedna was discovered, its extreme orbit raised questions about
its origin. Its perihelion is so distant (approximately 75 AU) that no
currently observed mechanism can explain Sedna's eccentric distant
orbit. It is too far from the planets to have been affected by the
Neptune or the other giant planets and too bound to the Sun
to be affected by outside forces such as the galactic tides .
Hypotheses to explain its orbit include that it was affected by a
passing star, that it was captured from another planetary system , or
that it was tugged into its current position by a trans-Neptunian
planet. The most obvious solution to determining Sedna's peculiar
orbit would be to locate a number of objects in a similar region,
whose various orbital configurations would provide an indication as to
their history. If Sedna had been pulled into its orbit by a
trans-Neptunian planet, any other objects found in its region would
have a similar perihelion to Sedna (around 80 AU). Elongated orbits
of group of
Kuiper belt objects
In 2012, Rodney Gomes modelled the orbits of 92
Kuiper belt objects
and found that six of those orbits were far more elongated than the
model predicted. He concluded that the simplest explanation was the
gravitational pull of a distant planetary companion, such as a
Neptune-sized object at 1500 AU or a Mars-sized object at around 53
AU. Discovery of
2012 VP113 and the orbital clustering of Kuiper
In 2014, astronomers announced the discovery of
2012 VP113 , a large
object with a Sedna-like 4200-year orbit and a perihelion of roughly
80 AU, which led them to suggest that it offered evidence of a
potential trans-Neptunian planet. Trujillo and Sheppard argued that
the orbital clustering of arguments of perihelia for VP113 and other
extremely distant TNOs suggests the existence of a "super-
Earth " of
between 2 and 15
Earth masses beyond 200 AU and possibly on an
inclined orbit at 1500 AU.
In 2014 astronomers at the
Universidad Complutense in Madrid
suggested that the available data actually indicate more than one
trans-Neptunian planet. Further analysis proposing a super-Earth
Planet Nine ) based on a statistical clustering of the
arguments of perihelia (noted before) near zero and also ascending
nodes near 113° of six distant trans-Neptunian objects . They
estimated it to be ten times the mass of
Earth (about 60% the mass of
Neptune) with a semimajor axis of approximately 400–1500 AU .
Even without gravitational evidence, Mike Brown, the discoverer of
Sedna, has argued that Sedna's 12,000-year orbit means that
probability alone suggests that an Earth-sized object exists beyond
Neptune. Sedna's orbit is so eccentric that it spends only a small
fraction of its orbital period near the Sun, where it can be easily
observed. This means that unless its discovery was a freak accident,
there is probably a substantial population of objects roughly Sedna's
diameter yet to be observed in its orbital region. Mike Brown noted
that "Sedna is about three-quarters the size of Pluto. If there are
sixty objects three-quarters the size of
Pluto then there are
probably forty objects the size of
Pluto ... If there are forty
objects the size of Pluto, then there are probably ten that are twice
the size of Pluto. There are probably three or four that are three
times the size of Pluto, and the biggest of these objects ... is
probably the size of
Mars or the size of the Earth." However, he
notes that, should such an object be found, even though it might
Earth in size, it would still be a dwarf planet by the
current definition, because it would not have cleared its
Additionally, speculation of a possible trans-Neptunian planet has
revolved around the so-called "Kuiper cliff ". The Kuiper belt
terminates suddenly at a distance of 48 AU from the Sun. Brunini and
Melita have speculated that this sudden drop-off may be attributed to
the presence of an object with a mass between that of
Mars and Earth
located beyond 48 AU. The presence of an object with a mass similar
to that of
Mars in a circular orbit at 60 AU leads to a
trans-Neptunian object population incompatible with observations. For
instance, it would severely deplete the plutino population.
Astronomers have not excluded the possibility of an object with a mass
similar to that of
Earth located further than 100 AU with an eccentric
and inclined orbit. Computer simulations by Patryk Lykawka of Kobe
University have suggested that an object with a mass between 0.3 and
Earth masses, ejected outward by
Neptune early in the Solar
System's formation and currently in an elongated orbit between 101 and
200 AU from the Sun, could explain the Kuiper cliff and the peculiar
detached objects such as Sedna and
2012 VP113 . Although some
astronomers, such as Renu Malhotra and David Jewitt, have cautiously
supported these claims, others, such as Alessandro Morbidelli, have
dismissed them as "contrived". In 2017, Malhotra and Kat Volk argued
that an unexpected variance in inclination for KBOs farther than 50 AU
provided evidence of a possible Mars-sized planet residing at the edge
of the Solar System.
OTHER PROPOSED PLANETS
Tyche was a proposed gas giant to be located in the
Solar System 's
Oort cloud . It was first proposed in 1999 by astrophysicists John
Matese, Patrick Whitman and Daniel Whitmire of the University of
Louisiana at Lafayette . They argued that evidence of Tyche's
existence could be seen in a supposed bias in the points of origin for
long-period comets . In 2013, Matese and Whitmire re-evaluated the
comet data and noted that Tyche, if it existed, would be detectable in
the archive of data that was collected by
NASA 's Wide-field Infrared
Survey Explorer (WISE) telescope. In 2014,
NASA announced that the
WISE survey had ruled out any object with Tyche's characteristics,
indicating that Tyche as hypothesized by Matese, Whitman, and Whitmire
does not exist.
The oligarch theory of planet formation states that there were
hundreds of planet-sized objects, known as oligarchs, in the early
stages of the Solar System's evolution. In 2005, astronomer Eugene
Chiang speculated that although some of these oligarchs became the
planets we know today, most would have been flung outward by
gravitational interactions. Some may have escaped the Solar System
altogether to become free-floating planets , whereas others would be
orbiting in a halo around the Solar System, with orbital periods of
millions of years. This halo would lie at between 1,000 and 10,000 AU
from the Sun, or between a third and a thirtieth the distance to the
Oort cloud .
In December 2015, astronomers at the Atacama Large Millimeter Array
(ALMA) detected a brief series of 350 GHz pulses that they concluded
must either be a series of independent sources, or a single, fast
moving source. Deciding that the latter was the most likely, they
calculated based on its speed that, were it bound to the Sun, the
object, which they named "Gna" after a fast-moving messenger goddess
in Norse mythology, would be about 12–25 AU distant and have a
dwarf planet -sized diameter of 220 to 880 km. However, if it were a
rogue planet not gravitationally bound to the Sun, and as far away as
4000 AU, it could be much larger. The paper was never formally
accepted, and has been withdrawn until the detection is confirmed.
Scientists' reactions to the notice were largely sceptical; Mike Brown
commented that, "If it is true that ALMA accidentally discovered a
massive outer solar system object in its tiny, tiny, tiny, field of
view, that would suggest that there are something like 200,000
Earth-sized planets in the outer solar system ... Even better, I just
realized that this many Earth-sized planets existing would destabilize
the entire solar system and we would all die."
CONSTRAINTS ON ADDITIONAL PLANETS
As of 2016 the following observations severely constrain the mass and
distance of any possible additional
Solar System planet:
* An analysis of mid-infrared observations with the WISE telescope
have ruled out the possibility of a
Saturn -sized object (95 Earth
masses) out to 10,000 AU , and a Jupiter-sized or larger object out to
* Using modern data on the anomalous precession of the perihelia of
Saturn, Earth, and Mars, Lorenzo Iorio concluded that any unknown
planet with a mass of 0.7 times that of
Earth must be further than
350–400 AU; one with a mass of 2 times that of Earth, further than
496–570 AU; and finally one with a mass of 15 times that of Earth,
further than 970–1111 AU. Moreover, Iorio stated that the modern
ephemerides of the
Solar System outer planets has provided even
tighter constraints: no celestial body with a mass of 15 times that of
Earth can exist closer than 1100–1300 AU. However, work by another
group of astronomers using a more comprehensive model of the Solar
System found that Iorio's conclusion was only partially correct. Their
analysis of Cassini data on Saturn's orbital residuals found that
observations were inconsistent with a planetary body with the orbit
and mass similar to those of Batygin and Brown's
Planet Nine having a
true anomaly of −130° to −110° or −65° to 85°. Furthermore,
the analysis found that Saturn's orbit is slightly better explained if
such a body is located at a true anomaly of
−10°. At this location, it would be approximately 630 AU from the
* Fictional planets of the
* List of hypothetical
Solar System objects
Large Synoptic Survey Telescope (LSST)
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer (WISE)
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Pluto". Astronomical Society of the Pacific Leaflets. 5: 73–80.
Planet X Discovered??" by PBS SpaceTime
* SEDS on
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* Andrew Coates provides a simplified summary of the history behind
the search ;background:none
* "Brass Knuckles "
Coleta de Dados Colles
Coleta de Dados Colles
Definition of planet
IAU definition of planet
IAU definition of planet
International Astronomical Union
International Astronomical Union
Michael E. Brown
Neil deGrasse Tyson
Neil deGrasse Tyson
* Solar eclipses on
Hubble Space Telescope
Hubble Space Telescope
Pluto Fast Flyby/
Pluto Kuiper Express
Mariner Mark II
* Terrestrial planets
* Giant planets
* Dwarf planets
* Saturnian (Rhean )
* 1 Terrestrial (Moon)
* Other near-
* 2 Martian
* 69 Jovian
* 62 Saturnian
* 27 Uranian
* 14 Neptunian
* 5 Plutonian
* 2 Haumean
* 1 Makemakean (S/2015 (136472) 1)
* 1 Eridian (Dysnomia)
Solar System objects
* By size
* By discovery date
* Minor planets
* Gravitationally rounded objects
* Possible dwarf planets
* Natural satellites
* Minor planets
* first discovered: Ceres
* Notable asteroids
* Main-belt comets
* Cis-Neptunian objects
* Trans-Neptunian objects
* Detached objects
* Fifth giant
* Planets beyond Neptune
* List of probes
* Deep space
Outline of the
Solar System Portals
Solar System →
Local Interstellar Cloud →
Local Bubble → Gould
Orion Arm →
Milky Way →
Milky Way subgroup → Local
Virgo Supercluster →
Laniakea Supercluster → Observable
Each arrow (→) may be read as "within" or "part of".
* Physics portal
* Solar Syst