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 proposed the 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 perturbed
Uranus enough to account for the
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 (IAU) reclassified
Pluto and its
largest neighbours as dwarf planets, leaving
Neptune the farthest
known planet in the Solar System.
While the astronomical community widely agrees that Planet X, 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 26,000
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 farther than about 1200 AU from the Sun. The orbit
is predicted to be anti-aligned to the clustered extreme
trans-Neptunian objects. Because
Pluto is no longer considered a
planet by the IAU, this new hypothetical object has become known as
1 Early speculation
2.1 Discovery of Pluto
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
4.1.1 Sedna's orbit
4.1.2 Elongated orbits of group of
Kuiper belt objects
4.1.3 Discovery of
2012 VP113 and the orbital clustering of Kuiper
4.1.4 Further analysis &
Planet Nine hypothesis
4.3 Kuiper cliff
4.4 Other proposed planets
5 Constraints on additional planets
6 See also
6.1 Survey telescopes
9 Further reading
See also: Discovery of Neptune
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 Bouvard to
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
Observatory in 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, "[There is] 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
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 meaningful results. George Forbes
is today considered to be the first describing
In 1900 and 1901,
Harvard College Observatory
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.
In 1909, 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
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
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 observatory in Arequipa, 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", [for it] 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
Main article: 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 search for 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 to 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 Pluto
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 for 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
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
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, 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
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
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 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
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
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
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 & Harrington 
0.00218 (1/459 Earth)
Buie et al.
Pluto's size was finally determined conclusively in 1978, when
James W. Christy
James W. Christy discovered its moon Charon. This
enabled him, together with
Robert Sutton Harrington of the U.S. Naval
Observatory, to measure the mass of the Pluto–Charon system directly
by observing the moon's orbital motion around Pluto. They
determined Pluto's mass to be 1.31×1022 kg; 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 not Pluto.
Further searches for
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
Neptune from 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
pointed out that these discrepancies were a hundredth the size of
those noticed by Le Verrier, and could easily be due to observational
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 beyond
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
Planet X would have severely affected the orbits of known outer
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 planet
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
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
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
Six months before,
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
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, Makemake, Haumea, Sedna, 2002 MS4,
2007 OR10, Quaoar, Salacia, Orcus, and
Earth along with the Moon.
See also: History of the Kuiper belt
After the discovery of
Pluto and Charon, no more trans-Neptunian
objects (TNOs) were found until
15760 Albion 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
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 Haumea, 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 "Planet X",
although the conception of these objects may differ considerably from
that proposed by Lowell.
Orbits of distant objects
The orbit of Sedna (red) set against the orbits of
Neptune (blue), and
Prediction of hypothetical
Planet Nine's orbit based on unique
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
2012 VP113 and the orbital clustering of Kuiper belt
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
that the available data actually indicate more than one
trans-Neptunian planet; subsequent work further suggests that the
evidence is robust enough.
Further analysis &
Planet Nine hypothesis
On January 20, 2016, Brown and
Konstantin Batygin published an article
corroborating Trujillo and Sheppard's initial findings; proposing a
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
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 [out there] 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 approach
Earth in size, it would still be
a dwarf planet by the current definition, because it would not have
cleared its neighbourhood sufficiently.
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 those of
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 farther than 100 AU with an
eccentric and inclined orbit. Computer simulations by Patryk Lykawka
Kobe University have suggested that an object with a mass between
0.3 and 0.7
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 the cliff at 50 AU provided evidence of a possible
Mars-sized planet residing at the edge of the Solar System.
Other proposed planets
Tyche was a hypothetical gas giant proposed 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
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
out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000
AU. The WISE telescope has continued to take more data since then,
NASA has invited the public to help search this data for evidence
of planets beyond these limits, via the Backyard Worlds:
citizen science project.
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 farther than
350–400 AU; one with a mass of 2 times that of Earth, farther
than 496–570 AU; and finally one with a mass of 15 times that
of Earth, farther 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
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 7000205599785884932♠117.8°+11°
−10°. At this location, it would be approximately 630 AU from the
Fictional planets of the Solar System
List of hypothetical
Solar System objects
Large Synoptic Survey Telescope
Large Synoptic Survey Telescope (LSST)
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer (WISE)
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the National Academy of Sciences Board on Physics and Astronomy.
Andrew Coates provides a simplified summary of the history behind the
search & claims for
Coleta de Dados Colles
Definition of planet
IAU definition of planet
International Astronomical Union
Michael E. Brown
Neil deGrasse Tyson
James W. Christy
Solar eclipses on Pluto
Hubble Space Telescope
Pluto Fast Flyby/
Pluto Kuiper Express
Mariner Mark II
The Solar System
S/2015 (136472) 1
Solar System objects
By discovery date
Gravitationally rounded objects
Possible dwarf planets
first discovered: Ceres
Planets beyond Neptune
List of crewed spacecraft
List of probes
Outline of the Solar System
Solar System → Local Interstellar Cloud → Local
Bubble → Gould Belt → Orion Arm → Milky
Milky Way subgroup → Local Group → Virgo
Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each arrow (→) may be read as "within" or "part of".