Ever since the discovery of Pluto in 1930, kids grew up learning that the solar system has nine planets. That all changed in the late 1990s, when astronomers started arguing about whether Pluto was indeed a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to designate Pluto as a "dwarf planet," reducing the list of the solar system's true planets to just eight.
Astronomers, however, are still hunting for another possible planet in our solar system, a true ninth planet, after mathematical evidence of its existence was revealed on Jan. 20, 2016. The alleged "Planet Nine," also called "Planet X," is believed to be about 10 times the mass of Earth and 5,000 times the mass of Pluto.
The order of the planets in the solar system, starting nearest the sun and working outward is the following: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and then the possible Planet Nine.
If you insist on including Pluto, it would come after Neptune on the list. Pluto is truly way out there, and on a wildly tilted, elliptical orbit (two of the several reasons it was demoted).
Ever since the discovery of Pluto in 1930, kids grew up learning that the solar system has nine planets. That all changed in the late 1990s, when astronomers started arguing about whether Pluto was indeed a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to designate Pluto as a "dwarf planet," reducing the list of the solar system's true planets to just eight.
Astronomers, however, are still hunting for another possible planet in our solar system, a true ninth planet, after mathematical evidence of its existence was revealed on Jan. 20, 2016. The alleged "Planet Nine," also called "Planet X," is believed to be about 10 times the mass of Earth and 5,000 times the mass of Pluto.
The order of the planets in the solar system, starting nearest the sun and working outward is the following: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and then the possible Planet Nine.
If you insist on including Pluto, it would come after Neptune on the list. Pluto is truly way out there, and on a wildly tilted, elliptical orbit (two of the several reasons it was demoted).
This near-infrared view of Uranus reveals its otherwise faint ring system, highlighting the extent to which the planet is tilted. (Image credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory)
The seventh planet from the sun, Uranus is an oddball. It has clouds made of hydrogen sulfide, the same chemical that makes rotten eggs smell so foul. It rotates from east to west like Venus. But unlike Venus or any other planet, its equator is nearly at right angles to its orbit — it basically orbits on its side. Astronomers believe an object twice the size of Earth collided with Uranus roughly 4 billion years ago, causing Uranus to tilt. That tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years at a time.
The collision is also thought to have knocked rock and ice into Uranus' orbit. These later became some of the planet's 27 moons. Methane in the atmosphere gives Uranus its blue-green tint. It also has 13 sets of faint rings.
· Discovery: 1781 by William Herschel (was originally thought to be a star)
· Named for the personification of heaven in ancient myth
· Diameter: 31,763 miles (51,120 km)
· Orbit: 84 Earth years
· Day: 18 Earth hours
More about Uranus:
· Uranus Facts
· Uranus Pictures
· NASA Solar System Exploration: Uranus
Neptune's winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system. (Image credit: NASA/JPL)
The eighth planet from the sun, Neptune is about the size of Uranus and is known for supersonic strong winds. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. Neptune was the first planet predicted to exist by using math, before it was visually detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other planet might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth and has a rocky core.
· Discovery: 1846
· Named for the Roman god of water
· Diameter: 30,775 miles (49,530 km)
· Orbit: 165 Earth years
· Day: 19 Earth hours
More about Neptune:
· Neptune Facts
· Neptune Pictures
· NASA Solar System Exploration: Neptune
New Horizons' photo of Pluto showing the heart-shaped area now named 'Tombaugh Regio'. (Image credit: NASA/JHUAPL/SWRI)
Once the ninth planet from the sun, Pluto is unlike other planets in many respects. It is smaller than Earth's moon; its orbit is highly elliptical, falling inside Neptune's orbit at some points and far beyond it at others; and Pluto's orbit doesn't fall on the same plane as all the other planets — instead, it orbits 17.1 degrees above or below.
From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was redefined as a dwarf planet. It's a cold, rocky world with a tenuous atmosphere. Scientists thought it might be nothing more than a hunk of rock on the outskirts of the solar system. But when NASA's New Horizons mission performed history's first flyby of the Pluto system on July 14, 2015, it transformed scientists' view of Pluto. Pluto is a very active ice world that's covered in glaciers, mountains of ice water, icy dunes and possibly even cryovolcanoes that erupt icy lava made of water, methane or ammonia.
· Discovery: 1930 by Clyde Tombaugh
· Named for the Roman god of the underworld, Hades
· Diameter: 1,430 miles (2,301 km)
· Orbit: 248 Earth years
· Day: 6.4 Earth day
More about Pluto:
· Pluto Facts
· Pluto Pictures
· NASA Solar System Exploration: Dwarf Planets
Planet Nine
The orbits of distant Kuiper Belt objects and the hypothesized Planet Nine around the sun are shown in this image.. Orbits in purple are primarily controlled by Planet Nine's gravity and exhibit tight orbital clustering. Green orbits are strongly coupled to Neptune and exhibit a broader orbital dispersion. Planet Nine is an approximately 5-Earth-mass planet that resides on a mildly eccentric orbit with a period of about 10,000 years. (Image credit: James Tuttle Keane/Caltech)
In 2016, researchers proposed the possible existence of a ninth planet, for now dubbed "Planet Nine" or Planet X. The planet is estimated to be about 10 times the mass of Earth and to orbit the sun between 300 and 1,000 times farther than the orbit of the Earth.
Scientists have not actually seen Planet Nine. They inferred its existence by its gravitational effects on other objects in the Kuiper Belt, a region at the fringe of the solar system that is home to icy rocks left over from the birth of the solar system. Also called trans-Neptunian objects, these Kuiper Belt objects have highly elliptical or oval orbits that align in the same direction.
Scientists Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena described the evidence for Planet Nine in a study published in the Astronomical Journal. The research is based on mathematical models and computer simulations using observations of six other smaller Kuiper Belt Objects with orbits that aligned in a similar matter.
A recent hypothesis proposed September 2019 on the pre-print server arXiv suggests Planet Nine might not be a planet at all. Instead, Jaku Scholtz of Durham University and James Unwin of the University of Illinois at Chicago speculate it could be a primordial black hole that formed soon after the Big Bang and that our solar system later captured, according to Newsweek. Unlike black holes that form from the collapse of giant stars, primordial black holes are thought to have formed from gravitational perturbations less than a second after the Big Bang, and this one would be so small (5 centimeters in diameter) that it would be challenging to detect.