Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of its co-proposer, Mike Brown.
Ever since William Herschel discovered Uranus in 1781, astronomers have been eager to find new planets on the outer edge of the solar system. But after the discovery of Neptune in 1846, we’ve found no other large planets. Sure, we discovered Pluto and other dwarf planets beyond it, but nothing Earth-sized or larger. If there is some planet nine, or “Planet X” lurking out there, we have yet to find it.
Artist’s impression shows the distant dwarf planet Eris. Credit: ESO
In 2005, astronomer Mike Brown and his colleagues Chad Trujillo and David Rabinowitz announced the discovery of a previously unknown planetoid in the Kuiper Belt beyond Neptune’s orbit. The team named this object Eris after the Greek personification of strife and discord, which was assigned by the IAU a year later. Along with Haumea and Makemake, which they similarly observed in 2004 and 2005 (respectively), this object led to the “Great Planet Debate,” which continues to this day. Meanwhile, astronomers have continued to study the Trans-Neptunian region to learn more about these objects.
While subsequent observations have allowed astronomers to get a better idea of Eris’ size and mass, there are many unresolved questions about the structure of this “dwarf planet” and how it compares to Pluto. In a recent study, Mike Brown and University of California Santa Cruz professor Francis Nimmo presented a series of models based on new mass estimates for Eris’ moon Dysnomia. According to their results, Eris is likely differentiated into a convecting icy shell and rocky core, which sets it apart from Pluto’s conductive shell.
Artist's concept of the New Horizons spacecraft encountering a Kuiper Belt object, part of an extended mission after the spacecraft’s July 2015 Pluto flyby. Credits: NASA/JHUAPL/SwRI
In the near future, astronomers will benefit from the presence of next-generation telescopes like the James Webb Space Telescope (JWST) and theNancy Grace Roman Space Telescope (RST). At the same time, improved data mining and machine learning techniques will also allow astronomers to get more out of existing instruments. In the process, they hope to finally answer some of the most burning questions about the cosmos.
For instance, the Dark Energy Survey (DES ), an international, collaborative effort to map the cosmos, recently released the results of their six-year survey of the outer Solar System. In addition to gathering data on hundreds of known objects, this survey revealed 461 previously undetected objects. The results of this study could have significant implications for our understanding of the Solar System’s formation and evolution.
Artist's impression of the the possible Planet 9 at the edge of the Solar System. Credit: Robin Dienel/Carnegie Science
In 2014, Scott Sheppard of the Carnegie Institution for Science and Chadwick Trujillo of Northern Arizona University proposed an interesting idea. Noting the similarities in the orbits of distant Trans-Neptunian Objects (TNOs), they postulated that a massive object was likely influencing them. This was followed in 2016 by Konstantin Batygin and Michael E. Brown of Caltech suggesting that an undiscovered planet was the culprit.
Since that time, the hunt has been on for the infamous “Planet 9” in our Solar System. And while no direct evidence has been produced, astronomers believe they are getting closer to discerning its location. In a paper that was recently accepted by The Astronomical Journal, Sheppard and Trujillo present their latest discoveries, which they claim are further constraining the location of Planet 9.
For the sake of their study, Sheppard and Trujillo relied on information obtained by the Dark Energy Camera on the Victor Blanco 4-meter telescope in Chile and the Japanese Hyper Suprime-Camera on the 8-meter Subaru Telescope in Hawaii. With the help of David Tholen from the University of Hawaii, they have been conducting the largest deep-sky survey for objects beyond Neptune and the Kuiper Belt.
An illustration of the orbits of the new and previously known extremely distant Solar System objects – showing the clustering in orbits that indicates that possible presence of Planet X. Credit: Robin Dienel/Carnegie Science
This survey is intended to find more objects that show the same clustering in their orbits, thus offering greater evidence that a massive planet exists in the outer Solar System. As Sheppard explained in a recent Carnegie press release:
“Objects found far beyond Neptune hold the key to unlocking our Solar System’s origins and evolution. Though we believe there are thousands of these small objects, we haven’t found very many of them yet, because they are so far away. The smaller objects can lead us to the much bigger planet we think exists out there. The more we discover, the better we will be able to understand what is going on in the outer Solar System.”
Their most recent discovery was a small collection of more extreme objects who’s peculiar orbits differ from the extreme and inner Oort cloud objects, in terms of both their eccentricities and semi-major axes. As with discoveries made using other instruments, these appear to indicate the presence of something massive effecting their orbits.
All of these objects have been submitted to the International Astronomical Union’s (IAU) Minor Planet Center for designation. They include 2014 SR349, an extreme TNO that has similar orbital characteristics as the previously-discovered extreme bodies that led Sheppard and Trujillo to infer the existence of a massive object in the region.
Another is 2014 FE72, an object who’s orbit is so extreme that it reaches about 3000 AUs from the Sun in a massively-elongated ellipse – something which can only be explained by the influence of a strong gravitational force beyond our Solar System. And in addition to being the first object observed at such a large distance, it is also the first distant Oort Cloud object found to orbit entirely beyond Neptune.
Artist’s impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Credit: ESO/Tomruen/nagualdesign
And then there’s 2013 FT28, which is similar but also different from the other extreme objects. For instance, 2013 FT28 shows similar clustering in terms of its semi-major axis, eccentricity, inclination, and argument of perihelion angle, but is different when it comes to its longitude of perihelion. This would seem to indicates that this particular clustering trend is less strong among the extreme TNOs.
Beyond the work of Sheppard and Trujillo, nearly 10 percent of the sky has now been explored by astronomers. Relying on the most advanced telescopes, they have revealed that there are several never-before-seen objects that orbit the Sun at extreme distances.
And as more distant objects with unexplained orbital parameters emerge, their interactions seem to fit with the idea of a massive distant planet that could pay a key role in the mechanics of the outer Solar System. However, as Sheppard has indicated, there really isn’t enough evidence yet to draw any conclusions.
“Right now we are dealing with very low-number statistics, so we don’t really understand what is happening in the outer Solar System,” he said. “Greater numbers of extreme trans-Neptunian objects must be found to fully determine the structure of our outer Solar System.”
Alas, we don’t yet know if Planet 9 is out there, and it will probably be many more years before confirmation can be made. But by looking to the visible objects that present a possible sign of its path, we are slowly getting closer to it. With all the news in exoplanet hunting of late, it is interesting to see that we can still go hunting in our own backyard!
The Universe is a very big place, and we occupy a very small corner of it. Known as the Solar System, our stomping grounds are not only a tiny fraction of the Universe as we know it, but is also a very small part of our galactic neighborhood (aka. the Milky Way Galaxy). When it comes right down to it, our world is just a drop of water in an endless cosmic sea.
Nevertheless, the Solar System is still a very big place, and one which is filled with its fair share of mysteries. And in truth, it was only within the relatively recent past that we began to understand its true extent. And when it comes to exploring it, we’ve really only begun to scratch the surface.
Discovery:
With very few exceptions, few people or civilizations before the era of modern astronomy recognized the Solar System for what it was. In fact, the vast majority of astronomical systems posited that the Earth was a stationary object and that all known celestial objects revolved around it. In addition, they viewed it as being fundamentally different from other stellar objects, which they held to be ethereal or divine in nature.
Although there were some Greek, Arab and Asian astronomers during Antiquity and the Medieval period who believed that the universe was heliocentric in nature (i.e. that the Earth and other bodies revolved around the Sun) it was not until Nicolaus Copernicus developed his mathematically predictive model of a heliocentric system in the 16th century that it began to become widespread.
Galileo (1564 – 1642) would often show people how to use his telescope to view the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain
During the 17th-century, scientists like Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance that the Earth revolves round the Sun. The development of theories like gravity also led to the realization that the other planets are governed by the same physical laws as Earth.
The widespread use of the telescope also led to a revolution in astronomy. After Galileo discovered the moons of Jupiter in 1610, Christian Huygens would go on to discover that Saturn also had moons in 1655. In time, new planets would also be discovered (such as Uranus and Neptune), as well as comets (such as Halley’s Comet) and the Asteroids Belt.
By the 19th century, three observations made by three separate astronomers determined the true nature of the Solar System and its place the universe. The first was made in 1839 by German astronomer Friedrich Bessel, who successfully measured an apparent shift in the position of a star created by the Earth’s motion around the Sun (aka. stellar parallax). This not only confirmed the heliocentric model beyond a doubt, but revealed the vast distance between the Sun and the stars.
In 1859, Robert Bunsen and Gustav Kirchhoff (a German chemist and physicist) used the newly invented spectroscope to examined the spectral signature of the Sun. They discovered that it was composed of the same elements as existed on Earth, thus proving that Earth and the heavens were composed of the same elements.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.
Then, Father Angelo Secchi – an Italian astronomer and director at the Pontifical Gregorian University – compared the spectral signature of the Sun with those of other stars, and found them to be virtually identical. This demonstrated conclusively that our Sun was composed of the same materials as every other star in the universe.
Further apparent discrepancies in the orbits of the outer planets led American astronomer Percival Lowell to conclude that yet another planet, which he referred to as “Planet X“, must lie beyond Neptune. After his death, his Lowell Observatory conducted a search that ultimately led to Clyde Tombaugh’s discovery of Pluto in 1930.
Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the MIT discovered the Trans-Neptunian Object (TNO) known as (15760) 1992 QB1. This would prove to be the first of a new population, known as the Kuiper Belt, which had already been predicted by astronomers to exist at the edge of the Solar System.
Further investigation of the Kuiper Belt by the turn of the century would lead to additional discoveries. The discovery of Eris and other “plutoids” by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers would lead to the Great Planet Debate – where IAU policy and the convention for designating planets would be contested.
The Sun contains 99.86% of the system’s known mass, and its gravity dominates the entire system. Most large objects in orbit around the Sun lie near the plane of Earth’s orbit (the ecliptic) and most planets and bodies rotate around it in the same direction (counter-clockwise when viewed from above Earth’s north pole). The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at greater angles to it.
It’s four largest orbiting bodies (the gas giants) account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson
Astronomers sometimes informally divide this structure into separate regions. First, there is the Inner Solar System, which includes the four terrestrial planets and the Asteroid Belt. Beyond this, there’s the outer Solar System that includes the four gas giant planets. Meanwhile, there’s the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune (i.e. Trans-Neptunian Objects).
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites (or moons). In the case of the four giant planets, there are also planetary rings – thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. The terrestrial planets of the Inner Solar System are composed primarily of silicate rock, iron and nickel. Beyond the Asteroid Belt, planets are composed mainly of gases (such as hydrogen, helium) and ices – like water, methane, ammonia, hydrogen sulfide and carbon dioxide.
Objects farther from the Sun are composed largely of materials with lower melting points. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (hence why they are sometimes referred to as “ice giants”) and the numerous small objects that lie beyond Neptune’s orbit.
Together, gases and ices are referred to as volatiles. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, which lies roughly 5 AU from the Sun. Within the Kuiper Belt, objects and planetesimals are composed mainly of these materials and rock.
Formation and Evolution:
The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud composed of hydrogen, helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System (known as the pre-solar nebula) collapsed, conservation of angular momentum caused it to rotate faster.
The center, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.
Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the frost line).
The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.
Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved.
At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium. This will occur roughly 5 billion years from now and mark the end of the Sun’s main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present.
The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion and the Sun will burn helium for a time, after which nuclear reactions in the core will start to dwindle.
At this point, the Sun’s outer layers will move away into space, leaving a white dwarf – an extraordinarily dense object that will have half the original mass of the Sun, but will be the size of Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.
Inner Solar System:
In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.
For starters, the inner planets are rocky and terrestrial, composed mostly of silicates and metals, whereas the outer planets are gas giants. The inner planets are also much more closely spaced than their outer Solar System counterparts. In fact, the radius of the entire region is less than the distance between the orbits of Jupiter and Saturn.
Generally, inner planets are smaller and denser than their counterparts, and have few to no moons or rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.
The terrestrial inner planets are composed largely of refractory minerals such as the silicates, which form their crusts and mantles, and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather. All of them have impact craters and tectonic surface features as well, such as rift valleys and volcanoes.
Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. Its magnetic field is only about 1% that of Earth’s, and it’s very thin atmosphere means that it is hot during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out. It has no moons of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System.
Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other gases. Dense clouds within Venus’ atmosphere are composed of sulphuric acid and other corrosive compounds, with very little water. Much of Venus’ surface is marked with volcanoes and deep canyons – the biggest of which is over 6400 km (4,000 mi) long.
Earth is the third inner planet and the one we know best. Of the four terrestrial planets, Earth is the largest, and the only one that currently has liquid water, which is necessary for life as we know it. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight and warmth in, which is also essential for life to survive.
Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only one we have, is comprised of a mixture of various rocks and minerals.
Mars, as it appears today, Credit: NASA
Mars is the fourth and final inner planet, and is also known as the “Red Planet” due to the oxidization of iron-rich materials that form the planet’s surface. Mars also has some of the most interesting terrain features of any of the terrestrial planets. These include the largest mountain in the Solar System (Olympus Mons) which rises some 21,229 m (69,649 ft) above the surface, and a giant canyon called Valles Marineris – which is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi).
Much of Mars’ surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.
Mars’ thin atmosphere has led some astronomers to believe that the surface water that once existed there might have actually taken liquid form, but has since evaporated into space. The planet has two small moons called Phobos and Deimos.
Outer Solar System:
The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas that have rings and plenty of moons. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers that the solar system was bigger than previously thought.
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot – a raging storm that has happening for the past 400 years at least.
Saturn is best known for its prominent ring system – seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years).
Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. The only spacecraft to visit this planet was the Voyager 2 spacecraft in 1986.
Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. NASA’s Voyager 2 spacecraft also visited this planet and its system by 1989 during its transit of the outer Solar System.
How many moons are there in the Solar System? Image credit: NASA
Trans-Neptunian Region:
There have been more than a thousand objects discovered in the Kuiper Belt, and it’s theorized that there are as many as 100,000 objects larger than 100 km in diameter. Given to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine.
However, spectrographic studies conducted of the region since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice – a composition they share with comets. Initial studies also confirmed a broad range of colors among KBOs, ranging from neutral grey to deep red.
This suggests that their surfaces are composed of a wide range of compounds, from dirty ices to hydrocarbons. In 1996, Robert H. Brown et al. obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of Pluto (as well as Neptune’s moon Triton) in that it possessed large amounts of methane ice.
Water ice has been detected in several KBOs, including 1996 TO66, 38628 Huya and 20000 Varuna. In 2004, Mike Brown et al. determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs, 50000 Quaoar. Both of these substances would have been destroyed over the age of the Solar System, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.
Keeping Pluto company out in the Kuiper belt are many other objects worthy of mention. Quaoar, Makemake, Haumea, Orcus and Eris are all large icy bodies in the Belt and several of them even have moons of their own. These are all tremendously far away, and yet, very much within reach.
Oort Cloud and Farthest Regions:
The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).
The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.
Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.
Exploration:
Our knowledge of the Solar System also benefited immensely from the advent of robotic spacecraft, satellites, and robotic landers. Beginning in the mid-20th century, in what was known as “The Space Age“, manned and robotic spacecraft began exploring planets, asteroids and comets in the Inner and Outer Solar System.
All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all the planets. In the case of landers and rovers, tests have been performed on the soils and atmospheres of some.
Photograph of a Russian technician putting the finishing touches on Sputnik 1, humanity’s first artificial satellite. Credit: NASA/Asif A. Siddiqi
The first artificial object sent into space was the Soviet satellite Sputnik 1, which was launched in space in 1957, successfully orbited the Earth for months, and collected information on the density of the upper atmosphere and the ionosphere. The American probe Explorer 6, launched in 1959, was the first satellite to capture images of the Earth from space.
Robotic spacecraft conducting flybys also revealed considerable information about the planet’s atmospheres, geological and surface features. The first successful probe to fly by another planet was the Soviet Luna 1 probe, which sped past the Moon in 1959. The Mariner program resulted in multiple successful planetary flybys, consisting of the Mariner 2 mission past Venus in 1962, the Mariner 4 mission past Mars in 1965, and the Mariner 10 mission past Mercury in 1974.
By the 1970’s, probes were being dispatched to the outer planets as well, beginning with the Pioneer 10 mission which flew past Jupiter in 1973 and the Pioneer 11 visit to Saturn in 1979.The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.
Launched on January 19th, 2006, the New Horizons probe is the first man-made spacecraft to explore the Kuiper Belt. This unmanned mission flew by Pluto in July 2015. Should it prove feasible, the mission will also be extended to observe a number of other Kuiper Belt Objects (KBOs) in the coming years.
Orbiters, rovers, and landers began being deployed to other planets in the Solar System by the 1960’s. The first was the Soviet Luna 10 satellite, which was sent into lunar orbit in 1966. This was followed in 1971 with the deployment of the Mariner 9 space probe, which orbited Mars, and the Soviet Venera 9 which orbited Venus in 1975.
The Galileo probe became the first artificial satellite to orbit an outer planet when it reached Jupiter in 1995, followed by the Cassini–Huygens probe orbiting Saturn in 2004. Mercury and Vesta were explored by 2011 by the MESSENGER and Dawn probes, respectively, with Dawn establishing orbit around the asteroid/dwarf planet Ceres in 2015.
The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, probes have landed on or impacted on the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3 and Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn’s moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005.
Curiosity Rover self portrait mosaic, taken with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop in Feb. 2013. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer
To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first robotic rover to land on another planet was the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was Sojourner, which traveled 500 meters across the surface of Mars in 1997, followed by Spirit(2004), Opportunity (2004), and Curiosity (2012).
Manned missions into space began in earnest in the 1950’s, and was a major focal point for both the United States and Soviet Union during the “Space Race“. For the Soviets, this took the form of the Vostok program, which involved sending manned space capsules into orbit.
The first mission – Vostok 1 – took place on April 12th, 1961, and was piloted by Soviet cosmonaut Yuri Gagarin (the first human being to go into space). On June 6th, 1963, the Soviets also sent the first woman – Valentina Tereshvoka – into space as part of the Vostok 6 mission.
In the US, Project Mercury was initiated with the same goal of placing a crewed capsule into orbit. On May 5th, 1961, astronaut Alan Shepard went into space aboard the Freedom 7mission and became the first American (and second human) to go into space.
After the Vostok and Mercury programs were completed, the focus of both nations and space programs shifted towards the development of two and three-person spacecraft, as well as the development of long-duration spaceflights and extra-vehicular activity (EVA).
Bootprint in the moon dust from Apollo 11. Credit: NASA
This took the form of the Voshkod and Gemini programs in the Soviet Union and US, respectively. For the Soviets, this involved developing a two to three-person capsule, whereas the Gemini program focused on developing the support and expertise needed for an eventual manned mission to the Moon.
These latter efforts culminated on July 21st, 1969 with the Apollo 11 mission, when astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon. As part of the Apollo program, five more Moon landings would take place through 1972, and the program itself resulted in many scientific packages being deployed on the Lunar surface, and samples of moon rocks being returned to Earth.
After the Moon Landing took place, the focus of the US and Soviet space programs then began to shift to the development of space stations and reusable spacecraft. For the Soviets, this resulted in the first crewed orbital space stations dedicated to scientific research and military reconnaissance – known as the Salyut and Almaz space stations.
The first orbital space station to host more than one crew was NASA’s Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then.
Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA
The United States’ Space Shuttle, which debuted in 1981, became the only reusable spacecraft to successfully make multiple orbital flights. The five shuttles that were built (Atlantis, Endeavour, Discovery, Challenger, Columbiaand Enterprise) flew a total of 121 missions before being decommissioned in 2011.
During their history of service, two of the craft were destroyed in accidents. These included the Space Shuttle Challenger – which exploded upon take-off on Jan. 28th, 1986 – and the Space Shuttle Columbia which disintegrated during re-entry on Feb. 1st, 2003.
In 2004, then-U.S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. These goals have since been maintained by the Obama administration, and now include plans for an Asteroid Redirect mission, where a robotic craft will tow an asteroid closer to Earth so a manned mission can be mounted to it.
All the information gained from manned and robotic missions about the geological phenomena of other planets – such as mountains and craters – as well as their seasonal, meteorological phenomena (i.e. clouds, dust storms and ice caps) have led to the realization that other planets experience much the same phenomena as Earth. In addition, it has also helped scientists to learn much about the history of the Solar System and its formation.
As our exploration of the Inner and Outer Solar System has improved and expanded, our conventions for categorizing planets has also changed. Our current model of the Solar System includes eight planets (four terrestrial, four gas giants), four dwarf planets, and a growing number of Trans-Neptunian Objects that have yet to be designated. It also contains and is surrounded by countless asteroids and planetesimals.
Given its sheer size, composition and complexity, researching our Solar System in full detail would take an entire lifetime. Obviously, no one has that kind of time to dedicate to the topic, so we have decided to compile the many articles we have about it here on Universe Today in one simple page of links for your convenience.
There are thousands of facts about the solar system in the links below. Enjoy your research.
An artist's conception of 2007 OR10, nicknamed Snow White. Astronomers suspect that its rosy color is due to the presence of irradiated methane. [Credit: NASA]
Over the course of the past decade, more and more objects have been discovered within the Trans-Neptunian region. With every new find, we have learned more about the history of our Solar System and the mysteries it holds. At the same time, these finds have forced astronomers to reexamine astronomical conventions that have been in place for decades.
Consider 2007 OR10, a Trans-Neptunian Object (TNO) located within the scattered disc that at one time went by the nicknames of “the seventh dwarf” and “Snow White”. Approximately the same size as Haumea, it is believed to be a dwarf planet, and is currently the largest object in the Solar System that does not have a name.
Discovery and Naming:
2007 OR10 was discovered in 2007 by Meg Schwamb, a PhD candidate at Caltech and a graduate student of Michael Brown, while working out of the Palomar Observatory. The object was colloquially referred to as the “seventh dwarf” (from Snow White and the Seven Dwarfs) since it was the seventh object to be discovered by Brown’s team (after Quaoar in 2002, Sedna in 2003, Haumea and Orcus in 2004, and Makemake and Eris in 2005).
Comparison of Sedna with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
At the time of its discovery, the object appeared to be very large and very white, which led to Brown giving it the other nickname of “Snow White”. However, subsequent observation has revealed that the planet is actually one of the reddest in the Kuiper Belt, comparable only to Haumea. As a result, the nickname was dropped and the object is still designated as 2007 OR10.
The discovery of 2007 OR10 would not be formally announced until January 7th, 2009.
Size, Mass and Orbit:
A study published in 2011 by Brown – in collaboration with A.J. Burgasser (University of California San Diego) and W.C. Fraser (MIT) – 2007 OR10’s diameter was estimated to be between 1000-1500 km. These estimates were based on photometry data obtained in 2010 using the Magellan Baade Telescope at the Las Campanas Observatory in Chile, and from spectral data obtained by the Hubble Space Telescope.
However, a survey conducted in 2012 by Pablo Santos Sanz et al. of the Trans-Neptunian region produced an estimate of 1280±210 km based on the object’s size, albedo, and thermal properties. Combined with its absolute magnitude and albedo, 2007 OR10 is the largest unnamed object and the fifth brightest TNO in the Solar System. No estimates of its mass have been made as of yet.
2007 OR10 also has a highly eccentric orbit (0.5058) with an inclination of 30.9376°. What this means is that at perihelion, it is roughly 33 AU (4.9 x 109 km/30.67 x 109 mi) from our Sun while at aphelion, it is as distant as 100.66 AU (1.5 x 1010 km/9.36 x 1010 mi). It also has an orbital period of 546.6 years, which means that the last time it was at perihelion was 1857 and it won’t reach aphelion until 2130. As such, it is currently the second-farthest known large body in the Solar System, and will be farther out than both Sedna and Eris by 2045.
Composition:
According to the spectral data obtained by Brown, Burgasser and Fraser, 2007 OR10 shows infrared signatures for both water ice and methane, which indicates that it is likely similar in composition to Quaoar. Concurrent with this, the reddish appearance of 2007 OR10 is believed to be due to presence of tholins in the surface ice, which are caused by the irradiation of methane by ultraviolet radiation.
The presence of red methane frost on the surfaces of both 2007 OR10 and Quaoar is also seen as an indication of the possible existence of a tenuous methane atmosphere, which would slowly evaporate into space when the objects are closer to the Sun. Although 2007 OR10 comes closer to the Sun than Quaoar, and is thus warm enough that a methane atmosphere should evaporate, its larger mass makes retention of an atmosphere just possible.
Also, the presence of water ice on the surface is believed to imply that the object underwent a brief period of cryovolcanism in its distant past. According to Brown, this period would have been responsible not only for water ice freezing on the surface, but for the creation of an atmosphere that included nitrogen and carbon monoxide. These would have been depleted rather quickly, and a tenuous atmosphere of methane would be all that remains today.
However, more data is required before astronomers can say for sure whether or not 2007 OR10 has an atmosphere, a history of cryovolcanism, and what its interior looks like. Like other KBOs, it is possible that it is differentiated between a mantle of ices and a rocky core. Assuming that there is sufficient antifreeze, or due to the decay of radioactive elements, there may even be a liquid-water ocean at the core-mantle boundary.
Classification:
Though it is too difficult to resolve 2007 OR10’s size based on direct observation, based on calculations of 2007 OR10’s albedo and absolute magnitude, many astronomers believe it to be of sufficient size to have achieved hydrostatic equilibrium. As Brown stated in 2011, 2007 OR10 “must be a dwarf planet even if predominantly rocky”, which is based on a minimum possible diameter of 552 km and what is believed to be the conditions under which hydrostatic equilibrium occurs in cold icy-rock bodies.
That same year, Scott S. Sheppard and his team (which included Chad Trujillo) conducted a survey of bright KBOs (including 2007 OR10) using the Palomar Observatory’s 48 inch Schmidt telescope. According to their findings, they determined that “[a]ssuming moderate albedos, several of the new discoveries from this survey could be in hydrostatic equilibrium and thus could be considered dwarf planets.”
Currently, nothing is known of 2007 OR10’s mass, which is a major factor when determining if a body has achieved hydrostatic equilibrium. This is due in part to there being no known satellite(s) in orbit of the object, which in turn is a major factor in determining the mass of a system. Meanwhile, the IAU has not addressed the possibility of accepting additional dwarf planets since before the discovery of 2007 OR10 was announced.
Alas, much remains to be learned about 2007 OR10. Much like it’s Trans-Neptunian neighbors and fellow KBOs, a lot will depend on future missions and observations being able to learn more about its size, mass, composition, and whether or not it has any satellites. However, given its extreme distance and fact that it is currently moving further and further away, opportunities to observe and explore it via flybys will be limited.
However, if all goes well, this potential dwarf planet could be joining the ranks of such bodies as Pluto, Eris, Ceres, Haumea and Makemake in the not-too-distant future. And with luck, it will be given a name that actually sticks!
Artist's impression of the Trans-Neptunian Object (TNO) 90482 Orcus. Credit: NASA
Since the early 2000s, more and more objects have been discovered in the outer Solar System that resemble planets. However, until they are officially classified, the terms Kuiper Belt Object (KBO) and Trans-Neptunian Object (TNO) are commonly used. This is certainly true of Orcus, another large object that was spotted in Pluto’s neighborhood about a decade ago.
Although similar in size and orbital characteristics to Pluto, Orcus is Pluto’s opposite in many ways. For this reason, Orcus is often referred to as the “anti-Pluto”, a fact that contributed greatly to the selection of its name. Although Orcus has not yet been officially categorized as a dwarf planet by the IAU, many astronomers agree that it meets all the requirements and will be in the future.
Discovery and Naming: Orcus was discovered on February 17th, 2004, by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Although discovered using images that were taken in 2004, prerecovery images of Orcus have been identified going back as far as November 8th, 1951.
Provisionally known as 90482 2004 DW, by November 22nd, 2004, the name Orcus was assigned. In accordance with the IAU’s astronomical conventions, objects with a similar size and orbit to that of Pluto are to be named after underworld deities. Therefore, the discovery team suggested the name Orcus, after the Etruscan god of the underworld and the equivalent of the Roman god Pluto.
90482 Orcus. The location of Orcus is shown in the green circle (top, left). Credit: NASA
Size, Mass and Orbit: Given its distance, estimates of Orcus’ diameter and mass have varied over time. In 2008, observations made using the Spitzer Space Telescope in the far infrared placed its diameter at 958.4 ± 22.9 km. Subsequent observations made in 2013 using the Herschel Space Telescope at submillimeter wavelengths led to similar estimates being made.
In addition, Orcus appears to have an albedo of about 21% to 25%, which may be typical of trans-Neptunian objects approaching the 1000 km diameter range. However, these estimates were based on the assumption that Orcus was a singular object and not part of a system. The discovery of the relatively large satellite Vanth (see below) in 2007 by Brown et al. is likely to change these considerably.
The absolute magnitude of Vanth is estimated to be 4.88, which means that it is about 11 times fainter than Orcus itself. If the albedos of both bodies are the same at 0.23, then the diameter of Orcus would be closer to 892 -942 km, while Vanth would measure about 260 -293 km.
In terms of mass, the Orcus system is estimated to be 6.32 ± 0.05 ×1020 kg, which is about 3.8% the mass of the dwarf planet Eris. How this mass is partitioned between Orcus and Vanth depends of their relative sizes. If Vanth is 1/3rd the diameter Orcus, its mass is likely to be only 3% of the system. However, if it’s diameter is about half that of Orcus, then its mass could be as high as 1/12 of the system, or about 8% of the mass of Orcus.
Orcus compared to Earth and the Moon. Credit: Wikipedia Commons
Much like Pluto, Orcus has a very long orbital period, taking 245.18 years (89552 days) to complete a single rotation around the Sun. It also is in a 2:3 orbital resonance with Neptune and is above the ecliptic during perihelion. In addition, it’s orbit has a similar inclination and eccentricity as Pluto’s – 20.573° to the ecliptic, and 0.227, respectively.
In short, Orcus orbits the Sun at a distance of 30.27 AU (4.53 billion km) at perihelion and 48.07 AU (7.19 billion km) at aphelion. However, Pluto and Orcus are oriented differently. For one, Orcus is at aphelion when Pluto is at perihelion (and vice versa), and the aphelion of Orcus’s orbit points in nearly the opposite direction from Pluto’s. Hence why Orcus is often referred to as the “anti-Pluto”.
Composition: The density of the primary (and secondary assuming they have the same density) is estimated to be 1.5 g/cm3. In addition, spectroscopic and near-infrared observations have indicated that the surface is neutral in color and shows signs of water. Further infrared observations in 2004 by the European Southern Observatory and the Gemini Observatory indicated the possible presence of water ice and carbonaceous compounds.
This would indicate that Orcus is most likely differentiated between a rocky core and an icy mantle composed of water and methane ices as well as tholins – though not as much as other KBOs which are more reddish in appearance. The water and methane ices are believed to cover no more than 50% and 30% of the surface, respectively – which would mean the proportion of ice on the surface is less than on Charon, but similar to that on Triton.
Another interesting feature on Orcus is the presence of crystalline ice on its surface – which may be an indication of cryovolcanism – and the possible presence of ammonia dissolved in water and/or methane/ethane ices. This would make Orcus quite unique, since ammonia has not been detected on any other TNO or icy satellite of the outer planets (other than Uranus’ moon Miranda).
Moon: In 2011, Mike Brown and T.A. Suer detected a satellite in orbit of Orcus, based on images taken by the Hubble Space Telescope on November 13th, 2005. The satellite was given the designation S/2005 (90482) before being renamed Vanth on March 30th, 2005. This name was the result of an opinion poll where Mike Brown asked readers of his weekly column to submit their suggestions.
The name Vanth, after the Etruscan goddess who guided the souls of the dead to the underworld, was eventually chosen from among a large pool of submissions, which Brown then submitted to the IAU. The IAU’s Committee for Small Body Nomenclature assessed it and determined it fit with their naming procedures, and officially approved of it in March of 2010.
Vanth orbits Orcus in a nearly face-on circular orbit at a distance of 9030 ± 89 km. It has an eccentricity of about 0.007 and an orbital period of 9.54 days. In terms of how Orcus acquired it, it is not likely that it was the result of a collision with an object, since Vanth’s spectrum is very different from that of its primary.
Therefore, it is much more likely that Vanth is a captured KBO that Orcus acquired in the course of its history. However, it is also possible that Vanth could have originated as a result of rotational fission of the primordial Orcus, which would have rotated much faster billions of years ago than it does now.
Much like most other KBOs, there is much that we still don’t know about Orcus. There are currently no plans for a mission in the near future. But given the growing interest in the region, it would not be surprising at all if future missions to the outer Solar System were to include a flyby of this world. And as we learn more about Orcus’ size, shape and composition, we are likely to see it added to the list of confirmed dwarf planets.
For more information on Orcus, Vanth, check out the Planetary Society’s page on Orcus and Vanth. To learn more about how they were discovered, consult Mike Brown’s Planets.
Astronomy Cast also has a great interview with Mike Brown from Caltech.
Discovered in 2005, Makemake, a Kuiper Belt Object (KBO) has . Credit: NASA
In 2003, astronomer Mike Brown and his team from Caltech began a discovery process which would change the way we think of our Solar System. Initially, it was the discovery of a body with a comparable mass to Pluto (Eris) that challenged the definition of the word “planet”. But in the months and years that followed, more discoveries would be made that further underlined the need for a new system of classification.
This included the discovery of Haumea, Orcus and Salacia in 2004, and Makemake in 2005. Like many other Trans-Neptunian Objects (TNOs) and Kuiper Belt Objects (KBOs) discovered in the past decade, this planet’s status is the subject of some debate. However, the IAU was quick to designate it as the fourth dwarf planet in our Solar System, and the third “Plutoid“.
Discovery and Naming:
Makemake was discovered on March 31st, 2005, at the Palomar Observatory by a team consisting of Mike Brown, Chad Trujillo and David Rainowitz. The discovery was announced to the public on July 29th, 2005, coincident with the announcement of the discovery of Eris. Originally, Brown and his team had been intent on waiting for further confirmation, but chose to proceed after a different team in Spain announced the discovery of Haumea on July 27th.
The provisional designation of 2005 FY9 was given to Makemake when the discovery was first made public. Before that, the discovery team used the codename “Easterbunny” for the object, because it was observed shortly after Easter. In July of 2008, in accordance with IAU rules for classical Kuiper Belt Objects, 2005 FY9 was given the name of a creator deity.
Photograph of Makemake taken by the Hubble Space Telescope. Credit: NASA/Mike Brown
In order to preserve the object’s connection with Easter, the object was given a name derived from the mythos of the Rapa Nui (the native people of Easter Island) to whom Makemake is the creator God. It was officially classified as a dwarf planet and a plutoid by the International Astronomical Union (IAU) on July 19th, 2008.
Size, Mass and Orbit:
Based on infrared observations conducted by Brown and his team using the Spitzer Space Telescope, which were compared to similar observations made by the Herschel Space Telescope, an estimated diameter of 1,360 – 1,480 km was made. Subsequent observations made during the 2011 stellar occulation by Makemake produced estimated dimensions of 1502 ± 45 × 1430 ± 9 km.
Estimates of its mass place it in the vicinity of 4 x 10²¹ kg (4,000,000,000 trillion kg), which is the equivalent of 0.00067 Earths. This makes Makemake the third largest known Trans-Neptunian Object (TNOs) – smaller than Pluto and Eris, and slightly larger than Haumea.
Makemake has a slightly eccentric orbit (of 0.159), which ranges from 38.590 AU (5.76 billion km/3.58 billion mi) at perihelion to 52.840 AU ( 7.94 billion km or 4.934 billion miles) at aphelion. It has an orbital period of 309.09 Earth years, and takes about 7.77 Earth hours to complete a single sidereal rotation. This means that a single day on Makemake is less than 8 hours and a single year last as long as 112,897 days.
A selection of dwarf planets, sometimes considered trans-Neptunian objects depending on their interactions with the planet Neptune. Credit: NASA/STSci
As a classical Kuiper Belt Object, Makemake’s orbit lies far enough from Neptune to remain stable over the age of the Solar System. Unlike plutinos, which can cross Neptune’s orbit, classical KBOs are free from Neptune’s perturbation. Such objects have relatively low eccentricities (below 0.2) and orbit the Sun in much the same way the planets do. Makemake, however, is a member of the “dynamically hot” class of classical KBOs, meaning that it has a high inclination compared to others in its population.
Composition and Surface:
With an estimated mean density of 1.4–3.2 g/cm³, Makemake is believed to be differentiated between an icy surface and a rocky core. Like Pluto and Eris, the surface ice is believed to be composed largely of frozen methane (CH4) and ethane (C2H6). Though evidence exists for traces of nitrogen ice as well, it is nowhere near as prevalent as with Pluto or Triton.
Javier Licandro and his colleagues at the Instituto de Astrofisica de Canarias performed examinations of Makemake using the William Herschel Telescope and Telescopio Nazionale Galileo. According to their findings, Makemake has a very bright surface (with a surface albedo of 0.81) which means it closely resembles that of Pluto.
In essence, it appears reddish in color (significantly more so than Eris), which also indicates strong concentrations of tholins in the surface ice. This is consistent with the presence of methane ice, which would have turned red due to exposure to solar radiation over time.
Atmosphere:
During it’s 2011 occultation with an 18th-magnitutde star, Makemake abruptly blocked all of its light. These results showed that Makemake lacks a substantial atmosphere, which contradicted earlier assumptions about it having an atmosphere comparable to that of Pluto. However, the presence of methane and possibly nitrogen suggests that Makemake could have a transient atmosphere similar to that of Pluto when it reaches perihelion.
Artist’s impression of the surface of Makemake. Credit: NASA
Essentially, when Makemake is closest to the Sun, nitrogen and other ices would sublimate, forming a tenuous atmosphere composed of nitrogen gas and hydrocarbons. The existence of an atmosphere would also provide a natural explanation for the nitrogen depletion, which could have been lost over time through the process of atmospheric escape.
Moon:
In April of 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.
Exploration:
Currently, no missions have been planned to the Kuiper Belt for the purpose of conducting a survey of Makemake. However, it has been calculated that – based on a launch date of August 21st, 2024, and August 24th, 2036 – a flyby mission to Makemake could take just over 16 years, using a Jupiter gravity assist. On either occasion, Makemake would be approximately 52 AU from the Sun when the spacecraft arrives.
Makemake is now the fourth designated dwarf planet in the solar system, and the third Plutoid. In the coming years, it is likely to be joined several more objects in the Trans-Neptunian region that are similar in size, mass, and orbit. And assuming we mount a flyby to the region, we may discover many similar objects, and learn a great deal more about this one.
