Our Solar System
The Solar System is our home in the universe. Scientifically speaking the Solar System is a gravitationally bound planetary system of the Sun and the objects that orbit it around it. The largest of these objects are the eight planets, followed by five confirmed dwarf planets, (Pluto being one of them) small celestial bodies such as asteroids, comets, and moons, the Kuiper Belt and the hypothesized Oort Cloud. These celestial bodies that orbit the Sun are more specifically named “planetary bodies”.
A planetary body is any secondary body (the Sun is a “primary body”) in the Solar System that is gravitationally rounded by hydrostatic equilibrium and has planet-like geology. This definition includes planets, dwarf planets, large moons, and some asteroids. According to the Planetary Habitability Laboratory at the Arecibo Observatory in Puerto Rico, there are 82 planetary bodies with only 9 of which are not cold miniterrans, or small terrestrial planetary bodies too far from the Sun to be hospitable for life as we know it. Those nine bodies that are not cold miniterrans are the eight planets and of course, our Moon which would be a miniterran if it didn’t inhabit the habitable zone with Earth.
What Is a Planet?
A planet is a type of planetary body that is in orbit around the Sun, has sufficient mass to have a nearly round shape (hydrostatic equilibrium) and has “cleared the neighborhood” around its orbit. The definition owes it’s origin to the discovery of trans-neptunium objects approximately the size of Pluto that was discovered by astronomer Mike Brown at the California Institute of Technology. This discovery meant that “Pluto-like” objects are common and shouldn’t share the same designation as the eight major planets. These bodies were categorized as”dwarf planets.”
The Planetary Habitability Laboratory has one the most scientifically accurate classifications of planets created. Their planetary index is based on mass since it is the most measured planetary parameter for confirmed exoplanets. It makes sense to include planets of our Solar System within the same categories as the exoplanets with their ongoing discoveries. The Habitable Exoplanets Catalog classifies planets as either Terrans, or terrestrial planets and Giant planets.
A terran planet, (other names include terrestrial planet, telluric planet, or rocky planet) is a planet composed mostly of silicate rocks and metals. The word “Terran” means “of or about Earth.” The terms “terrestrial planet” and “telluric planet” are derived from the Latin words for Earth (Terra and Tellus). Our Solar System consists of four Terran planets which are also the inner planets of the Solar System, Mercury, Venus, Mars, and our world, Earth. These planets are referred to as “inner planets” because they are the closest planets to the Sun and are separated from the “outer planets” by the Asteroid Belt. There are three classifications of terran planets, miniterrans, subterrans, terrans and superterrans.
Giant planets (also known as gas planets since the majority of these planets are gaseous) are significantly more massive than Terran planets. There are four the Giant planets in our Solar System, Jupiter, Saturn, Uranus, and Neptune. The principal components of these giant planets are hydrogen and helium in the case of Jupiter and Saturn, and water, ammonia, and methane in the case of Uranus and Neptune. We have also discovered many extrasolar giant planets orbiting other stars. Since other star systems are so far away, the vast majority of exoplanets we have found are Giant planets. These planets are categorized as is either Neptunians and Jovians. Jovian planets, Jupiter and Saturn, are often referred too as “gas giants” and Neptunians planets, in our case, Uranus and Neptune are also referred to as “ice giants.”
Moons of the Solar System
Moons are natural satellites of planets, dwarf planets and other planetary bodies in our Solar System. There are approximately 158 confirmed moons in our solar system, with nearly 50 more objects believed to be moons waiting to be confirmed (these are called “provisional moons”). 19 of these moons are gravitationally rounded by hydrostatic equilibrium. Moons usually are named after mythological characters that have a subordinate relationship with with the mythological characters their planet (or primary) was named after. The only exception to this rule is Uranus’ moons which were named for characters from Shakespearean plays. The majority of moons are classified as either regular moons or irregular moons.
Regular moons have prograde orbits (or prograde motion) and usually orbits the planet close to the planets equatorial plane. For reference, the Moon is unusual as it orbits the Earth closer to the Earth’s ecliptic plane than it’s the equatorial plane. The Earth’s equatorial plane is the imaginary plane that extends out into space from the Equator. The Earth’s ecliptic plane is the imaginary plane in which the Earth and most other planets orbit the Sun. This plane is also the apparent path the Sun travels through the sky throughout the year if you mark the Sun’s location in the sky at the same time every day over a certain time period.
Irregular moons orbit their planets or “primary” in retrograde orbits. A retrograde orbit is a moon’s orbit that moves in the opposite direction as the planet’s rotation. Retrograde orbits are similar to observational retrograde motion which is the apparent change in the movement of a celestial body, usually a backward movement. These moons are usually extremely small, non-spherical, orbit their primary at extreme angles and most likely captured minor planets of the Solar System.
Galileo Galilei was the first person to discover moons orbiting another planet in 1610 when he discovered the four Galilean moons, Ganymede, Callisto, Io and Europa orbiting Jupiter. These moons are visible with binoculars or a telescope with a few completing orbits around Jupiter in the same night. Along with these Jovian moons, Saturn’s seven large moons are also visible via telescope although they are a little harder to spot.
Moons by Planet
- Earth has one Moon, Luna with a couple of co-orbitals or quasi-satellites: the asteroids 3753 Cruithne and 2002 AA29.
- Mars has two moons, Phobos and Deimos (“fear” and “dread”, after attendants of Ares, the Greek god of war.
- Jupiter has the largest number of known moons in our Solar System coming in at 72. Of these moons, 52 have been named. 8 of these moons are regular moons which include Galilean moons and the smaller Amalthea group. Jupiter’s moons were named after lovers of Zeus (Greek equivalent of Jupiter. Jupiter’s moons can be classified in several ways. There are 5 groups based on their origins and the way they orbit the planet. The moons also can be separated into two groups, irregular moons, and regular moons. The groups consist of the Galilean moons, and the Himalia, Carme, Ananke and Pasiphae groups. The Galilean moons, Himalia group, and 3 groupless moons are regular moons and the Carme, Ananke and Pasiphae groups are irregular moons.
- Saturn has 62 moons, of which 53 have names. Seven moons of there moons are large enough to reach hydrostatic equilibrium. Saturn’s moon, Titan is the second-largest moon in our Solar System. 24 of Saturn’s moons are regular and are named after the mythological Titans or other characters associated with the Roman god, Saturn. Identifying moons of Saturn can be tricky since Saturn’s rings are composed of millions of small rocks and icy material that can otherwise be classified as a moon. At least 150 “moonlets” have been identified.Moonlets are smaller, minor satellites and usually serve as “shepherd moons”, moons that clears gaps in planetary-ring material and keeps particles within a ring contained.
Saturn’s moons are grouped into the following groups: Shepherd Moons, Norse moons, Tethys Trojans, Inuit group, the Gallic group, the Dione Trojan moons, the Alkyonides group and two subgroups of the Norse moons, Narvi and Skathi. Trojan moons are moons that share an orbit of a larger moon. Trojan moons orbit their primary in a stable orbit approximately 60° ahead or behind the main secondary. The points at 60° ahead or behind the other orbiting body are referred too as Lagrangian points L4 and L5. The term “trojan” also applies to planetary objects other than moons as well.
- Uranus has 27 moons and is divided into four groups, the 5 major moons, 13 inner moons, and 9 irregular moons. Uranus’ moons are named after characters from the works of Shakespeare and Alexander Pope’s work The Rape of the Lock. Titania, Uranus’s largest moon is the eighth-largest moon in the Solar System. All of Uranus’s moons The orbit Uranus in a coplanar fashion, parallel to Uranus’s equator, which is tilted 97.77° to its orbit.
- Neptune has 14 moons with one large moon, Triton. Triton is unique for a large moon in that it orbits Neptune a retrograde orbit. Neptune’s smaller moons can be classified into 7 inner, regular satellites and 6 outer, irregular satellites. Neptune’s moon Neso has an orbital period of about 26 Julian years and orbits further from its planet than any other known moon in the Solar System.
- Pluto has five moons with it’s the largest moon, Charon, being more than half as large as Pluto. This fact makes Pluto and Charon a binary or double dwarf planetary system, since Charon and Pluto orbit a point outside of Pluto’s system. The only other system close to this configuration is the Earth-Moon system. The Moon orbits a point miles away from the Earth’s center. Pluto’s 4 other moons are Nix, Hydra, Kerberos, and Styx. These moons are far smaller and orbit the Pluto–Charon system.
- Haumea has two moons, Hiʻiaka and Namaka, named after Hawaiian goddesses since Haumea was named after the Hawaiian goddess of childbirth. Haumea and its moons are thought to be apart of a collision family. This would explain both Haumea’s usual shape, rotation and the extremely eccentric orbits of its moons.
- Eris has one moon, Dysnomia. Dysnomia was named after the daughter of the Greek goddess Eris.
Eris is a dwarf planet, plutoid, and trans-neptunium object that orbits the Sun in a highly eccentric orbit with an inclination of approximately 44 degrees to the ecliptic. The ecliptic is the plane parallel to the Sun’s equator and the plane in which most planets orbit the Sun. Eris takes approximately 558 years to orbit the Sun. Eris has a mass of roughly 0.27% of the Earth’s mass — making it almost 27% heavier than Pluto. Similar to most of the known dwarf planets, Eris can be found in the Kuiper Belt ringing around the outer solar system.
Eris circles our star approximately three times farther away than Pluto. It takes an incredible 561 years to make a single trip around the Sun and Eris rotates once every 25 hours — making a day on the distant dwarf planet very similar to what we’d experience here on Earth.
Eris was discovered by Mike Brown and his team on January 5th, 2005. Various other large trans-neptunium objects, including Ocrus and Sedna, were also found at the time. Upon discovery, Eris was commonly referred to as the “tenth planet” by NASA and media outlets. Upon discovery, Eris was widely referred to as the “tenth planet” by NASA and media outlets. Eris’s discovery sparked the intense debate that eventually led to Pluto being reclassified as a planet.
Eris’s surface is quite reflective, bouncing back 96% of the light that hits it, making it one of the most reflective bodies in the solar system. Researchers believe it’s nitrogen-rich ice mixed with frozen methane.
Haumea’s discovery is claimed by two teams, Jose Luis Ortiz Moreno and his team in 2005 and Mike Brown and his team in 2004. Haumea was classified as a classical Kuiper belt object by the Minor Planet Center shortly after it’s discovery. This designation makes it a dwarf planet, plutoid, and a trans-neptunium object.
Jose Luis Ortiz Moreno and his team announced their own discovery of the dwarf planet in an email to the Minor Planet Center on July 27th, 2005 after spotting it in images taken on March 7-10th, 2003. Although Brown initially allowed Ortiz to take credit for the discovery, he eventually suspected the team of fraud.
According to Brown, his logs of observation were accessed from Jose’s observatory the day prior to the announcement of their discovery. IAU typically gives credit to whoever submits their report to the Minor Planet Center first, but in this case, the IAU announcement about the dwarf planet did not mention a discoverer.
Haumea has a radius of approximately 385 miles — almost 1/14 of the radius of Earth. It’s roughly 4,010,000,000 miles or 43 astronomical units away from the Sun, which means it takes nearly 6 hours for sunlight to travel from the Sun to Haumea.
Haumea is thought to be highly dense. Haumea’s proposed density hints at the presence of silicate minerals, such as pyroxene or olivine. These silicate minerals that make up a lot of the rock-strewn objects found in the solar system.
Makemake’s discovery on March 31st, 2005 by M.E. Brown and his team was a part of the chain of events that lead to Pluto’s reclassification as a dwarf planet. Makemake (minor-planet designation 136472 Makemake) is known as a plutoid and dwarf planet. ‘It’s thought to be the second biggest Kuiper belt object with a diameter roughly two-thirds the magnitude of Pluto. Makemake is a member of the “dynamically hot” class of classical Kuiper belt objects due to its high inclination compared to other planetary bodies in the same area. Also, Makemake is the third major dwarf planet found in the solar system and the second furthest dwarf planet away from the Sun.
Makemake has a radius of approximately 444 miles, which means ‘it’s 1/9 the size of the radius here on Earth. It’s 4,253,000,000 miles or 45.8 astronomical units away from the Sun. This is quite far considering one astronomical unit is the distance between the Sun and Earth. It takes almost 6 hours and 20 minutes for sunlight to reach Makemake from the Sun. This dwarf ‘planet’s day length is very similar to ours. It makes a complete rotation around the Sun every 22.5 hours. Makemake takes 305 years as we know them to complete an entire trip around the Sun.
Makemake appears to be reddish-brown, similar to Pluto, in the visible spectrum. Scientists believe the near-infrared spectrum is marked by the presence of something known as broad methane absorption bands. Pluto and Eris also show significant signs of methane. However, the spectral signature on Pluto and Eris is much weaker than it is on Makemake. The surface appears to have methane in the form of large grains roughly one centimeter in size. Also, tholins and ethane are thought to be present in large amounts. Other factors, such as acetylene, ethylene, and high-mass alkanes, might be present on the dwarf planet as well.
Makemake gets its name from the myths of the Rapa Nui, the native people of Easter Island, Makemake is known as the creator of humanity and god of fertility. The name was chosen due to its discovery taking place near Easter.
Ceres is the earliest known and smallest of the current category of dwarf planets. It is also the only dwarf planet within the inner Solar System and not a transneptunian object. However, it is the only dwarf planet that is also an asteroid. Working with the hypothesis that a world should exist in between Mars and Jupiter, Sicilian astronomer Giuseppe Piazzi discovered Ceres in 1801.
Ceres was quite small, coming in at around 590 miles in diameter with a mass of just 0.015 percent that of Earth. This size made it easy to classify Ceres as both an asteroid and a dwarf planet. Unlike many asteroids, Ceres has been pounded into shape by hydrostatic equilibrium.
NASA’s Dawn space probe visited Ceres along with the asteroid Vesta in 2015. The Dawn mission spotted interesting features on Ceres surface included shiny bright spots within craters that sparked the imaginations of a few conspiracy theorists. Those high albedo spots turned out to be consistent with a large amount of sodium carbonate-based on near-infrared spectra analysis of these bright areas. Here on Earth sodium carbonate is often left when water evaporates. This suggests that the salts formed in the wet conditions beneath the crust.
It’s also interesting to note that the bright regions are almost always associated with craters. This association could mean that the formation of bright areas may have a lot to do with impacts and also gives credence to the idea that Ceres could contain large amounts of water in the form of water-ice meters beneath its surface. Ceres could have more water than we have here on Earth. Ceres’s density is 2.09 grams per cubic centimeter. Which also suggests approximately a quarter of its weight is water. Along with water, organic compounds or “tholins” are also found on Ceres.
Pluto is a dwarf planet, plutoid (ice dwarf) and trans-neptunium object in the Kuiper belt. It was the first Kuiper belt object to be discovered. Clyde Tombaugh discovered Pluto in 1930. At the time, Pluto was recognized as the ninth planet from the Sun.
Pluto is the largest known trans-Neptunian object by volume but is less massive than Eris. Pluto is primarily made of ice and rock and is relatively small like most Kuiper belt objects, about one-sixth the mass of the Moon and one-third its volume. Pluto has an eccentric and inclined orbit which ranges from 30 to 49 astronomical units or AU from the Sun. This means Pluto periodically comes closer to the Sun than Neptune. A stable orbital resonance with Neptune prevents them from colliding. Orbital resonance is the push and pull of gravitational forces between two or more bodies. It occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because of their orbital periods.
Unlike most dwarf planets, Pluto has five moons, Charon, Styx, Nix, Kerberos, and Hydra. Pluto and Charon together form a binary system or double planetary system since they both orbit a barycenter outside of both bodies.
Small Bodies of the Solar System
A Trans-Neptunian Object, also known as a TNO, is any minor planet that orbits the Sun in our solar system beyond the orbit of Neptune. Neptune is around 30.1 astronomical units from the Sun or 30 times as far from the Sun as Earth. Trans-Neptunian objects such as Haumea and Varuna have somewhat distinctly elongated shapes. Some even host small moons. These small systems are believed to be collision families or remnants of a larger body that suffered collisions dating back the origin of our Solar System. TNOs are thought to be the most primitive remnants of the planet-forming era of our Solar System. These factors often make TNO excellent candidates for the study of how the solar system evolved.
Pluto was the first TNO discovered back in 1930 as it happened to be the easiest to find due to its high apparent magnitude. The next TNO, 15760 Albion wasn’t discovered until 1992. The discovery of Albion sparked a systematic search for more TMO’s. A broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Upon reviewing and digitally evaluating a photograph of a broad strip of the sky around the ecliptic, hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers. Eris, the most massive TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, and whether objects like Pluto can be considered planets.
As of 2018, there are over 528 numbered and 2,000 unnumbered TNOs in the catalog of minor planets. TNOs can vary significantly in terms of color — ranging from a grey-blue to a red color. The European Space Agency’s Herschel Space Observatory has studied many TNOs — finding a diverse range of shapes, colors, and sizes. Herschel Space Telescope was able to measure the size, albedo, and reflected visible light from the surface of 132 TNOs.
TNOs are classified into two main groups: the Kuiper Belt Objects (KBOs) and the scattered disc objects (SDOs). Different classes can be found in these two main groups, including resonant objects and classical objects. There are approximately 70,000 TNOs, each at least 100 km across, ranging from 30 to 50 astronomical units from the Sun.
KBOs are an average distance of 30 to 55 AU from the Sun. They typically have close-to-circular orbits with a slight inclination from the ecliptic. SDOs, on the other hand, are farther from the Sun with very eccentric and inclined orbits that are non-resonant and non-planetary-orbit crossing.
A plutoid, (also known as an ice dwarf) is a planetary body orbiting the Sun beyond Neptune, has a semi-major axis greater than Neptune, has sufficient mass to round itself into a spherical shape, and has not cleared the neighborhood around their orbit. These objects are different from TMOs in that they are rounded into shape by hydrostatic equilibrium. There are potentially thousands of plutoids, but only four that have been designated as “plutoids” by the IAU. These plutoids include Pluto, Eris, Haumea, and Makemake.
Although the term and definition were accepted, many astronomers refer plutoids them as “ice dwarfs”. It seems that the IAU and astronomers alike love to complicate naming conventions. This is especially the case when it comes to trans-neptunium objects, plutoids and dwarf planets. Dwarf planets are planets that do not meet all the three criteria set out to define planets. Tran-Neptunium Objects are just that, objects beyond the orbit of Neptune. Plutiods are dwarf planets that orbit the Sun beyond the orbit of Neptune and are rounded by hydrostatic equilibrium.
Centaurs are small, icy planetoid bodies orbiting the Sun in between Jupiter and Neptune. These objects orbit the Sun at highly inclined, elliptic orbits, crossing the orbits of the Giant planets, thereby making their orbits highly unstable. Centaurs are thought to be transitory objects that originated in the Kuiper Belt and have over time migrated to outer Solar System. A lot of Centaurs are destined to either be thrown from the Solar System by the gravitational forces of the Giant planets or become comets. These objects bare the characteristics of asteroids, comets, and even planets.
The first Centaur was discovered in 1920. Although it wasn’t originally designated as a Centaur, 944 Hidalgo, was the first of many mysterious objects that displayed the characteristics of both asteroids and comets.
Centaurs weren’t recognized officially as a category or population until 1977 when 2060 Chiron was discovered. Chiron was originally classified as an asteroid. However, Chiron developed a cometary coma and has since been classified as both an asteroid and a comet. This potential for dual classification (as an asteroid and as a comet) prompted the name, Centaur, as in the half-man-half-horse beast from Greek mythology.
Various institutions use different criteria to classify an object as a centaur. For example, the Minor Planet Center describes centaurs as holding a semi-major axis that is less than that of Neptune and a perihelion beyond that of the orbit of Jupiter. The Deep Ecliptic Survey, on the other hand, delineates centaurs using a dynamical classification scheme. They state that centaurs are non-resonant objects whose instantaneous perihelia are not more than the semi-major axis of Neptune throughout any point in the simulation.
It’s believed that Saturn’s moon, Phoebe, might be a centaur captured by Saturn’s gravity. Any centaur that comes close enough to the Sun is expected to turn into a comet. Since the initial discovery, there have been 474 centaurs cataloged of an estimated 44,000 such objects.
Chiron appears to be the most perplexing of all centaurs so far. The spectra of Chiron shows a water ice signature during periods of low activity, but this signature disappears during high activity.
A sednoid is a TNO that has a semi-major axis greater than 150 AU and perihelion greater than 50 AU. The “perihelion” is the point in an orbit in which the secondary or orbiting body is closest to the Sun. A semi-major axis of an elliptical orbit is its longest diameter, from the center to the edge. There are only three objects the IAU has designated as a Sednoid: 90377 Sedna, 2015 TG387, and 2012 VP113. There may be many more sednoids that have yet to be discovered. These three objects are said to have no significant interaction with the planets — lying outside an empty gap in the solar system.
The three known sednoids have a remarkably similar orientation or tilt according to the Solar System’s ecliptic plane. The orbital tilt of these objects should be random. This suggests that there may be a “perturber” or an undiscovered massive planetary body that has “herded” the sednoids into their current orbital configuration. A hypothetical “Super-Earth” has been proposed and dubbed “Planet Nine” by many researchers. Other possible explanations could be that a passing star perturbed the objects or that they were captured by the Sun from a passing star.
Sedna was discovered on November 2003. The planetoid is located in the outer reaches of the solar system approximately 86 astronomical units from the Sun, roughly three times as far as Neptune. 2015 TG387 was discovered next in October 2018 with an aphelion of 2123 astronomical units — bringing it much further than the first discovery.
Asteroids are known as minor planets as they’re much smaller than planets or dwarf planets but still orbit the Sun. They’re typically found in the inner solar system, featuring volatile-rich surfaces very similar to comets.
There are millions of asteroids in our Solar System, many of which are thought to be fragments of planetesimals. Planetesimals were elements inside of the young Sun’s solar nebula that wasn’t large enough to develop into planets. Most asteroids can be found in the main Asteroid Belt — a region right between the orbits of Jupiter and Mars.
The Asteroid Belt is estimated to contain between 1.1 to 1.9 million asteroids, more massive than 1 kilometer in diameter. It’s also expected to hold millions of smaller asteroids as well. Asteroid are classified into three main groups: C-type, S-type, and M-type. These were chosen based on various compositions, including carbon-rich, metallic, and silicate (stony).
Ceres is the most massive asteroid at 1,000 kilometers across, but the size of asteroids varies significantly across asteroids. Asteroids that have orbits passing close to Earth are known as Near-Earth asteroids. These can also be referred to as Earth-crossers. There are approximately 14,464 of these asteroids known. A few well-known asteroids are Vesta, Juno, and Pallas.
Ceres was the first asteroid discovered in 1801, but initially, it was thought to be a new planet. It happens to be the most massive object found in the asteroid belt — leaning slightly closer to Mar’s orbit than Jupiter’s orbit. It has a diameter of 1,000 kilometers across and takes place as the 33rd-largest known body found in the solar system. After this was discovered, many other similar bodies were found appearing to be points of light, similar to stars, without a planetary disc.
As celestial objects, asteroid’s apparent motion differ from stars since they are much closer to us and orbit the Sun. This is why they’re called asteroids — a name derived from the Ancient Greek term meaning “star, planet.” Over the past two centuries, methods for finding asteroids have dramatically improved — allowing us to see many more.
Asteroids are said to be created from leftovers from the formation of our solar system an incredible 4.6 billion years ago. We believe the formation of Jupiter prevented any other planetary bodies from forming between the gap of Jupiter and Mars. This resulted in small objects colliding into each other and fragmenting into asteroids as we know them now.
Comets are icy, small solar system bodies of frozen gases, rock, and dust. They pass close by the Sun, warm up, and begin to release gases. This process is known as outgassing, which produces a visible coma or atmosphere due to the effects of solar radiation. There may be billions of comets orbiting the Sun in the Kuiper belt, and more than likely, even more, found in the Oort Cloud. How do comets differ from asteroids?
They have a protracted, gravitationally boundless atmosphere that covers their central nucleus. This consists of two parts: the tail and the coma. Extinct comets that may have come too close to the Sun resemble small asteroids as they have lost their formation of volatile ices and dust.
Short-period comets or periodic comets take less than 200 years to orbit the Sun fully. They usually orbit in the ecliptic plane following the direction of the planets. When a comet’s aphelia is located near a major planet’s orbit, it’s referred to as its “family.” Aphelia is the furthest point in a planetary bodies orbit from the Sun.
Short-period comets that take less than 20 years to orbit the Sun and are known as Jupiter-family comets (JFCs). Short-period comets that take between 20 and 200 years to orbit the Sun are known as Halley-type comets (HTCs). Long-period comets, on the other hand, take between 200 to thousands of years to fully orbit the Sun. They tend to go far beyond the outer planets at aphelia. Comet West and C/1999 F1 follow an aphelion distance of close to 70,000 AU, taking almost 6 million years to complete an orbit around the Sun.
Comets that travel close to the Sun are known as sungrazers, often crashing into the Sun. If they don’t crash into the Sun, they get close to it and break up — evaporating upon impact.
When a comet is found outside of the solar system, they’re known as exocomets. Beta Pictoris is the first exocomet system discovered in 1987. As of 2013, there have been a total of 10 exocomets found to date. These are identified using the absorption spectrum that results from comets that expel gas when they get too close to their stars.
Kreutz Sungrazer Comets
Kreutz Sungrazers, also known as sungrazing comets, are a group of comets that travel incredibly close to the Sun at perihelion, thereby garnering them the name “sungrazers.” These comets also have an aphelion of about 170 AU from the Sun. The name comes from Heinrich Kreutz, a German Astronomer who first demonstrated that these objects were related to comets.
The Kreutz family of comets have become Great Comets. Great Comets are comets that are occasionally visible near the Sun in the daytime sky. The most recent Great Coment was Comet Ikeya–Seki. Comet Ikeya–Seki was seen in1965, making it one of the brightest comets seen in the last millennium. Some scientists believe that there may be another cluster of bright Kreutz system comets may entering the inner Solar System within the decade.
Throughout history, there have been observations of Kreutz Sungrazers. In fact, the Great Comet of 1680 is a well-known example as it was extremely close to the Sun. It’s the first known sungrazing comet, and it went approximately 200,000 kilometers above the Sun’s surface — making it roughly 50% of the distance between the Moon and Earth.
Edmond Halley, an Astronomer, believed that the Great Comet of 1680 was the same comet that was seen in 1106. Other astronomers believed that all sungrazing comets were actually pieces of an earlier sungrazing comet. This theory became more realistic when the Great Comet of 1882 completed its perihelion passage and broke up into various fragments.
The three most visible, well-known sungrazers include the Great Comet of 1843, Comet Ikeya-Seki, and the Great Comet of 1882. All three of these sungrazers were incredibly impressive to anyone who was able to catch a glimpse.