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Monday December 11th 2017

‘Moons’

The Moons of the Solar System

More Questions Than Answers

Europa Moon of Jupiter Europa – Moon of Jupiter. Credit NASA

Think of our solar system, and for most of us the first thing to come to mind would be the eight planets orbiting our Sun. But perhaps even more interesting are the moons which orbit the planets. Our Moon seems to be a lifeless body, with hardly any atmosphere and no dynamic activity. But the more data we collect, and the more we learn about its formation, the more fascinating it becomes. The same is true of the moons of the outer gas giants, some of which are comparable in size to the Moon. Little was known about these bodies until we began to send spacecraft armed with a host of sensors in order to analyse them in much greater detail. Each has its own uniquely distinctive characteristics in terms of overall composition, and their widely diverse range of surface features give tantalizing clues to what is going on beneath. The image below shows the size of the Earth’s Moon in relation to other moons and planets in the Solar System.

Moons of the Solar System Moons of the Solar System / Credit NASA

Galileo and the Beginning of Modern Astronomy

In 1610 Italian astronomer Galileo Galilei discovered the four largest moons of Jupiter; Io, Europa, Ganymede and Callisto. He used a homemade telescope to observe the motion of these bodies, which he first took to be stars. But after a few weeks he realized that they never left the vicinity of Jupiter, and they changed position in relation to each other and the planet. Galileo therefore concluded that they must be planetary bodies in orbit around Jupiter. This discovery, along with his measurements of the phases of Venus, proved that not everything in the Universe revolves around the Earth, and led to conflict with the Catholic Church by refuting the geocentric view of the Solar System. But importantly for science, his work also marked the beginning of modern astronomy.

Jupiter-moons_475px All that’s required to see the Galilean moons of Jupiter is either a good pair of binoculars or a small telescope. Credit: Jan Sandberg, www.desert-astro.com

The Inner Solar System

Phobos_Deimos_475px Phobos (left) and Deimos (right), Moons of Mars. Credit NASA

Both Mercury and Venus have no moons at all. One theory for this is that if they ever possessed a moon in the past, it would have eventually been stolen by either the gravitational pull of the Sun or the gravitational pull of the host planet. Then there’s Mars which has two moons, Phobos and Deimos. These are very small bodies, with Phobos having a diameter of 22.2 km and Deimos 12.4 km, so they haven’t got enough mass to enable them to form into the roughly spherical shapes that larger bodies are able to do. The orbital radius of Phobos is just 9,377 km, with an orbital period of 0.32 days. It is being pulled 1.8 m closer to Mars every century, and so will eventually either crash into the planet or break up and form a ring of material around it. Deimos, on the other hand, has an orbital radius of 23,460 km, and is gradually moving in the opposite direction, away from Mars, just like our own Moon is moving away from the Earth. So one day both Deimos and the Moon will cease to be influenced by the pull of their host planets and be set free into space.

The Outer Solar System

Io, moon of Jupiter shown with plume. Credit: NASAIt seems reasonable to assume that the further from the Sun the colder the temperature of the bodies that exist there. At Jupiter, which lies five times further from the Sun than the Earth, the average surface temperature of its moons is -170°C. At Saturn the average surface temperature is -200°C, at Uranus -210°C, and at Neptune -235°C. Before it was possible to closely inspect the moons of the outer planets in any great detail, scientists assumed that, due to these low temperatures, they would be cold, lifeless, inactive bodies. But since Voyager 1, launched in 1977, the Galileo mission, launched in 1989, and Cassini-Huygens in 1997, it has become clear that their theories were very far from the truth. Io for example, the innermost of the four Galilean moons of Jupiter, is the most volcanically active body in the solar system. Whilst having an average surface temperature of -130°C, its volcanoes can reach 1,650°C. It is covered in sulfur due to the hundreds of active volcanoes on its surface, and has lava lakes, floodplains of liquid rock and plumes of sulfur reaching as high as 300km. The above image (credit: NASA) of Io showing an active plume from a volcano. Io’s colorful appearance is due to various materials produced by its volcanism, including silicates, sulfur and sulfur dioxide.

Resonance

The reason for Io’s volcanic activity is due to the orbital resonance of three of the four Galilean moons, meaning that their orbital periods are multiples of each other. Io revolves around Jupiter four times in the same period it takes Europa to revolve twice and Ganymede to revolve once. This regular alignment results in a gravitational pull which has caused their orbital paths around Jupiter to become elliptical. This in turn creates immense tidal forces, causing the physical rock on Io’s surface to rise up and down a hundred meters during the course of each Io day, or about every 42 hours. A huge amount of heat is therefore generated, which is enough to melt a large proportion of Io’s interior and bring about the conditions we have observed on its surface from pictures relayed by Voyager 1 in the late seventies and the Galileo mission in the late 1990’s and early 2000’s. Resonance is common in the solar system, and accounts for the geysers and the jets on Enceladus for example, and the liquid water ocean beneath the surface of Europa.

Cryovolcanism

Image taken by NASA’s Cassini probe of jets of water ice being emitted from the surface of Enceladus. Credit: NASA/JPL/SSI Image taken by NASA’s Cassini probe of jets of water ice being emitted from the surface of Enceladus. Credit: NASA/JPL/SSI

Volcanism does not only occur on rocky bodies like Io. In March 2006 the Cassini probe observed icy jets being emitted from the south pole of Enceladus, a moon of Saturn. The volcanoes erupting these jets were however not spewing out molten rock. When Cassini flew through the plume of one of these emissions, it detected predominantly salty water-ice, with small amounts of carbon dioxide, ammonia, methane and other hydrocarbons. The contaminants lower the melting temperature of the ice on the crust of Enceladus, allowing the generation of cryomagma, which can be erupted in plumes reaching hundreds of kilometers above its surface. Other icy moons exhibiting cryovolcanism include Ariel and Miranda orbiting Uranus, and Triton, the largest moon of Neptune.

Is There Life in the Solar System?

Mars has traditionally been the place to look for alien life in the solar system, but the icy moons of the outer planets are exciting for scientists to study because their surfaces are comprised of large amounts of water ice. Although solid at the surface, it has been proven that vast amounts of liquid water can exist underneath. The Galileo mission to Jupiter gave evidence that Europa has an ocean of water beneath its icy surface totaling more than all the oceans, rivers and lakes existing on the Earth. And where there’s water there’s the potential for life, at least life as we know it. Tidal distortion has created cracks on the surface of Europa, enabling liquid water to escape to the surface. These could be the places where life is most likely to occur, as sunlight could create the conditions for photosynthesis to take place. Other places with the potential for life are Ganymede, Enceladus and Titan, Saturn’s biggest moon.

Plate tectonics on Europa. Image Credit: NASA Plate tectonics on Europa. Image Credit: NASA

But life can also be created without the need for sunlight. At the bottom of our oceans on Earth, hydro-thermal vents exist that harbor microbes which, in a process called chemosynthesis, convert chemicals from the vent into usable energy. So why can’t this same process take place elsewhere in the solar system, or elsewhere in the Universe for that matter?

Future Exploration

NASA’s New Horizons spacecraft is one of the latest ongoing missions to explore the solar system. It is on its way to a rendezvous with the newly termed dwarf planet Pluto and its moon Charon, both members of the vast region beyond Neptune known as the Kuiper Belt. Launched in 2006, and moving at almost one million miles per day, it will reach Pluto in summer 2015, armed with much more state of the art sensors than earlier missions. For instance, it will include LORRI, one of the highest resolution telescopes ever sent into space. Scientists believe that the Kuiper Belt contains a totally different class of world than the rest of the Solar System, so to better understand it, we need to understand worlds like Pluto and Charon.
In 2022 the European Space Agency is planning to launch the Jupiter Icy Moons Explorer (JUICE), which will study Ganymede, Europa and Callisto, and their potential for containing life. NASA too is hoping to launch its own mission to Europa in 2022, called the Europa Clipper. It will fly down to within 25km of its surface, and may even include a lander. So the future of space exploration is alive and well, with plenty of exciting encounters to look forward to.

Further Sources

NASA JPL Live http://ustre.am/onbo. NASA Scientists discuss the future mission to Europa

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Planets, Dwarf Planets and Small Solar System Bodies

Questions and Answers

By: International Astronomical Union
Source:  http://www.iau.org/public/pluto/

Pluto Dwarf PlanetQ: What is the origin of the word planet?
A: The word planet comes from the Greek word for “wanderer”, meaning that planets were originally defined as objects that moved in the night sky with respect to the background of fixed stars.

Q: Why is there a need for a new definition for the word planet?
A: Modern science provides much more information than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable with and larger than Pluto. Historically Pluto has been recognised as the ninth planet. Thus these discoveries have rightfully called into question whether or not the newly found Trans-Neptunian Objects should also be considered as new planets.

Q: How did astronomers reach a consensus for a new definition of planet?
A: The world’s astronomers, under the auspices of the International Astronomical Union, deliberated on a new definition for the word planet for nearly two years. The results of these deliberations were channelled to a Planet Definition Committee and ultimately proposed to the IAU General Assembly. Continued evolution of the definition through debate and further discussion allowed a final consensus and vote.

Q: What new terms are used in the official IAU definition?
A: There are three new terms adopted as official definitions by the IAU. The terms are: planet, dwarf planet and small Solar System body.

Q: In plain language, what is the new definition of planet?
A: A planet is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or near-spherical) shape. In addition a planet orbits in a clear path around the Sun. If any object ventures near the orbit of a planet, it will either collide with the planet, and thereby be accreted, or be ejected into another orbit.

Q: What is the exact wording of the official IAU proposed definition of planet?
A: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Q: Does a body have to be perfectly spherical to be called a planet?
A: No. For example, the rotation of a body can slightly distort the shape so that it is not perfectly spherical. Earth, for example, has a slightly greater diameter measured at the equator than measured at the poles.

Q: Based on this new definition, how many planets are there in our Solar System?
A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.

Q: Is that all, only eight planets?
A: No. In addition to the eight planets, there are also five known dwarf planets. Many more dwarf planets are likely to be discovered soon.

Q: What is a dwarf planet?
A: A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.

Q: How many dwarf planets are there?
A: Currently there are five objects accepted as dwarf planets. Ceres, Pluto, Eris, Makemake and Haumea.

Q: What is Ceres?
A: Ceres is (or now we can say it was) the largest asteroid, about 1000 km across, orbiting in the asteroid belt between Mars and Jupiter. Ceres now qualifies as a dwarf planet because it is now known to be large enough (massive enough) to have self-gravity pulling itself into a nearly round shape. (Thomas, 2005) Ceres orbits within the asteroid belt and is an example of the case of an object that does not orbit in a clear path. There are many other asteroids that can come close to the orbital path of Ceres.

Q: Didn’t Ceres used to be called an asteroid or minor planet?
A: Historically, Ceres was called a planet when it was first discovered in 1801, orbiting in what is known as the asteroid belt between Mars and Jupiter. In the 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet.

Q: Why is Pluto now called a dwarf planet?
A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region.

Q: Is Pluto’s satellite Charon a dwarf planet?Pluto System with Charon
A: For now, Charon is considered just to be Pluto’s satellite. The idea that Charon might qualify to be called a dwarf planet in its own right may be considered later. Charon may receive consideration because Pluto and Charon are comparable in size and orbit each other, rather than just being a satellite orbiting a planet. Most important for Charon’s case as a dwarf planet is that the centre of gravity about which Charon orbits is not inside of the system primary, Pluto. Instead this centre of gravity, called the barycentre, resides in free space between Pluto and Charon.

Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called dwarf planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the “barycentre”) that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be satellites rather than planets. [Actually, there has been no official recognition that the location of the barycenter is involved with the definition of a satellite.]

Q: What was 2003 UB313?
A: 2003 UB313 was a provisional name given to a large object discovered in 2003 that resides in an orbit around the Sun beyond Neptune. It is now called Eris and is recognised as a dwarf planet.

Q: Why is Eris a dwarf planet?
A: Hubble Space Telescope images have resolved the size of Eris showing it to be as large as, or larger than Pluto, Brown (2006).More important, Eris was found to have a satellite, which was later named Dysnomia, after the Greek demon of lawlessness who was Eris’ daughter. In 2007, the mass of Eris was determined to be (1.66 ± 0.02)×1022 kg, 27% greater than Pluto, based on observations of the orbit of Dysnomia. Eris also orbits within the transneptunian region – a region that has not been cleared out. Therefore Eris is a dwarf planet.

Q: What is an object called that is too small to be either a planet or dwarf planet?
A: All objects that orbit the Sun that are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape are now defined as being small Solar System bodies. This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars and Jupiter Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs) and comets.

Q: What is a small Solar System body?
A: The term “small Solar System body” is a new IAU definition to encompass all objects orbiting the Sun that are too small (not sufficiently massive) to satisfy the definition of planet or dwarf planet.

Q: Is the term minor planet still to be used?
A: The term “minor planet” may still be used. But generally the term small Solar System body will be preferred.

Q: How will an official decision be reached on whether or not to call a newly discovered object a planet, dwarf planet, or a Solar System body?
A: The decision on how to classify newly discovered objects will be made by a review committee within the IAU. The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definitions. It is likely that for many objects, several years may be required to gather sufficient data.

Q: Are there additional planet candidates currently being considered?
A: No. None appear likely in our Solar System. But there are planet discoveries galore around other stars.

Q: Are there additional dwarf planet candidates currently being considered?
A: Yes. Some of the largest asteroids may be candidates for dwarf planet status and some additional dwarf planet candidates beyond Neptune will soon be considered.

Q: When will additional new dwarf planets likely be announced?
A: Probably within the next few years.

Q: How many more new dwarf planets are there likely to be?
A: There may be dozens or perhaps even more than a hundred waiting to be discovered.

Q: What are plutoids?
A: Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

Q: Can a satellite orbiting a plutoid be a plutoid too?
A: No, according to the IAU Resolution B5 a dwarf planet can not be a satellite, even if they are massive enough that their shape is dictated by self-gravity.

References

Brown, M. et al. 2006, Astrophysical Journal, 643, L61

Thomas, P. et al. 2005, Nature, 437, 224

Further reading: Pluto and the Developing Landscape of our Solar System

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Pluto and the Developing Landscape of Our Solar System

By: International Astronomical Union
Source:  http://www.iau.org/public/pluto/

The discovery of Pluto

Nearly eighty years ago an astronomer working at the Lowell Observatory in the United States made a discovery that would ultimately initiate a dramatic change in the way we look at our Solar System. The young astronomer was Clyde Tombaugh, an observing assistant working at the observatory made famous by the great astronomer Percival Lowell. Tombaugh was continuing the search for an elusive planet – planet X – that Lowell had believed (incorrectly) to be responsible for perturbing the orbits of Uranus and Neptune.

Within a year, after spending numerous nights at the telescope exposing photographic plates and months tediously scanning them for signs of a planet, Tombaugh saw what he was looking for. At around 4pm on the afternoon of 18 February 1930 Tombaugh began comparing two plates taken in January that year showing a region in the constellation of Gemini. As he flicked from one plate to the other, trying to see if something moved slightly between the two (the tell-tale sign of the planet he was hunting), he spotted something. In one part of the frame a small object flitted a few millimetres as he switched between the two plates. Tombaugh had found his new planet! (Stern & Mitton, 2005).

The changing landscape of the solar system

Solar systemThe object Tombaugh had discovered was named Pluto, a name officially adopted by the American Astronomical Society, the Royal Astronomical Society in the UK and the IAU. It is a frigid world, billions of kilometres from Earth, and 30 times less massive than the then-smallest known planet, Mercury. But Pluto was not alone. It was found to have three satellites. The largest, Charon, was discovered in 1978. The smaller two were discovered using the Hubble Space Telescope in 2005 and officially named Nix and Hydra by the IAU in early 2006

The view of our solar system’s landscape began to change on August 30, 1992 with the discovery by David Jewitt and Jane Luu from the University of Hawaii of the first of more than 1000 now known objects orbiting beyond Neptune in what is often referred to as the transneptunian region. More generally these bodies are often simply labelled as Trans-Neptunian Objects (TNOs). 

With so many Trans-Neptunian Objects being found, it seemed inevitable that one or more might be found to rival Pluto in size. On the night of the 21 October 2003, Mike Brown from Caltech, Chad Trujillo from the Gemini Observatory and David Rabinowitz from Yale University were using a telescope and camera at the Palomar Observatory in the US to search the edge of the Solar System. That night they imaged a region of sky showing an object moving relative to the background stars. Later analysis showed that they had discovered another cold world, around 2500 km across, orbiting the Sun. Subsequent observations showed that the new object, initially named 2003 UB313 according to the International Astronomical Union’s protocol on the initial designation of such objects, was more massive than Pluto and that it too had a satellite (read more). With an object larger and more massive than Pluto now beyond Neptune and ever more of these Trans-Neptunian Objects being discovered, astronomers were beginning to ask: “Just what constitutes a planet?”

A new class of objects and how to define a planet

The IAU has been responsible for the naming and nomenclature of planetary bodies and their satellites since the early 1900s. As Professor Ron Ekers, past president of the IAU, explains:

 Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.

The IAU decided to create a committee to gather opinions from a broad range of scientific interests, with input from professional astronomers, planetary scientists, historians, science publishers, writers and educators. Thus the Planet Definition Committee of the IAU Executive Committee was formed and quickly went about preparing a draft resolution to put to the members of the IAU. After the final meeting in Paris the draft resolution was completed. One crucial aspect of the resolution is described by Professor Owen Gingerich, Chair of the IAU Planet Definition Committee: On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects”.

The final resolution

The first draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly in Prague and a new version slowly took shape. This new version was more acceptable to the majority and was put to the members of the IAU for a vote at the Closing Ceremony on the 24 August 2006. By the end of the Prague General Assembly, its members voted that the resolution B5 on the definition of a planet in the Solar System would be as follows:

A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Dwarf Planets, plutoids and the Solar System today

The IAU Resolution means that the Solar System officially consists of eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called dwarf planets was also decided on. It was agreed that planets and dwarf planets are two distinct classes of objects. The first members of the dwarf planet category are Ceres, Pluto and Eris, formerly known as 2003 UB313. Eris was named after the IAU General Assembly in 2006 Eris is the Greek god of discord and strife, a name which the discoverer Mike Brown found fitting in the light of the academic commotion that followed its discovery.

The dwarf planet Pluto is recognised as an important prototype of a new class of Trans-Neptunian Objects. The IAU has put given a new denomination for these objects: plutoids.

Today the resolution remains in place and is a testament to the fluid nature of science and how our view of the Universe continues to evolve with changes made by observations, measurements and theory.

References:

Stern, A., & Mitton, J., 2005, Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, Wiley-VCH 1997


 

Planets, Dwarf Planets and Small Solar System Bodies

Questions and Answers

 

Q: What is the origin of the word planet?
A: The word planet comes from the Greek word for “wanderer”, meaning that planets were originally defined as objects that moved in the night sky with respect to the background of fixed stars.

Q: Why is there a need for a new definition for the word planet?
A: Modern science provides much more information than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable with and larger than Pluto. Historically Pluto has been recognised as the ninth planet. Thus these discoveries have rightfully called into question whether or not the newly found Trans-Neptunian Objects should also be considered as new planets.

Q: How did astronomers reach a consensus for a new definition of planet?
A: The world’s astronomers, under the auspices of the International Astronomical Union, deliberated on a new definition for the word planet for nearly two years. The results of these deliberations were channelled to a Planet Definition Committee and ultimately proposed to the IAU General Assembly. Continued evolution of the definition through debate and further discussion allowed a final consensus and vote.

Q: What new terms are used in the official IAU definition?
A: There are three new terms adopted as official definitions by the IAU. The terms are: planet, dwarf planet and small Solar System body.

Q: In plain language, what is the new definition of planet?
A: A planet is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or near-spherical) shape. In addition a planet orbits in a clear path around the Sun. If any object ventures near the orbit of a planet, it will either collide with the planet, and thereby be accreted, or be ejected into another orbit.

Q: What is the exact wording of the official IAU proposed definition of planet?
A: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Q: Does a body have to be perfectly spherical to be called a planet?
A: No. For example, the rotation of a body can slightly distort the shape so that it is not perfectly spherical. Earth, for example, has a slightly greater diameter measured at the equator than measured at the poles.

Q: Based on this new definition, how many planets are there in our Solar System?
A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.

Q: Is that all, only eight planets?
A: No. In addition to the eight planets, there are also five known dwarf planets. Many more dwarf planets are likely to be discovered soon.

Q: What is a dwarf planet?
A: A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.

Q: How many dwarf planets are there?
A: Currently there are five objects accepted as dwarf planets. Ceres, Pluto, Eris, Makemake and Haumea.

Q: What is Ceres?
A: Ceres is (or now we can say it was) the largest asteroid, about 1000 km across, orbiting in the asteroid belt between Mars and Jupiter. Ceres now qualifies as a dwarf planet because it is now known to be large enough (massive enough) to have self-gravity pulling itself into a nearly round shape. (Thomas, 2005) Ceres orbits within the asteroid belt and is an example of the case of an object that does not orbit in a clear path. There are many other asteroids that can come close to the orbital path of Ceres.

Q: Didn’t Ceres used to be called an asteroid or minor planet?
A: Historically, Ceres was called a planet when it was first discovered in 1801, orbiting in what is known as the asteroid belt between Mars and Jupiter. In the 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet.

Q: Why is Pluto now called a dwarf planet?
A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region.

Q: Is Pluto’s satellite Charon a dwarf planet?
A: For now, Charon is considered just to be Pluto’s satellite. The idea that Charon might qualify to be called a dwarf planet in its own right may be considered later. Charon may receive consideration because Pluto and Charon are comparable in size and orbit each other, rather than just being a satellite orbiting a planet. Most important for Charon’s case as a dwarf planet is that the centre of gravity about which Charon orbits is not inside of the system primary, Pluto. Instead this centre of gravity, called the barycentre, resides in free space between Pluto and Charon.

Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called dwarf planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the “barycentre”) that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be satellites rather than planets. [Actually, there has been no official recognition that the location of the barycenter is involved with the definition of a satellite.]

Q: What was 2003 UB313?
A: 2003 UB313 was a provisional name given to a large object discovered in 2003 that resides in an orbit around the Sun beyond Neptune. It is now called Eris and is recognised as a dwarf planet.

Q: Why is Eris a dwarf planet?
A: Hubble Space Telescope images have resolved the size of Eris showing it to be as large as, or larger than Pluto, Brown (2006).More important, Eris was found to have a satellite, which was later named Dysnomia, after the Greek demon of lawlessness who was Eris’ daughter. In 2007, the mass of Eris was determined to be (1.66 ± 0.02)×1022 kg, 27% greater than Pluto, based on observations of the orbit of Dysnomia. Eris also orbits within the transneptunian region – a region that has not been cleared out. Therefore Eris is a dwarf planet.

Q: What is an object called that is too small to be either a planet or dwarf planet?
A: All objects that orbit the Sun that are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape are now defined as being small Solar System bodies. This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars and Jupiter Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs) and comets.

Q: What is a small Solar System body?
A: The term “small Solar System body” is a new IAU definition to encompass all objects orbiting the Sun that are too small (not sufficiently massive) to satisfy the definition of planet or dwarf planet.

Q: Is the term minor planet still to be used?
A: The term “minor planet” may still be used. But generally the term small Solar System body will be preferred.

Q: How will an official decision be reached on whether or not to call a newly discovered object a planet, dwarf planet, or a Solar System body?
A: The decision on how to classify newly discovered objects will be made by a review committee within the IAU. The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definitions. It is likely that for many objects, several years may be required to gather sufficient data.

Q: Are there additional planet candidates currently being considered?
A: No. None appear likely in our Solar System. But there are planet discoveries galore around other stars.

Q: Are there additional dwarf planet candidates currently being considered?
A: Yes. Some of the largest asteroids may be candidates for dwarf planet status and some additional dwarf planet candidates beyond Neptune will soon be considered.

Q: When will additional new dwarf planets likely be announced?
A: Probably within the next few years.

Q: How many more new dwarf planets are there likely to be?
A: There may be dozens or perhaps even more than a hundred waiting to be discovered.

Q: What are plutoids?
A: Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

Q: Can a satellite orbiting a plutoid be a plutoid too?
A: No, according to the IAU Resolution B5 a dwarf planet can not be a satellite, even if they are massive enough that their shape is dictated by self-gravity.

 

References

Brown, M. et al. 2006, Astrophysical Journal, 643, L61

Thomas, P. et al. 2005, Nature, 437, 224

check also the article  Dwarf Planets, plutoids and the Solar System today

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Solar Dynamics Observatory 2017-12-11T09:30:53Z
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