What is the term for astronomical objects outside the solar system that are smaller than dwarf planets?

What is the term for astronomical objects outside the solar system that are smaller than dwarf planets?

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Asteroids and comets only seem to refer to masses within the solar system, and it seems unclear whether planetesimal does.

Just add the prefix "exo-". At least exocomet and exomoon seem to be established.

I have to disagree with the accepted answer. The terms Planets, moons, asteroids, planetesimals are generic and are not implied to mean objects in the Solar system. So, these terms are completely correct when dealing with the generic objects.

When specifically emphasizing the fact that a certain objects is not in the Solar system, then you may want to add 'exo' for disambiguation, but this should be reserved for this purpose only.

A planet is a planet no matter where. Same with a house, which is a house no matter whether it's in your home town or elsewhere on the Earth.

Note in edit: My answer is based on the practice among professional astronomers and not on what some dictionaries say. I reckon the latter are somewhat behind the times, when all planets, asteroids etc. known were those in orbiting the Sun. Wikipedia is a great resource, but anybody can change its contents and, not surprisingly, some of its pages are quite biased.

Small body

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Small body, also called small solar system body (SSSB), any natural solar system object other than the Sun and the major planets and dwarf planets and their satellites (moons). The small bodies populate the solar system in vast numbers and include the mostly rocky asteroids, or minor planets, the predominantly icy comets, and the fragments of such bodies—commonly called meteoroids—over a continuum of sizes down to microscopic grains known as interplanetary dust particles or micrometeoroids. The term small body is sometimes restricted to objects that can be observed telescopically while still outside Earth’s atmosphere. In practice, that results in a lower limit of a few metres on the diameters of small bodies.

The use of meteoroid has expanded to describe any small chunk of matter in interplanetary space, especially one less than a few tens of metres in size. The term formerly was limited to small bodies that were near enough to Earth to eventually enter its atmosphere. In that narrower meaning, meteoroid is usually linked with the terms meteor, meteorite, or both. As a meteoroid travels through the atmosphere, friction heats it up, creating a glowing trail of hot ionized gases (plasma) called a meteor. If the object survives both its passage through the atmosphere and its subsequent impact on the ground, it is called a meteorite. Over 99 percent of meteorites are fragments of asteroids. A small group is known to be of lunar origin, and a second group is generally acknowledged to have come from Mars (see Mars: Meteorites from Mars). There is also reason to believe that some are fragments of the rocky remnants of comets, although that remains to be firmly established.

Why do we have the category "Dwarf Planets"?

Eris and Eris' moon, Dysnomia. © Photo by Francis Reddy

Pluto was originally considered one of the nine planets in our solar system. This theory was turned on its head with the discovery of other planetoids farther out that were as large or possibly even larger than Pluto.

The discovery of Eris, a dwarf planet the same size as Pluto, occurred in 2003. Eris triggered the debate about what was really a planet.

There was astronomical society meetings debating whether or not to add several new bodies to the formal list of planets or create a new category for bodies smaller than major planets but considered separate from asteroids. In order to avoid constantly amending the formal list of “planets“, the category of “dwarf planets” was created. The category of dwarf planets was officially adopted by the International Astronomical Union in 2006.

Pluto was demoted from planet to dwarf planet and put in the new category with recently discovered bodies.

Astronomical Terms

Aphelion: see Orbit.

Apogee: see Orbit.

Black hole: the theoretical end-product of the total gravitational collapse of a massive star or group of stars. Crushed even smaller than the incredibly dense neutron star, the black hole may become so dense that not even light can escape its gravitational field. In 1996, astronomers found strong evidence for a massive black hole at the center of the Milky Way. Recent evidence suggests that black holes are so common that they probably exist at the core of nearly all galaxies.

Conjunction: the alignment of two celestial objects at the same celestial longitude. Conjunction of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be in conjunction with the Sun when Saturn and Earth are aligned on opposite sides of the Sun.

Mercury and Venus, the two planets with orbits within Earth's orbit, have two positions of conjunction. Mercury, for example, is said to be in inferior conjunction when the Sun and Earth are aligned on opposite sides of Mercury. Mercury is in superior conjunction when Mercury and Earth are aligned on opposite sides of the Sun.

Dwarf planet: see Planet.

Elongation: the angular distance between two points in the sky as measured from a third point. The elongation of Mercury, for example, is the angular distance between Mercury and the Sun as measured from Earth. Planets whose orbits are outside Earth's can have elongations between 0 and 180. (When a planet's elongation is 0, it is at conjunction when it is 180, it is at opposition.) Because Mercury and Venus are within Earth's orbit, their greatest elongations measured from Earth are 28 and 47, respectively.

Galaxy: gas and millions of stars held together by gravity. All that you can see in the sky (with a very few exceptions) belongs to our galaxy?a system of roughly 200 billion stars. The exceptions you can see are other galaxies. Our own galaxy, the rim of which we see as the ?Milky Way,? is about 100,000 light-years in diameter and about 10,000 light-years in thickness. Its shape is roughly that of a thick lens more precisely, it is a spiral nebula, a term first used for other galaxies when they were discovered and before it was realized that these were separate and distinct galaxies. Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. In 2004, the Hubble Space Telescope and observers at the Keck Observatory in Hawaii discovered a new galaxy 13 billion light-years from Earth.

Neutron star: an extremely dense star with a powerful gravitational pull. Some neutron stars pulse radio waves into space as they spin these are known as pulsars.

Occultation: the eclipse of one celestial object by another. For example, a star is occulted when the Moon passes between it and Earth.

Opposition: the alignment of two celestial objects when their longitude differs by 180. Opposition of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be at opposition when Saturn and the Sun are aligned on opposite sides of Earth. Only the planets whose orbits lie outside Earth's can be in opposition to the Sun.

Orbit: the path traveled by an object in space. The term comes from the Latin orbis, which means ?circle? or ?disk,? and orbita, ?orbit.? Theoretically, there are four mathematical figures, or models, of possible orbits: two are open (hyperbola and parabola) and two are closed (ellipse and circle), but in reality all closed orbits are ellipses. Ellipses can be nearly circular, as are the orbits of most planets, or very elongated, as are the orbits of most comets, but the orbit revolves around a fixed, or focal, point. In our solar system, the Sun's gravitational pull keeps the planets in their elliptical orbits the planets hold their moons in place similarly. For planets, the point of the orbit closest to the Sun is the perihelion, and the point farthest from the Sun is the aphelion. For orbits around Earth, the point of closest proximity is the perigee the farthest point is the apogee. See also Retrograde.

Perigee: see Orbit.

Perihelion: see Orbit.

Planet: the International Astronomical Union (IAU) issued the definition for planet (from the Greek planetes, ?wanderers?) at their General Assembly in August 2006. A planet is a body that (a) is in orbit around the Sun, (b) is massive enough that its self-gravity gives it a nearly-spherical shape, and (c) has cleared the neighborhood around its orbit. A body that fulfills the first two criteria but not the third is a dwarf planet, provided that it (d) is not a satellite.

While the exact definition of ?clearing the neighborhood? was not established at press time, the eight planets from Mercury through Neptune have either assimilated or repulsed most other objects in their orbits, and each has more mass than the combined total of everything else in its area. The same cannot be said for Pluto, which has now been reclassified as a dwarf planet. There are currently eight planets and three dwarf planets recognized in the solar system, and more dwarf planets are expected to be admitted.

In 1994, Dr. Alexander Wolszcan, an astronomer at Pennsylvania State University, presented convincing evidence of the first known planets to exist outside our solar system. These particular extrasolar planets circle a pulsar, or exploded star, in the constellation Virgo.

In 1995, several of these extrasolar planets were discovered orbiting stars similar to our Sun. Swiss astronomers found the first extrasolar planet (HD 209458b, nicknamed ?Osiris?) to circle a normal Sun-like star. As of May 2006, 170 such planets have been discovered.

In Feb. 2004, using the Hubble Space Telescope, a team of scientists at the Institut d'Astrophysique de Paris announced that they had discovered oxygen and carbon in the atmosphere of ?Osiris.?

In Aug. 2004, NASA and the National Science Foundation announced the discovery of two new planets, the smallest yet found, about the size of Neptune. The discovery opens up the possibility of smaller, Earth-sized extrasolar planets.

In April 2005, a team of American and European astronomers reported that the first image of an extrasolar planet had been made. The planet is orbiting a brown dwarf near the constellation Hydra, 230 million light-years from Earth.

Pulsar: a celestial object, believed to be a rapidly spinning neutron star, that emits intense bursts of radio waves at regular intervals.

Quasar: ?quasi-stellar? object. Originally thought to be peculiar stars in our own galaxy, quasars are now believed to be the most remote objects in the universe.

Quasars emit tremendous amounts of light and microwave radiation. Although they are not much bigger than Earth's solar system, quasars pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars. It is believed that quasars are powered by massive black holes that suck up billions of stars.

Retrograde: describes the clockwise orbit or rotation of a planet or other celestial object, which is in the direction opposite to Earth and most celestial bodies. As viewed from a position in space north of the solar system (from some great distance above Earth's North Pole), all the planets revolve counterclockwise around the Sun, and all but Venus, Uranus, and Pluto rotate counterclockwise on their own axes. These three planets have retrograde motion.

Sometimes retrograde is also used to describe apparent backward motion as viewed from Earth. This motion happens when two objects rotate at different speeds around another fixed object. For example, the planet Mars appears to be retrograde when Earth overtakes and passes by it as they both move around the Sun.

Satellite (or moon): an object in orbit around a planet. Until the discovery of Jupiter's four main moons by Galileo Galilei, celestial objects in orbit around a planet were called moons. However, upon Galilei's discovery, Johannes Kepler (in a letter to Galileo) suggested satellite (from the Latin satelles, which means ?attendant?) as a general term for such objects. The word satellite is used interchangeably with moon, and astronomers speak and write about the moons of Neptune, Saturn, etc. The term satellite is also used to describe man-made devices of any size that are launched into orbit.

Small Solar System Objects: at the 2006 IAU General Assembly, solar system bodies not defined as planets, dwarf planets, or satellites were placed in this category. These include most asteroids, most Trans-Neptunian Objects, comets, and other small bodies.

Star: a celestial object consisting of intensely hot gases held together by gravity. Stars derive their energy from nuclear reactions going on in their interiors, generating heat and light. Stars are very large. Our Sun has a diameter of 865,400 mi?a comparatively small star.

A dwarf star is a small star that is of relatively low mass and average or below-average luminosity. The Sun is a yellow dwarf, which is in its main sequence, or prime of life. This means that nuclear reactions of hydrogen maintain its size and temperature. By contrast, a white dwarf is a star at the end of its life, with low luminosity, small size, and very high density.

A red giant is a star nearing the end of its life. When a star begins to lose hydrogen and burn helium instead, it gradually collapses, and its outer region begins to expand and cool. The light we see from these stars is red because of their cooler temperature. There are also red super giants, which are even more massive.

A brown dwarf lacks the mass to generate nuclear fusion like a true star, but it is also too massive and hot to be a planet. A brown dwarf usually cools into a dark, practically invisible object. The existence of brown dwarfs, also called failed stars, was confirmed in Nov. 1995 when astronomers at Palomar Observatory in California took the first photograph of this mysterious object.

Supernova: a celestial phenomenon in which a star explodes, releasing a great burst of light. There are two basic types of supernova. Type Ia happens when a white dwarf star draws large amounts of matter from a nearby star until it can no longer support itself and collapses. The second more well-known kind of supernova, type IIa, is the result of the collapse of a massive star. (Massive is a classification for a star that is at least eight times the size of our Sun.) Once the star's nuclear fuel is exhausted, if its core is heavy enough, the star will collapse in on itself, releasing a huge amount of energy (the supernova), which may be brighter than the star's host galaxy.

On Feb. 24, 1987, Canadian astronomer Ian Shelter at the Las Campanas Observatory in Chile discovered a supernova?an exploding star?from a photograph taken on Feb. 23 of the Large Magellanic Cloud, a galaxy some 160,000 light-years away from Earth. Astronomers believe that the dying star was Sanduleak ?69202, a 10-million-year-old blue supergiant.

Supernova 1987A was the closest and best-studied supernova in almost 400 years. One was previously observed by Johannes Kepler in 1604, four years before the telescope was invented.

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What’s in a name?

“I’m a little sceptical of the size measurement,” says Brown, as Pál’s readings show it could be between 1310 and 1610 km. “I think with the uncertainty it could easily be smaller than Makemake.”

As discoverers, Brown’s team have first dibs at naming the object. Originally they gave 2007 OR10 the unofficial nickname “Snow White” as it was thought it must be particularly bright to show up in the team’s survey, and was the “seventh dwarf” discovered. But later observations revealed it is actually one of the reddest objects in the solar system, likely due to methane ice on the surface.

As such, we need to find out just what makes 2007 OR10 special before we can give it a name, says Brown. “All of the other objects out there we’ve named have had some sort of appropriate name based on some feature,” he says. “But we will learn more soon, I have hope.”

Pál agrees the error bars on their measurement may end up making 2007 OR10 smaller, but it still deservers a name, even if it means getting a bit creative. “If you consider that there are more and more objects known by astronomers, it’s much harder to find some property in which it is dominant.” 2007 OR10 is one the darkest objects of its size, which combined with its redness could provide a few pointers, he says.

Journal reference: The Astronomical Journal, DOI: 10.3847/0004-6256/151/5/117

What is Solar System? Basics of Astronomy

Solar system is a gravitationally bound system that contain sun and other objects that orbit around sun, either directly or indirectly. The bodies orbiting around sun directly, include eight planets and there are moons that orbit indirectly. The solar system was formed 4.6 billion years ago by the gravitational collapse of a massive interstellar molecular cloud. The majority part of system ‘mass is possessed by sun, Jupiter. Inner planets: mercury, Venus, earth and mars are terrestrial planets that are made up of rock and metal. Outer planets are giant which are considerably massive than inner planets. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic. This all defines what is solar system.

Jupiter and Saturn

These are the gas giants that are made up of hydrogen and helium.

Uranus and Neptune

These are known as ice giants which are composed by the substances called volatiles like water, ammonia and methane. These volatiles have higher melting points than hydrogen and helium.

What are planets?

Planet is an astronomical object that is enough massive that it can get folded by its own gravity and is not massive that it can cause thermo-nuclear fusion. Planet rotates in its own axis (spinning) and revolves around sun too. It is in circular shape and has a core too.

A few properties of the planets in our solar system are given here.

  • Mercury: since it is near to sun so its surface temperature is 426.7 degree Celsius. However the opposite side to sun has temperature of 173 degree Celsius.
  • Venus: due to density of atmosphere, air pressure on its surface is 90 times more than that of earth, making it not suitable for life.
  • A planet of life, where we are living.
  • Mars: it has red color, it is made up of rock and experiences many wind storms.
  • Jupiter: it is largest of all planets.
  • Saturn: it contains many asteroids which cause rings surrounding it.
  • Uranus: its spinning axis is parallel to its orbit so seems to slide on axis.
  • Neptune: it is farthest planet.

Planets are categorized into 2 types.

  • Terrestrials: they are small with low density and rocky. They include Mercury, Venus, Earth, and Mars.
  • Giant: they are large in size and also known as gas giants including Jupiter, Saturn, Uranus, and Neptune.

What is star?

Star is a massive ball made of plasma that emits light throughout the space. Size, brightness, color, temperature and compositions are the points, on which properties of star can be determined.

Stars are formed within the dust clouds and are scattered in most galaxies. The Orion Nebula is a familiar example of such a dust cloud. Now the question arises that how they shine? So let’s understand it, well it is too interesting…

All stars, including sun are hot balloons of hot glowing plasma which is held together by its own gravity. Gravity of star is too intense and they are continuously crushing themselves inward. Now due to the gravitational friction, interior of star heats up. A star like sun has 5800 K temperature at its surface but temperature of core can be 15 million K.

Due to this high temperature in core, nuclear fusion takes place where hydrogen atoms gets fused into helium atoms. As a result, an enormous amount of energy gets released in form of gamma rays. Against the gravitational contraction, gamma rays push the star outward as they are trapped inside star. This is the reason why stars don’t contract. Now these gamma rays jump outside after being absorbed by an atom. It happens many times in a second and a single photon takes 100,000 years to reach the surface of star from its core. So as they reach on surface, they have lost some of their energy, becoming visible photons instead of gamma rays, from which it was started. After being emitted from star, they travel in straight line in space and they can travel forever if they don’t run into anything.

How moon shines?

Moon is a body that is smaller than planets and revolves around planets. Moon is the natural satellite of our planet as earth has 1 moon, mars has 2 and Jupiter has 67 moons. It is the closest celestial body to earth. It is 239, 000 miles away from earth. It completes its one round around earth in 27.3 days. Since there is no atmosphere of moon so eve after years the dust present on surface of moon is in same position as before. Its size is ¼ of earth. Due to small mass and size, it has gravity that is 1/6 of earth’s gravity. That’s why you can jump on moon so high, because your body has only mass, it has no weight there. (W = mg) It is not a planet as it does not revolves around sun. It looks brighter in night because of the sun’s light reflected from it.

What is a satellite?

Satellite is defined as an object which is launched into space to orbit or can be defined as it is a moon, machine or planet which orbits a planet or star. As earth revolves around sun so it is a satellite, similarly moon revolves around earth so it is also. Both of them are natural satellites. However many artificial satellites have also been sent into space that are revolving around earth. The functions of these satellites can be different, some of them are used to take pictures of that planet which helps meteorologists to predict about weather and to monitor the cloud patterns. Pictures of other planets, black hole, dark matter and sun help scientist to understand solar system in better way.

The path followed by a satellite is called orbit. Orbits can be at different altitudes and may have different rotation and orientation relative to earth. Satellites at higher altitudes of 36,000 Km facing same side of earth all the time are called geostationary satellites. Their speed is same as the rotation of earth so it seems to be stationary. This helps them to collect information over specific areas, continuously. Usually communication and weather satellites have this type of orbits.

What is galaxy?

A galaxy is a huge collection of dust particles, gas and billions of stars and their solar systems. The thing that keeps galaxy bound, is its gravity. Our galaxy, Milky Way also has a super massive black hole in its core that causes gravity effect. In night, when you see stars on sky, then actually you see them in Milky Way. There are countless galaxies around us. You can get estimate of total number of galaxies by knowing that Hubble Space Telescope has seen 10,000 galaxies in a small patch of space in 12 days. Our galaxy is spiral shaped and it has curved arms which makes its look like pinwheel. Other galaxies are smooth and ovel shaped, called elliptical galaxies. Others are irregular shaped and look like blobs. Galaxies look bright, due to the brightness of the stars inside it.

What is light year?

Light year is the unit of distance and is defined as the distance travelled by light in one year. Since light travels at speed of 300,000 km/s, so in 1 year it can travel 10 trillion Km. So 1 light year = 9,500,000,000,000 Km.

Actually this unit is used for very very large distances, because we are not discussing about cities or countries, we are discussing planets and stars. We appear to define distances in our solar system in terms of an Astronomical Unit (AU). The AU is known as the Earth’s mean distance from the Sun. It is about 150 million km (93 million miles) away. It may be said that Mercury is about 1/3 of an AU from the Earth, and that Pluto is about 40 AU from the Earth.

Asteroid belt definition

It lies between Mars and Jupiter. Asteroid belt is defined as a torus shaped (a surface that is generated by revolving a circle in 3D in the axis that is coplanar with circle) area in solar system which is located b/w the orbits of mars and Jupiter. This region contains solid and irregular shaped bodies that are smaller than planets, called asteroids.

Define kuiper belt

Kuiper belt, also known as edgeworth- Kuiper belt, is a disc in outer solar system which is extended from the orbit of Neptune to 50 Au of sun. It is similar to asteroid belt but it is more wider, 20 times wider and 20 to 200 times massive. It also consists of small bodies like asteroid belt. Since many asteroids are composed of rock and metal, so most objects of Kuiper belt are made of large frozen volatiles (named as ices) including methane, ammonia and water.

What are dwarf planets?

These are the massive objects that are not satellites and does not have any specific region in space, unlike other planets. It is in direct orbit of sun. They have sufficient mass for their self-gravity to overcome the forces of rigid bodies. Currently, five dwarf planets have been found. Ceres is located in the asteroid belt while remaining are present in or near to the Kuiper belt.

What is solar wind?

It is defined as a stream of charged particles that are released from the upper atmosphere of the sun, which is known as corona (it is the outer layer of sun, when it stretches far in space then its particles reach on the surface of earth). The stream with which solar wind is made up of, consists of electrons, protons and alpha particles having kinetic energy ranging from 0.5 to 10 KeV. Composition of solar wind also includes heavy ions and atomic nuclei like carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur and iron, in a little amount.

Define comet

Comets are snowballs made of frozen gases, rock and dust and they orbit around sun. Initially its size is smaller, when it is in frozen state, but when it reaches near sun, it releases gas or dust due to the heat of sun. These emitted dust particles and gases made a tail that stretches away from sun for millions of miles. These icy bodies are also known as snowy dirtballs or dirty snowballs. They are composed of dust, ice, carbon dioxide, ammonia, methane and many more.


Nebula is a giant cloud of gas and dust present in space. Its plural is nebulae. Some nebula come from the dust and gas which is resulted by the explosion of a dying star. Nebula is also known as star nurseries as many stars are formed over here. They are found in the space b/w stars (interstellar space). The closest nebula to our earth is named as “Helix Nebula” which is approximately 700 light years away from us. Mean it will take 700 years to bring you there, if you can move with speed of light.

What is supernova?

A supernova is the largest ever observed eruption by humans. Each blast is a star’s incredibly bright, super powerful blast. There are two types of supernova.

One of them is caused by the death of a massive star. Massive stars have huge amount of fuels in their core/center which causes tons of energy, that’s why center is too hot. This heat produces pressure and this pressure prevents star o collapse. There is 2 types of force inside a star, both are opposite. Star’s gravity tries to make it smaller by squeezing it, as much possible, but the nuclear fuel in core, creates too pressure that produces strong outward pressure.

When fuel ends in a massive star then it cools off due to which pressure also drops and gravity wins and star is dead.

Second type of supernova occurs when two stars orbit one another and one of them is earth sized dwarf.

Supernova event is so bright that they outshine entire galaxy for few days or even months and are visible across whole universe.

What are black holes?

Black hole is defined as a place in space where grip of gravity is so strong that even light can’t come out of it. The reason of strong gravity is that matter has been squeezed into tiny space which happens during death of star. Black holes are invisible as no light can come out of them.

They can be of different sizes. Small black holes can be of the size of an atom, since these are very tiny but have mass (amount of matter in object) equivalent to large sized mountain.

Second type of black holes is termed as “stellar”. Their mass can be upto 20 times of sun or even more. Largest black hole is called supermassive with the mass of 1 million suns together. Research tells that our galaxy, Milky Way has a super massive black hole at its center named as Sagittarius A.

Black hole does not vender in space to eat stars and planets. Moreover, our earth will not fall into black hole, because there is no black hole near o our earth in solar system.

What is dark matter?

Dark matter is a substance with which 27 percent of universe is made up. It is invisible and it can’t emit, absorb or reflect any electromagnetic radiation like x rays and radio waves even light. It can interact with ordinary matter. This is fact due to which astronomers are able to make maps of its distribution even it is invisible.

Falling star or meteoroids

A body moving in solar system that changes its identity and becomes meteor when it enters earth’s atmosphere. When a meteoroid enters in earth’s atmosphere at speed of 20 km/s then streak of light is produced due to aerodynamic heating. This streak of light is seen for both the glowing object and tail of glowing particles that it leaves. This phenomenon is called meteor or shooting star. If many meteors appears after seconds or minutes and appear to get originated from same point in sky then it is called meteor shower. Now if the object bears ablation and falls or touches ground then it is called meteorite. Falling star is nothing to do with star, it is just a term used to identify the meteor on sky.

Planets of the Solar System and the International Astronomical Union Definition

On August 24, 2006, the International Astronomical Union (IAU), having evaluated proposals submitted at its plenary meeting, resolved to officially adopt definitions for planets and other bodies in the Solar System, according to which:

  1. A planet is an object: a) in orbit around the Sun, b) that 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 neighborhood around its orbit.
  2. A dwarf planet is an object: a) in orbit around the Sun, b) that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, c) that has not cleared the neighborhood around its orbit of other objects, d) is not a satellite of another planet.
  3. Any other objects, except satellites, must be called small Solar System bodies, including most trans-Neptunian objects.

Therefore, within the Solar System there are only eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune). Pluto, Ceres and Eris (discovered in 2005, previously known as 2003 UB313 and the cause of the definition change) are now dwarf planets. Subsequently Haumea and Makemake were also included in this category. In particular, Pluto lost its status as a planet on account of several factors, including: its orbit, which is highly elliptical and outside the ecliptic plane the presence of a companion, Caronte (discovered in 1978), with a size and mass of the same order of magnitude, and the company of a further four smaller-mass objects (Nix and Hidra, glimpsed for the first time in 2005 Cerbero, announced in 2011 and Estigia, seen one year later). Thus, Pluto fails to meet condition 2c of the official IAU definition.

The planets of the Solar System according to the new IAU definition and three dwarf planets. Credits IAU.

Meanwhile, Pluto, Eris, Makemake and Haumea, found within the confines of the Solar System, belong to a new category of objects, plutoids, tens of which are expected to be identified over the next few years. There is another category that includes trans-Neptunian objects. These are objects that orbit the Sun at a greater distance than Neptune (technically, the semi-major axis of the TNO will be more distant than that of Neptune). More than 1,000 such objects have been identified in our planetary system, of very diverse sizes. Eris has a satellite, also discovered in 2005, called Disnomia, named after the daughter of the Greek goddess that lends her name to the dwarf planet. Haumea, named after the patron goddess of Hawaii, has two satellites, Hi’iaka and Namaka, who are the goddess’ daughters.

The more than 1,000 exoplanets that have been confirmed discovered since 1995 (and earlier if we include those orbiting neutron stars) are not subject to this definition. For these the IAU agreed a provisional definition in 2003, which many specialists found unsatisfactory and is subject to review. Thus, an exoplanet is an object that:

Orbits around a star or remnant of a star (white dwarf, neutron stars) and has a mass of less than 13 Jupiter masses. This means they cannot fuse deuterium, a hydrogen isotope, nor any other chemical element. Therefore, they do not produce energy from this source.

Artistic illustration of four dwarf planets located beyond Neptune’s orbit (Eris, Pluto, Makemake and Haumea), along with their satellites, and another two trans-Neptunian objects (Sedna and Quaoar). The sizes are proportional, with the Earth included for comparative purposes.

Substellar objects, with greater masses than exoplanets but less than approximately 8% of the mass of the Sun, are called brown dwarves. These are characterized by the absence of hydrogen fusion.

Isolated planetary mass objects (IPMOs, planemos, xebarcos, or orions), with a mass below the threshold of 13 Jupiter masses, must be referred to as sub-brown dwarfs or any other suitable name, except for that of planet.

This text is an updated summary of the book Visiones de Gaia: la Tierra desde el espacio, written by David Barrado and published by INTA (National Aerospace Technology Institute “Esteban Terradas”) in 2011. To access the complete book in Spanish, click here.

Mike Brown's Planets

A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

What is a dwarf planet?

Now that the IAU has officially declared the fifth dwarf planet (in order of size: Eris, Pluto, Makemake, Haumea, Ceres), we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.

Now is a good time, then, to remind ourselves what a dwarf planet really is.

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet. The official definition is that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction.

Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object which is geologically dead and one which might have interesting processes worthy of study.

[Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.]

So how many dwarf planets are there? Five, of course. The IAU says so.

But let's ask the more scientifically interesting question: how many (non-planet) objects in the solar system are large enough to be round due to their own gravitational pull?

Well, no. Here is where the IAU and reality part ways.

There are many more objects that precisely fit the definition of dwarf planet but that the IAU chosen not to recognize. But if the category of dwarf planet is important, then it is the reality that is important, not the official list. So let's examine reality.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round.

The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. We might be a bit off in one direction or another, but 400 km seems like a good estimate.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of now, about 50 objects this size or larger are known in the Kuiper belt (including, of course, Eris, Pluto, Makemake, and Haumea). Our best estimate is that a complete survey of the Kuiper belt would double this number, so there are roughly 100 dwarf planets in the Kuiper belt, of which 50 are currently known.

The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

Does it matter how many dwarf planets we say there are?

I think the answer is "yes." If you believe that there are only 4 dwarf planets in the Kuiper belt then you place an oversized importance on those 4 objects and you get an exceedingly warped picture of what the outer solar is like. The important thing about the Kuiper belt is that beyond Neptune there are many many many objects with hundreds being large enough to be round. The four "IAU Dwarf Planets" in the outer solar system are all fascinating objects -- hey! I discovered 3 of them, I must think there are at least a little interest -- but it would be a gross exaggeration to think of them as the only objects, or even the only important objects, in the fascinating region of space beyond Neptune.

Astronomers discover coolest sub-stellar body outside our solar system

An international team, led by astronomers at the University of Hertfordshire, has found what may be the coolest sub-stellar body ever found outside our own solar system.

Using the United Kingdom Infrared Telescope (UKIRT) in Hawaii, the team discovered an object which is technically known as a brown dwarf.

What has excited astronomers are its very peculiar colours, which actually make it appear either very blue or very red, depending on which part of the spectrum is used to look at it.

The object is known as SDSS1416+13B and it is in a wide orbit around a somewhat brighter and warmer brown dwarf, SDSS1416+13A. The brighter member of the pair was detected in visible light by the Sloan Digital Sky Survey. By contrast, SDSS1416+13B is only seen in infrared light. The pair is located between 15 and 50 light years from the solar system, which is quite close in astronomical terms.

"This looks like being the fourth time in three years that the UKIRT has discovered made a record breaking discovery of the coolest known brown dwarf, with an estimated temperature not far above 200 degrees Celsius," said Dr Philip Lucas at the University of Hertfordshire's School of Physics, Astronomy and Mathematics.

"We have to be a bit careful about this one because its colours are so different than anything seen before that we don't really understand it yet. Even if it turns out that the low temperature is not quite record breaking, the colours are so extreme that this object will keep a lot of physicists busy trying to explain it."

SDSS1416+13B was first noticed by Dr Ben Burningham of the University of Hertfordshire as part of a dedicated search for cool brown dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS). The object appeared far bluer at near infrared wavelengths than any brown dwarf seen before. A near infrared spectrum taken with the Japanese Subaru Telescope in Hawaii showed that it is a type of brown dwarf called a T dwarf, which has a lot of methane in its atmosphere, but with peculiar features including a big gap at certain wavelengths.

Dr Burningham soon noticed that a previously observed brighter star (SDSS1416+13A) which appears close by in the UKIDSS discovery image was also a brown dwarf. Team member Dr Sandy Leggett, of the Gemini Observatory, then used the orbiting Spitzer Space Telescope to investigate SDSS1416+13B at longer wavelengths. She measured its colour at mid-infrared wavelengths, which are thought to be the most reliable indicator of temperature, and found that it is the reddest known brown dwarf at these wavelengths by some margin. Comparison with theoretical models of the brown dwarf atmospheres then provided a temperature estimate of about 500 Kelvin (227 degrees Celsius).

"The fact that it is a binary companion to a warmer brown dwarf that also has an unusual spectrum is helping us to fill in some gaps in our understanding," says Dr Burningham. "It seems likely that both brown dwarfs are somewhat poor in heavy elements. This can be explained if they are very old, which also fits with the very low temperature of the faint companion."

Too small to be stars, brown dwarfs have masses smaller than stars but larger than gas giant planets like Jupiter. Due to their low temperature these objects are very faint in visible light, and are detected by their glow at infrared wavelengths. They were originally dubbed "brown dwarfs" long before any were actually discovered, to describe the idea of bodies that were cooler, fainter and redder than red dwarf stars, with the colour brown representing the mix of red and black.

The team's findings have been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

Story Source:

Materials provided by University of Hertfordshire. Note: Content may be edited for style and length.

Watch the video: Terms used in astronomical Survey (July 2022).


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