# Up to 384 minor planets (including Pluto) qualify as planets?

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In 2006, the International Astronomical Union redefined the definition of planet in order to exclude Pluto, Eris, and several other objects whose category was disputed. This new definition of a planet featured 3 criteria:

1. A planet must be massive enough to achieve hydrostatic equilibrum and become ellipsoidal.
2. A planet must orbit a star or brown dwarf (with the exception of planemo).
3. A planet must be massive enough to clear any nearby objects.

Pluto and the other objects apparently failed to meet the 3rd criteria, as they had not cleared the nearby Kuiper belt objects, so they were reclassified under the term "dwarf planets".

But what if Pluto actually has met the third criteria? It's probably well-known by now that Pluto has 5 moons. The largest moon Charon is over half the size of Pluto, and is frequently classed as a dwarf planet in its own right. The other 4 minor moons Nix, Styx, Hydra, and Kerberos, are much smaller then Charon, and are similar to the many asteroid-like moons found around gas planets.

The leading theory for how these smaller moons formed is that they are leftover debris from a collision between Pluto and another object, but it's just as likely that they were Kuiper belt objects that were captured by Pluto (just like the moons of Jupiter and Saturn). If they were captured, this means that Pluto has technically cleared 4 objects from its neighborhood, and so it meets the 3rd criteria.

The same logic can be applied to Eris, 2007 OR10, and the 400 other dwarf planets known to host moons. So should we have 392 planets, keep the current definition, or scrap or redefine the term "planet" entirely?

That actual IAU Resolution B5 adopted at the IAU General Assembly in 2006 states:

(1) 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.

The last part being the neighbourhood around its (whole) orbit is the important part; it's not enough for Pluto to dominate its family of moons (and this last part is debatable as Pluto and Charon orbit a common barycenter (center of mass) which is between Pluto and Charon since Charon is so massive, compared to Pluto).

Since Pluto orbits in the Trans-Neptunian region with a similar orbit to other bodies, some of which are similar or larger in size and mass (Eris), and cross the orbit of Neptune, it can't really be said have to "cleared the neighbourhood around its orbit". Indeed Pluto is the prototype of the Plutinos; object which have a 3:2 mean motion resonance with Neptune (orbiting twice for every three orbits Neptune makes).

No, Pluto is not a planet by that definition. However, that definition has a serious issue: Earth is also not a planet. There are objects that have not been cleared from the Earth's orbit. (see https://www.nationalgeographic.com/news/2011/7/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/)

## 139 Minor Planets Found in our Solar System

Astronomers have discovered 139 new minor planets orbiting the sun beyond Neptune by searching through data from the Dark Energy Survey. The new method for spotting small worlds is expected to reveal many thousands of distant objects in coming years — meaning these first hundred or so are likely just the tip of the iceberg.

Taken together, the newfound distant objects, as well as those to come, could resolve one of the most fascinating questions of modern astronomy: Is there a massive and mysterious world called Planet Nine lurking in the outskirts of our solar system ?

## Comets

Comets, like asteroids, are debris left over from the solar nebula and the formation of the Solar System. While asteroids are rocky bodies found (for the most part) in the Asteroid Belt between Mars and Jupiter, comets are mainly icy bodies that are found (for the most part) in the outer reaches of the Solar System in the Kuiper Belt, beginning just beyond the orbit of Neptune, and in the spherical Oort Cloud, far beyond at 50,000 AU, which effectively defines the edge of the Solar System. Most cometary bodies never come into the inner Solar System. Occasionally, however, a comet&rsquos orbit is altered by some gravitational encounter and it zooms towards the Sun. Its orbit typically has an eccentricity of e=99%, a semi-major axis of a=50,000 AU, and an orbital period of P

10$^<6>$ years. A few comets that enter the inner reaches of the Solar System in this way are redirected into shorter orbits and become &ldquoperiodic&rdquo comets the most famous of these is Halley&rsquos Comet with an orbital period P=76 years. Below are some photos from the last passage of Comet Halley near Earth in 1986. We now have close-up images of several comets from robotic probes: they are irregularly shaped, loosly packed bodies of water ice, rocks, and dust.

Images of Comet Halley. Image credits: NASA/W. Liller (left). Halley Multicolor Camera Team, Giotto Project, ESA (right).

In summer 2011, NASA&rsquos Dawn spacecraft visited Vesta, one of the largest asteroids in the Asteroid Belt. See video overviews of this mission here, here and here.

Amazingly, astronomers witnessed the collision of a comet with Jupiter in 1994! The comet was broken into fragments by Jupiter&rsquos gravitational tidal forces on its first close passage by the planet in July 1992 and impacted the planet on its next orbit over July 16 to 22, 1994. These impacts resulted in huge disruptions to the atmosphere of Jupiter which were observed from Earth-based observatories, the Hubble Space Telescope, and the Galileo spacecraft, then on its way to Jupter. Here are pictures before and after the collision taken by the Hubble Space Telescope.

Images from Hubble Space Telescope of Jupiter before and after impacts by fragments of Comet Shoemaker-Levy 9. Image credits: NASA/HST.

## The five dwarf minor planets in Astrology and their meanings:

### Makemake

Makemake is the second dwarf planet to be officially named after it was discovered on March 31, 2005, by Michael E. Brown, Chad Trujillo, and David L. Rabinowitz. The name "Makemake" refers to apa Nui, the creation god of Easter Island.

#Makemake and its moon (circled) imaged by @Alex_Parker using the Hubble Space Telescope in 2018, over a period of 2 months. Not much is known about the moon's physical and orbital properties as there haven't been any publications yet. pic.twitter.com/ChbP85dO7M

— Nrco0e (@nrco0e) June 16, 2020

Makemake symbolizes a connection to environmental wisdom and relates to a love of nature and the beauty around you. It also correlates to environmental activism and protecting the Earth.

When Makemake is activated in your natal chart, you will be better able to manifest your desires through the power of your mind and thoughts. Visualizations, positive thinking, and the power of thought are all examples of active Makemake.

If Makemake is on your natal chart, you have a natural ability to manifest what you want into reality, simply by setting your mind to it and visualizing your desires. When the planet crosses your Sun, Moon, Mercury, or Venus this power is amplified and heightened.

### Quaoar

Quaoar is named after the creation force of the Native American Tongva tribe that originated near Los Angeles. It was discovered by Mike Brown and Chadwick Trujillo on June 4, 2002.

It is very important and applicable in people’s daily lives. It emphasizes the sacredness of life and the natural order of things (not man’s law).

Similarly to Makemake, if Quaoar is on your natal chart, you have that natural ability to manifest, visualize, and think your desires into being.

Your power of thought is naturally stronger than others. Additionally, when the planet crosses your Sun, Moon, Mercury, or Venus this power is amplified and heightened.

### Varuna

Varuna was discovered November 28, 2000, by Robert S. McMillan and has similar traits to Makemake and Quaoar. In Indian mythology, Varuna is the all-knowing creator god. He is merciful and just known as a protector of people. He is said to control the order of the universe and the natural world.

Varuna is all about nature taking its natural course and letting what needs to happen occur.

Man-made rules and facts hold no place in the face of cosmic law and the will of the universe. Varuna can instill ethics within people and encourage people to turn away from the laws of man that conflict with nature.

On a natal chart, it is similar to Makemake and Quaoar, as it shows a gift for visualizations and thought to achieve desired outcomes in the world.

Again, when the planet crosses your Sun, Moon, Mercury, or Venus this power is more prominent.

### Sedna

Sedna is named for the Inuit goddess of the sea who is said to live at the bottom of the Arctic Ocean. Sedna was discovered by Michael, E. Brown, Chadwick, Has. Trujillo and David L. Rabinowitz on November 14, 2003.

It is activated when you are dreaming of a better life or a better world where your hopes and dreams are true.

This causes you to follow a path of false promises to try and achieve this desire for something different and ultimately leaves you in a worse position than when you started. It primarily deals with naivety.

Discovered January 5, 2005, by Michael E. Brown, Chad Trujillo, David L. Rabinowitz, Eris is a minor dwarf planet named for the Greek goddess of chaos.

Strong and self-reliant, yet full of resentment and bitterness, Eris is activated when you feel left out. It can be anything from a major life event to a small snub, but Eris can make you frustrated and annoyed easily when you are left out or feel resentful.

in 2005 astronomers discovered a new dwarf planet — Eris. astronomers found it to be larger and more significant than pluto. this alludes to this concept in a relationship where the othwr partner has found someone new for them, that’s caused them to withdraw their other relation

— the rasgullahification of yoongi (@TAEMOR0US) February 24, 2021

To counter this bitterness, it is important to create your own social circles and groups where you are not waiting on invitations from others you control when you are with others. Being the leader of your own circles is vital.

On a natal chart, it can indicate how rebellious you are and where this will appear in your life.

## What's a Planet?

By: Richard Tresch Fienberg August 16, 2006 0

### Get Articles like this sent to your inbox

This week our solar system has nine planets. Next week, if astronomers approve a new definition of the word "planet," there will be 12 — with more to come. Newcomers to the list include Ceres, the largest asteroid Charon, Pluto's largest moon and 2003 UB313, an icy body more than twice as far from the Sun as Pluto and a little bigger (and not yet graced with an official name).

If the International Astronomical Union approves a proposed new definition of the word "planet," our solar system will include at least 12 of them, including the formerly minor planet Ceres, Pluto's moon Charon, and the soon-to-be-renamed (thankfully) 2003 UB313.

International Astronomical Union / Martin Kornmesser

International Astronomical Union (IAU), the arbiter of solar-system nomenclature since its inception in 1919. Here's the actual wording: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

The new scheme includes eight so-called classical planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These are large objects in nearly circular orbits close to the ecliptic plane (that is, the plane of Earth's orbit). Planets smaller than Mercury — including Ceres, Pluto, Charon, and 2003 UB313 — are collectively referred to as dwarf planets. Pluto becomes the prototype of a new class of planets called plutons, small objects with orbital periods longer than 200 years and highly elongated paths tipped steeply with respect to the ecliptic.

If you're already getting confused, especially about Charon, perhaps this footnote will help: "For two or more objects comprising a multiple-object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycenter [center of mass] resides outside the primary. Secondary objects not satisfying these criteria are 'satellites.' Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet" with two tiny moons, Nix and Hydra, discovered last year. Pluto-Charon is the only known pair in the solar system whose center of mass lies in free space between the two objects. Even though some moons are bigger than some planets (for example, Jupiter's Ganymede and Saturn's Titan), they're still just satellites, not planets, because the center of mass of the system lies within the parent planet.

The proposed new planets 2003 UB313, Charon, and Ceres are much smaller than Earth (seen an right), but still massive enough for gravity to pul them into spherical shapes.

## Up to 384 minor planets (including Pluto) qualify as planets? - Astronomy

Any definition of a planet must be recognized as subjective to some degree: distinguishing between small planets and large minor bodies is to some extent arbitrary. Astronomers have not had to deal with this continuum before, but now they must. Further, any planet definition is in some sense just a name. Pluto is the same physical object regardless of which category we put it in.

Despite the limitations on a planet definition, it is still desirable. Such groupings assist us in looking at general characteristics of objects. Astronomers deal with large numbers of objects: currently, astronomers have assigned designations to hundreds of moons, thousands of comets, hundreds of thousands of asteroids, and millions of stars. Astronomers long ago agreed to give the IAU certain roles in regard to nomenclature, such as overseeing the naming of small solar system objects.

• A clear scientific basis: we should strive for logical scientific criteria to distinguish planets from non-planets, so as to have a term useful to scientists.
• Generalizable to other planetary systems: we are already learning about planets beyond our solar system, including planets very different from those we are familiar with. A planet definition should be general enough to apply, in a meaningful way, to these objects.
• Easy to apply: it would be helpful if the criteria do not depend on characteristics that may not be known for years or decades after an object is discovered.
• It is scientifically ambiguous. Pluto along with hundreds of other known TNOs cross the orbit of Neptune, so Neptune fails this definition. Thousands of known asteroids cross the Earth's orbit, so the Earth is apparently not a planet either. Jupiter even shares its orbit with thousands of known Trojan asteroids.
• It may not be applicable in other planetary systems. Many of the Jupiter-sized planets known are in highly eccentric orbits. Should their neighborhoods prove to be "uncleared", disqualification of objects larger than Jupiter will erode the significance of the definition. (Such extrasolar planets are a profound objection to one proposal, that planents be in near-circular orbits.)
• It is very difficult to apply, since it requires knowledge about smaller bodies in the neighborhood of the object in question. It took 62 years to begin identifying objects in the neighborhood of Pluto's orbit. Suppose Pluto was missed in those photographs taken in 1930, but that 2003 UB313 Eris was not missed in the photographs taken of it in 1954: Eris, in an orbit well beyond Neptune's, would have apparently qualified as a planet. For extrasolar planets as well, such knowledge may be generations away.

A criterion of similar spirit but more scientifically robust than the IAU "cleared neighourhood" criterion is that a planet be the dominant object gravitationally in its neighborhood. This would clarifies why Neptune is a planet and Pluto is not: Pluto's orbit is a consequence of resonances driven by perturbations from Neptune. But in some sense the inner planets are dominated by Jupiter the Earth's "clear" neighborhood is partly due to Jupiter. This definition remains difficult to apply in extrasolar planetary systems, dependent on observations decades away.

The definition adopted by the IAU was hastily offered at the end of the 2006 conference (after most attendees had departed) to replace the controversial definition produced by an IAU-designated committee after lengthy deliberation. In general, this definition used only parts (a) and (b) of the adopted defintion. By counting any Sun-orbiting body nearly in hydrostatic equilibrium, it would tend to include asteroids and TNOs down to diameters of 300-800 km. This would likely have included 2-4 asteroids and 10-40 known TNOs, even apart from yet-to-be-discovered TNOs. The "inclusive" nature of the original proposal was its downfall. The significance is clearly that it did not capture the essence of what astronomers have in mind with the term "planet".

There are significant differences between rocky or icy bodies with sufficient self-gravity to be round, and the larger objects traditionally counted as planets--perhaps including Pluto. Basri and Brown recently reviewed several such differences. Around a size of 3000 km, gravitational energy is sufficient to significantly modify the object's internal chemistry. A bit larger and solid state convection becomes important. Internal pressures significantly compress the internal material of a rocky body larger than 6000 km diameter, or an icy body larger than 1000 km diameter. Gravitational differentiation into a denser core and less dense mantle/crust occurs for objects as small as Pluto, perhaps as small as 400 km. Ice-rock bodies like Pluto and even smaller may have subsurface oceans, according to some models.

Pluto is the best studied of objects that some would lump together as less than planets. These studies show a variety of phenomena more associated with planets than with small solar system bodies. Pluto has an atmosphere, apparently particular to when Pluto is near perihelion, but at which time it drives surface changes: frost deposits form and evaporate over large regions. When Pluto is finally examined close up in 2014, astronomers would not be surprised to find similarities to Neptune's moon Triton, which shows geologic activity including active geysers or cryovolcanoes.

The manner in which the new IAU definition has been implemented is arguably clumsy. Following opposition to the committee proposal, the IAU instead hastily adopted a less than fully robust definition that excluded Pluto. Within weeks, after 76 years as a planet, Pluto was relegated to minor planet number 134,340, sandwiched in the MPC list between two main belt asteroids each smaller than New York's Central Park. The newly created "dwarf planet" category, explicitly not a planet despite the grammatically construction, in practice overlaps with minor planets since the distinction between dwarf planets and small solar system bodies is difficult to apply and not particularly useful at this time, it is likely this term will not make it into the astronomical lexicon.

This comes to a defect in the IAU's adoption of a planet definition: it is a term, not a name, and was imposed by a minority of the scientific community. The scientific community has established usage of the term "planet", and in fact many in the planetary science community have signed a petition rejection the new IAU definition and promised to produce a better one.

I've been asked what definition I would offer, given my issues with the IAU definition. I concede that no definition is perfect, and that it will likely prove necessary to revisit the issue in the near future. All things considered, I would suggest simply using a size threshold to discriminate between planets and smaller solar system bodies. A diameter cutoff of 2000 km would count Pluto and Eris as planets a cutoff of 1,300 km would include three more TNOs that already appear distinct from the other 1,122 smaller ones (there is in fact a fundamental argument in favor of this smaller threshold). This threshold is approximately the size range where some of the structural and surface pheonomena we identify with planets start to come into play.

A threshold-based definition has the disadvantage of not having as clear a scientific premise as one based on hydrodynamic equilibruim or cleared orbital neighborhood. It is significantly more straightforward to apply, comes closer to the spirit of what astronomers are trying to capture with the term planet, and is generalizable to other planetary systems. It is also somewhat intended as a stop-gap measure: we are attempting to impose a definition on objects we have yet to examine as more than points of light. The adoption of a definition excluding Pluto is ironic given that a NASA mission is en route to Pluto to provide our first closeup look in 2014 this look might have a bearing on how we think about this object, and we may want to keep an open mind.

Related information on this web site:

© 2006 by Wm. Robert Johnston.

## Researchers find new minor planets beyond Neptune

Voyager 2 took this picture of Neptune in 1989.

Using data from the Dark Energy Survey (DES), researchers have found more than 300 trans-Neptunian objects (TNOs), minor planets located in the far reaches of the solar system, including more than 100 new discoveries. Published in The Astrophysical Journal Supplement Series, the study also describes a new approach for finding similar types of objects and could aid future searches for the hypothetical Planet Nine and other undiscovered planets. The work was led by graduate student Pedro Bernardinelli and professors Gary Bernstein and Masao Sako.

The goal of DES, which completed six years of data collection in January, is to understand the nature of dark energy by collecting high-precision images of the southern sky. While DES wasn't specifically designed with TNOs in mind, its breadth and depth of coverage made it particularly adept at finding new objects beyond Neptune. "The number of TNOs you can find depends on how much of the sky you look at and what's the faintest thing you can find," says Bernstein.

Because DES was designed to study galaxies and supernovas, the researchers had to develop a new way to track movement. Dedicated TNO surveys take measurements as frequently as every hour or two, which allows researchers to more easily track their movements. "Dedicated TNO surveys have a way of seeing the object move, and it's easy to track them down," says Bernardinelli. "One of the key things we did in this paper was figure out a way to recover those movements."

Using the first four years of DES data, Bernardinelli started with a dataset of 7 billion "dots," all of the possible objects detected by the software that were above the image's background levels. He then removed any objects that were present on multiple nights—things like stars, galaxies, and supernova—to build a "transient" list of 22 million objects before commencing a massive game of "connect the dots," looking for nearby pairs or triplets of detected objects to help determine where the object would appear on subsequent nights.

With the 7 billion dots whittled down to a list of around 400 candidates that were seen over at least six nights of observation, the researchers then had to verify their results. "We have this list of candidates, and then we have to make sure that our candidates are actually real things," Bernardinelli says.

To filter their list of candidates down to actual TNOs, the researchers went back to the original dataset to see if they could find more images of the object in question. "Say we found something on six different nights," Bernstein says. "For TNOs that are there, we actually pointed at them for 25 different nights. That means there's images where that object should be, but it didn't make it through the first step of being called a dot."

Bernardinelli developed a way to stack multiple images to create a sharper view, which helped confirm whether a detected object was a real TNO. They also verified that their method was able to spot known TNOs in the areas of the sky being studied and that they were able to spot fake objects that were injected into the analysis. "The most difficult part was trying to make sure that we were finding what we were supposed to find," says Bernardinelli.

After many months of method-development and analysis, the researchers found 316 TNOs, including 245 discoveries made by DES and 139 new objects that were not previously published. With only 3,000 objects currently known, this DES catalog represents 10% of all known TNOs. Pluto, the best-known TNO, is 40 times farther away from the sun than Earth is, and the TNOs found using the DES data range from 30 to 90 times Earth's distance from the sun. Some of these objects are on extremely long-distance orbits that will carry them far beyond Pluto.

Now that DES is complete, the researchers are rerunning their analysis on the entire DES dataset, this time with a lower threshold for object detection at the first filtering stage. This means that there's an even greater potential for finding new TNOs, possibly as many as 500, based on the researchers' estimates, in the near future.

The method developed by Bernardinelli can also be used to search for TNOs in upcoming astronomy surveys, including the new Vera C. Rubin Observatory. This observatory will survey the entire southern sky and will be able to detect even fainter and more distant objects than DES. "Many of the programs we've developed can be easily applied to any other large datasets, such as what the Rubin Observatory will produce," says Bernardinelli.

This catalog of TNOs will also be a useful scientific tool for research about the solar system. Because DES collects a wide spectrum of data on each detected object, researchers can attempt to figure out where the TNO originated from, since objects that form more closely to the Sun have are expected to have different colors than those that originated in more distant and colder locations. And, by studying the orbits of these objects, researchers might be one step closer to finding Planet Nine, a hypothesized Neptune-sized planet that's thought to exist beyond Pluto.

"There are lots of ideas about giant planets that used to be in the solar system and aren't there anymore, or planets that are far away and massive but too faint for us to have noticed yet," says Bernstein. "Making the catalog is the fun discovery part. Then when you create this resource you can compare what you did find to what somebody's theory said you should find."

## Category Archives: astronomy

The last 12 months have certainly been a great year for those who search for asteroids, study asteroids, write about asteroids, and now – of course – want to mine asteroids. I did a bit of tweeting the last week, as Planetary Resources, Inc. announced not only their existence as a corporation but also their plans to turn science fiction into reality. Yes, Virginia (and Poul Anderson, and Larry Niven, and…), there are people with deep pockets who want to move us permanently into space. And that means business.

But wait…let’s not get into the asteroid-mining area just yet. I’ll have more to say, in other posts. What I want to do here is a) start posting in this little blog again and b) highlight several very interesting scientific reports and areas of research in the world of asteroids and other small solar system bodies from the last several months.

For this post, I’ll present some quick summaries of research that’s been posted on www.arxiv.org, that wonderful compendium of preprints and just-published research in fields ranging from physics and astronomy to cosmology and mathematics. Here are three items for starters.

In just the last several years, astronomers have been finding some rather unusual objects: asteroids that suddenly start acting like comets. It’s long been suspected that some comets – or even comet groupings or families, like the Damocloids – may actually be “dead” comets. What a surprise, though, to spot asteroids that start acting like comets, with outgassing, comas, and even tails. David Jewitt (co-discoverer of the first Kuiper Belt/Trans-Neptunian Object…if you don’t count Pluto) has a great summary of what’s known so far. The abstract begins: “Some asteroids eject dust, unexpectedly producing transient, comet-like comae and tails. First ascribed to the sublimation of near-surface water ice, mass losing asteroids (also called “main-belt comets”) can in fact be driven by a surprising diversity of mechanisms. In this paper, we consider eleven dynamical asteroids losing mass, in nine of which the ejected material is spatially resolved. We address mechanisms for producing mass loss….”

Click the paper’s title above to see the full abstract, and if you’re interested in the whole paper, click the PDF link. The paper itself was “in press” for The Astronomical Journal as of December.

Trojan asteroids are asteroids that occupy the L4 or L5 Lagrange points of a planet’s orbit. Those two quasi-stable gravitational areas are located 60 degrees behind or ahead of the planet. The first and best-known Trojans are in Jupiter’s orbit there are hundreds of them known so far. But Trojan asteroids have also been discovered for Saturn, Mars, Earth (the first one was found just last year) and Neptune.

As its designation indicates, this Neptunian Trojan was discovered in 2008. In this paper, Jonathan Horner of the University of New South Wales (Australia) and his colleagues explore how stable – or unstable – 2008 LC18 is in its current location. They conclude that there’s some evidence that this object may be “either a temporary Trojan capture, or a representative of a slowly decaying Trojan population (like its sibling the L4 Neptunian Trojan 2001 QR322), and that it may not be primordial.” Click the title above to see the full abstract, and you can get to the full paper from there.

## Physical Properties 2

Planet Mass
(Earth = 1)
Density
(吆 3 kg m -3 )
Surface
Gravity
(Earth = 1)
Escape
Velocity
(km s -1 )
Escape
Velocity
(Earth = 1)
Mercury 0.0553 5.43 0.378 4.3 0.384
Venus 0.815 5.25 0.907 10.36 0.926
Earth 1.0 5.52 1.000 11.19 1.0
Mars 0.107 3.95 0.377 5.03 0.450
Jupiter 317.83 1.33 2.364 59.5 5.32
Saturn 95.159 0.69 0.916 35.5 3.172
Uranus 14.536 1.29 0.889 21.3 1.903
Neptune 17.147 1.64 1.120 23.5 2.10
Pluto 0.002 2.03 0.059 1.1 0.0983

The mass of planets with satellites can be measured by observing the motions of the satellites and applying Kepler's Law. For Mercury and Venus, the mass used to be measured by detecting these planet's influence of the Earth, asteroids or comets. Recently, their masses have been measured by probes.

The very low mass of Pluto is one of the reasons why it is no longer considered to be a major planet. Since the 1980s other Pluto-sized bodies have been found in the distant part of the Solar System. These are the Kuiper Belt Objects and Pluto is now counted as one of them.

### Density

The density of a planet is its mass divided by its volume. The units are kilograms per cubic meter.

### Surface Gravity

Only Jupiter and Neptune have a stronger surface gravity than the Earth. The Surface Gravity of a planet is proportional to the planet's mass and inversely proportional to the square of the planet's radius.

• a gravity is the acceleration of gravity (metres per second per second),
• G is the Gravitational Constant (6.673 × 10 -11 N m 2 kg -2 ),
• M is the mass of the planet (kg),
• R is the radius of the planet (metres).

### Escape Velocity

• v esc is the escape velocity (metres per second),
• G is the Gravitational Constant (6.673 × 10 -11 N m 2 kg -2 ),
• M is the mass of the planet (kg),
• R is the radius of the planet (metres).

## Up to 384 minor planets (including Pluto) qualify as planets? - Astronomy

In order to answer this question, we need to know what criteria we will use to make our decision. We previously established reasonable criteria for the definition of a planet. Does Pluto qualify?

1. orbits the Sun at a distance of 39 AU with a period P = 247.7 yr
2. has a very eccentric (elliptical) orbit: perihelion = 29 AU, aphelion = 49 AU at high inclination (i = 17) [most eccentric, highly inclined planet]
3. mass = 0.0021 M_Earth (1/500th M_moon) [smallest mass of any planet Mercury is 0.05 M_Earth]
4. diameter = 2274 km (0.18 of Earth 2/3 of Moon)
5. surface gravity = 1/14th of Earth (moon = 1/6)
6. rotation period = 6.39 days backwards (or upside down)
7. has a moon (Charon). D = 1300 km. distance = 20,000 km. orbital period = 6.39 days. rotation period = 6.39 days.
8. was closer to Sun than Neptune from 1989-1999 this repeats once per orbit. But when this occurs, Pluto is always at least 60 degrees away from Neptune so no collision is possible due to 3:2 resonance (P_Nep = 164.8), so P_pluto/P_Neptune = 247.7 / 164.8 = 1.503 = 3 : 2
9. has an atmosphere for 40 years out of each orbit, when temperature is warm enough for CO and CH4 and Argon to sublimate off surface during day, freeze back out at night.
• whose primary orbit is around a star.
• is not massive enough to permit nuclear fusion to occur.
• is round.

A brief history of relevant events:

1543: Copernicus argues that the Earth is a planet orbiting the Sun. This establishes the Earth as a planet, not as the center of the universe, and the Moon and Sun as the Moon and Sun, not as planets orbiting the Earth.

1609: Kepler's 1st and 2nd laws. The beginning of mathematical physics.

1609: Galileo invents telescope.

1619: Kepler's 3rd law.

1685: Newton publishes Principia. Gives mathematical formula for law of gravity. Kepler's laws can be derived from Newton's law.

1695: Edmund Halley uses law of gravity to predict that comets are not one-time objects, but they orbit the sun and return periodically. Predicts the return of a comet in 1758.

1758: Halley's comet returns, found on Dec 25, proving Newton correct in very dramatic fashion.

In 1766, Johann Titius, and then in 1772, Johann Bode, proposed similar versions of what became known as the Titius-Bode law. This law "predicted" the spacings of planets' orbits around the sun, as follows:

 Planet number Add 4 Divide by 10 Actual a (AU) Planet? 0 4 0.4 0.39 Mercury 3 7 0.7 0.72 Venus 6 10 1.0 1.00 Earth 12 16 1.6 1.52 Mars 24 28 2.8 . . 48 52 5.2 5.2 Jupiter 96 100 10.0 9.56 Saturn 192 196 19.6 . . 384 388 38.8 . . 768 772 77.2 . .

One can even develop a formula for this "law": after 0, "double" the number, add 4, and divide by 10:

distance = 0.4 + 0.3 * 2 n where n stands for the first (n=0), second (n=1), etc. planet.

Is this a law of physics on par with Kepler's laws and Newton's laws? How would we determine this? This "law" predicts the existence of planets at 2.8 and 19.6 AU.

William Herschel. Born in Germany, emigrated to England as a child. From a musical family, worked as organist at a chapel in Bath. Studied mathematics to aid his musical composing and teaching. Discovered books on optics. Built a telescope. Was good at it. Built bigger and better telescopes than anyone else. Learned the sky well. Decided to search for and measure parallax.

March 13, 1781, discovered a "star" bigger than all the others. Thought it was a comet. Other astronomers soon demonstrated
that it was a planet in orbit around Sun.

Named it "Georgium Sidus" [George's star] after King George III. This outraged French and other continental astronomers who instead called it "Herschel's planet." Eventually, name of "Uranus" stuck.

Where is Uranus? It has a semi-major axis a = 19.22 AU. Let's add it to our table:

 Planet number Add 4 Divide by 10 Actual a (AU) Planet? 0 4 0.4 0.39 Mercury 3 7 0.7 0.72 Venus 6 10 1.0 1.00 Earth 12 16 1.6 1.52 Mars 24 28 2.8 . . 48 52 5.2 5.2 Jupiter 96 100 10.0 9.56 Saturn 192 196 19.6 19.22 Uranus 384 388 38.8 . . 768 772 77.2 . .

So is Uranus a confirmation of the Titius-Bode "law"? It appears to be close enough to be tantalizing.

The Discovery of the Ceres

So, in 1796, an an astronomical conference, the astronomy community decided that a systematic search should be undertaken for other planets.

In 1800, six German astronomers, the so-called "celestial police," decided to put the Titius-Bode law to the test by looking for the "missing planet" at 2.8 AU. But they were scooped.

On January 1, 1801, Giuseppe Piazzi, a Sicilian monk, announced the discovery of an unknown body in the heavens. He discovered this object during routine observations of stars he was making. He was not searching for new or unknown objects. He named the object "Ceres" (from the Roman goddess of the harvest hence "cereal"). Piazza at first thought it was a comet. He observed it for 41 days.

In 1801, Gauss figured out how to take a few observations, such as those made by Piazza, and calculate an entire orbit to predict the future positions of an object. Using Gauss' was, Ceres was re-found on December 7, 1801 (in another great triumph for mathematical physics). And what is the answer? Ceres is located at a = 2.77 AU.

 Planet number Add 4 Divide by 10 Actual a (AU) Planet? 0 4 0.4 0.39 Mercury 3 7 0.7 0.72 Venus 6 10 1.0 1.00 Earth 12 16 1.6 1.52 Mars 24 28 2.8 2.77 Ceres 48 52 5.2 5.2 Jupiter 96 100 10.0 9.56 Saturn 192 196 19.6 19.22 Uranus 384 388 38.8 . . 768 772 77.2 . .

Now the Titius-Bode law is looking very strong, indeed!

The Discovery of the Asteroid Belt

The euphoria didn't last though. Quickly, more objects were discovered. Note in this table their semi-major axes:

 asteroid number/name discovery date diameter semi-major axis mass 1 Ceres 1801, Jan 1 925 km 2.768 AU 1/10,000 mass of Earth 2 Pallas 1802, Mar 28 583 km 2.773 AU 3 Juno 1804, Sept 1 249 km 2.671 AU 4 Vesta 1807, Mar 29 555 km 2.362 AU 5 Astraea 1845, Dec 8 (3 more) 1847 (1 more) 1848 (1 more) 1849 (47 more) 1850-1859 (52 more) 1860-1869 (102 more) 1870-1879

Now, we know of 3500 asteroids with well known orbits and another 6000 with less well known orbits. Of these, the biggest are those four listed above and #10 with d = 443 km and #65 with d = 311 km. Most have diameters smaller than 150 km. All have orbits between Jupiter and Mars.

By 1802, with the discovery of Pallas, the astronomy community was calling these objects "minor planets," not planets. Soon they would be renamed "asteroids" and the region where they are located would be called the asteroid belt. Thus, within a year of the "missing planet" being discovered, it became clear that there was no missing planet to be found. There is no planet in the asteroid belt.

Or is there? Do any of the asteroids meet our criteria for planet status? They all have primary orbits around the sun. They all are too small to be stars or brown dwarfs. But are they round? Images of Gaspra, Ida and Dactyl, Phobos, Deimos, Eros, Toutatis, and Mathilde reveal that these asteroids are not round. But these are small objects, the biggest being about 60 km across its longest axis. [click here for a rotation movie of Eros click here for information on NEAR landing on Eros]

How big must an object be to be round? Jupiter's moons are all large spherical objects, the smallest one being Europa with a diameter of 3130 km. These are quite big compared to asteroids. What about Saturn's moons? Enceladus is perfectly spherical, with a diameter of 498 km. Mimas, with a diameter of 398 km, is spherical, albeit with one enormous crater. But Hyperion at 370 km clearly is not spherical. It's triaxial dimensions are 370 x 280 x 225 km. So we might conclude, fairly safely, that 400 km is a good boundary, above which an object is spherical, below which objects become less so.

What about our asteroids? At least three of them are more than large enough "to be round." Therefore, Ceres, Pallas and Vesta all meet our criteria for being planets. Yet we don't consider them planets!

Before leaving behind the asteroids and the asteroid belt, for the time being, we should note one other aspect of the discovery of these objects: which ones were discovered first? The biggest ones. In fact, the very biggest was discovered first. This isn't surprising because the biggest would reflect the most sunlight and therefore would appear brightest in our telescopes, thereby being easiest to discover.

The Discovery of Neptune

Soon after discovery of Uranus, astronomers realized that this planet had been observed numerous times, as far back as 1690,
but was never recognized as a planet. By early in the 19th century, it was clear that the historical and new observations of Uranus could not be reconciled with a single elliptical orbit, as demanded by Kepler's and Newton's laws. The errors in the observed position of Uranus were never more than 2 minutes of arc off from those predicted, but even this small error is much larger than observational error. Clearly the observations were not the source of the error. So either the law of gravity is wrong (or incomplete) or there is some other physical law waiting to be discovered or .

In the period from1843- to 845, the English astronomer John Couch Adams analyzed all the existing Uranus data. In October, 1845, he proposed an answer: there is another planet out there. The gravitational tug of that planet influences Uranus' orbit. He predicted where and how big that planet must be. British Astronomer Royal, Sir George Airy, didn't take the prediction very seriously and didn't push his staff to make the necessary observations.

In 1846, Frenchman Urbain Leverrier independently made the same mathematical prediction as Adams. He sent his calculations to Johann Galle at the Berlin Obsevatory. Galle received the letter from Leverrier on the afternoon of Sept 23, 1846. Within 30 minutes of looking,
he found Neptune that very night, exactly where he'd been advised to look.

(As it turns out, Galileo himself observed Neptune twice (12/24/1612, 1/28/1613) but didn't recognize that it wasn't a star when it was in the same field of view for him as Jupiter.)

Again, we have an incredible triumph of the law of Gravity. Newton is king. Long live Newton! And what of the Titius-Bode "law"?
Neptune has a semi-major axis of 30.11 AU.

 Planet number Add 4 Divide by 10 Actual a (AU) Planet? 0 4 0.4 0.39 Mercury 3 7 0.7 0.72 Venus 6 10 1.0 1.00 Earth 12 16 1.6 1.52 Mars 24 28 2.8 2.6-2.8 Ceres,Pallas, Vesta, Juno, etc. 48 52 5.2 5.2 Jupiter 96 100 10.0 9.56 Saturn 192 196 19.6 19.22 Uranus 384 388 38.8 30.11 Neptune 768 772 77.2 . .

This result is inauspicious for the Titius-Bode "law." Neptune is in the wrong place by quite a large margin. At this point, a sensible approach would be to toss out the Titius-Bode "law" as a curious piece of numerology. It clearly is not a fundamental law of physics. It is not a good theory with predictive power. However, it does lead us to ask interesting questions, such as "why are the planets spaced the way that they are?"

The Discovery of Pluto

Although the discovery of Neptune rang the death knell for one proposed law of physics, it cemented the status of Newton's law of gravity as paramount. In addition, the method by which Neptune was discovered clearly suggested that the same method could be used to predict the existence and location of the next planet. All we have to do is observe the location of Neptune over a period of years, note the discrepancies between the observed and predicted positions, and use those discrepancies to determine the mass and location of the 9th planet.

In 1848, J. Babinet made the first such prediction of a planet with a mass 12 times that of the Earth. Note that at this time, Neptune, which has an orbital period of 165 years, had only moved about 4 degrees in the sky, out of one 360 orbit, and even today has not yet completed one orbit around the Sun since it was discovered!

Percival Lowell, a rich gentlemen from Boston and a self-taught astronomer, built himself an observatory in Flagstaff, AZ, dedicated to observing Mars and proving that Martians existed. (But that is another story). He also used the Adams/Leverrier method to predict that a planet of 7 earth masses exists out there beyond Neptune. He searched in vain for this planet until his death in 1916.

In 1929, the director of Lowell Observatory decided to hire someone to take up the search for the predicted planet. He hired Clyde Tombaugh, a self taught amateur from Kansas. The method he used to search for this planet was straightforward: take two images of same part of sky, 2-3 days apart. Use a "blink comparator" to compare images (up to 400,000 stars per image) on each 14" x 17" photographic plate. Look for "moving" star. After about a year, on Feb 18, 1930, he found Pluto, about 6 degrees from Lowell's predicted position.

Pluto was about 10 times fainter than Lowell had predicted. So mass must be

10 times smaller than predicted (i.e. less than one Earth
Mass).

Thinking carefully about Pluto, or Pluto reconsidered

For a long time, we knew very little about Pluto. In fact, at a minimum distance of more than 4 billion km from Earth, we still know very little. We know the shape of Pluto's orbit. It is very elliptical, with a perihelion of 29 AU and an aphelion of 49 AU. This is by far the most elliptical of the known planets. Pluto's orbit is inclined by 17 degrees to the ecliptic plane. The next most inclined planetary orbit is that of Mercury, at 7 degrees and Venus at 3 degrees.

What about Pluto's mass? Initial estimates were based on predictions from how massive the predicted Pluto must be in order to perturb Neptune's orbit such that it would produce noticable discrepancies in the position of Neptune. These estimates had dropped from about 12 Earth masses to 7 Earth masses by the time Pluto was discovered.

After the discovery, Pluto's mass was based on guesses about how big it was and what it was made out of. A big object reflects more sunlight than a small object a dark object (carbon rich rock dirty ice) reflects less sunlight than a bright object. Initial estimates of Pluto's mass, post discovery, began at 1 Earth mass. By the 1960s, the estimated mass had dropped to about 0.1 Earth masses and by the early 1970s to 0.003 Earth Masses. Finally, in 1978, the matter was settled: Christy and Harrington discovered Pluto's moon, Charon. From
straightforward observations of the orbital period of the moon and the planet-moon separation, one can determine the mass of the moon and planet from Kepler's and Newton's laws.

The mass of Pluto = 0.0021 Earth Masses. The diameter of Pluto = 2390 km. Charon has a mass equal to 1/6 that of Pluto and a diameter = 1186 km. It's a good thing that Charon was discovered, because, had it not it appears that Pulto's mass would have become negative by now.

Does Pluto's known mass raise any questions in your mind about it's discovery?

If not, it should. Pluto was discovered because Percival Lowell, among others was convinced that Neptune was continually in the wrong place. Why? Some massive object, more distant that Neptune, was tugging on it, competing with the Sun gravitationally for control over Neptune. This was how Neptune was discovered. And Neptune has a mass about 18 times that of the Earth. It is a big object, as it must be to exert that kind of influence on Uranus despite the enormous distances between those two planets.

But wait! Pluto, we now know, has a mass 500 times smaller than the Earth's, 9000 times smaller than that of Neptune. How could Pluto possibly mess with Neptune's orbit? Must there still be another large planet out there waiting to be discovered, the one that is really responsible for the so-called perturbations in Neptune's orbit?

In 1995, Miles Standish, of the Jet Propulsion Laboratory, the acknowledged NASA guru of planetary positions and orbits, took on the task of figuring this out. He used all the data collected by all NASA spacecraft to refine the masses and orbits of all the known objects in the solar system. He used this information to work backwards and predict where Neptune should have been at each time it was observed in the historical record. He found that all the historical observations of Neptune were fine, accurate to well within the observational errors. From the historical observations, one would be led to predict that no other planet existed. Therefore, the theoretical work that led to the prediction of the existence of Pluto was wrong! There never should have been any such prediction. A previous generation of astronomers misread and misinterpreted their data and found evidence for what they wanted to see, not for what the data really revealed. Percivial Lowell never should have been looking for Pluto, Clyde Tombaugh never should have found Pluto.

But Clyde Tombaugh did found Pluto. And he found it within 5 degrees of where Lowell predicted it should be.

Remember Neptune's discovery? It was found on the first night by Johann Galle within minutes of arc of the predicted position. Pluto was hundreds of times further from the predicted position than was Neptune. Yet it was found, relatively close to the predicted position, given the huge swath of sky that exists to look in.

Should this give us pause to wonder: Why did Clyde Tombaugh find Pluto in the same general part of the sky that Percival Lowell said he should be searching when there was absolutely no reason, on the basis of the data, to have ever predicted the existence of Pluto?