# What is the minimum mass of a celestial object so that it can have a moon?

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I was wondering how massive something have to be so that it it can attract moons by pop culture standards (ellipsoid/round shape).

Could a planetoid have a moon? What is the relation between the mass of a moon and planet?

Planetoids can have moons and the minimum size is "pretty small". For example 2003 SS84is a small Near-Earth asteroid, with a diameter of 120m and a moon of about 60m in diameter, which orbits at a distance of 270m ever 24 hours. It probably didn't form by "attracting the moon" but the moon probably formed as a result of impact splitting a "rubble pile" asteroid. The moon, in this case, is similar in size to the main object. There is a wide distribution of size ratios from close to 1:1 to very small objects orbiting much larger ones.

However one aspect of your question is different: you ask for a "round" shape. That makes things harder, since a round shape requires a fairly large size to pull the object into a sphere. There is probably only one object except for the major planets, with a spherical moon, and that is Pluto with its moon Charon (and even Charon isn't perfectly rounded, but it is close)

So it is quite easy for a planetoid to have a moon. But it is hard for a planetoid's to be big enough to pull itself into ellipsoidal equilibrium

In Kollmeier & Raymond (2018), it is stated that a moon can have its own submoon, and that submoon can have a subsubmoon, etc. It doesn't matter if the object is round or not, and there's no minimum mass per se, provided the secondary object has $$10^{−5}$$ times the primary body's mass, using the rule of thumb from Reid (1973). So, for example for a submoon of the Moon, which has mass of $$7.342×10^{22}$$ kg, could have a maximum mass of $$7.3420×10^{17}$$ kg, and that submoon could have a subsubmoon of $$7.3420×10^{12}$$ kg. There could be, also, a subsubsubmoon, with a mass of $$7.3420×10^{7}$$ kg, or $$10000$$ tons. As you can see, there is a moment that the smallest object has very little mass, so it can be gravitationally attached to almost anything.

Edit: the smallest moon in the Solar System is Deimos, with a mass of $$1.4762×10^{15}$$ kg and a diameter of 12.4 km. So, theoretically, it could have a submoon of $$1.4762×10^{7}$$ tons.

The good, the bad, and the ugly

All it takes to prove the existence of a planet, even one that is visibily hidden, are observation, time, and mathematics. We've been proving the existence of "bodies of mass" for a long time because orbits don't make sense unless everything is taken into account. Therefore, while your planet may be visibily hidden, it cannot be mathematically hidden. If your light side peoples have calculus, they can prove the existence of the planet. Which might not be that ugly, as it could be an interesting plot point. Things you can't see are easily forgotten/ignored, even when a small group of scientists keep reminding people, "there's gotta be something there, and here's how massive it is. "

The limitation is that the mass of the two objects will need to be nearly identical (I think) to minimize instabilities. The greater the difference, either the greater the distance between the two worlds (making travel very difficult) or the greater the possibility one will want to orbit the other rather than both orbiting the star.

while your proposed orbit is theoretically possible, the reality is that it is unstable. Any change in mass, distance from the star, even the rotation of either or the worlds (or a passing comet, for that matter), and the orbital sync would fall apart. If either planet has a moon, it's probably impossible (but only an astrophysicist could confirm that statement).

However, that might not be a show stopper becasue things can take a honking long time to change when it comes to stellar phenomena, so it might be that the two orbits are not actually sync'd, that one is only just faster than the other, and we happen to be in a. say. 1,000 year period when your hidden planet is actually hidden. That might actually be a useful plot point for you as the existence of the planet would be common enough knowledge for people to not actually think about it (how often do you think about the back of your knees?) and, better still, it's existence would have basically become myth. Of course, those pesky scientists are still reminding people it's there, but your average response might be, "yeah, and according to popular fairy tales, dragons live there. don't we have something more important to talk about?"

The Good (or, at least, the really cool)

It might be a bit more complicated, but with a bit of handwavium, you could set up an argument for a Lissajous orbit around Lagrange Point L2. Lagrange points are (simplistically) gravitational eddies that can be orbited like an actual body of mass. L2 is directly in line "behind" a planet such that the planet always shields the point from the sun. A Lissajous orbit is one that requires no artificial propulsion to maintain. Granted, it ususally requires a loop around earth to maintain momentum, but I mentioned handwavium, right?

This concept could allow you to create a moon that is never seen by the light side. Note that those pesky scientists are still yammering about some mass that keeps affecting the tides (and it would be a wild and wooly affectation, too. did I mention plot point?), but there are ways to discredit/silence/ignore scientists.

No matter what solution you come up with, it will always be mathematically visible. Tides for moons. Orbital perturbations for planets. Observation, time, and mathematics will detect them all. For the sake of realism, you'll need to deal with this nasty problem in your story. But it's a cool problem, wouldn't you say?

## What is the minimum mass required for a celestial object to become spherical in shape?

Moons, planets, and stars are usually spheres and many asteroids tend to be big rocks. How massive does an object have to be before nature makes it a sphere? And why do large masses become spherical?

AFAIK the smallest round moon in the solar system is Mimas (about 3.8 x 10 19 kg), although it slightly tidally distorted ovoid by Saturn's gravity.

Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."

## Aonther massive celestial object, with a companion star in tow, November 19, 2002 1:29 PM Subscribe

Posted this question on Slashdot, but figured I'd throw it out here. Any physicists in the house?

The conclusion about this being kicked by a supernova doesn't make sense to me. Can anyone help me understand? Two problems:

1) The black hole has a companion star, so wouldn't a kick of that magnitude tear it away from its companion and preclude it from acquiring another until it slows?

2) Even ignoring the mass of the companion, the estimates are that the BH is about 7 solar masses. That means that the BH has acquired a kinetic energy of 1/2 * 7 * (2^30 kg) * (10^5 m/s)^2 = about 10^41 J of energy, which is about 1/1000 of the energy of the SN explosion (10^51 erg = 10^44 J). To me, that seems like an exceedingly large fraction of a roughly isotropic explosion converted into motion. It gets even worse if you throw in the mass of the companion.

Anyone have any insights into how this can happen?
posted by ptermit at 3:22 PM on November 19, 2002

ptermit, there's others who could phrase it more eloquently than I could, given my general knowledge of physics:

1) The companion star was snared by the BH long after the bow shock otherwise, the BH would've absorbed it long before anyone on Earth could observe it.

2) The brunt of the BH's kinetic energy is within the event horizon. The massively dense output is the result of the collapse of the star which formed the BH.

1) A supernova occurs, with the magnitude of a gamma-ray burster the shockwave of the explosion extends out toward a neighboring star.

2) Affected by the force of the gamma-ray burster, the neighboring star collapses as it is wrenched from its prior trajectory depending upon the age of the neighboring star, and its proximity to the nova, it either becomes a neutron star, BH/gravistar, or itself goes nova.

3) The waywardly-traveling survivor of the gamma-ray burster eventually passes yet another star, one which is weaker than the first two. The third object follows the rogue until it is eventually absorbed. (If the third star were closer, and/or larger, it would've collapsed/ been flung, etc.)

Hope that clears things up.
posted by Smart Dalek at 4:24 PM on November 19, 2002

Smart Dalek: Thanks for answering me, but I have to confess that your post didn't clear things up at all.

I didn't understand what you meant by most of (or any of) the black hole's kinetic energy being inside the event horizon. It's just a moving mass, and so has kinetic energy that it can transfer via gravity outside its event horizon, so I can't figure out what you're saying there. Also, companions need not be absorbed by black holes at all they can orbit indefinitely, just as planets can orbit stars. And I don't see what bow shocks have to do with this, either, as they're produced by shock waves that have already left the star to be accelerated.

Maybe I wasn't terribly clear, or I'm not understanding what's going on here. I thought that these guys saw a fast-moving BH and companion. They concluded that the BH was so fast because it was born in a supernova, which propelled the BH.

My objection is that given the amount of energy in a supernova, the BH is moving too fast, and shouldn't have a companion. It's moving too fast because the SN shouldn't be able to pump so much energy into the star to get it moving like that. It shouldn't have a companion because its original companion should have been torn away, and it's probably moving too fast to acquire a new companion. (And if it did acquire a new one, it must have been moving considerably faster before it did, making the first point even more troublesome.)
posted by ptermit at 8:07 PM on November 19, 2002

#### Next stop: space

To leave the planet, a vehicle called a climber could attach to the ribbon. It would grip the ribbon on both sides with a pair of wheels or belts, much like a treadmill. They would move and pull people or cargo up the ribbon. You might think of it, says Bradley Edwards, as being “essentially like a vertical railroad.” Edwards is physicist based in Seattle, Wash. He wrote reports for NASA in 2000 and 2003 about the likelihood of developing space elevators.

A person could reach low-Earth orbit in around an hour, Edwards says. Traveling to the end of the tether would take a couple of weeks.

“You get in and you barely feel it move … it’d be sort of like a normal elevator,” Edward says. Then you’d see the anchor station, where the ribbon is tied to Earth, dropping away. You might start slow, but the elevator could reach speeds of between 160 to 320 kilometers per hour (100 to 200 miles per hour).

The view would change from watching clouds and lightning over the Earth’s surface to seeing the curve of the Earth. You’d pass the International Space Station. “And by the time you get to geosynchronous [orbit], you can put your hand up and cover the Earth,” Edwards says.

But you wouldn’t have to stop there. Because of how the end of the elevator is being flung around, you could use it to slingshot yourself to another planet. This is just like swinging a rock on a string around your head. If you let go of the string, the rock goes flying. “The same thing works with a space elevator,” Edwards says. In this case, the destination could be the moon, Mars or even Jupiter.

#### Spinning a yarn

The biggest challenge of building a space elevator may be the 100,000-kilometer-long tether. It would have to be incredibly strong to handle the gravitational and centrifugal forces pulling on it.

The steel used in tall buildings wouldn’t work for a space elevator cable. You’d need a higher mass of steel than all the mass in the universe, Landgraf noted in a 2013 TEDx talk.

#### Scientists Say: Graphene

Instead, physicists are looking to carbon nanotubes. “Carbon nanotubes are one of the strongest materials we know about,” says chemical engineer Virginia Davis. Davis works at Auburn University in Alabama. Her research focuses on carbon nanotubes and graphene, another carbon material. These are nanoscale materials, with at least one dimension around one thousandth the thickness of a human hair.

The structure of carbon nanotubes resembles a chain link fence that’s been rolled into a tube. Instead of being made of wire, carbon nanotubes are made only of carbon atoms, Davis explains. Carbon nanotubes and graphene are “way stronger than most other materials, especially given that they’re really super lightweight,” she says.

“We already can make fibers and cables and ribbons out of carbon nanotubes,” Davis says. But no one has made anything out of carbon nanotubes or graphene that even approaches tens of thousands of kilometers yet.

Edwards estimated that the strength the cable would need to have a strength of around 63 gigapascals. That’s a huge number, thousands of times higher than the strength of steel. It’s dozens of times more than some of the toughest materials known, such as the Kevlar used in bulletproof vests. In theory, carbon nanotubes’ strength reaches far past 63 gigapascals. But only in 2018 did researchers make a bundle of carbon nanotubes that surpassed that.

The strength of a massive ribbon, though, would not only depend on the material used but also on how it is woven. Defects, such as missing atoms in the carbon nanotubes could also affect overall strength, Davis says, as well as other materials used in the ribbon. And, if successfully built, the space elevator would have to withstand all manner of threats from lightning strikes to collisions with space junk.

“Certainly, there’s a long way to go,” says Davis. “But a lot of things that we used to think of a science fiction, which is where this idea started, have become science fact.”

### Power Words

antenna (plural: antennae) In biology: Either of a pair of long, thin sensory appendages on the heads of insects, crustaceans and some other arthropods. (in physics) Devices for picking up (receiving) electromagnetic energy.

astronaut Someone trained to travel into space for research and exploration.

atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon nanotube A nanoscale, tube-shaped material, made from carbon that conducts heat and electricity well.

centrifugal force A force that seems to pull a rotating body — or something on a rotating object (such as a rider of an amusement park ride) — away from the center of rotation.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical engineer A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.

consortium A group or association of independent organizations.

dimension Descriptive features of something that can be measured, such as length, width or time.

Earth’s crust The outermost layer of Earth. It is relatively cold and brittle.

fiber Something whose shape resembles a thread or filament.

graphene A superthin, superstrong material made from a single-atom-thick layer of carbon atoms that are linked together.

gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

International Space Station An artificial satellite that orbits Earth. Run by the United States and Russia, this station provides a research laboratory from which scientists can conduct experiments in biology, physics and astronomy — and make observations of Earth.

Kevlar A super-strong plastic fiber developed by DuPont in the 1960s and initially sold in the early 1970s. It’s stronger than steel, but weighs much less, and won’t melt.

laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

mechanical Having to do with the devices that move, including tools, engines and other machines (even, potentially, living machines) or something caused by the physical movement of another thing.

NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

peer (verb) To look into something, searching for details.

physicist A scientist who studies the nature and properties of matter and energy.

planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

science fiction A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.

solar cell A device that converts solar energy to electricity.

star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

strategy A thoughtful and clever plan for achieving some difficult or challenging goal.

tether A tie or cord that loosely anchors some object to a semi-fixed position. Or the process of tying some object to a cord that will keep it loosely affixed to that position. (Consider the child’s game tether ball, whereby a cord it attached to a ball on one end and an anchoring pole on the other end.)

universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

vertical A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

### Citations

Report: B.C. Edwards and Eureka Scientific. NIAC phase II study. March 1, 2003.

Journal: B.C. Edwards et al. Design and deployment of a space elevator. Acta Astronautica Vol. 47, November 2000. doi: 10.1016/S0094-5765(00)00111-9

Report: B.C. Edwards and Eureka Scientific. NIAC phase I study. 2000.

Carolyn Wilke is a former staff writer at Science News for Students . She has a Ph.D. in environmental engineering. Carolyn enjoys writing about chemistry, microbes and the environment. She also loves playing with her cat.

## A Celestial Object That Defies Description

An object discovered by astrophysicists at the University of Toronto nearly 500 light years away from the sun may challenge traditional understandings about how planets and stars form.

The object is located near &ndash and likely orbiting &ndash a very young star about 440 light years away from the sun, and is leading astrophysicists to believe that there is not an easy-to-define line between what is and is not a planet.

&ldquoWe have very detailed measurements of this object spanning seven years, even a spectrum revealing its gravity, temperature, and molecular composition. Still, we can&rsquot yet determine whether it is a planet or a failed star &ndash what we call a &lsquobrown dwarf&rsquo. Depending on what measurement you consider, the answer could be either,&rdquo said Thayne Currie, a post-doctoral fellow in U of T&rsquos Department of Astronomy & Astrophysics and lead author of a report on the discovery published this week in Astrophysical Journal Letters.

Named ROXs 42Bb for its proximity to the star ROXs 42B, the object is approximately nine times the mass of Jupiter, below the limit most astronomers use to separate planets from brown dwarfs, which are more massive. However, it is located 30 times further away from the star than Jupiter is from the sun.

&ldquoThis situation is a little bit different than deciding if Pluto is a planet. For Pluto, it is whether an object of such low mass amongst a group of similar objects is a planet,&rdquo said Currie. &ldquoHere, it is whether an object so massive yet so far from its host star is a planet. If so, how did it form?&rdquo

Most astronomers believe that gas giant planets such as Jupiter and Saturn formed by core accretion, whereby the planets form from a solid core that then develops a massive gaseous envelope. Core accretion operates most efficiently closer to the parent star due to the length of time required to first form the core.

An alternate theory proposed for forming gas giant planets is disk instability &ndash a process by which a fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet. This mechanism works best farther away from the parent star.

Of the dozen or so other young objects with masses of planets observed by Currie and other astronomers, some have planet-to-star mass ratios less than about 10 times that of Jupiter and are located within about 15 times Jupiter&rsquos separation from the sun. Others have much higher mass ratios and/or are located more than 50 times Jupiter&rsquos orbital separation, properties that are similar to much more massive objects widely accepted to not be planets. The first group would be planets formed by core accretion, and the second group probably formed just like stars and brown dwarfs. In between these two populations is a big gap separating true planets from other objects.

Currie says that the new object starts to blur this distinction between planets and brown dwarfs, and may lie within and begin to fill the gap.

&ldquoIt&rsquos very hard to understand how this object formed like Jupiter did. However, it&rsquos also too low mass to be a typical brown dwarf disk instability might just work at its distance from the star. It may represent a new class of planets or it may just be a very rare, very low-mass brown dwarf formed like other stars and brown dwarfs: a &lsquoplanet mass&rsquo brown dwarf.

&ldquoRegardless, it should spur new research in planet and star formation theories, and serve as a crucial reference point with which to understand the properties of young planets at similar temperatures, masses and ages,&rdquo Currie said.

The discovery is reported in a study titled, &ldquoDirect imaging and spectroscopy of a candidate companion below/near the deuterium-burning limit in the young binary star system, ROXs 42B&rdquo. Currie will present these and other findings at the annual meeting of the American Astronomical Society in Washington, DC, this week.

The observational data used for the discovery was obtained using the telescopes of the Keck Observatory and Subaru Observatory on Mauna Kea, Hawaii (pictured right) and the telescopes of the European Southern Observatory in Chile.

The international research team includes scientists from: the Space Telescope Science Institute in Baltimore, MD the University of Montreal the University of Hyogo in Kobe, Japan the Universitats-Sternwarte Munchen and the Ludwig-Maximilians-Universitat, Munchen, Germany and the University of Hawaii.

Publication of press-releases or other out-sourced content does not signify endorsement or affiliation of any kind.

## Missing Link Of Neutron Stars? Bizarre Hibernating Stellar Magnet Discovered

Astronomers have discovered a most bizarre celestial object that emitted 40 visible-light flashes before disappearing again. It is most likely to be a missing link in the family of neutron stars, the first case of an object with an amazingly powerful magnetic field that showed some brief, strong visible-light activity.

This weird object initially misled its discoverers as it showed up as a gamma-ray burst, suggesting the death of a star in the distant Universe. But soon afterwards, it exhibited some unique behaviour that indicates its origin is much closer to us. After the initial gamma-ray pulse, there was a three-day period of activity during which 40 visible-light flares were observed, followed by a brief near-infrared flaring episode 11 days later, which was recorded by ESO's Very Large Telescope. Then the source became dormant again.

"We are dealing with an object that has been hibernating for decades before entering a brief period of activity", explains Alberto J. Castro-Tirado, lead author of a new paper in the journal Nature.

The most likely candidate for this mystery object is a 'magnetar' located in our own Milky Way galaxy, about 15 000 light-years away towards the constellation of Vulpecula, the Fox. Magnetars are young neutron stars with an ultra-strong magnetic field a billion billion times stronger than that of the Earth. &ldquoA magnetar would wipe the information from all credit cards on Earth from a distance halfway to the Moon,&rdquo says co-author Antonio de Ugarte Postigo. "Magnetars remain quiescent for decades. It is likely that there is a considerable population in the Milky Way, although only about a dozen have been identified."

Some scientists have noted that magnetars should be evolving towards a pleasant retirement as their magnetic fields decay, but no suitable source had been identified up to now as evidence for this evolutionary scheme. The newly discovered object, known as SWIFT J195509+261406 and showing up initially as a gamma-ray burst (GRB 070610), is the first candidate. The magnetar hypothesis for this object is reinforced by another analysis, based on another set of data, appearing in the same issue of Nature.

Forty-two scientists used data taken by eight telescopes worldwide, including the BOOTES-2 robotic telescope at EELM-CSIC, the WATCHER telescope at Boyden Observatory (South Africa), the 0.8-m IAC80 at Teide Observatory (Spain), the Flemish 1.2-m Mercator telescope at Observatorio del Roque de los Muchachos (Spain), the Tautenburg 1.34-m telescope (Germany), the 1.5-m at Observatorio de Sierra Nevada (IAA-CSIC), the 6.0-m BTA in Russia, the 8.2-m VLT at ESO in Chile and the IRAM 30-m Pico Veleta y Plateau de Bure telescopes, together with the SWIFT (NASA) and XMM-Newton (ESA) satellites.

Neutron stars is the bare, condensed remain of a massive star with between eight and fifteen times the mass of the Sun, which has expelled its outer layers following a supernova explosion. Such stars are only around 20 kilometres in diameter, yet are more massive than the Sun. Magnetars are neutron stars with magnetic fields hundreds of times more intense than the average neutron star fields. The energy release during one flare in the course of a period of activity can amount to the energy released by the Sun in 10 000 years.

Story Source:

Materials provided by ESO. Note: Content may be edited for style and length.

## Can Moons Have Moons?

Can moons have moons? More technically, could a natural satellite of a major planetary body have its own natural satellite? In short…kind of.

The answer to this question is not black and white. To answer it more completely, let’s back up. What is a moon? The International Astronomical Union Assembly of 2006 yielded this definition for a planet:

“A ‘planet’ is defined as 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.”

Unfortunately there is no official definition of a moon. Most astronomers just refer to them as natural satellites. So throughout this article, when I refer to a moon I am speaking of a natural satellite of a planet as defined by the IAU. So picture looking up on a clear night, and you see the moon, full and bright. After watching it for some time, a second, smaller sphere begins to slowly peek its way around from the other side of the moon. We’ll call this hypothetical object a sub-moon.

It would be quite a sight, and it seems plausible. After all, the sun is a satellite of the galactic center, and the earth orbits the sun, the moon orbits the earth…couldn’t nature just go one step further? The moon, like all other massive objects, produces it’s own gravity. That’s what allowed us to land on the moon and put things into orbit around it.

So we can orbit the moon, doesn’t that prove sub-moons are a possibility? Again, not exactly. As of right now there are 174 known moons in our solar system orbiting the eight major planets, and none of them have sub-moons of their own. It was thought that Saturn’s moon, Rhea, might have its own ring system, but such claims have been since dismissed. If true this would have been the closest thing to a sub-moon within our solar system.

So it could be possible, but with so many moons why don’t we see it happening? Believe it or not, the reason is likely because of the age of the solar system. All objects within orbit have something called a Hill Sphere, which is an area around an object (say, the Earth) where it’s gravity is more powerful than the object it orbits around. An object outside of the moon’s Hill Sphere would orbit (or fall to) the Earth. Outside the Earth’s sphere you would orbit the sun. The trouble is that the smaller the object, the smaller the sphere.

On top of this, the more levels of orbits you have, the more unstable the orbit is. For example, it would take a lot to pull our sun out of orbit, but to pull the Earth out of orbit would only require another star to pass too close to our solar system. On the level of a sub-moon, the orbit would be horribly unstable for a celestial object. In the case of the Earth-moon system, their are tidal forces between the moon and the Earth, which cause the moon to be gravitationally locked with the Earth. If you combine all of these factors, a possible sub-moon would fall out of orbit on a cosmologically short time scale. A sub-moon could last long enough for us to enjoy it, perhaps thousand of years (its lifespan depends on lots of different factors), but most, if not all, moons probably formed in the early solar system. So a sub-moon would most likely not last long enough for humans to see it 5 billion years down the line. It’s not impossible, it just requires very special circumstances, ones we were not blessed with.

## Scientist proposes yet another new definition of a planet

It used to be easy to know what was and wasn’t a planet. Planets were big, bigger than any smaller moons that happened to be orbiting them. They were round. They orbited our sun. Then, in 2006, the erstwhile planet Pluto lost its major planet status, becoming a dwarf planet. Around that time, astronomers were discovering a plethora of small bodies in our own solar system, so that there are now half a million known asteroids and over a thousand Kuiper Belt Objects, including five recognized dwarf planets counting Pluto. Astronomers also now know several thousand exoplanets orbiting other stars. The dramatic expansion in the number of known objects orbiting our sun and other suns has caused some astronomers to try to override or re-define the 2006 planet definition from the International Astronomical Union (IAU), which caused Pluto to lose full planet status. The most recent new planet definition comes from a Johns Hopkins astronomer, Kevin Schlaufman. Read three earlier planet definitions below.

Schlaufman’s definition is based on mass. In a paper published January 22, 2018, in the peer-reviewed Astrophysical Journal, Schlaufman has set the upper boundary of planet mass between four and 10 times the mass of the planet Jupiter.

Astrophysicist Kevin Schlaufman proposed the new definition of a planet. Image via Johns Hopkins.

Schlaufman said in a statement that setting a limit is possible now mainly due to:

… improvements in the technology and techniques of astronomical observation. The advancements have made it possible to discover many more planetary systems outside our solar system and therefore possible to see robust patterns that lead to new revelations.

The conclusions in the new paper are based on observations of 146 solar systems … Defining a planet, distinguishing it from other celestial objects, is a bit like narrowing down a list of criminal suspects. It’s one thing to know you’re looking for someone who is taller than 5-foot-8, it’s another to know your suspect is between 5-foot-8 and 5-foot-10.

Schlaufman said his definition will help distinguish between two “suspects:” a giant planet and a celestial object called a brown dwarf. Brown dwarfs are more massive than planets, but less massive than the smallest stars. They are thought to form as stars do. His statement described his thinking:

For decades brown dwarfs have posed a problem for scientists: how to distinguish low-mass brown dwarfs from especially massive planets? Mass alone isn’t enough to tell the difference between the two … the missing property is the chemical makeup of a solar system’s own sun. [Schlaufman] says you can know your suspect, a planet, not just by his size, but also by the company he keeps. Giant planets such as Jupiter are almost always found orbiting stars that have more iron than our sun. Brown dwarfs are not so discriminating.

That’s where his argument engages the idea of planet formation. Planets like Jupiter are formed from the bottom-up by first building-up a rocky core that is subsequently enshrouded in a massive gaseous envelope. It stands to reason that they would be found near stars heavy with elements that make rocks, as those elements provide the seed material for planet formation. Not so with brown dwarfs.

Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.

Schlaufman’s idea was to find the mass at which point objects stop caring about the composition of the star they orbit. He found that objects more massive than about 10 times the mass of Jupiter do not prefer stars with lots of elements that make rocks and therefore are unlikely to form like planets.

For that reason, and while it’s possible that new data could change things, he has proposed that objects in excess of 10 Jupiter mass should be considered brown dwarfs, not planets.

Want to be mad at somebody about Pluto’s demotion to dwarf planet status in 2006? These astronomers drafted the IAU resolution that did that. From Upper Left: Andre Brahic, Iwan Williams, Junichi Watanabe, Richard Binzel, Catherine Cesarsky, Dava Sobel (author), Owen Gingerich. Even within this small group, by the way, there was disagreement.

Will Schlaufman’s definition be accepted by other astronomers? Well … the field of planet definitions has gotten crowded. It’s becoming hard to imagine a definition that will be acceptable to every astronomer. For example: here are three more planet definitions:

1. The International Astronomical Union (IAU) planet definition in 2006:

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 neighborhood around its orbit.

Because the IAU took it upon itself early in the 20th century to be the body that names and defines things in space, this definition is why Pluto is no longer considered a major planet. In other words, Pluto – though it has a moon, an atmosphere, weather and many planet-like qualities – has not yet cleared the neighborhood of its orbit in space.

Jean-Luc Margot (@jeanlucmargot on Twitter) is a professor and chair of UCLA’s Department of Earth, Planetary, and Space Sciences. He proposed a new planet definition in 2015.

2. Planet definition from Jean-Luc Margot at UCLA, offered in 2015. He devised a formula to tell if a body has cleared its orbit of debris (part of the IAU definition for a planet), just by knowing a body’s mass, its orbital period, and the mass of the star it orbits. This formula can be worked out via readily available data, even for most exoplanets. Hence, according to Margot: a body is a planet when it is in orbit around one or more stars, it dominates its orbit as per the formula, and has a mass below 13 Jupiters. There’s no need to require an object to be spherical, as required by the IAU definition, because, Margot has said, bodies that can clear their orbits will almost certainly be round. Read more about Margot’s planet definition.

Astronomer Alan Stern (@AlanStern on Twitter). The New Horizons mission to Pluto was his brainchild. Stern has also founded a private company called UWingU in an attempt to give the public more access to naming and defining things in space.

3. Planet definition via NASA scientists, led by Alan Stern, in early 2017. Stern is the instigator and principal investigator of NASA’s New Horizons mission, which performed a first-ever flyby of Pluto in 2015. Pluto was a major planet when Stern launched his New Horizons mission in January, 2006, but Pluto was demoted to dwarf planet in August of that same year. Not surprisingly, the team’s planet definition focuses on getting rid of the aspect of the IAU’s definition that requires clearing the orbit of debris (the same aspect that caused Pluto’s demotion). The jargon-laden version of their definition is:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

Their layman’s version is simply:

Round objects in space that are smaller than stars.

According to Stern’s planet definition, our solar system has not 8 major planets, but more than 100, including Pluto, of course, and including Earth’s moon.

So you see that this formerly simple question – “what is a planet?” – is not so simple anymore.

And, by the way, what we’re seeing in this community of professional astronomers is an interesting microcosm, isn’t it? It parallels the larger world of incredibly divided and divisive political thinking in, for example, the U.S. One wonders how much the fast increase in human population in recent decades has contributed to the crazy political situation (human population doubled between 1950 and 1987, from 2.5 to 5 billion people now there are an estimated 7.6 billion people on Earth). Meanwhile, the number of professional astronomers has increased, too. I couldn’t find any solid numbers on how much, but here’s a discussion.

Since both world population and the number of astronomers isn’t likely to decrease dramatically any time soon, it’ll be interesting to see how this ongoing divisiveness (in planet definitions, and, well, everything else) gets resolved … if it does, anytime soon.

Maybe astronomers will act honorably and wisely and set an example for the rest of us! Maybe …

Bottom line: What’s a planet? What’s a dwarf planet? What’s a brown dwarf? In recent years, astronomers have grappled with these definitions. The newest proposal comes from Johns Hopkins.