19

In 2006 the IAU had a trilemma. They could decide that Eris was a planet, and potentially allow for future discoveries of tens of new planets. They could be inconsistent, declare that Pluto was a planet, but Eris (and Ceres) wasn't They could come up with a definition of "planet" that would exclude Eris, and consequently also exclude Pluto. Each option ...


16

To first order, the relative abundances of the heavier elements to iron (for instance) are constant. So the metal content of a star is shorthand for the content of any element heavier than He. (NB: we now know this is not true in many circumstances and elements can be grouped by synthesis process - for example we can talk about "alpha elements" - O,...


11

Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?) and it is big enough for us to have seen it as an independent body. This contrasts with the "moonlets" that have been detected in Saturn's rings by their ...


10

If we're going to get technical, Asteroids are not really an official name anymore. In 2006, when the IAU redefined what a planet was (and thus demoted Pluto), they also decided to more formally define other terms to identify objects in our solar system. You can see a diagram of all the official terms and how they relate below. Notice the important factor ...


10

Singling out one point: Moons must have "cleared the neighbourhood" of their own orbit, i.e. in their particular orbit around the host they are by far the most massive body This would mean that co-orbital moons won't exist anymore; e.g. Telesto and Calypso share their orbit around Saturn with the much more massive Tethys. With the proposed ...


9

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I like this statement by Steven Soter: The IAU definition of a planet as a heliocentric body that "has cleared the neighborhood around its orbit” is problematic....


8

We have only discovered one such object so far: ʻOumuamua. (Although a second, 2I/Borisov, was discovered in 2019) The general practice has been to call it an "interstellar asteroid" or "interstellar comet", even though it is not in orbit. You could just call it an "interstellar object".


7

You are mixing the rotation of the body around a barycenter with its moons, and the rotation of the body around its own center. For a bound rotation like Pluto and Charon both have to have the same rotation period - yet both have to be present. Thus Pluto's axis of rotation of course intersects its surface - it rotates around its own axis at exactly the same ...


6

In 1812 Fraunhofer measured a set of absorption lines in the sun. It wasn't until later that Kirchoff and Bunsen figured out that these absorption lines matched emission features from metals they were burning in the lab. (http://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html) In chemistry, most things in the periodic table are known as "metals"( ...


6

There is no 'history' behind it. Stellar physics is less than a 100 years old. Thus, it is not a terminology that came to be due to some anecdotal reason. This is the way it has been since the start. Why? Because we don't care. Why, really? Hydrogen and Helium are more important and abundant, so we need something to measure the others, which can be grouped ...


5

This is really a question of Language rather than Astronomy. The meaning of words is defined by their use. The IAU has proposed one way of using the word "planet", this group of scientists have proposed another. Anyone can propose a definition of any word. This group proposes that a "planet" is any astronomical body which 1) Does not have, and never has had ...


5

I'd like to expand on @James K's good answer. The question you need to answer before your question can be answered is: why do you want to have a definition of "moon"? What purpose does it serve other than pedantry? (Remember that there is a continuum of objects from the largest stars down to the smallest bits of space dust -- whatever you do, you're ...


5

I'll just add a supplement to @planetmaker's answer. As long as a body is distinct and not connected to anything else, it will have a center of mass. If the body is roughly spherical its center of mass will be near it's middle. The body's rotational axis by definition passes through its center of mass, and is parallel to it's own angular momentum vector. If ...


4

That is galaxy NGC 1097, a barred spiral galaxy in the constellation Fornax. It has been distorted by interactions with neighbouring galaxies and has a weakly active galactic nucleus, with jets coming from the nucleus. The jets contain stars from a galaxy that has recently (in astronomical terms) been cannibalized. The ring in the centre is a region of star ...


4

The IAU website has this draft definition of "planet" and "plutons" page, published on 16 August 2006 in Prague. It says (emphasis mine): The world's astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between "planets" and the smaller "...


4

What was the definition of a planet before August 24, 2006? I think this deserves a bit more of an answer because it's not quite so simple. Yes, it's true that there was no formal definition of what a planet was prior to the 2006 vote, but essentially everyone knew what was and wasn't a planet, just as we know what is and isn't a star, though the boundary ...


4

I've noted before that the IAU naming critera are guidelines rather than laws of nature. If applied to Jupiter's moons the four Galilean satellites would probably be "planets" (they don't share their orbits with anything else of comparable size and are large enought to be in hydrostatic equilibrium) There is a mean-motion resonance 4:2:1 between ...


4

a) What is the definition of completeness? Completeness is the number of objects in a data set that are detected over the number that exist. In astronomy, completeness is often estimated for a particular apparent magnitude or flux density. As an example, for sources that are as bright as the Sun (-27 magnitude), we have a completeness of 1. That is, we’...


3

There is no lower limit, and as you say, all stars are somewhat variable. However catalogues of variable stars exist, and they can record a wide range of levels of variability. For example, the general catalogue of variable stars lists stars like Alpha Triangulum, with a variability of 0.01 magnitudes. Ultimately a variable star is a star which has had ...


3

I'm inclined to say a hard no using the casual definition of a "great comet" A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which during certain appearances are bright enough to be noticed by casual observers who are not looking for them, and ...


3

Even though it arised for historical reasons outlined in the other answers, the distinction between metals and non-metals as defined by astronomers does continue to make sense today. Metals are formed in stars and supernovas, whereas non-metals preexist stars. Therefore, the distinction is relevant when considering nucleosynthesis.


3

Your question: There also are satellites in between the rings. What distinguishes a moon from any other chunk in orbit around a planet? Wikipedia should be taken with a grain of salt, but they define moons or moonlets within a ring system as creating a gap or partial gap, sometimes described as looking like a propeller. In 2006, four tiny moonlets ...


2

A diameter of 0.5 kilometer will get you classified as a moonlet. See Aegaeon. Apparently at 1.4 km × 0.5 km × 0.4 km, it is on the edge of the definition.


2

I think that @JamesK's answer summarizes the situation nicely. Here I can summarize the way that I see space commonly sliced and diced by people in Space Exploration SE. I can't defend these as "right", but recollect seeing them typically used this way. Earth orbit LEO or low Earth orbit MEO or medium Earth orbit GEO or geostationary orbit (though it is ...


2

It it wherever is convenient for the article you are writing. There is no single consenus definintion of "interplanetary space". If you are in orbit around a planet then you are not in interplanetary space, if you are between planets, then you are. But there is no substantial change in the matter filling space as you move from planetary orbit to ...


2

According to NASA - What is a Moon?: Planets and asteroids orbit the Sun. Moons—also known as natural satellites—orbit planets and asteroids. Moons come in many shapes, sizes and types. Most are airless, but a few have atmospheres and even hidden oceans. There are hundreds of moons in our solar system—even a few asteroids have small companion moons. The ...


2

Small objects like comets and asteroids are generaly not round. The roundness of bigger bodies is a consequence of the body own gravity. So, to answer your question, no, not all celestial bodies would become a planet with this definition as small objects like comets and asteroids doesn't have a sufficent mass to become round.


2

Try plotting absolute G magnitude (i.e. corrected for the fact tthat all the stars are at different distances) on the y-axis.


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