82

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition. When Eris was discovered, it turned out to be really, really similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the ...


73

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises. The people The most trusted source in Astronomy would have to be the people that set ...


71

There are no undiscovered planets between the sun and Neptune. Objects closer (to Sun) than Neptune that are large enough to be considered planets (and not dwarf planets) can't remain 'hidden'. If it's there, the light from the sun will bounce off it and we will see it. As it moves in its orbit, we will notice the position in the sky change, so we will know ...


69

New Horizons has just passed the Kuiper Belt Object (KBO) 2014 MU69 also known as Ultima Thule. KBOs form a belt of asteroids (the Kuiper Belt) from Neptune's orbit outwards and of which Pluto is the largest member of the Belt. During the encounter with Ultima Thule, all of the 7 instruments on New Horizons were gathering data (although not all at the same ...


54

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail is not left trailing behind the comet, it is pushed away from the comet by the sun. When the comet is moving away from the sun, the tail is in front of the ...


51

Brown and Batygin, the authors of the paper on the possible planet, have a webpage addressing this. A few reasons not already covered: It moves quite slowly - the authors estimate 0.2-0.6 arc seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object. Eris, which is the most distant confirmed ...


46

The other answer mentions it, but this gives a bit more theory as to the why. It's effectively for the same reason that your phone or Wi-Fi don't work as well and slow down when that they are far from the hotspot or cannot get a clear line of access to the cell tower, more commonly known as having "few bars": the signal gets weaker and as a result the ...


45

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative zone is around 20,000 - 200 kg/m$^3$ (already less dense than water). Eventually at the edge is the convective zone - the density at the part that we see is much ...


44

The same reason (almost) all of them rotate in the same direction: because of the conservation of angular momentum. Before a star and its planets exist, there’s just a cloud of disorganized gas and small molecules. The Solar System formed from such a cloud around 4.6 billion years ago. On that scale, there is some small amount of rotation within the cloud....


44

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the gravitational attraction of the additional rock changes their course. Those near misses are the beginning of rotation. Multiply that effect by trillions, and you have ...


42

It is not true that "objects float around" in the solar system. Perhaps you have seen video from the space station, and you can see things floating. This is not because there is no gravity, but because everything in the space station going at the same speed in the same direction. This makes it look as if things are floating. In fact the space station and ...


41

No, the sun won't ever become a black hole. The choice between the three fates of stars (white dwarf, neutron star, black hole) is entirely determined by the star's mass. A star on the main sequence (like most stars, including our sun) is constantly in a balance between the inward pressure of gravity and the outward pressure of the energy generated by the ...


41

No. Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas Giants. Gas Giants are planets, that form via processes in their parent protoplanetary disc. Contrasting this, Brown Dwarves form via direct fragmentaion of the ...


41

The sun is the nearest star to Alpha Centauri (unless you count Proxima Centauri, which is really part of the same system). There is a very small and dim pair of brown dwarfs, called Luhman 16 that are closer, at about 3.6 light years from Alpha Centauri. Brown dwarfs are not true stars, but they do glow from their own heat. They were only discovered in ...


39

You're probably asking the wrong question - which I am going to answer anyway, and after that I am going to answer the question you should have asked instead. As a general rule, there isn't much point in pushing the magnification above 2x the diameter of the instrument, measured in mm. 3 inch, that's 75mm, that's 150x max. Beyond that limit, even under ...


38

In our solar system, it is possible for one planet to partially eclipse the sun, but it is not possible for any planet to cause a full solar eclipse as seen from another planet. The sun is too big and the planets are too small and too far apart. Transits occur, and can occur for any pair of planets, but they are very rare. As seen from earth, Mercury only ...


36

This graph from XKCD says a lot about why that is the case The bottom line is, the 9th planet is too small to be detected through WISE, and too far/small to have been detected through visible observation. Most likely this hypothetical planet is a long ways away, possibly as far as 1200 AU, and not particularly large, making it difficult to see. WISE was ...


35

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the temperature they start to vaporise and how they are affected by solar winds. I think this picture shows it in a good way. https://community.dur.ac.uk/physics....


33

To help with James K's excellent answer, a visual representation might help. Let's look at a thought experiment - Newton's Cannonball. Let's say you have a cannon, high enough that it's being held above Earth's atmosphere. You fire it, and it falls to Earth a little ways away ("D" in the below diagram). You fire another one with more power so it's moving ...


32

Awesome question, especially since we know so little of the answer. Nobody knows for sure how the Oort Cloud formed - I'll put that out there right now - but the current hypothesis is that it was originally part of the Sun's protoplanetary disk. All of the ice and rock coalesced into small bodies - proto-comets, if you will. While these bodies were much ...


30

It's too dim to be seen during a normal survey during the majority of its orbit. Update: Scientists at the University of Bern have modeled a hypothetical 10 Earth mass planet in the proposed orbit to estimate its detectability with more precision than my attempt below. The takeaway is that NASAs WISE mission would have probably spotted a planet of at ...


29

The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all. If you took some of Neptune's outer layer back to earth and mixed it with our air, it could burn. Even very cold hydrogen can burn (it soon heats up!) This couldn't happen on ...


28

Fusion inside of a star affects the sun's density (which does not happen with a planet). It produces an outward pressure that balances against the attraction of gravity, thereby reducing the density as long as the star is burning. Once a star the mass of the sun is no longer able to sustain fusion, what is left is a white dwarf which is in fact much denser ...


26

From an exoplanet-finding point of view, the Sun has between one and three planets. The major exoplanet-finding techniques in current use involve watching for either periodic Doppler shifts as the planet's gravitational pull causes the star to wobble, or periodic brightness shifts as the planet transits the star. Both require that the planet is large ...


25

No, Pluto is a so called resonant trans-neptunian object; the orbital period of Pluto is almost exactly 3:2 (1.5) times that of Neptune. This means that every time Pluto nears perihelion and is therefore closest to the Sun and also closest to the orbit of Neptune, Neptune is always at a specific angle (50° according to Wikipedia) in front or behind Pluto. (...


24

The solar system contains very little of elements heavier than Helium - less than 2% by mass. This is reflected in the chemical abundances measured in the photosphere of the Sun. i.e. The Sun does contain heavier elements. Your question is the wrong way around; it is not that the heavier elements have not sunk into the middle, it is that the vast majority ...


23

In the protostar stage of the Sun, it was surrounded by a (spinning) gas cloud. This cloud behaved like a fluid (well, a gas is a fluid), so it flattened out into an accretion disk due to conservation of angular momentum. The planets eventually formed from the dust/gas in the disk from compression of the dust in the disk. This process won't end up moving the ...


23

There are many normal methods that we use to detect exoplanets, but none of them work well in the case of the 9th planet. Here are some of the main ones. Radial velocity. The Sun is not moving significantly with respect to Earth, and the hypothetical planet is too far away from the Sun to have much of an impact. Transit. This is obviously impossible, as the ...


23

Stars twinkle because they are effectively a point of light. This point of light can be distorted and magnified by movement of patches of varying density in the atmosphere. These act as lenses causing the twinkle effect. If you get to space, the stars don't twinkle. Stars appear as a point only because they are so very very distant. Planets twinkle less ...


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