73 votes
Accepted

Considering our methods of exploration, how likely is it that there are unfound planets (not dwarf planets) in our solar system?

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'...
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  • 3,958
69 votes

Why does it take so long to transmit an image from New Horizons to Earth?

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 ...
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  • 7,250
57 votes

How can comets have tails if there's no air resistance in space?

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 ...
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  • 91k
52 votes
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Why hasn't the "9th Planet" been detected already?

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 ...
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  • 1,231
52 votes
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Why won't the Sun set for days at N66.2 which is below the arctic circle?

From the PyEphem Quick Reference Guide: Rising and setting are sensitive to atmospheric refraction at the horizon, and therefore to the observer’s temp and pressure; set the pressure to zero to turn ...
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  • 3,369
48 votes
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What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

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 ...
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47 votes

Why does it take so long to transmit an image from New Horizons to Earth?

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 ...
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45 votes

Why is the Sun's density less than the inner planets?

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 ...
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  • 2,377
45 votes
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Why do the planets orbit in the same direction?

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 ...
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42 votes
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How much magnification is needed to see the planets of solar system?

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 ...
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42 votes

Is it odd that our Sun has so many planets?

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 ...
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  • 2,993
42 votes

Why do things float in space, though the gravity of our star is always present?

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 ...
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  • 91k
42 votes
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Closest star system to Alpha Centauri?

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 ...
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  • 91k
41 votes
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Can our Sun become a black hole

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 (...
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  • 534
40 votes

Is Jupiter a failed star?

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 ...
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39 votes

Can a planet in our system eclipse the sun as seen from another one?

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 ...
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  • 14.1k
39 votes
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Which planet, viewed from which other planet would appear the largest?

Here is a table with planets, their minimum/maximum distances from the Sun, their minimum distance (assuming the planet's aphelion and its neighbour's perihelion coincide, which they don't, so the ...
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  • 4,700
36 votes

Why hasn't the "9th Planet" been detected already?

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 ...
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36 votes
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Which planet came first in our Solar system?

The planets (probably) formed by the accumulation of planetesimals, and they all formed over roughly the same time span. Inner protoplanets had access to more matter, so they probably were able to ...
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  • 10.4k
35 votes

How can comets have tails if there's no air resistance in space?

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 ...
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34 votes
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How can Io be tidally heated while it is in tidal lock?

How can Io be tidally heated while it is in tidal lock? It is tidally locked in a mean motion sense of "tidally locked". That Io is in an eccentric orbit rather than a circular orbit means ...
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  • 28.3k
33 votes

Why do things float in space, though the gravity of our star is always present?

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 ...
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  • 421
32 votes
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Do all stars have an Oort cloud or is it a rare occurence?

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 ...
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  • 34.1k
31 votes
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9th planet location?

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 ...
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31 votes

Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?

The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar ...
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30 votes
Accepted

How is it possible for a collision to be responsible for Uranus's axial tilt?

For objects on the scale of a planet, the state of matter doesn't really matter much. A colliding planet or planetesimal would not "just pass through" The amount and density of the gas ...
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  • 91k
29 votes

Why is the Sun's density less than the inner planets?

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 ...
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  • 6,104
29 votes
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Starting a fire in a cold planet that was full of flammable gas

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 ...
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  • 91k
29 votes
Accepted

Do the axes of rotation of most stars in the Milky Way align reasonably closely with the axis of galactic rotation?

There is very likely to be a random scatter. Unlike planets orbiting the Sun in the Solar System, most of the stars in the Galaxy did not form at the same time as the Galaxy itself. There is therefore ...
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