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Despite being known to humans for many thousands of years, the planets in the Solar System are still active areas of study for astronomers. Each of the planets is unique in some way, allowing us to see how different conditions can produce very different results just within our own astronomical back yard. When you add in things like asteroids and comets, the Solar System is a very dynamic place!

In addition to our own Solar System, astronomers now study planets around other stars, which are called exoplanets. These planets are so small and far away that it is very difficult to learn anything about them, or if they are even there at all. However, astronomers have developed some complex techniques for studying them, allowing us to learn more about what our Solar System could have been like if things had been slightly different.

 

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Solar System

What planet is closest to Earth? (Beginner)
This is actually a very interesting question and the answer depends on how you ask it.
If you want to know the planet that gets the closest to Earth, the answer is Venus. Venus’s average orbital radius is about 108 million km compared to Earth’s 150 million km, meaning that when the planets are exactly lined up, they are around 42 million km from each other, which is closer than Earth ever gets to its other neighbor Mars, which has an average orbital radius of about 227 million km.
If we want to know what planet is closest to the Earth at any given time, then the answer gets more complicated and depends on the specific positions of the inner planets. Over time, this shifts between Mercury, Venus, and Mars (currently it’s Venus), but Mercury is actually the planet that is closest on average to Earth. Here are some very cool and informative YouTube videos giving the reasons why, but the summary is that since Mercury orbits so close to the Sun, it just can’t ever get that far away from Earth. Venus and Mars are very far away when they are on the opposite side of the Sun, but Mercury is on a tight leash with its small orbit so it spends more time closer to the Earth. This is a cool and unexpected result that was only really discovered in the past year or so.
Is Planet 9 real? Is its proposed orbit weirdly shaped because it ran into Pluto? (Intermediate)
The existence of Planet 9 is currently a large and unanswered question in solar system astronomy. The evidence for it boils down to weirdness we have observed in the orbits of comets and other bodies orbiting past Pluto in a region called the Kuiper Belt. Essentially, the orbits of these Kuiper Belt Objects seem to have all been tugged in the same direction by a single object, leading to the theories on the position and size of the planet. The current best guess is that it is larger than the Earth and on a elliptical (ie elongated) orbit that is inclined (ie slanted) relative to the orbits of all of the other planets. However, the distance and size of Planet 9 mean that it is almost impossible to find. We would have to point one of the biggest telescopes on Earth in exactly the right spot multiple times in order to see its movement against the stars in the background, which would require us being incredibly lucky since it could be almost anywhere in the sky. It is possible that an upcoming telescope survey by the Vera Rubin Telescope (formerly the Large Synoptic Survey Telescope) would find it as it scans the entire sky every few days, but we don’t know for sure.
Your question about planetary collisions is also complicated. In the early solar system, things ran into each other all the time, resulting in large piles of materials that became the planets we have today. You can think of planets as snowballs rolling through the snow gathering up more debris and becoming larger and larger. Over time, planets attract all of the material near them, absorbing it all to make the main planet larger. Planet-size things don’t bounce off each other like pool balls, they merge together like clay. Given that this process has been happening for over 4 billion years, it seems surprising then that Neptune and Pluto haven’t run in into each other yet since their orbits cross. However, Neptune and Pluto are in something called an orbital resonance, meaning that their orbits are in sync with each other. For every two times Neptune goes around the Sun, Pluto goes around three times, and this perfect synchronization means that they always end up missing each other. These kinds of orbital resonances are common in places where different objects are close enough together to interact gravitationally, like the moons of Jupiter. Pluto likely ended up in its current position because it’s a stable orbit, so it’s unlikely that it would have been knocked there by accident.
Planet 9 (if it exists) is definitely not so close that it would risk running into Pluto. If it was, its size (which we expect to be pretty large) would mean that we would be able to see it better than Pluto, and its gravity would have a noticeable impact on Pluto’s orbit as well. Currently, the only evidence we see if it is some possible disturbances of Kuiper Belt Objects, and since scientists are required to be skeptical of any bold new claims until they are proven conclusively, we must wait to see some more evidence. Hopefully this answers your questions.
If a new comet passes Earth (like Neowise), does that mean a new meteor shower will occur? (Beginner)

You are correct that many meteor showers are the result of the Earth passing through the remains of a comet tail, but the conditions have to be just right for that to actually happen. Most comets are not actually aligned with the orbits of the planets, meaning that while all of the major planets more or less orbit in one flat circle, comets can go far outside that circle. This is the case for Neowise, which, as you can see in this 3D model of the Solar System, travels completely above/below the Earth’s orbit. So it’s tail may leave some debris in the orbit of Mercury (which can’t really have meteor showers since it doesn’t have an atmosphere) but it won’t really do anything for the Earth.

What would happen to the Moon if the Earth disappeared? (Beginner)
If the Earth disappeared, the Moon would mostly just keep on orbiting around the Sun the way the Earth was before. Depending on where the Moon was in its orbit, it would get flung off in a random direction, but it wouldn’t go fast enough to change the shape of its orbit much from where the Earth orbits now (it would probably just make it a little more elliptical). So essentially, if the Earth disappeared, then the Moon would basically just take its place as a new planet. 
Is the Earth at risk of being destroyed by a rogue planet, asteroid, or black hole? (Beginner)
The good news is that you have nothing to worry about. Of course, it’s not as easy as being told, “don’t worry,” to stop worrying, so let’s look at some science.
The first thing to recognize is that an asteroid is the only sort of rogue object that has any chance of hitting Earth and causing damage to it.
Black holes are caused by the collapse of stars which are at least three times bigger than our sun. The nearest star, Proxima Centauri, is 4.24 light years away and far too small to ever become a black hole. Even if Proxima Centauri could become a black hole, it would stay right where it formed: 25,000,000,000,000 miles away.
Asteroids are a bit of a different story. We know they do move around in our solar system, because we witness rocks falling to Earth in beautiful meteor showers all of the time.
Most objects that enter Earth’s atmosphere are going to burn up before they ever reach the ground. That is what creates those bright streaking tails of meteors. However, sometimes, an object is big enough that it won’t burn up and will impact with the surface of the Earth. As I’m sure you have heard, an asteroid 6 miles across is what ended the reign of the dinosaurs and allowed mammals like us to take over the Earth. So, if a large enough asteroid were to hit Earth, it would have a pretty big effect.
That’s where the good news comes in. We, as humans, happen to be a bit smarter and more technologically savvy than dinosaurs. By using telescopes and advanced computer tracking software, we can detect and predict the movement of objects in space that are heading towards Earth. In fact, there are experts and space agencies across the world devoted to watching the skies and keeping us safe. The Guardians of the Galaxy may be the stuff of comic books, but we do have real life Guardians of Earth in the form of NASA’s Center for Near Earth Orbit Studies.
Scientists are constantly working to evaluate the best ways to deal with an asteroid if it approaches Earth. Everything from breaking it up with explosives to redirecting it. And, an asteroid large enough to cause an extinction-level event is predicted to occur only once every 100 million years. So, the chances of it occurring in our lifetime, or even the lifetime of our children, is 0.00000001%!
Do you think there could be a volcano underneath the Great Red Spot? (Intermediate)
Astronomers can be pretty sure that there is no volcano below the Great Red Spot on Jupiter. There has been a lot of study of the internal structure of Jupiter (including by the Juno space probe that is currently orbiting it) and we can be reasonably sure that if there is a solid surface anywhere below Jupiter’s clouds, it’s so far down that there is no way it could ever directly influence what is going on on the surface. The clouds we see are just a thin layer on top of thousands of miles of ultra compressed hydrogen and helium that are forced into a weird metallic fluid form by the super high pressures (see below). The Great Red Spot could be a result of some “volcano” of atmospheric gases (kind of like a thunderstorm) but it’s not anything related to the surface. Also, the storm moves around the planet at a slightly different rotation rate compared to the rest of the planet, so we know it’s not tied to anything solid.
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Why does everything in the Solar System seem to orbit on a plane? Or is that’s just the way it’s portrayed due to looking at it in a 2D manner on videos? (Intermediate)
The Solar System started as a big spherical cloud of gas. The individual particles in this gas were moving around essentially randomly, but on average, the cloud was rotating slightly. You can think of this like a basketball spinning on someone’s finger: most of the air molecules in the basketball are just bouncing around randomly inside the basketball, but since the basketball as a whole is rotating, the average of all of the velocities of the air molecules will show some rotation.
The particles in this rotating spherical cloud of gas also had gravity though, so over time, they pulled themselves towards each other, making the gas cloud shrink. As these particles were attracted to each other, they would sometimes hit each other and stick together, forming the beginnings of what would become the planets and asteroids. When these particles stuck together, the velocity of the new particle would be the average of the velocities of the two particles that made it up:
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This means that if you had a particle that was going up collide with a particle that was going down, their velocities would average out when they stuck together and the new particle wouldn’t be going up or down. This process happens countless times as small particles combine into entire planets, and at every step, the velocities get averaged out more and more. We said that on average, all of the particles were rotating slightly in one direction, so after we have averaged out all of the randomness, all we are left with is this rotation.
So all of the planets are orbiting in the direction of the original rotation of the cloud of gas that created the Solar System. The various rocks and particles that combined to make them all had their randomness averaged out, leaving a set of flat orbits that only deviate from each other by a few degrees.
What evidence is there for nebular theory, the idea that the Solar System formed from a cloud of gas? (Intermediate)
Nebular theory is the model of star formation where solar systems collapse out of a cloud of interstellar gas, resulting in a central star and a disc of debris that forms itself into planets. This theory is so dominant in modern astronomy that I have never heard any theory besides it, so it’s kind of hard to talk about what “evidence” would be in this case (since it’s not really an argument) but I’ll tell you a few things about our solar system and others that let us know that this is what happened.
The most conspicuous and convincing piece of supporting evidence is that all of the planets orbit in basically the same plane. Out of the 8 major planets, the largest angle that one deviates from the same plane is around 7 degrees for Mercury. This is because as the cloud of gas condensed to create the solar system, it had some dominant direction of rotation that was passed on to all of the things that it formed. If the planets were just assembled randomly by some other process, we would not expect this, so nebular theory makes much more sense. In addition, random orbits would likely be much more elliptical rather than circular.
Another supporting piece of evidence along the same lines is that the rotations of most bodies in the solar system go in the same direction as well. This is not as overwhelmingly true as their orbital directions because there are exceptions (Uranus rotates sideways and Venus rotates backwards) and these are open questions in solar system dynamics, but it is still much better than random chance.
One more piece of evidence that is a bit less obvious is the composition of the solar system. We have seen that the planets in the solar system tend to be made of similar mixtures of elements, which wouldn’t necessarily be true if they weren’t all formed out of the same cloud. Furthermore, every piece of material we have found in the solar system can be radiologically dated to basically the same time, meaning everything in the solar system must have formed concurrently, pointing to a single collapse of a gas cloud rather than separate formation.
Are infrared telescopes sophisticated enough to peer through the thick atmospheres of gas giants like Jupiter and Saturn since infrared light can pass through gas and dust better? (Advanced)
This is a very interesting connection to make. One of the appeals of IR telescopes is that IR light is not blocked by gas and dust as much, enabling observations of things that would otherwise be hidden (this is one of the main motivations of the James Webb Space Telescope and many other infrared telescopes). And you’re also correct that the thick cloud layers in gas giants prevent us from knowing very much about their internal structures. So one might think that this would be helpful for studying Jupiter’s interior, but these two ideas don’t really fit together in the way you might hope.
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You may have seen a picture like the one above before. This is a dense gas cloud called a Bok globule (specifically Barnard 68), a very dense cloud of gas and dust that are thought to be in the process of collapsing down to form a star, observed at wavelengths ranging from blue to mid infrared. Visible light (shown in the 0.44 and 0.55 micron images) is almost entirely blocked by the center of the gas cloud, but the cool thing about this collage of pictures is that the cloud gets so much more transparent at the longer infrared wavelengths because infrared light can pass through more easily, allowing us to see the stars that are behind it. However, I want to point out that this process is not magic. There is still a noticeable amount of dimming from the gas and dust even in the longest wavelength image in the bottom left, so even infrared imaging has its limits.
The other thing to point out is that this cloud of gas, which is extremely dense by galactic standards, is still essentially a vacuum. The average density of this cloud is about 500 quintillion times smaller than the average density of Jupiter, despite Jupiter only being about 100 million times smaller. So a beam of light travelling through Jupiter would have to get through significantly more gas and dust if it wanted to escape the planet, meaning that even with the benefits of IR observations, we still wouldn’t be able to see that far. This isn’t really a question of “sophistication” of the telescopes, it’s just that there isn’t anything to look at.
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This is compounded by the fact that Jupiter is not all just gas clouds. Eventually the pressure of the hydrogen atmosphere gets so high that it condenses the hydrogen down into a liquid metal, which would reflect basically all light. So any core would be below thousands of miles of dense gas followed by thousands of miles of even denser liquid metal. Realistically, no light is going to be able to make it through that.
So although astronomers are excited about upcoming IR instruments for a number of reasons, looking at Jupiter’s interior is not really one of them. Currently, the best information about Jupiter’s interior we have is from the very precise gravity readings from the JUNO probe which can give us a rough idea of how lumpy it is.

Exoplanets

On a tidally-locked planet, what sort of conditions would have to be in place for both sides of the planet to be inhabitable? (Beginner)

The short answer is essentially that it is very unlikely that a tidally locked planet could ever be habitable all the way around without some weather condition being unfavorable. Usually, astronomers don’t consider tidally locked planets to be inhabitable at all.

For a more detailed explanation, tidally locked planets have a number of problems, mainly due to the fact that one side is very hot all the time. The temperatures would be extreme, with temperatures far too hot for life under the sun and far too cold on the dark half of the planet. Favorable temperatures would only be found somewhere in the “sunset zone” around the edge of the planet where the sun is low on the horizon. If you imagine an ocean facing the sun all the time, it would eventually evaporate and have its moisture carried off elsewhere, likely to the other side of the planet where the temperatures are hundreds of degrees colder and the moisture would freeze out, depositing a large amount of water on the other side. Beyond this, heating up rock continuously leads to it expelling gas, leading to a Venus-like runaway greenhouse effect, further messing with the planet’s weather.

The only way I could see the temperature being somewhat consistent would be if there were extremely strong winds going in bands around the planet. These types of winds exist in some form on the Earth (I’m not a meteorologist so I don’t know the details), but if you had winds that distributed the sun’s energy more equally around the planet, it might extend the habitable zone a bit. Of course, this would cause lots of other habitability issues and extreme weather, so this probably wouldn’t be nice.

How do astronomers determine the rotations of celestial-bodies outside of our solar system? (Beginner)

It is very difficult to measure rotation for planets outside the solar system (exoplanets) and stars, but mostly astronomers rely on looking for regular fluctuations in brightness. If an exoplanet or star has a dark cloud or surface feature on it, then you can expect that once per rotation, that dark cloud would reduce the overall amount of light that you see from that object. If astronomers see regular dips in brightness like this, then you can guess that the time in between the dips is one day for that thing.

How perfectly aligned to exoplanet orbits have to be in order to be observed by the transit method? (Advanced)
This is actually something I’ve been thinking about in the back of my mind for a while, and this question made me sit down and do the research and math to answer it properly. You are correct in your calculation of the angle of inclination that is necessary to see the Earth in front of the Sun, but let’s be a bit more careful about figuring out your chances to observe this. First, it is necessary to double the angle since you can observe the Earth from above or below its orbit so we actually have a window of about 0.5 degrees. Next, we have to realize that we can observe this from any angle around the Earth’s orbit, so we have to revolve this around 360 degrees and divide by the area of the sky. After all of this, we land with a probability of about 0.5% of being in the region of sky that could observe a transit of the Earth.
This is indeed a very small chance, and this is why exoplanet observers typically monitor large numbers of stars to see which ones line up. For instance, the Kepler space telescope observed approximately 150,000 stars simultaneously to look for transits, so with the chances from above, it would be able to see about 750 earths.
However, Kepler ended up observing over 2500 planets (more are still being confirmed) for a couple of reasons. First is that the vast majority of known exoplanets are closer to their star than the Earth is, meaning that the possible viewing angles are much larger (for a typical “hot Jupiter” 0.1 AU, the fraction of the sky increases to 5%, and for the first exoplanet ever discovered 51 Pegasi b which is only 0.05 AU away, the fraction is almost 10%). So these planets are much more visible than further planets, and their larger size also means their transit dips are easier to detect. The other reason Kepler detected so many planets is that many stars have multiple planets in their system, and if one planet is aligned properly, then the others have a higher probability of being aligned too. About a third of Kepler’s planets are in multiplanetary systems.
So even though astronomers can’t confirm that every star has planets using transits, they can make statistical arguments based on how many planets they see versus how many they would have expected to see probabilistically. Also, orbital planes don’t tend to align with the plane of the Milky Way (ours doesn’t, it’s about 60 degrees off) because the orbital plane is determined by the specific way the gas cloud that made the solar system collapsed.
Would it not make sense that maybe we would have more rocky planets like earth in nearby solar systems with similar elemental makeup? (Advanced)

This is an interesting question that I had to spend a bit thinking about. There are a number of factors at play here that make the answer to your question a bit complicated. The first thing to note is that usually when astronomers talk about what a given star is made of, they talk about its “metallicity.” Conceptually, this is the amount of heavy elements (heavier than helium) present in a star, and practically this is usually defined based off of the ratio of iron to hydrogen in a star (Fe/H). You have correctly pointed out that younger stars tend to have higher metallicities than older stars because they incorporate metals from previous generations of stars. 

Typically, stars form in large clusters from a single cloud of gas that condenses down into numerous stars in close proximity. These stars stay together for a period of time (the Pleiades star cluster in the night sky is a good example of what this looks like) but after a few hundred million years, their movement through the galaxy carries them away from each other, scrambling up the stellar population throughout the galaxy. So to answer your immediate question, the Sun’s “siblings” i.e. stars that were born from the same gas cloud and thus have the same composition, are likely very far from the Sun now so there should be no local grouping of “similar planets.”

However, in a broader sense, we do expect there to be roughly similar planet trends around our Sun because there is a trend in stellar metallicity as you go radially outwards from the center of the galaxy. Since there is more stuff (i.e. stars, gas, dust, etc.) in the center of the galaxy, more stars are formed and more metals are created compared to the more sparse outskirts of the galaxy. Astronomers have also observed a correlation between the metallicity of a star and the number of planets that form around it, so at a given distance from the galactic center (i.e. for a certain metallicity), there will actually be some similarity in the number of planets in a given solar system. If you’re interested in some more technical reading, I recommend this paper from Fischer & Valenti in 2005 about the subject, specifically Figure 4 which shows that stars with more iron are more likely to have planets. 

So the Sun isn’t particularly related to the stars immediately around it, but it does fit in with an overall trend through the whole galaxy that makes it so nearby stars may be somewhat similar planet-wise.