This post goes into the field of science, or possibly pseudo-science (which you might expect from someone who names himself after a creature whose existence is not universally accepted). There are some interesting but strange ideas out there, some of which I've decided are a bit too strange.
I do not believe:
That there is a planet, often called Nibiru, which is headed for a close encounter with Earth.
This idea seems to be quite popular on YouTube, and is based on the work of Zechariah Sitchin, who in his book The Twelfth Planet, alleges that the solar system includes a planet with a highly elliptical orbit. About every 3,600 years this planet, called Nibiru or Marduk, passes through the inner solar system. According to various YouTube users, Nibiru will soon either collide with Earth or pass closely thereto, which either way will be catastrophic. The trouble is, predictions of this planetary encounter have been going on for over a year, with the predicted times of Nibiru's arrival coming and going without it ever showing up. Another problem is that the Nibiru theorists don't agree with each other as to the planet's characteristics and trajectory. Some say that it's coming from the south, which is why nobody in the northern hemisphere can see it. Others say that it's behind the sun, either directly or off to one side, and show pictures of the sun with a second smaller luminous sphere nearby. I've also heard that Nibiru has an extremely dark surface that reflects almost no light from the sun, which is why it can't be seen. Is this object very dark, or does it reflect so much sunlight that it looks like a second sun? Both can't be true. For those of you who believe in an imminent encounter with Nibiru, could you please decide what it is, where it is, and how it's moving?
I would think that if such a planet were on its way into the inner solar system, and is only months away from a close encounter with Earth, astronomers would be able to detect it. This would be especially true if it were larger than the Earth, as some have claimed. Even if it were very dark, its presence could still be detected because it would pass in front of other celestial bodies, such as distant stars. On the other hand, if it is near the sun but offset to one side, it should be easily seen by people in general, which doesn't seem to have happened yet. If Nibiru or some other object is near the sun, it should be moving rather quickly, since objects near the sun travel faster than objects farther away, and we should be able to see its change in position. Some videos I've seen appear to conflate Nibiru with Planet 9, which has been postulated, due to its alleged effects on certain Kuiper belt objects, but not yet actually found. This conflation is nonsense, because Planet 9, if it exists, is located far outside the orbit of Pluto, thus being far too distant to constitute any threat to the Earth, and has an orbital period between 10,000 and 20,000 Earth years, far more than the 3,600 years alleged for Nibiru.
That there is a satellite of alien origin, known as the Black Knight Satellite, in orbit around the Earth.
The Black Knight Satellite is an alleged object in polar orbit around Earth, placed there about 13,000 years ago by space aliens. According to Wiki, the BNS "is most likely a conflation of several disconnected stories". According to Ancient Code, both the United States and Soviet Union have shown an interest in this satellite, and was asserted by a ham radio operator to have transmitted data believed to be a star chart centered on the Epsilon Boötes system. However, as pointed out by Ancient Aliens Debunked, the radio operator later found flaws in his own data and "recanted". The AAD article does a good job reviewing and debunking the "evidence" for the BNS. I'll add just one more reason that I don't believe that a 13,000-year-old satellite is in orbit, polar or otherwise, around the Earth. Humans have been sending satellites into orbit since the late 1950's. The orbits of many satellites have deteriorated, resulting in them falling down into the atmosphere, where they meet a fiery end. It thus seems very unlikely that an artificial satellite could stay in orbit for 13 millennia.
That airplanes are being used to spray "chemtrails" into the atmosphere, for some type of nefarious purpose.
The relatively opaque nature and duration of some jet contrails, this particular word being derived from the longer term "condensation trails", is sometimes alleged to be evidence that something other than combusted jet fuel is being emitted from jet engines during flight. Because this is allegedly done for some sinister reason, such as weather modification or to adversely affect human health, these particular contrails are called "chemtrails".
I do not believe that any sort of chemical is being released from airplanes, other than the normal jet exhaust, for several reasons. First, due to the high altitudes of normal jet flights, any chemical released therefrom would be very difficult to direct toward any particular place on the ground, and would dissipate before reaching the ground, in part because they would have to pass through layers of air which might have different wind directions. Second, any chemical added to jet fuel would be burned up in the engine, and thus be converted into the oxides of its constituent elements. Anyone desiring to release chemicals, for nefarious reasons, by means of jet exhaust, would have to take into account the reactions which occur when the fuel is burned. Alternatively, if the chemical is added to the exhaust downstream from the engine, no one that I know of has pointed out how and by what means this is done. Third, jet exhaust is made mainly of water vapor and carbon dioxide, the former condensing, depending on the temperature of the surrounding air, into water droplets or ice crystals. This is precisely the composition of a natural atmospheric phenomenon known as (wait for it, wait for it) clouds. That's right, a contrail is essentially an artificial cloud, which would mean that a contrail should be expected to dissipate no more quickly than a natural cloud.
As pointed out by HowStuffWorks, engine exhaust can have components other than carbon dioxide and water, including solid materials such as sulfate particulates and soot, which indeed "can have negative environmental effects". This isn't too different from the exhaust of land-based vehicles, which means that "chemtrails" would be no more of a problem than the stuff coming out of your truck, car, or SUV. So when you look up and see lines of cloud-like trails in the sky, don't worry. It's probably no worse than what's coming out of whatever you're driving.