A star is flinging comets at Earth.
A long-standing theory known as Nemesis supposes that there is some sort of "death star" on the outer edge of the solar system, whose orbital motions perturb comets in an icy region of objects known as the Oort Cloud. According to the myth, the star's gravity throws these comets toward the inner solar system, and these comets collide with Earth and cause mass extinctions once every 27 million years.
However, 2011 study concluded that this idea is unlikely, because the comet strikes in recorded history haven't happened with any regularity. The pattern that was recorded in the hoax is actually a statistical artifact, or the result of researchers trying to find patterns in nature where they do not exist, the study's authors found.
There's life on Venus.
Back in the 1970s and 1980s, the Soviet Union sent several uncrewed missions to study Venus. Ten of these Venera probes landed on the surface of Venus and were able to transmit data and images for a few minutes before succumbing to the planet's extreme atmosphere. In 2012, the Russian news service RIA Novosti reported that Leonid Ksanfomaliti, a scientist who worked on the Venera missions, suggested that the photographs showed living objects moving around on the planet's surface. (RIA Novosti ceased operations in 2013.) [Photos of Venus, the Mysterious Planet Next Door]
These alleged life-forms on Venus are just an example of "letting your mind see patterns in low-resolution data that simply aren't real," Jonathon Hill, a research technician who processes images taken during NASA's Mars missions, explained to Space.com's sister site, LiveScience, in 2012. According to NASA, the objects that appeared to be moving were actually camera-lens covers that automatically popped off of the cameras after landing LiveScience reports. These half-circle objects were seen in images from Venera-13 and Venera-14, two identical spacecraft that landed about 590 miles (950 km) apart. Both had two identical cameras — one in the front and one in the back — so it makes sense that the covers would appear in different places. Another photograph that Ksanfomaliti said was a scorpion is actually a blur in the image.
An asteroid is about to crash into Earth.
This recurring rumor claims that a threatening "doomsday" asteroid is about to slam into our planet. An example from 2015 had an asteroid purported to hit Earth in late September, when it would supposedly wreak devastation from its impact point near Puerto Rico. NASA quickly dismissed the reports — which turned out, as usual, to be false. But that's not to say that asteroids will never hit our planet.
NASA and a network of monitoring telescopes across the world are cataloging all known asteroids wider than 459 feet (140 meters) across in line with a 2005 congressional mandate. (Smaller asteroids, if found, are also cataloged.) Of the space rocks discovered so far, NASA has not found a single asteroid that has a high probability of hitting Earth in the foreseeable future. [Gallery: Potentially Dangerous Asteroids]
Aliens landed in Roswell, New Mexico
On a ranch in Roswell, New Mexico, so the story goes, an alien spacecraft crashed in 1947. While the accounts of exactly what happen vary, the legend claims that a disc or some sort of spacecraft was found on a ranch, and that the government quickly covered up the evidence.
While rumors of aliens circulated, some people speculated that the crash was just a plain old weather balloon that might not have been recognized by the local community. The U.S. military has since acknowledged the "spacecraft" was actually a weather balloon sent aloft as part of Project Mogul, which involved flying microphones on high-altitude balloons to listen for sound waves generated by possible Soviet Union nuclear tests.
Climate change isn't real.
Earth is on an abnormal warming trend. Arctic ice is melting, the sea level is rising and temperatures are going to extremes in many locations around the world. Why is this happening? Anti-climate-change conspirators have many explanations: solar activity, radiation, the Earth's (and sun's) movements around the Milky Way, among other theories.
While there are many components of climate change, the fact that humans have contributed to it is indisputable. Temperature graphs show that the climate has not warmed this much, this quickly in all of Earth's history (as seen in geological records), and that the increase correlates with increased industrialization. [The Reality of Climate Change: 10 Myths Busted]