The Rays, They Raineth Every Day

Another great question from Papou: How many different particles from beyond our Universe reach the earth and penetrate our human bodies? What particles are they and where did they emanate from?

There's an easy answer to the question as written, since "the Universe" usually refers to everything that has or ever will exist: None! That's not very satisfying though, so instead, let's look at some of the particles that arrive on Earth from space.

Solar Neutrinos
Starting off close to home, aside from light, the Sun also puts out about 9 x 10^28 neutrinos per second that stream through the Earth. Neutrinos only rarely interact with other matter, which should relax you about the sheer number hitting us right now. My friend Kevin studies these neutrinos, and the only way to detect them is to burrow deep into the Earth, to filter out other particles, and then build a huge tank of ultra-pure water surrounded by photodetectors. Now and then, a neutrino will interact with one of the water molecules and emit a flash of light. There are several types of neutrino, and Kevin studies how these different types change into each other in a process called oscillation.

Cosmic Rays
Moving outside the Solar System now, we also get cosmic rays from distant objects. These are massive particles, like protons or electrons, that hit the atmosphere at high speed. Unlike neutrinos, these do interact with the molecules in the atmosphere, and cause a cascade of reactions, similar to what happens in a particle collider:

PhysicsOpenLab

The main source of these cosmic rays is believed to be supernovae. When stars explode, they send out particles in all directions at high speed. These particles can travel a long distance, since space is big, and there's not a lot to run into. When they reach Earth, they're going so fast that entering the atmosphere produces Cherenkov radiation. This type of radiation occurs when a massive particle enters a substance faster than light can travel in that substance. Massive particles cannot go faster than light in a vacuum, but light can go much slower in other materials. This type of radiation is analogous to a sonic boom. I actually experienced Cherenkov radiation as a static fuzz over my vision during my proton radiation treatments.

Gamma Rays
The cosmic rays I just described are high-energy massive particles, but there are also high-energy massless photons that hit the Earth. These are referred to as gamma rays, due to the symbol for photons used in particle physics, 𝛾. In the 1960s, the US launched the Vela satellites to look for gamma rays coming not from space, but the surface of the Earth. They hoped to use the satellites to monitor Russian nuclear tests, which also produce gamma rays, but were surprised to find signals coming from space instead. When the LIGO/Virgo collaborations detected a neutron star merger in 2017, scientists discovered it was correlated with a gamma ray burst in the same direction, suggesting the burst was produced in the collision.

Shields Up!
All this talk of high-energy particles hitting Earth might have you a bit worried. Lucky for us, we have some defenses against them. The ozone layer absorbs most of the UV light from the Sun, and the Earth's magnetic field protects us from most charged particles. Astronauts who spend time outside these protective fields though, are exposed to numerous risks from exposure to particles like these. NASA has studied the effects of space flight by comparing the twins, Mark and Scott Kelly, and while some differences are temporary, the increased radiation exposure can cause permanent DNA damage. We should be glad for the protections our planet offers us, and do what we can to protect it in turn!