What's the fastest thing on Earth?

A spacecraft launching, a race car zipping around a track, a cheetah hurtling toward its prey — our home planet is full of high-speed feats.

But what's the fastest thing on Earth? The answer depends on how you define "thing" and "on Earth." And the top candidates — neutrinos and photons — cannot be seen with the naked eye.

The most obvious answer appears to be light. In a vacuum, light travels approximately 186,000 miles per second (300,000 kilometers per second). Nothing in the universe that we know of can exceed this speed.

Related: What is the smallest particle in the universe? (What about the largest?)

Is light a thing, though? Physicists don't fully agree on this. Some say no, because light has no mass. Others say yes — due to the quirks of quantum mechanics, light is simultaneously a wave and a particle. And most physicists agree that particles are things.

In the vacuum of space, those particles, called photons, are the fastest things, according to John Matthews, a physicist at the University of Utah.

Down on Earth, it gets a little more complicated, unless you're in a vacuum chamber. Once a photon hits Earth's atmosphere, it slows down slightly. And then, under the right circumstances, it could have some competition. That's because not all particles are slowed by the atmosphere the way photons are, Matthews told Live Science.

Matthews is part of a team that has detected several very fast particles originating from ultra-high-energy cosmic rays, showers of subatomic particles that rain down on Earth from space. One of these particles, dubbed the Oh-My-God particle, was detected by his colleagues in 1991, from the highest-energy cosmic ray that's yet to be spotted.

Particles like these start out going extremely close to the speed of light in a vacuum. But when they hit Earth's atmosphere, "by their nature, they just continue to barrel ahead," Matthews said. "So they're exceeding the speed of light in the atmosphere."

That makes the Oh-My-God particles among the fastest things with mass on Earth, but they're not at the very top. Instead, the neutrino takes that prize, per Justin Vandenbroucke, a particle physicist at the University of Wisconsin-Madison.

The Oh-My-God particle is probably a proton, or at least proton-like — fairly massive on the scale of subatomic particles. A neutrino's mass is at least 10 billion times smaller than a proton's, so thanks to the basic laws of physics, it can travel even faster if given the same amount of energy.

But while neutrinos can, in theory, go very fast, it's quite hard to pin them down in practice. "Neutrinos are famously shy," Vandenbroucke told Live Science.

In a long-term experiment at the South Pole — aptly named IceCube — physicists have placed detectors inside a 0.2-cubic-mile (1 cubic kilometer) chunk of ice, hoping to find high-energy neutrinos. Inside ice, a neutrino with enough energy can travel faster than light can. When that high-energy neutrino collides with the nucleus of an atom in the ice, it can produce charged subatomic particles also traveling faster than light. These speedy particles emit a flash of light known as Cherenkov radiation, making the neutrino indirectly detectable.

And in 2016, IceCube scientists detected the highest-energy neutrino that's been carefully evaluated yet.

"As far as we know, these are the fastest particles ever seen," Bill Louis, a physicist at Los Alamos National Laboratory, told Live Science. No particle with mass can reach the vacuum speed of light, but if it's very light and has a lot of energy, it can get very close.

Just how close? To keep the speeds straight, Vandenbroucke likes to think in terms of 9s. Something moving at 99.99% the speed of light would have four 9s. The superfast neutrino detected in 2016 would have 33 9s, Vandenbroucke said. And the Oh-My-God particle would have somewhere between 20 and 24 9s, per his calculations. This particle had more energy, but it was also much more massive.

For comparison, the fastest speeds reached by particles in human-made particle accelerators like CERN's Large Hadron Collider would have just seven 9s.

Meanwhile, these record-setting superfast particles "are produced by naturally occurring particle accelerators out in the universe," Vandenbroucke said. "We don't know how they work, but it really tells you how amazing nature is compared to humans."