However, certain gases in our atmosphere, such as carbon dioxide, methane and water vapor, work like a blanket to retain much of that heat. This helps to warm our atmosphere. The gases do this by absorbing the heat and radiating it back to Earth’s surface. These gases are nicknamed “greenhouse gases” because of their heat-trapping effect. Without the “greenhouse effect,” Earth would be too cold to support most forms of life.

But you can have too much of a good thing. Carbon dioxide is released when we use fossil fuels, such as coal, oil and natural gas. We burn these fuels, made from the ancient remains of plants and animals, to run electricity-generating plants that power factories, homes and schools. Products of these fossil fuels, such as gasoline and diesel fuel, power most of the engines that drive cars, airplanes and ships.

By examining air bubbles in ice cores taken from Antarctica, scientists can go back and calculate what the concentrations of carbon dioxide in the atmosphere have been throughout the last 650,000 years. The amount of carbon dioxide in the atmosphere has been climbing to where today it is 30 percent greater than 650,000 years ago. That rise in carbon dioxide “is essentially entirely due to the burning of fuels,” Susan Solomon says. She’s a senior scientist with the National Oceanic and Atmospheric Administration, in Boulder, Colo., and studies factors that affect climate.

Humans have further increased the levels of greenhouse gases in the air by changing the landscape. Plants take up carbon dioxide to make food in a process called photosynthesis. Once cut down, they can no longer take in carbon dioxide, and this gas begins building up in the air instead of fueling the growth of plants. So by cutting down trees and forests for farmland and other human uses, more carbon dioxide is also added into the atmosphere.

“We’ve always had some greenhouse gases in the atmosphere,” Solomon says. “But because we’ve burned a lot of fossil fuels and deforested parts of the planet, we’ve increased the amount of greenhouse gases, and as a result have changed the temperature of the planet.”

“The seven planets according to the Greeks were the seven planets at the time of the Copernicus, and those seven included the sun and the moon,” he says.

Nicolaus Copernicus was the Polish astronomer who suggested in the early 1500s that the sun, and not the Earth, was at the center of what we today call the solar system. He removed the sun from the planet tally. Then, in 1610, the Italian mathematician Galileo Galilei pointed a telescope to the sky. He saw, for the first time, the objects we know today as the four moons of Jupiter.

Later that century, the astronomers Christiann Huygens and Jean-Dominique Cassini spotted five additional objects orbiting around Saturn. At the end of the 1600s, astronomers agreed that the objects orbiting Jupiter and Saturn, along with those two planets, Mercury, Venus, Earth, Earth’s moon and Mars, should all be called planets. This brought the grand total of objects called planets to sixteen.

Between that time and the early 1900s, the objects astronomers called planets fluctuated from a high of 16, back to six when the objects circling planets were reclassified as moons, up to seven when Uranus was discovered, and back up to 13 after the initial discovery of several objects lying between Mars and Jupiter — objects we know today as asteroids.

Clearly, scientists have been naming, re-naming and categorizing the various parts of the solar system ever since people began looking at and documenting the objects in the night sky thousands of years ago. In 2006, the International Astronomical Union defined “planet” in a way that kicked Pluto out of the planet tribe.

Atoms, which used to be considered the smallest unit of matter, are made from particles too. Just how small is an atom? That’s a tricky question, since different atoms have different sizes and atoms are mostly empty space. But here’s one way to think about it: Let’s say you wanted to fill up a baseball-sized bowl with gold atoms. You’d need roughly twice as many of these atoms as it would take to fill an Earth-sized bowl with baseballs.

Particles that are even smaller than an atom are called “subatomic.” The main subatomic particles that make up atoms are protons, electrons and neutrons. But some of these particles are also made of even smaller particles. Protons and neutrons, for example, are made of subatomic particles called “quarks.” There are six kinds of quarks, each with a weird name: up, down, strange, charm, top and bottom.

Dozens of types of subatomic particle exist, and scientists suspect there may be still more to discover. When a new type emerges, scientists tend to give them pretty odd-sounding names. To date, we’ve got bosons, fermions, leptons, muons, pions, neutrinos, photons, gluons, and gravitons.

Neutrinos are unusually weird because they have almost no mass and they fly through space at almost the speed of light. Three types exist: muon neutrinos, electron neutrinos and tau neutrinos.

And the strangest particle of all: the tachyon. It’s considered “hypothetical,” which means it might not even exist. If it does, it can go faster than the speed of light and travel back in time.

No wonder some physicists refer to these — the smallest inhabitants of our universe — as their “particle zoo.”

Going Deeper:

Credit: NASA/Imagine the Universe

Energy travels throughout the universe at the speed of light in the form of electromagnetic radiation. What that radiation is called depends on its energy level.

At the really high-energy end of the spectrum, you’ve got gamma rays. You’re probably familiar with a close cousin to these: X-rays. They’re the ones doctors and dentists use to probe for unusual structures inside your body. Radio waves fall at the extreme other end. Those radio waves are the ones that deliver music and news broadcasts to your home radios.

Credit: NASA/Imagine the Universe

Ultraviolet rays, visible light, infrared radiation, and microwaves fall at energy levels in between.

Together, all of these types of radiation make up one long, continuous electromagnetic spectrum. Its energy travels in what’s usually referred to as waves.

What separates one type of electromagnetic radiation from another is its wavelength. That’s the length of a wave of that type of radiation. To identify the length of a wave of water in the sea, you would measure the distance from the crest (upper part) of one wave to the crest of another. Or you could measure from one trough (bottom part of a wave) to another.

It’s more difficult to do, but scientists measure electromagnetic waves the same way—from crest to crest or from trough to trough. In fact, each segment of the energy spectrum is defined by this wavelength. Even what we refer to as the heat given off by radiators is a type of radiation—one that has wavelengths in the infrared portion of the spectrum.

Sometimes these segments of the electromagnetic spectrum are also described in terms of frequency. A radiation’s frequency will be the inverse of its wavelength. So the shorter the wavelength, the higher its frequency. That frequency is typically measured in hertz, a unit which stands for cycles per second.

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Scientists are still trying to figure out a lot about the sea’s gooey creatures, and the different kinds of jellies can be hard to tell apart. The true jellyfish are called scyphozoans (sigh-fuh-ZOH-unz) and are a kind of jelly. Then there are two groups of close relatives—box jellies and hydrozoans (HIGH-druh-ZOH-unz). While they are very close relatives of the true jellyfish and they have the same stinging cells, scientists don’t consider them true jellyfish.