• Greenhouse effect: Atmosphere traps some of the energy radiated from surface, heating planet
• Tidal forces: Gravity pulls more strongly on closest part of object, distorting shape
• Magnetic field: Strength depends on rotation rate and amount of conducting material in core
• Surface gravity: Determines weight; proportional to mass, inversely proportional to radius squared
• Rocky planets: Inner planets; slow rotators, heavy atmosphere if any, rocky composition
• Gas giants: Outer planets; fast rotators, hydrogen/helium atmosphere, gas/liquid with rocky core, moons/rings

• Core: Interior of planet, typically rocky or metallic, dense and hot
• Mantle: Region between core and surface of planet
• Plate tectonics: Large plates on surface of Earth interact, forming mountains and ridges
• Troposphere/stratosphere/mesosphere/ionosphere: Levels in the Earth's atmosphere, in increasing height
• Maria: Dark areas on lunar surface, less heavily cratered, probably cooled lava flows

Using NASA, Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that our galaxy's central black hole let loose a powerful flare three centuries ago.

The finding helps resolve a long-standing mystery: why is the Milky Way's black hole so quiescent? The black hole, known as Sagittarius A* (pronounced "A-star"), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."