1. The artificial manipulation of the environments of the earth; especially, as a means of counteracting global warming.
2. Proposals to deliberately manipulate the earth's climate to counteract the effects of global warming from greenhouse gas emissions.

So far, no large-scale geoengineering projects have been undertaken, nor has a consensus been reached that geoengineering is desirable.

President Obama’s science adviser, Dr. John Holdren, after giving his first round of interviews (April 10, 2009) immediately caused a ruckus by airing his thoughts on geoengineering; the large-scale tinkering with the earth’s climate to chill runaway global warming climate changes that could potentially slow or reverse global warming.

Holdren emphasized that even if he personally thinks it prudent to start evaluating geoengineering options, he still believes that the most pressing concern should be curbing greenhouse gas emissions to prevent global warming from reaching catastrophic proportions.

The fraction (approximately 30 percent) of incident solar radiation that is reflected by the Earth-atmosphere system: Planetary albedos are returned to space, mostly by scattering from clouds in the atmosphere.

Someone who studies planets, moons, and planetary systems, in particular those of the solar system and the processes that form them: A planetary astronomer studies objects ranging in size from micrometeoroids to gas giants, with the objectives of determining their compositions, dynamics, formations, interrelations, and histories.

The investigation of the solar system and planetary systems using both ground and spacecraft-based observations: Planetary astronomy includes the study of Kuiper belt objects, dwarf planets, Titan, other icy satellites, extra-solar planetary systems (from brown dwarfs to stellar disks), and the occasional inner solar system body.

The lower part of the earth's atmosphere that is directly influenced by conditions on the surface, such as evapotranspiration: The height of the planetary boundary layer is variable and ranges from 100 to 3,000 meters.

The situation when two or more planets look as if they are very close together, but in reality they are far apart from each other: A planetary conjunction takes place when a planet is closely aligned with another celestial object, such as the moon, a star, or another planet.

The cloud of expanding gas surrounding a star that has blown off its outer layers, possibly in a nova stage: A planetary nebula is a shell of gas thrown off by a star at the end of its life.

Planetary nebulae have nothing to do with planets. They were named by William Herschel, who thought their rounded shape resembled the disk of a planet.

After a star, such as the Sun, has expanded to become a red giant, its outer layers are ejected into space to form a planetary nebula, leaving the core as a white dwarf at the center.