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.
It includes the studies of objects ranging in size from micrometeoroids to gas giants, with the objectives of determining their compositions, dynamics, formations, interrelations, and histories.
It 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.
Its height is variable and ranges from 100 to 3,000 meters.
2. 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.