As observation techniques have advanced, however, scientists have begun discovering a newer, smaller type of planet - the tantalizingly named "Super Earth."
"A 'Super Earth' is generally considered to be a planet that's up to about 10 times the mass of the Earth," explains JPL scientist Steve Edberg. "Planets bigger than that tend to be gaseous, like Uranus or Neptune."
Super Earths are notable because, unlike gas giant planets, they're small enough to have terrestrial surfaces or liquid oceans that could support life as we know it.
And while none of the Super Earths discovered so far would be a good place to take a vacation, scientists are hopeful that they'll find one with the right chemical composition and at the right distance from its star to support living things.
So what's life on a Super Earth like? First of all, cautions Edberg, it's important to remember that a planet's mass and size are two different things. "The relationship between a planet's size and its mass isn't linear," Edberg explains. "A world that's 10 times the mass of Earth will not be 10 times as big in diameter; it'll actually be quite a bit smaller than that."
You might also feel a bit heavier than normal if you were to visit a Super Earth, says Edberg, because "a bigger planet is going to have more gravity...it's also probably going to have a thicker, more dense atmosphere than Earth's."
Despite these differences, Edberg points out that under the right conditions, a Super Earth could harbor living things. "You might not get redwood trees and basketball players," he says, "but you'd still have the right ingredients for life to get established."
And as the Super Earth discoveries begin to pile in, chances are that the discovery of such an Earthlike planet may be just around the corner.
Terrestrial or Not?
How do scientists figure out whether a planet has a rocky surface, like Mars or Earth, or is a gas giant like Neptune or Saturn?
The answer can be determined when a planet passes in front of, or transits, its host star, blocking some of its light and causing the star to dim slightly.
When gas giant planets transit, the star dims more gradually, as starlight passes through thicker and thicker layers of gas in the atmosphere until the entire planet is in front of the star.
Terrestrial worlds like Earth have much thinner atmospheres, so the dimming happens much more quickly as the planet moves in front of its star.
Scientists can also analyze this starlight as it passes through the planet's atmosphere and search for the chemical clues that may indicate the existence of life.