That's all changed in the past few years, and the plenitude of possible worlds to explore was confirmed Wednesday when a California Institute of Technology team reported there are billions and billions of planets, just within this galaxy.
They're known as exoplanets, and the recent spate of discoveries has changed astronomers' beliefs about the nature of planetary systems. Rocky, Earth-sized planets very close to their star are now considered the most common kind in the universe.
"If you look up at the night sky, probably every star, statistically speaking, has a planet or two," said John Johnson, who leads Caltech's planet-hunting Exolab. "That is a big new discovery that's ushered in this new era of exoplanets. It was charging forward before that, but now it's almost overwhelming."
Johnson led the study that determined there are at least 100 million planets in the Milky Way, based on the characteristics of a star named Kepler-32. There are five planets orbiting the star, and all of them are visible because the disc-shaped planetary system faces Earth on its edge, so astronomers can see the planets blocking the star's light as they orbit.
Detecting planets that way is known as the transit method, and it's the bread-and-butter of NASA's Kepler telescope, an orbiting spacecraft that trains its eye at a specific patch of the Milky Way containing about 150,000 stars. It launched in 2009, and in November began an extended mission for another four years.
The extra time will give astronomers greater chance to find Earth-sized, terrestrial planets in longer orbits around their stars, and many of those could be in the habitable zone, the sweet spot in planetary systems where liquid water could exist.
"Even in the next few months, several new discoveries like this are going to pour out," said Nick Gautier, the Kepler project scientist at Jet Propulsion Laboratory.
Some scientists already believe a big discovery will be just around the corner.
"I'm very positive that the first Earth twin will be discovered next year," Abel Mendez of the Planetary Habitability Laboratory in Puerto Rico told space.com in December.
The possibility of finding life on other planets is the potential gold mine of habitable zone exoplanets, and ideas about how to overcome light years to visit another star no longer seem so far-fetched. Reality before long could mirror the planet-hopping depictions in science fiction such as "Star Trek" and "Star Wars."
"I grew up with visions of Tatooine, Hoth, Dagobah," Johnson said. "Now we're finding things that could be like those planets."
A Tatooine-style planet orbiting two stars was among last year's exoplanet discoveries. Researchers also found that one planet is made largely of diamonds. It's discoveries like those that have contributed to an exoplanet boom, not only because the tools are improving but because more people are interested in exploring space.
At Caltech, the search for exoplanets has prompted the November formation of the Center for Planetary Astronomy, which combines two distinct camps: astronomers, who mostly look beyond our own solar system, and planetary scientists, who mostly look within it.
The center would rival the Harvard-Smithsonian Center for Astrophysics as one of the only university environments where students could study planets such as Jupiter along with the gas giants elsewhere in the universe.
"This field moves so fast," Johnson said. "If you try to do old-fashioned astronomy where you do a paper on any topic, you're going to get left behind. When we announced the three smallest planets that had ever been discovered (in January 2012), for us in our group it was old news."
Johnson also leads Project Minerva, a planned telescope array at Palomar Mountain that will the first observatory dedicated exclusively to exoplanets.
Johnson called it a low-cost expressway for finding exoplanets, thought up by his Exolab students.
"If we lived in an ideal world, we wouldn't do Minerva because we'd have money from our funding agencies," he said.
The European Space Agency also has plans to seek super-Earths, recently announcing the Cheops telescope, a spacecraft similar to Minerva. The HARPS telescope in Chile is also among the most trusted planet-finding tools, though it uses the Doppler method, measuring how a star's mass wobbles as a planet circles around it. That has an advantage over the transit method because planets don't have to pass directly in front of the stars.
NASA's Hubble telescopes and other mission can also find exoplanets, but none like Kepler. Since its launch in 2009, the Kepler telescope has led to a flood of data and discoveries, now listing 105 confirmed planets and more than 2,000 other candidates.
NASA's Ames Research Center in Mountain View manages the mission, while Caltech's Exoplanet Science Institute maintains a database of information.
Kepler might not last much longer than the end of its mission in 2016, because it's operating without one its reaction wheels, used to keep it in orbit and turn it to send data back to Earth, Gautier said. But the spacecraft can still run well on three wheels.
"I'm confident that Kepler, under any circumstances, was producing so much interesting science that we would have kept it going," Gautier said.
Once planets are identified, the next step is characterizing them beyond just their mass and orbit.
"Can we answer questions like what kind of planet is it, is it a gas giant, is it mostly rocky, can we answer the question of what temperature it is, does it have an atmosphere, what the atmosphere is made of, what's the climate like, things like that," said Heather Knutson, an assistant professor of geological and planetary science at Caltech. "I try to fill in the more detailed picture."
That includes detecting atmospheric molecules such as methane, carbon dioxide and water, mostly on gas giants. Characterizing the small, rocky planets that have emerged in the past year is more difficult because current telescopes don't yet have enough capabilities, Knutson said.
However, astronomers can look toward our own solar system to find some of those answers. While the sun isn't a common type of star in the universe, it's Jupiter that tends to define the solar system, Caltech astronomer Mike Brown said.
Brown, known as the man who killed Pluto, has helped astronomers change their understanding of how the solar system formed, with the planets previously much closer together. Jupiter and Saturn likely entered an orbital resonance at one point early in their history, circling around each other and causing a cataclysmic event, he said. It's one explanation for the late heavy bombardment that caused many of the craters on Earth's moon, and a source of debris in the Kuiper belt beyond Neptune, which can serve as a fossil record for the planets.
"It's like the giant planets are a big shipwreck that happened four and a half billion years ago," Brown said. "What we see is debris washed up on the shore. We don't know what the ships did four and a half billion years ago, but we see the little piles. There's a pile over here, there's a bunch of underwear stuck over here, and there's some shoes, and we can try to figure out what must have happened by looking at all these things that are still stuck in the ocean."
Brown now focuses his attention on Sedna, a stray body in a 12,000-year orbit around the sun that doesn't conform to the usual rules of a circular orbit. Brown hopes to find more of them, which could validate the thought that another star may have once passed close enough to influence it.
"If this is true, if Sedna is in this orbit because of other stars passing by, then it really is this fossil record not of the early solar system, but of the birth of the sun itself." Brown said. "If you can put this fossil record together, then you have the earliest possible history that you can construct around here."