However, just like the other planets, we haven't seen or tested the atmospheres of any of these planets, so whether they're habitable remains theoretical.
Often, when you find an example of a kind of planet, you start seeing a lot of them, said Sara Seager, professor of planetary science at Massachusetts Institute of Technology. She used to be part of the Kepler team but is now independent.
"Planets in the habitable zones of stars must be everywhere," she said.
Seager called the discovery of the three planets "a huge milestone," but her excitement is tempered because potentially habitable planets have been announced in the past and there's currently no way to get more details, given how far away they are.
"We'll possibly never know if these particular ones do have water oceans or signs of life," she said.
Identifying planets, not atmospheres
The goal of the Kepler mission wasn't to find alien lifeforms, however. The satellite is supposed to investigate Earth-sized planets around stars that resemble our sun.
The Kepler telescope, launched in 2009, allows scientists to measure changes in brightness of individual stars over time; these dimming events signal that a planet is nearby. Scientists undertake sophisticated calculations to verify that such signals are planets and not passing rocks.
The satellite finds planets that are very far away because it's surveying so many stars. Imagine, said Seager, that you are in Times Square and you want to see 150,000 people at once. You wouldn't be able to do it in such a way that you could see any details of the people; you'd have to get far enough away that that many people would fit into your field of view.
In total, Kepler has found 122 confirmed planets and more than 2,700 planet candidates. A total of seven confirmed planets were announced Thursday -- the three in the habitable zone, and four others that are not.
This isn't the only planet-finding technique. The Gliese-581 system was found with the Keck I Telescope in Hawaii, using what is called the radial velocity method. The telescope's spectrometer allows scientists to look for wobbles in the motion of a star, which happens in response to the gravity of nearby planets.
It would take a different kind of mission to investigate the atmosphere of one of these distance planets to find out answers to the most-pressing questions. Is there carbon dioxide and water? Is there oxygen?
"Future NASA missions are going to focus on more nearby stars that we can look at in much more detail," Barclay said.
You can read more about the Kepler discoveries at the mission website.