Kepler Space Telescope observations have been utilized to find the most Earth-like pair of exoplanets ever discovered by astronomers searching for alien worlds. In addition, researchers also uncovered evidence for six other worlds with potentially moderate temperatures.

Kepler-438b and Kepler-442b each appear to be rocky worlds, located in the so-called "habitable zone" around their parent stars. This is the distance at which liquid water is likely to be found on planets.

"Most of these planets have a good chance of being rocky, like Earth," Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics (CfA) said.

Kepler-438b is roughly 12 percent larger than the Earth, and receives 40 percent more sunlight than our home planet. Kepler-442b is 30 percent dimmer than our world, but 30 percent larger.

Two previously discovered Earth-like worlds, Kepler-62f, 40 percent larger than Earth, and Kepler-186f, with a diameter 10 percent greater than our home world, each receive significantly less light than the Earth, roughly equivalent to the amount seen on Mars.

The Kepler spacecraft launched in 2009, on a mission to discover exoplanets orbiting alien suns. The observatory searches for a dimming of light seen when a planet orbits in front of its parent star, as seen from Earth.

Several events, not just planets, can cause stars to appear to dim. Because of this, astronomers must witness three such dimming events before they declare a new planet may have been discovered. By carefully measuring the degree and timing of these events, it is possible to deduce the size of the world, the distance it keeps from its sun, and the amount of sunlight the body receives.

These recent discoveries drive the number of confirmed exoplanets spotted by the Kepler observatory to over 1,000. They also double the number of potentially habitable alien worlds known to mankind.

Four of the eight newly confirmed planets orbit in multi-star systems, but in each case, one stellar companion is distant enough not to influence the planetary orbit to a great extent.

The space observatory was designed to study stars much like our own sun. After launch, however, astronomers discovered that these stellar bodies fluctuate more than expected, making observations challenging. Researchers then decided to focus their attention on smaller, dimmer stars with steadier levels of light output.

"We don't know for sure whether any of the planets in our sample are truly habitable. All we can say is that they're promising candidates," David Kipping of the Harvard-Smithsonian Center for Astrophysics stated.

In May 2013, the second of four gyroscopic wheels failed, preventing accurate orientation of the vehicle. Kepler was declared inoperable in August 2013. In 2014, NASA announced the K2 mission, extending the life of the crippled observatory.

Discovery of the eight newly recognized planets will be detailed in a future issue of the Astrophysical Journal.