Astronomers announced that they have found eight new planets in the "Goldilocks" zone of their stars, orbiting at a distance where liquid water can exist on the planet's surface.
"Most of these planets have a good chance of being rocky, like Earth," said lead author Guillermo Torres of the Harvard-Smithsonian Centre for Astrophysics (CfA).
The eight new worlds confirmed by NASA's venerable Kepler space telescope boost the number of Kepler's confirmed planets to over 1,000.
The two most Earth-like planets of the group are Kepler-438b and Kepler-442b. Both orbit red dwarf stars that are smaller and cooler than our Sun.
Kepler-438b circles its star every 35 days, while Kepler-442b completes one orbit every 112 days.
With a diameter just 12 per cent bigger than Earth, Kepler-438b has a 70-per cent chance of being rocky, according to the team's calculations.
Kepler-442b is about one-third larger than Earth, but still has a 60-per cent chance of being rocky.
To be in the habitable zone, an exoplanet must receive about as much sunlight as Earth. Too much, and any water would boil away as steam. Too little, and water will freeze solid.
"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," said Torres.
Kepler-438b receives about 40 per cent more light than Earth. In comparison, Venus gets twice as much solar radiation as Earth.
As a result, the team calculates it has a 70 per cent likelihood of being in the habitable zone of its star.
Kepler-442b get about two-thirds as much light as Earth.
The scientists give it a 97 per cent chance of being in the habitable zone.
"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," said second author David Kipping of the CFA.
Kepler-438b is located 470 light-years from Earth while the more distant Kepler-442b is 1,100 light-years away.
Prior to this, the two most Earth-like planets known were Kepler-186f, which is 1.1 times the size of Earth and receives 32 per cent as much light, and Kepler-62f, which is 1.4 times the size of Earth and gets 41 per cent as much light.
The team studied planetary candidates first identified by NASA's Kepler mission. All of the planets were too small to confirm by measuring their masses.
Instead, the team validated them by using a computer programme called BLENDER to determine that they are statistically likely to be planets.
The team spent another year gathering follow-up observations in the form of high-resolution spectroscopy, adaptive optics imaging, and speckle interferometry to thoroughly characterise the systems.
Those follow-up observations also revealed that four of the newly validated planets are in multiple-star systems.
However, the companion stars are distant and don't significantly influence the planets.
As with many Kepler discoveries, the newly found planets are distant enough to make additional observations challenging.
The finding was published in The Astrophysical Journal.
"Most of these planets have a good chance of being rocky, like Earth," said lead author Guillermo Torres of the Harvard-Smithsonian Centre for Astrophysics (CfA).
The eight new worlds confirmed by NASA's venerable Kepler space telescope boost the number of Kepler's confirmed planets to over 1,000.
The two most Earth-like planets of the group are Kepler-438b and Kepler-442b. Both orbit red dwarf stars that are smaller and cooler than our Sun.
Kepler-438b circles its star every 35 days, while Kepler-442b completes one orbit every 112 days.
With a diameter just 12 per cent bigger than Earth, Kepler-438b has a 70-per cent chance of being rocky, according to the team's calculations.
Kepler-442b is about one-third larger than Earth, but still has a 60-per cent chance of being rocky.
To be in the habitable zone, an exoplanet must receive about as much sunlight as Earth. Too much, and any water would boil away as steam. Too little, and water will freeze solid.
"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," said Torres.
Kepler-438b receives about 40 per cent more light than Earth. In comparison, Venus gets twice as much solar radiation as Earth.
As a result, the team calculates it has a 70 per cent likelihood of being in the habitable zone of its star.
Kepler-442b get about two-thirds as much light as Earth.
The scientists give it a 97 per cent chance of being in the habitable zone.
"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," said second author David Kipping of the CFA.
Kepler-438b is located 470 light-years from Earth while the more distant Kepler-442b is 1,100 light-years away.
Prior to this, the two most Earth-like planets known were Kepler-186f, which is 1.1 times the size of Earth and receives 32 per cent as much light, and Kepler-62f, which is 1.4 times the size of Earth and gets 41 per cent as much light.
The team studied planetary candidates first identified by NASA's Kepler mission. All of the planets were too small to confirm by measuring their masses.
Instead, the team validated them by using a computer programme called BLENDER to determine that they are statistically likely to be planets.
The team spent another year gathering follow-up observations in the form of high-resolution spectroscopy, adaptive optics imaging, and speckle interferometry to thoroughly characterise the systems.
Those follow-up observations also revealed that four of the newly validated planets are in multiple-star systems.
However, the companion stars are distant and don't significantly influence the planets.
As with many Kepler discoveries, the newly found planets are distant enough to make additional observations challenging.
The finding was published in The Astrophysical Journal.