Using reanalysed data from NASAs Kepler space telescope, a team of scientists has discovered an Earth-size exoplanet orbiting in its star's habitable zone, the area around a star where a rocky planet could support liquid water.
Out of all the exoplanets found by Kepler, this distant world -- located 300 light-years from Earth -- is most similar to Earth in size and estimated temperature, according to a study published in The Astrophysical Journal Letters.
Scientists discovered this planet, called Kepler-1649c, when looking through old observations from Kepler, which the agency retired in 2018.
While previous searches with a computer algorithm misidentified it, researchers reviewing Kepler data took a second look at the signature and recognised it as a planet.
This newly revealed world is only 1.06 times larger than our own planet. Also, the amount of starlight it receives from its host star is 75 per cent of the amount of light Earth receives from our Sun -- meaning the exoplanet's temperature may be similar to our planet's, as well.
But unlike Earth, it orbits a red dwarf.
This type of star is known for stellar flare-ups that may make a planet's environment challenging for any potential life.
"This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate in Washington.
"The data gathered by missions like Kepler and our Transiting Exoplanet Survey Satellite (TESS) will continue to yield amazing discoveries as the science community refines its abilities to look for promising planets year after year."
There is still much that is unknown about Kepler-1649c, including its atmosphere, which could affect the planet's temperature.
Current calculations of the planet's size have significant margins of error, as do all values in astronomy when studying objects so far away.
Rocky planets orbiting red dwarfs are of particular astrobiological interest. However, astrobiologists will need much more information about this planet in order to gage whether it is promising for life as we know it.
But based on what is known, Kepler-1649c is especially intriguing for scientists looking for worlds with potentially habitable conditions.
There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c.
Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d. But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system.
Kepler-1649c orbits its small red dwarf star so closely that a year on Kepler-1649c is equivalent to only 19.5 Earth days.
"Out of all the mislabeled planets we've recovered, this one's particularly exciting -- not just because it's in the habitable zone and Earth-size, but because of how it might interact with this neighbouring planet," said Andrew Vanderburg, a researcher at the University of Texas at Austin and first author on the paper.
"If we hadn't looked over the algorithm's work by hand, we would have missed it," Vanderburg said.
