Kepler-452b orbits at a very similar distance from its star, though its radius is 60% larger.
Mission scientists said they believed it was the most Earth-like planet yet.
Such worlds are of interest to astronomers because they might be small and cool enough to host liquid water on their surface – and might therefore be hospitable to life.
Nasa’s science chief John Grunsfeld called the new world the “closest so far” to Earth.
And John Jenkins, Kepler data analysis lead at Nasa’s Ames Research Center in California, added: “It’s a real privilege to deliver this news to you today. There’s a new kid on the block that’s just moved in next door.”
The new world joins other exoplanets such as Kepler-186f that are similar in many ways to Earth.
Determining which is most Earth-like depends on the properties one considers. Kepler-186f, announced in 2014, is smaller than the new planet, but orbits a red dwarf star that is significantly cooler than our own.
Kepler-452b, however, orbits a parent star which belongs to the same class as the Sun: it is just 4% more massive and 10% brighter. Kepler-452b takes 385 days to complete a full circuit of this star, so its orbital period is 5% longer than Earth’s.
The mass of Kepler-452b cannot be measured yet, so astronomers have to rely on models to estimate a range of possible masses, with the most likely being five times that of Earth. If it is rocky, the world would likely still have active volcanism and its gravity would be roughly twice that on our own planet.
The new world is included in a haul of 500 new possible planets sighted by the Kepler space telescope around distant stars.
Twelve of the new candidates are less than twice Earth’s diameter, orbiting in the so-called habitable zone around their star.
This zone refers to a range of distances at which the energy radiated by the star would permit water to exist as a liquid on the planet’s surface if certain other conditions are also met.
Of these 500 candidates, Kepler-452b is the first to be confirmed as a planet.
Dr Suzanne Aigrain, from the University of Oxford, who was not involved with the study, told BBC News: “I do believe the properties described for Kepler-452b are the most Earth-like I’ve come across for a confirmed planet to date.
“What seems even more significant to me is the number of planets in the habitable zone of their host stars with radii below two Earth radii; 12 is quite a few compared to the pre-existing Kepler planet catalogue.
“It bodes well for their attempts to provide a more robust measure of the incidence of Earth-like planets, which is the top-level goal of the Kepler mission.”
While similar in size and brightness to the Sun, Kepler-452b’s host star is 1.5 billion years older than ours. Scientists working on the mission therefore believe it could point to a possible future for the Earth.
“If Kepler-452b is indeed a rocky planet, its location vis-a-vis its star could mean that it is just entering a runaway greenhouse phase of its climate history,” explained Dr Doug Caldwell, a Seti Institute scientist working on the Kepler mission.
“The increasing energy from its aging sun might be heating the surface and evaporating any oceans. The water vapour would be lost from the planet forever.”
“Kepler-452b could be experiencing now what the Earth will undergo more than a billion years from now, as the Sun ages and grows brighter.”
Dr Don Pollacco, from Warwick University, UK, who was not involved with the latest analysis, told the BBC: “Kepler data allows you to estimate the relative size of a planet to its host star, so if you know the size of the host, hey presto, you know the size of the planet.
“However, to go further – i.e. is it rocky? – involves measuring the mass of the planets and this is much more difficult to do as the stars are too far away for these measurements (which are incredibly difficult) to make.
“So in reality they have no idea what this planet is made of: It could be rock but it could be a small gassy ball or something more exotic maybe.”
Dr Chris Watson, from Queen’s University Belfast, UK, commented: “Other Kepler habitable zone planets may well be more Earth-like in this respect. For example, Kepler-186f is approximately 1.17 Earth radii, and Kepler-438b is approximately 1.12 Earth radii.
“In fact, at 1.6 Earth radii, this would place Kepler-452b in a category of planet called a ‘Super-Earth‘ – our Solar System does not actually have any planet of this type within it! Super-Earths are hugely interesting for this reason, but one might then say, well, is it really ‘Earth-like’ given all this?”
He added: “When we look at the type of star Kepler-452b orbits, then it seems to be a star not too dissimilar to our Sun… The other Kepler habitable zone planets that have been discovered so far tend to be orbiting M-dwarfs – stars far cooler than our Sun, and therefore the planets need to orbit much closer to receive the same levels of heating.
“So it may be a potentially rocky super-Earth in an Earth-like orbit (in terms of host star and orbital distance). It’s this combination of the host star and orbit that set it apart in my opinion.”
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