Two studies of an Earth-sized planet circling the sun-like star Kepler-78 show it has Earth-like amounts of iron and rock, the first world of its size that scientists have been able to calculate both density and diameter.
But don't pack your bags yet. The planet, known as Kepler-78b, circles sizzlingly close to its parent star, far from an Earth-like water-friendly orbit, a condition that is believed to be necessary for life.
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"To me this means that planets like the Earth are probably not all that uncommon," astronomer Drake Deming, with the University of Maryland, told Discovery News.
"If one of the first measurements you're able to make that really pins down the density of a small, rocky planet gives you a density close to Earth's, Earth can't be that rare in terms of density," Deming said.
Two independent teams came up with nearly the same assessment of Kepler-78b, which was discovered last spring by another group of astronomers using data collected by NASA's now-defunct Kepler space telescope.
The observatory, which was sidelined by a positioning system failure in May, detected slight dips in the amount of light coming from about 150,000 target stars. Some dips were caused by orbiting planets passing by, or transiting, relative to the telescope's line of sight.
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Based on how much Kepler-78's light dimmed, astronomers knew they were looking at a planet that was about the same size as Earth. To learn about its density, they followed up with ground-based telescopes to try to tease out how much gravitational tugging the little world exerted on its parent star. From that information, scientists worked out how much rock and how much iron Kepler-78b contains.
The timing of the planet's transits also revealed a startling 8.5-hour orbit, meaning Kepler-78b is almost skimming the surface of its parent star. From that location, temperatures would be about 2,000 degrees hotter than Earth, making for a molten surface and little, if any, atmosphere.
Scientists are homing in on finding Earth-sized worlds in the so-called "habitable zones" of stars similar to the sun, but figuring out those planets' densities is beyond present-day technologies.
"If you were to take (Kepler-78b) and put it out in an Earth-like orbit, it would be undetectable with our current techniques," astronomer Andrew Howard, with the University of Hawaii at Manoa, told Discovery News.
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But the fact that one small rocky body exists means that some of the planets found in stars' habitable zones can be rocky too.
"A core of rock and iron means that processes similar to what formed the Earth are very common," Demming said.
Kepler-78b has a diameter of 9,200 miles -- about 20 percent larger than Earth's. Its density is similar to Earth's, which suggests an Earth-like mix of iron and rock, a pair of papers published in this week's Nature shows.
Kepler-78 is located about 400 light years away in the constellation Cygnus.