An artist's rendering shows a comparison between the Earth and planet Kepler-78b, which is located in the Cygnus constellation hundreds of light-years away. (David A. Aguilar/Associated Press)

Scientists have found a planet way out in the cosmos that’s close in size and content to Earth: an astronomical first.

But hold off on the travel plans. This rocky world is so close to its sun that it’s at least 2,000 degrees hotter than here, almost certainly too hot for life.

Astrophysicists reported last week in the journal Nature that the exoplanet (that’s a planet outside of our solar system) Kepler-78b appears to be made of rock and iron, just like Earth. They measured the planet’s mass to determine its density and content. It’s actually a little bigger than Earth and nearly double its mass, or the amount of matter it contains.

Kepler-78b is located in the Cygnus constellation, which is about 400 light-years away. (A light-year is the distance light travels in one year, or about 6 trillion miles.)

Incredibly, the planet orbits its sun every 81 / 2 hours, which is a mystery to astronomers. Until now, they hadn’t found a planet that formed or moved so close — less than 1 million miles — to a star. They agree the planet will be sucked up by the sun in a few billion years, so its time remaining, astronomically speaking, is short.

An illustration shows NASA’s Kepler Space Telescope, which found the planet. (Associated Press)

NASA’s Kepler Space Telescope discovered Kepler-78b while looking for planets that could possibly sustain life. The planet’s presumably liquid surface and searing temperatures make it unsuitable for life, but two independent teams of astronomers wanted to follow up the discovery with ground-based measurements to try to determine the density of Kepler-78b.

Using different telescopes, the teams zeroed in on how strongly the planet’s gravity tugs at its parent star, information that could be used to figure out Kepler-78b’s weight and composition.

In two papers in last week’s journal Nature, the teams report that not only were they successful, they also came to the same conclusion: Kepler-78b has roughly the same density as Earth, suggesting that it too is made primarily of rock and iron.

Scientists would like to be able to make the same measurements of Earth-size planets in more life-friendly orbits, but that is beyond today’s technology.

Kepler-78b is one of more than 1,000 exoplanets that have been confirmed so far.

The Kepler Space Telescope has identified 3,500 more potential candidates. The telescope lost its precise pointing ability earlier this year, and NASA has given up trying to fix it.

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