A newly discovered planet called HD 219134b isn't quite what we'd call Earth-like. But like Earth, it's a rocky planet -- and it's the closest planet of that type we've ever spotted outside of our own solar system.

At just 21 light years away, HD 219134b has something that many more "Earth-like" worlds don't: The potential for exploration.

In addition to being rocky, the newly found planet has the benefit of being a "transiting" world, which means that it crosses in front of its host star from our perspective. The planet itself can't be seen from Earth -- even using powerful telescopes -- but because we can see its star, we can measure it based on the changes in brightness caused by those transits. Knowing the size and mass of a planet tells us its density, and from there it's easy to infer whether it's rocky like Earth, gaseous like Jupiter, or icy like Pluto.

"Transiting exoplanets are worth their weight in gold because they can be extensively characterized," Michael Werner, the project scientist for the Spitzer mission at NASA's Jet Propulsion Laboratory, said in a statement. "This exoplanet will be one of the most studied for decades to come."

According to current data, which is set to be published soon in the journal Astronomy & Astrophysics, HD 219134b has a mass of about 4.5 times Earth's and is about 1.6 times as big across. That's much closer in size to Earth than the "Earth-like" planets scientists get so excited about. Unfortunately, this newly discovered world is too close to its sun (which is a bit smaller, cooler, and less massive than our own) to harbor life -- it orbits once every three days.

The planet was confirmed using observations from NASA's Spitzer Space Telescope, but researchers hope it will soon be the target of many telescopes. It's not much farther than the very closest known planet outside of our system (GJ674b) which is just 14.8 light years away and of an unknown composition. 

By training more telescopes on it, scientists may be able to spot tiny variations in the host star's dimming that reveal the planet's atmosphere, if it has one.

"Thanks to NASA's Kepler mission, we know super-Earths are ubiquitous in our galaxy, but we still know very little about them," study co-author Michael Gillon of the University of Liege in Belgium said in a statement. "Now we have a local specimen to study in greater detail. It can be considered a kind of Rosetta Stone for the study of super-Earths."

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