Spitzer Space Telescope confirms nearest rocky planet

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Artist's concept of HD 219134b(Credit: NASA/JPL-Caltech)

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NASA's Spitzer Space Telescope has confirmed the presence of the closest rocky planet to the Solar System. Orbiting a visible main-sequence star 21 light years away in the constellation of Cassiopeia, HD 219134b is larger than Earth and is uninhabitable.

NASA says that HD 219134b was first detected by the HARPS-North instrument, which is installed on the Italian 3.6-meter Galileo National Telescope in the Canary Islands. The discovery was made using the radial velocity technique, involving taking measurements of how an invisible planet tugs at its star as it orbits. From the HARPS-North data, the investigation team concluded that HD 219134b is 4.5 heavier than Earth, making it a super-Earth, and orbits its star every three days. This means it is extremely close to its star, and too hot for life.

So far, HD 219134b was just another big planet, though a close one. Until now, most of the exoplanets have been hundreds of light years away. According to NASA, the closest known one is GJ674b, which is 14.8 light years away, but its composition is unknown. What sets HD 219134b apart is that we now know something about its nature.

Location of the star HD 219134 (circle)(Credit: NASA/JPL-Caltech/DSS)

The Spitzer telescope looked at HD 219134b using the transiting method. That is, it discovered that HD 219134b is oriented in such away that, seen from Earth, it passes in front of its star. This causes the star to dim during transit and by measuring the light curve, astronomers can not only detect a planet's presence, but learn about its composition.

In the case of HD 219134b, infrared measurements revealed that it is 1.6 times the size of Earth. Now that its size and mass are known, the research team deduced that HD 219134b has a density of 3.5 oz/cubic in (6 g/cubic cm), which means it's a rocky world. In addition, the Spitzer data indicates that there are three more planets in the system.

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

Artist's concept shows the silhouette of HD 219134b as it passes in front of its star(Credit: NASA/JPL-Caltech)

According to team leader Ati Motalebi, HD 219134b could be a target for NASA's James Webb Space Telescope in 2018. In the meantime, the space agency believes that it will draw the attention of scientists, who will try to learn more about its chemical makeup and possible atmosphere by means of spectral analysis.

The team's results were accepted for publication in Astronomy & Astrophysics.

"Thanks to NASA's Kepler mission, we know super-Earths are ubiquitous in our galaxy, but we still know very little about them," says Michael Gillon of the University of Liege in Belgium, lead scientist for the Spitzer detection of the transit. "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."

The video below shows the location of HD 219134b.

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