Closest Rocky Exoplanet Discovered

Super-Earth HD 219134b is just 21 light-years away, orbiting a nearby orange star that you can see from your backyard.

HD 219134 in Cassiopeia

See HD 219134 for yourself - it's an orange, 5th-magnitude star in Cassiopeia. Cassiopeia is upside down in this image, with her feet pointing to the top left corner. (See more exoplanet-hosting stars.)
NASA / JPL-Caltech / DSS

If Cassiopeia in her celestial seat were to raise her head from her looking glass, she would see HD 219134, a 5th-magnitude, orange, -type star 21 light-years from Earth. And if she peered closely, Cassiopeia might make out the system of four planets orbiting this nearby star.

Of course, Cassiopeia is mythical and doesn’t see a thing, but Fatemeh Motalebi (University of Geneva, Switzerland) and colleagues do. Using  the ESO 3.6-meter telescope in the Canary Islands and the Spitzer Space Telescope, the astronomers discovered the signature of a transiting rocky planet. The planet itself is never seen, but its crossing briefly dims the star by a minuscule amount that was first detected as part of the HARPS-N survey, then confirmed in Spitzer follow-up observations.

Two additional super-Earths and a giant planet haven’t transited (yet), but HARPS-N revealed their presence by the gravitational tugs they exert on their parent star.

Jackpot: A Nearby Rocky Super-Earth

HD219134 Light Curve

HD 219134b transits its star every 3 days, briefly dimming its star's brightness very slightly. First spotted as part of the HARPS-N survey, Spitzer observations confirmed the transit and pinned down the planet's mass and radius.
NASA / JPL-Caltech

Here are the vital stats: The innermost planet, “b,” is a super-Earth between 4 and 5 times the mass of Earth and about 1.6 times the size, which whips around (and in front of) its star every 3 days.

Those numbers put the super-Earth’s density somewhere between 4.7 and 7.0 g/cm3. For reference, Earth’s density is 5.5 g/cm3 (that’s the highest density for any planet in the solar system), and models suggest that rocky super-Earths would literally pack on the pounds, becoming denser as they grow larger. So there’s little doubt this planet is rocky, and like Earth, its atmosphere is probably skin-thin in comparison to its bulk.

But that doesn’t mean astronomers can’t study the planet’s atmosphere. The authors are optimistic about attempting to obtain high-resolution spectra — as the planet transits, a sliver of starlight would shine through the planet’s atmosphere and might reveal the presence of molecules such as water. Whether or not the team detected anything, the attempt would guide future observations with James Webb Space Telescope or ground-based megatelescopes.

Companion Worlds

Two of the rocky planet’s companions might also be super-Earths. At minimum, they have 2.7 and 8.7 times Earth’s mass — “at minimum” because these planets haven’t transited (yet), and calculating mass via an object’s gravitational tug is a matter of perspective. If these planets also transit, then those minimum masses are pretty close to their real masses. But if they turn out to have inclined orbits, their masses could be much larger. They orbit every 6.8 and 46.8 days, respectively, so future missions such as CHEOPS (scheduled for a December 2017 launch) will be on the lookout for potential transits.

Farther out, at twice the Earth-Sun distance, a giant planet orbits the same star every three years.

The Kepler mission made clear that super-Earths are surprisingly common in the universe, if not in our own solar system. So even if it’s not habitable, HD 219134b will make an excellent test case for understanding these unique worlds.

Fatemeh Motalebi et al. "The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun." Accepted for publication in Astronomy & Astrophysics.

Astronomers are only beginning to learn about the weird weather on alien worlds. Read up on what we know so far in the May 2014 issue of Sky & Telescope.

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