They are not as similar to Earth as the planet announced just last week, but the newly discovered planets HD 219134b, c and d are much closer, and therefore easier to study. At 21 light-years away, these three planets are the closest rocky worlds identified outside our Solar System. Moreover, they orbit a star that, while hardly a solar twin, is more like the Sun than most.

Almost all known planets orbit stars too faint for the naked eye to see. Even the closer ones mostly circle red dwarves that require telescopes to observe. However, if you are in the Northern Hemisphere away from city lights, you can see HD219134 in the constellation Cassiopea. As a K-type star, HD219134 is 78% of the Sun’s mass and a quarter of its brightness.

^{HD29134 is circled here in a photograph of the constellation Cassiopea with the constellation lines drawn in. Credit: NASA / JPL-Caltech / DSS}

A team from Geneva University used the HARPS-N planet finder on La Palma Island to study this astronomical neighbor. In the finder’s first new detection, they registered a planet with an orbit lasting just three days. At 4% of the Earth-Sun distance from its star, such a planet must be far too hot to support life. However, planet finders believe that where there is one world, there are usually more.

Additional rocky worlds have been picked up with orbits lasting 6.8 and 46.8 days. Further out, there is a probable gas giant that takes three years to go around its star.

Even more excitement was in store for the team when they received time on the Spitzer Space Telescope to study the system in detail. "The idea was to check for a potential transit of the planet in front of the star, a mini eclipse, that would allow us to measure the size of the planet," said lead author and Ph.D. student Fatemeh Motalebi. "To do this, we needed to go to space to reach the required precision."

HARPS-N finds planets using the Doppler-wobble method. The Spitzer Space Telescope, on the other hand, can see a planet crossing the face of the star if the alignment is right. While the first method provides information on the planet’s orbit and mass, we can only guess its size. Transit observations indicate size, and therefore density, and can sometimes reveal an atmosphere, making transiting planets particularly prized by astronomers.

Spitzer confirmed that HD219134b does indeed transit from our perspective, Motalabi and her co-authors report in a forthcoming paper in Astronomy and Astrophysics (available in prepress on arXiv). This makes it easily the closest known transiting exoplanet.

HD219134b is 4.5 times the mass of the Earth and 1.6 times the radius, indicating a similar density to Earth. While the size of the outer planets are unknown, it is known that the two rocky worlds are 2.7 and 8.7 times the Earth's mass, while the gas giant is 62 times the Earth’s mass.

Planets usually orbit in similar planes, creating a distinct possibility that some of the other planets in the HD219134 system will turn out to be transiting, but so far we don’t have long enough observations to confirm.

HD219134 is so close, astronomically speaking, that future generations of telescopes should be able to study it in great detail.