A planetary system has been discovered in Cassiopeia that contains the closest transiting planet known today, 21 lightyears from Earth.


The system was found around HD219134, a star that is slightly colder and slightly smaller than our Sun, yet bright enough to be seen with the naked eye in dark skies, near one leg of the Cassiopeia constellation.

The planets in the system consist of three rocky super-Earths and an outer giant planet.

Astronomers from the University of Geneva and a team from NCCR PlanetS discovered the system with the aid of the HARPS-N planet searcher at the Telescopio Nazionale Galileo on La Palma Island in the Canary Islands.

“When the first HARPS-N radial-velocity measurements indicated the presence of a three-day planet around HD219134, we immediately asked NASA for Spitzer space telescope time,” says Ati Motalebi, astronomer at UNIGE and first author of the paper revealing the discovery.

“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.

To do this, we needed to go to space to reach the required precision.”

HD219134b does transit its star, and is the closest transiting planet to Earth known. It is 4.5 times more massive than Earth.

Two further planets were discovered during the same study, from HARPS-N radial velocities.

In the inner regions of the system, a planet weighing 2.7 times Earth has a 6.8-day orbit, while another planet 8.7 times the mass of Earth completes its orbit in 46.8 days.

The study says that if these two planets could be in coplanar configuration with the third, then all planets could potentially be transiting.

The system also includes a giant planet described as being like a small Saturn, which orbits the star in just over three years.

Future observations have already been planned to learn more about this newly-discovered system.


“Being able to characterise three transiting super-Earths in a single bright and close system would provide incomparable constraints for planet formation and composition models, in particular for super-Earths,” says Prof Stéphane Udry from the University of Geneva.


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.