A super-Earth, weighing in at 4.5 times the mass of our planet, just 21 light years away has been discovered, making it the nearest world to our Solar System. What’s more, its orange host star – which can be found just off the “W” shape of the constellation Cassiopeia – is visible to the unaided eye under dark skies, meaning that anyone with a star map can find the planetary system.
The newfound planet, designated HD 219134b, was uncovered by the 3.6-metre Telescopio Nazionale Galileo in the Canary Islands and its distance makes it an ideal target to scrutinise in the future. “Most of the known planets are hundreds of light years away. This one is practically a next door neighbour,” says Lars Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA). “The James Webb Space Telescope and future large ground-based observatories are sure to point at it and examine it in detail.”
The super-Earth made its appearance by crossing, or transiting, the face of its star. This caused the planet to block out a fraction of starlight, which gave away its existence. “When the first measurements indicated the presence of a planet [with a three-day orbit] around [its star], we immediately asked NASA for Spitzer space telescope time,” explains Ati Motalebi, an astronomer at the University of Geneva in Switzerland. “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 and to do this, we needed to go to space to reach the required precision.”
Motalebi and her team measured HD 219134b to be 1.6 times the size of Earth. But that’s not all they found.
Employing the so-called radial velocity method in order to measure the planet’s mass, the astronomers came across three additional worlds thought to be in the same system. Alongside HD 219134b is a planet weighing in at least 2.7 times Earth, another world that’s nine times the mass of our planet and, much further out from host star HD 219134, is a fourth world some 62 times more massive than Earth and believed to be a smaller version of ringed planet Saturn.
Being able to get key measurements of HD 219134, which is somewhat smaller, cooler and less massive than our Sun, has meant that Motalebi and her team have been able to take a stab at guessing the characteristics of its accompanying planets. For example, Motalebi speculates that HD 219134b has a molten surface with geological activity, including volcanoes. However, we won’t know for sure until we take further measurements of the 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 Stéphane Udry also from the University of Geneva.