"Most of the known planets are hundreds of light-years away. This one is practically a next-door neighbour," said Lars A. Buchhave, an astronomer at the Harvard-Smithsonian Center for Astrophysics, in a statement today. Buchhave is the co-author of a paper about the planet accepted for publication in the journal Astronomy & Astrophysics. The study was led by Ati Motalebi at the Geneva Observatory in Switzerland.
HD 219134b is a super-Earth just 21 light years away. It orbits an orange star that is a little smaller and cooler than our sun, in the constellation Cassiopeia, visible in the night sky near the North Star. It is not in the star's habitable zone – it's too close and too hot for liquid water, or life as we know it, to exist on its surface. Scientists predict that its surface is rocky and partially molten, and might have volcanoes on its surface.
The planet was first discovered by the HARPS-North, the University of Geneva's planet-hunting device on the 3.6-metre TelescopioNazionale Galileo in the Canary Islands. Data from HARPS-North showed it has a mass 4.5 times that of Earth and sped around its star once every three days.
HARPS-North and its sister instrument HARPS in Chile look for planets by precisely measuring the colour of a star. The gravity of a planet tugs on a star as it orbits, pulling it toward and away from a distant observer on Earth. That movement, in turn, slightly changes the colour of the star as seen from Earth due to the Doppler effect.
Follow-up studies using NASA's Spitzer telescope showed that the new planet has a diameter 1.6 times that of Earth. Based on its mass and diameter, it has the density of a rocky planet.
Closest 'transiting' planet
The researchers also discovered that the planet passes in front of its star, as viewed from Earth, each time it orbits. That makes it a "transiting planet" — a type of planet that is much easier to study with other telescopes. Based on changes in the colours of light detected by telescopes each time that happens, astronomers hope to be able to learn about the chemicals in the planet's atmosphere, if it has one. So far, it is by far the closest transiting planet ever found.
The planet is part of a system that includes three other planets:- A small one with a mass 2.7 times that of Earth a little farther from the star, orbiting once every 6.8 days.
- A Neptune-like planet with a mass nine times that of the Earth, which orbits once every 47 days.
- A giant planet with 62 times the mass of the Earth, orbiting about twice as far from its star as the Earth is from the Sun, once every 1,190 days.
HD 219134b may be the closest known rocky planet to Earth, but another planet has been discovered that's even closer. GJ674b is 14.8 light-years, but scientists don't know whether it is gaseous or rocky.