Closest-ever rocky, transiting planet to Earth discovered

Astronomers have discovered the closest-ever rocky and transiting planet outside our solar system. At a distance of barely 21 light-years, the planet - named HD 219134 - is by far the closest transiting planet to Earth which makes it ideal for follow-up studies.

Its host star is visible to the unaided eye in dark skies, meaning anyone with a good star map can see this record-breaking system.

"Most of the known planets are hundreds of light-years away. This one is practically a next-door neighbour," said astronomer Lars A Buchhave from the Harvard-Smithsonian Center for Astrophysics (CfA) in a statement.

HD 219134 weighs 4.5 times the mass of Earth, making it a super-Earth.

With such a close orbit, researchers realised that there was good possibility the planet would transit its star.

In April this year, they targeted the system with NASA's Spitzer Space Telescope and found that HD 219134b is a rocky world.

At the appropriate time, the star dimmed slightly as the planet crossed the star's face. Measuring the depth of the transit gave the planet's size, which is 1.6 times of Earth.

"Its proximity makes HD 219134 ideal for future studies. The James Webb Space Telescope and future large ground-based observatories are sure to point at it and examine it in detail," added lead study author Ati Motalebi from the Geneva Observatory.

The team detected three additional planets in the system using radial velocity data. A planet weighing at least 2.7 times the Earth orbits the star once every 6.8 days.

A Neptune-like planet with nine times the mass of Earth circles the star in a 47-day orbit.

And much further out, a hefty fourth world 62 times Earth's mass orbits at a distance of 2.1 astronomical units (200 million miles) with a "year" of 1,190 days.

Any of these planets might also transit the star, so the team plans to search for additional transits in the months ahead.

HD 219134 is an orange Type-K star somewhat cooler, smaller and less massive than our Sun.