Nearby rocky planet revealed close to Cassiopeia

Astronomers have discovered a planet, likely to be rocky, close to our own solar system.

The planet, which has a three-day orbit round a central star, is joined by a further three planets also newly discovered in the same system. The star at the centre of the system can be seen in the sky close to the Cassiopeia the Queen constellation, near the North Star, and is visible to the naked eye from dark skies.

Future research

At a distance of just 21 light years away, the rocky planet is the nearest confirmed outside our solar system. It is also the closest planet to Earth that has been found to transit across the front of its star which, together with its short orbit, makes it ideal for further study.

The newfound Earth-like planet, designated HD 219134b, was discovered by an international team of astronomers using data from the HARPS-North instrument on the 3.6-metre Telescopio Nazionale Galileo in the Canary Islands. Scientists found that it weighs 4.5 times the mass of Earth, making it what is termed a super-Earth.

"Although this planet is far too hot to be habitable, it is an amazing discovery given how close and bright the star is. This will provide amazing opportunities to learn about and characterise a rocky planet outside our solar system." Dr Eric Lopez, School of Physics & Astronomy, University of Edinburgh 

Images from space

Three additional planets in the system were also found - one planet with a mass at least 2.7 times that of the Earth orbits the star once every 6.8 days. A Neptune-like planet with nine times the mass of Earth circles in a 47-day orbit. Much further out from the star, a hefty fourth world with 62 times the mass of Earth orbits at a distance of about 200 million miles, with a year length of 1,190 days.

Astronomers used NASA’s Spitzer Space Telescope to capture the smallest planet crossing in front of the star. The star was seen to dim slightly as the planet crossed its face. Measuring the depth of the transit gave the planet’s size, enabling the team to calculate the planet’s density, which showed that it is a rocky world.

"This is a fascinating system, not only because it is around a relatively nearby star, but also because we are able to determine the compostion of the closest of the four planets, and because the architecture, with three inner low-mass planets with an outer, more massive companion, is similar to that of our own Solar System." Prof. Ken Rice, School of Physics & Astronomy, University of Edinburgh

Transiting planets

Any of the other three planets might also pass directly in front of the star, so the team plans to search for additional transits across the star in the months ahead. The star which the planets orbit is cooler, smaller and less massive than our Sun, and is known as HD 219134.

Transiting planets are ideal targets for astronomers wanting to know more about planetary compositions and atmospheres. As a planet passes in front of its star, it causes the starlight to dim, and telescopes can measure this effect. If molecules are present in the planet's atmosphere, they can absorb certain wavelengths of light, leaving imprints in the starlight. This type of technique will be used in the future to investigate potentially habitable planets and search for signs of life. 

The study will be published in the journal Astronomy & Astrophysics.


The HARPS-North project is led by the University of Geneva, and involves the Universities of St Andrews, Edinburgh and Queen’s University Belfast. Other partners are the Harvard-Smithsonian Center for Astrophysics, and the Italian National Institute for Astrophysics.

The St Andrews and Edinburgh contributions were part-funded by the Scottish Funding Council through the Scottish Universities Physics Alliance. Together with funds from Queen’s University Belfast, these contributions funded construction of the telescope interface and instrument control systems at the Science and Technology Facilities Council’s UK Astronomy Technology Centre in Edinburgh.

The UK Astronomy Technology Centre instrument team were responsible for delivering the HARPS-North instrument Front End Unit, Calibration Unit, Instrument control electronics and software, Detector control system and Sequencer software. In addition they helped to install and commission the instrument on the Telescopio Nazionale Galileo (TNG) in partnership with colleagues from the University of Geneva and the TNG.