Astronomers have discovered a super-Earth planet orbiting a star in our own cosmic neighborhood, and it lies in the Goldilocks zone, where conditions are just right to sustain liquid water and an Earth-like climate.

Artist's concept of HD 40307g, a super-Earth planet found in another star's habitable zone. Credit: J. Pinfield, for the RoPACS network at the University of Hertfordshire.
 
The planet is located about 42 light-years from Earth around a stable sun-like dwarf star.

"The star HD 40307 is a perfectly quiet old dwarf star, so there is no reason why such a planet could not sustain an Earth-like climate," said Guillem Angla-Escude, a researcher from the University of Goettingen in Germany.

Scientists knew the star HD 40307 was home to at least three planets, but they all resided too close to the star to harbor conditions conducive to life.

But analysis of spectral data from the European Southern Observatory's HARPS spectrograph uncovered three more worlds. The HARPS instrument is part of an 11.8-foot telescope at the La Silla Observatory in Chile.

"We pioneered new data analysis techniques including the use of the wavelength as a filter to reduce the influence of activity on the signal from this star," said Mikko Tuomi, an astronomer at the University of Hertfordshire. "This significantly increased our sensitivity and enabled us to reveal three new super-Earth planets around the star known as HD 40307, making it into a six-planet system."

Tuomi and Angla-Escude led the research team which made the discovery.

One of the new-found planets, HD 40307g, has at least seven times the mass of Earth and completes an orbit of its parent star every 198 days. The planet's average distance from its parent star is about 56 million miles, 90 million kilometres.

"The longer orbit of the new planet means that its climate and atmosphere may be just right to support life," said Hugh Jones of the University of Hertfordshire. "Just as Goldilocks liked her porridge to be neither too hot nor too cold but just right, this planet or indeed any moons that is has lie in an orbit comparable to Earth, increasing the probability of it being habitable."

Planets close to their parent star are too hot, causing water to evaporate and starving life of vital nutrients. Colder planets lie further away from the star.

In an email to Spaceflight Now, Tuomi said the estimated temperature of HD 40307g is about 9 degrees Celsius, or 48 degrees Fahrenheit. It also has days and nights like Earth.

"While it might have a rocky surface, or perhaps a surface covered with a giant ocean ... it is also possible the planet has a thick atmosphere and is, in reality, more similar to Neptune - only warmer," Tuomi said.

Scientists using data from NASA's Kepler space telescope announced in December 2011 they discovered a similar super-Earth planet around another star. But the Kepler discovery lies 600 light-years from Earth, while HD 40307g resides more than 15 times closer to Earth.

"Discoveries like this are really exciting, and such systems will be natural targets for the next generation of large telescopes, both on the ground and in space," said David Pinfield of the University of Hertfordshire, who leads a European network of scientists focused on studying rocky planets around other stars.

The findings are discussed in a paper accepted for publication in the journal Astronomy & Astrophysics. Astronomers still classify the HD 40307g super-Earth as a planet candidate until confirmation from follow-up observations.

Scientists measured tiny wobbles from HD 40307 to detect the three new exoplanets. This technique, called radial velocity, measures the effect of a planet's gravity on the parent star.

Future space telescopes could try to directly measure light from HD 40307g, raising the possibility astronomers could confidently say whether the super-Earth has liquid water or an hospitable atmosphere.