One of two new planets discovered in the Gliese 581 system is three Earth masses and lies square in the middle of the star's habitable zone, boosting its chances of hosting conditions suitable for life.

For our own Solar System, Earth lies in the Sun's habitable zone, where liquid water is stable and conditions are not too hot and not too cold for life to flourish, hence the "Goldilocks zone" moniker.

The red dwarf star Gliese 581, which lies 20 light years away in the constellation Libra, was already known to host four exoplanets, orbiting with periods ranging from 3 to 67 days. The new survey has turned up two more planets – one seven Earth mass planet orbiting in 433 days, and one 3.1 Earth mass planet orbiting in 36.6 days. As well as now boasting the most confirmed exoplanets of any system discovered to date (a system identified last month, HD 10180, had five confirmed planets with two more potential candidates) the three Earth mass planet, Gliese 581g, orbits in the crucial Goldilocks Zone of the star.

“[Previously] we had planets on both sides of the habitable zone – one too hot and one too cold – and now we have one in the middle that’s just right,” says Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. This bears striking resemblance to our own Solar System where Venus is too hot, and Mars, on the outer edge of the habitable zone, is too cold.

The finding comes from 11 years worth of observations made with the HIRES spectrometer on the Keck I telescope at the W. M. Keck Observatory in Hawaii. HIRES employs the radial velocity technique to detect exoworlds, looking out for the gravitational tug to the star's motion as seen along the line of sight from Earth that a companion planet imposes. Multiple planets induce complex wobbles, which can be translated into their orbits and masses.

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” said Paul Butler of the Carnegie Institution. “Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey and report the new findings in a paper to be published in the Astrophysical Journal. They combine their data for the Gliese 581 system with that attained by the High Accuracy Radial velocity Planetary Search project, HARPS. “It’s really hard to detect a planet like this,” says Vogt. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 metres per second to detect this planet.”

The mass of Gliese 581g suggest that it has enough gravity, perhaps a little greater than Earth's, to hold an atmosphere, which would be needed to maintain liquid on the surface. But the planet is tidally locked, meaning that one side is always facing its star, resulting in one fiercely hot side and one freezing cold. The planet-hunters suggest that the most stable surface conditions would be provided along the dividing line between the light and dark sides – a zone known as the terminator. Surface temperatures might range from a mild -31 to -12 degrees Celsius, but with actual temperatures encompassing a much wider range, at least some of the planet's water inventory will likely be in liquid form rather than ice or vapour. “Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” says Vogt.

If confirmed to have water and an atmosphere, Gliese 581g will be the most Earth-like exoplanet discovered yet, and sets a precedent for a new era of exoplanet research.

“This is an amazing discovery, we just might have the first habitable world, although the jury is still out until we analyze its light with a future space telescope – but it has great potential by being small and at the right distance from its star,” says exoplanet expert Lisa Kaltenegger who is affiliated with Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy in Germany, and not involved with this particular study. “It shows that we are already at the stage where we can find potentially habitable planets, the thing we have to do next is collect their light and analyze its spectral fingerprint so we can distinguish between a Neptune-, Venus- or Earth-like world, based on what we find in the planet's atmosphere. We also don't know how fast life gets started, if it does automatically, or if it needs just the right conditions. On our Earth we find life occupying almost every niche of the environment, many hostile to us, but life can survive there, so extrapolating from that, chances are high, that if the environment is habitable, life could exist.”