Welcome to Your New Earth! Beware of Red Sunlight

Forget Mars or Titan’s icy hopes of extra-terrestrial life: scientists claim they have discovered the first planet that could allow the existence of liquid water on its surface.

Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered something they called “a super-Earth”- about 5 times the mass of our planet-that orbits a star included in the category of red dwarfs. The star, dubbed Gliese 581, is already known to be orbited by a Neptune-mass planet (fifteen to seventeen earth masses). The Neptune-mass exo-planet orbits just 6 million kilometers (0.04 AU) from Gliese 581, and so is estimated to have a surface temperature of 150 °C, despite how dim the star is. It orbits its host star in 5.4 days.

Red dwarfs constitute the majority of stars in our Universe and are considered to be small and relatively cool stars, with a surface temperature of less than 3,500K. Red dwarfs have a mass of less than one-half that of our Sun, (down to about 0.075 solar masses, which are brown dwarfs). Due to the low temperatures in their core, fusion in red dwarfs proceeds slowly and as a consequence they emit little light. Red dwarfs have an enormous estimated lifespan because they burn their hydrogen slower; from tens of billions up to trillions of years depending upon mass; the lower the mass, the longer the lifespan.

Being so numerous, red dwarfs are more likely to “host” exo-planets, although it is now estimated that at least 10% of sunlike stars have planets orbiting them, and that the true proportion may actually be much higher.

An exo-planet is defined as a planet observed outside the borders of our solar system. Until now, astronomers have found that most of the 227 exo-planets (whose actual existence is deduced from their influence on nearby stars or through their effects on the light traveling towards Earth, rather than direct observations) are inhospitable for life, resembling more to gaseous giants like Jupiter. That means that their mass is at least comparable (if not bigger) to the one of the largest planet in our solar system.

Scientists are now claiming that the first planet far enough from the star it orbits and “light” enough to support life was discovered near Gliese 581, a red dwarf located 20.40 light years away from Earth, in the constellation Libra, with a mass estimated to be approximately a third of the Sun.

The Earth-like celestial body that determined astronomers’ enthusiasm is the smallest ever found up to now and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581, is smaller and colder than the Sun - and thus less luminous - the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid.

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. "Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky - like our Earth - or fully covered with oceans," he adds.

"Liquid water is critical to life as we know it," avows Xavier Delfosse, a member of the team from Grenoble University (France). "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."

Observations of the star also revealed the possibility of a third planet with a mass of roughly 8 Earths, and an orbit of 84 Earth days. Among the 100 closest stars to our Sun, 80 belong to the red dwarfs’ category.

"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun," emphasizes Xavier Bonfils, a co-worker from Lisbon University.

According to the astronomers from Europe, the planetary system surrounding Gliese 581 contains no fewer than 3 planets of 15 Earth masses or less, making it “a quite remarkable system.”

The outstanding discovery wouldn’t have been possible without HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. HARPS is one of the most successful instruments for detecting exo-planets and holds already several recent records, including the discovery of another 'Trio of Neptunes'.

"HARPS is a unique planet hunting machine," says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. "Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!"

HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets - HD 69830 and Gl 581 - were discovered by HARPS.

"And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach," affirms Mayor.