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New world holds the promise of plentiful super-earths
Posted: 12 September 2011

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A record-breaking planet-finding instrument at the European Southern Observatory (ESO) in Chile has opened up a treasure chest of exoplanet discoveries, including a bounty of 16 ‘super-earths’ amidst a haul of over 50 new alien worlds.

An artistŐs impression of a new potentially habitable exoplanet, HD 85512b, which orbits a star in the Southern Hemisphere constellation of Vela. Image: ESO/M Kornmesser.

The most successful instrument of its kind, the High Accuracy Radial velocity Planet Searcher (HARPS) attached to ESO’s 3.6-metre telescope measures the wobbles imparted on stars by the gravity of orbiting planets. The smaller the mass of the planet, the smaller the wobble. For example, at the limits of HARPS’ sensitivity it can detect a Doppler shift in the rotation of a star of just four kilometres per hour, which would be caused by the gravity of a two Earth-mass planet tugging on the star each orbit. Although none of the new planets are this small, five are less than five Earth masses, and one is shown to exist in its star’s habitable zone.

This potentially habitable world is a 3.6 Earth-mass planet (described as a ‘super-earth’, i..e a terrestrial world more massive than our planet) known as HD 85512b. Its orbit of around 39 million kilometres (0.26 astronomical units, or about a quarter of the distance from Earth to the Sun) from its star places it just within the inner edge of the habitable zone, the region where temperatures should be suitable for the existence of liquid water on a planet’s surface.

“HD 85512b is the only one that really lies in the habitable zone, but there are a few others that lie close to it,” says Francesco Pepe of Geneva Observatory, who led the discoveries. However, for a planet to be habitable more conditions are required to be met than simply existing in the habitable zone – it must have an atmosphere and a carbon cycle, for example – and there is certainly no guarantee that there would be life even if those conditions are met. Furthermore, a thicker atmosphere adept at trapping more heat may render planets just outside the traditional habitable zone more palatable to potential life forms.

“We’re not saying that planets outside of this zone cannot be habitable,” says Lisa Kaltenegger, a specialist in the habitability of exoplanets from the Max Planck Institute for Astronomy and the Harvard–Smithsonian Center for Astrophysics. “The habitable zone is where we can reasonably expect to find life, but whether there is life on cooler planets is an open question.”

The distance of the habitable zone from various stars, and the placement of planets around them. The red dwarf star Gliese 581 has two planets confirmed to be right on the edge of the zone (and a third, unconfirmed and not on this graphic, that may exist within the zone), while the newly discovered HD 85512b exists within the habitable zone. However, the fact that Venus and Mars also creep into our Sun’s habitable zone goes to show that location does not guarantee habitability. Image: ESO.

Overall, by considering the approximately 150 exoplanets discovered by HARPS during its eight-year history, including its latest haul, and applying that knowledge in broad strokes to the Galaxy, statistically between 30 and 50 percent of Sun-like stars have a planet less massive than Saturn, and most planets less massive than Neptune seem to reside in multiple planetary systems. So if you find one super-earth, there’s a good chance you’ll find several.

Furthermore, there seems to be a relative dearth of planets at 30 Earth masses around other stars. Lisa Kaltenegger points out that this is also evident in our Solar System, where there is a big gap between Uranus (14 times the mass of Earth) and the next planet up, Saturn (95 times). “There is a difference in population numbers above and below 30 Earth-masses, which may point to different formation mechanisms,” adds Pepe.

Formation mechanisms may explain another surprising finding. The stars around which these 50 exoplanets were found were not chosen for their composition, and so a range of metallicities (astronomy jargon for the abundance of elements heavier than hydrogen and helium) were represented.

“Jupiter mass planets are very common around high metallicity stars and much less common around low metallicity stars,” says Pepe. “This isn’t the same for low mass planets where we find low mass planets equally around stars of low and high metallicity.”

This may be down to the way gas giant planets form. Although they are made primarily from hydrogen and helium gas, the bottom-up theory of how they are created requires large rocky cores up to ten or twenty times the mass of the Earth to form quickly before gas can accrete onto them. A low metallicity star system would not have enough material for these cores to form rapidly, whereas there would be sufficient material for smaller super-earths to form at a more leisurely pace.

The results indicate a gradual turning point in our search for exoplanets. For 15 years lower mass planets were rarely discovered, and more easily-discovered ‘hot jupiters’ dominated. Now, with the advent of NASA’s Kepler spacecraft and instruments like HARPS, the balance is being redressed as we enter a new era of discovering that low mass terrestrial planets far outnumber more massive worlds. Finding another Earth may only be a matter of time.

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