Looking Up – Bob Eklund

A team of astronomers has combined new observations of the star Gliese 667C with existing data from the European Southern Observatory’s 3.6-meter telescope in Chile to reveal a system with at least six planets. A record-breaking three of these planets are “super-Earths” lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life.

Gliese 667C is a very well-studied star. With just over one third of the mass of the Sun, it is part of a triple-star system known as Gliese 667 (also referred to as GJ 667), 22 light-years away in the constellation of Scorpius (The Scorpion). This is quite close to us—much closer than the star systems investigated using telescopes such as the planet-hunting Kepler space telescope.

Viewed from one of these newly found planets, the two other suns would look like a pair of very bright stars visible in the daytime, and at night they would provide as much illumination as the full Moon. The new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it.

The astronomers knew that the star had three planets from previous studies, and they wanted to see whether there were any more. By adding some new observations and revisiting existing data they were able to confirm those three and confidently reveal several more.

Three of these planets are confirmed to be super-Earths—planets more massive than Earth, but less massive than planets like Uranus or Neptune—that are within their star’s habitable zone, an area around a star in which water may be present in liquid form if conditions are right. (In our own solar system, Venus orbits close to the inner edge of the habitable zone and Mars close to the outer edge.)

Compact systems around Sun-like stars have been found to be abundant in the Milky Way. Around such stars, planets orbiting close to the parent star are very hot and are unlikely to be habitable. But this is not true for cooler and dimmer stars such as Gliese 667C. In this case, the habitable zone lies entirely within an orbit the size of Mercury’s, much closer than for our Sun. The Gliese 667C system is the first example of a system where such a low-mass star is seen to host several potentially rocky planets in the habitable zone.

The team used data from the UVES spectrograph on ESO’s Very Large Telescope in Chile (to determine the properties of the star accurately); the Carnegie Planet Finder Spectrograph (PFS) at the 6.5-meter Magellan II Telescope at the Las Campanas Observatory in Chile; the HIRES spectrograph mounted on the Keck 10-meter telescope on Mauna Kea, Hawaii; and extensive previous data from HARPS (the High Accuracy Radial velocity Planet Searcher) at ESO’s 3.6-meter telescope in Chile.

The ESO scientist responsible for HARPS, Gaspare Lo Curto, remarks: “This exciting result was largely made possible by the power of HARPS and its associated software, and it also underlines the value of the ESO archive. It is very good to also see several independent research groups exploiting this unique instrument and achieving the ultimate precision.”

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