The star Gliese 667C is now the best candidate for harboring habitable worlds.
Our Solar System has only one habitable planet, or maybe two if you count Mars’ past when liquid water was running on its surface. More than one potentially habitable planet per star has been a very rare event in the known stars with planets. The well-known star Gliese 581 might have two and just recently the Kepler Space Telescope discovered two in the star Kepler-62. Now a team of scientists, led by Guillem Anglada of the University of Göttingen, reports the discovery of a star with three potentially habitable worlds.
Gliese 667C is part of a nearby triple system of stars 22 light years away. The team discovered that Gliese 667C has six planets, or even more, with three of them in the habitable zone and not much more massive than Earth. Gliese 667C is now the most interesting object for studying stellar systems with the potential for life.
About one hundred exoplanets, of the 898 detected so far, orbit their star at the right distance to sustain liquid water or the so-called ‘habitable zone’. However, most are big Jupiter-like worlds that are unable anyway to have a surface with liquid water. Only a few probably have the right size to be rocky worlds just like Earth.
Gliese 667C is the fourth known planetary system with at least six confirmed planets (Kepler-11, HD 10180, and HD 40307 also have six planets). Gliese 667C f and e will be added to the Habitable Exoplanets Catalog, now having a dozen objects of interest. Gliese 667C c was already added last year.
Figure 1. Artistic representations of the three potentially habitable planets around the star Gliese 667C as compared with Earth. The planets are shown assuming a rocky composition with surfaces mostly covered by water clouds. Credit: PHL @ UPR Arecibo.
Figure 2. The Habitable Exoplanets Catalog now list a dozen object of interest as potentially habitable worlds with the addition of two new planets, Gliese 667C e and f (Gliese 667C c was known since early 2012). Credit: PHL @ UPR Arecibo.
Figure 3. Simulated view of the star Gliese 667C from its three orbiting potentially habitable worlds. The Earth image is an unretouched photo of a beach sunset. The size of the star, and the brightness and color of the sky was carefully adjusted in subsequent frames to approximate the same view from each planet. The original photo is from a beach in Puerto Rico. Credit: PHL @ UPR Arecibo.
Figure 4. Gliese 667 is a nearby system of three stars that is easy to locate in the Scorpius constellation. The main two central stars Gliese 667AB are barely visible to the naked eye but easy to spot by binoculars or a small telescope. They are so close together that they appear as a single star. Gliese 667C is far enough from the central stars to be seen as separate star. However, it requires a larger telescope to be seem. Credit: PHL @ UPR Arecibo.
Figure 5. Orbits and approximate relative size of the planets around Gliese 667C (orbits and planets are not to scale with each other). All three potentially habitable planets (c, f, and e) orbit within the 'conservative habitable zone.' Credit: PHL @ UPR Arecibo.
Figure 6. This image shows the relative location of each potentially habitable planet of Gliese 667C with respect to the habitable zone. The location of Earth and Mars are shown for comparison. Note that Earth is not at the center of the habitable zone. Any planet close to the inner edge risk being a super-Venus while a frozen world closer to the outer edge. The star is not to scale with the planets.
Table 1: Planetary properties of the six planets around the star Gliese 667C plus one still unconfirmed (Gliese 667C h). Three of these (e, f, and e) are potentially habitable (highlighted in red). This table includes some estimated values that are not part of the original observations (see table notes below). Table 2 has similar values for the Solar System planets.
Planet Type - according to the PHL's Classification of Exoplanets.
Expected Mass - estimate assuming a probable inclination of 60°.
Expected Radius - estimate from an empirical mass-radius relationship.
Equilibrium Temperature - estimate assuming a bond albedo of 0.3.
Expected Surface Temperature - estimate assuming a scaled Earth-like atmosphere.
Habitable Zone Distance (HZD) - a measure of the orbital location of the planet with respect to the HZ. Planets with HZD values between -1 and +1. are within the HZ.
Earth Similarity Index (ESI) - using the new revised ESI which only considers observed parameters, stellar flux, and radius or mass.
Table 2: Planetary properties of the planets of the Solar System for comparison purposes. Description similar to Table 1.