First Potential Habitable Exoplanet in a Six-Planet Star System

Post date: Nov 08, 2012 2:59:28 AM

HD 40307g is the closest habitable planet candidate around a Sun-like star.

A team of European-American astronomers announced the discovery of a new potential habitable exoplanet around the star HD 40307, a star slightly smaller than the Sun and 42 light years away in the constellation Pictor. HD 40307 is the third known star system with as many as six planets(1), and the first one of such with a potentially habitable planet.

The star HD 40307 was previously known to have three planets. The research team discovered three additional ones, including the one potentially habitable, HD 40307g. They used a novel statistical tool, the HARPS-TERRA software(2), to extract the faint signals of these objects from data from ESO's HARPS Instrument.

HD 40307g is a superterran(3) at least seven times more massive than Earth orbiting the star at the right distance to support liquid water. The planet could have a size between 1.9 and 2.5 Earth radii depending on composition, either rocky or water-rich, respectively. HD 40307g receives on average about 67% of the light Earth receives from the Sun.

There is no information about the type and composition of the atmosphere of HD 40307g. Average temperatures might be near 9°C (48°F) assuming a similar scaled-up terrestrial atmosphere. It might also experience strong seasonal surface temperature shifts between -17° to 52°C (1.4° to 126°F) due to its orbital eccentricity. Nevertheless, these extremes are tolerable by most complex life, as we know it.

HD 40307g has been added to the Planetary Habitability Laboratory’s Habitable Exoplanets Catalog(4). There are seven potentially habitable exoplanets now in the catalog with HD 40307g being the new fourth target of interest based on an Earth Similarity Index of 0.79(5). HD 40307g also is the closest planet orbiting a Sun-like star, and probably not tidally locked, unlike most of the other listed planets around red dwarf stars.

The detection of HD 40307g is remarkable example of the use a Bayesian Monte Carlo Markov Chain technique and HARPS-TERRA tools developed by the authors. However, this kind of detection needs independent confirmation by other methods, as also cautioned by the research team. Meanwhile, this discovery is considered a potentially habitable exoplanet candidate and an interesting target for future observations.

The international research team was led by Mikko Tuomi (Hertfordshire University) and Guillem Anglada-Escude (University of Goettingen). The study also contains significant contributions from E. Gerlach (Technical University of Dresden), Hugh R. A. Jones (University of Hertfordshire), A. Reiners (University of Goettingen), S. Vogt, E. Rivera (UCO-Lick Observatory) and R. P. Butler (Carnegie Institution for Science).

The research paper Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307 was accepted for publication in Astronomy & Astrophysics.


    1. The other two stellar systems with as many as six planets are HD 10180 and Kepler-11. Up to three additional planets have been suggested for HD 10180 but are still unconfirmed. HD 10180 also has a planet in its stellar habitable zone but is too large, Neptune-size, to be considered potentially habitable. All the Kepler-11 planets are too hot for life.

    2. HARPS-TERRA is a new software tool that implements additional analyses and tests beyond those enabled by other methods to analyze exoplanets radial velocity data. The software can rule out the planetary nature of prominent signals correlated with stellar activity.

    3. Superterran, aka super-Earths, are planets with masses between 2 to 10 Earth masses regardless of composition.

    4. The Habitable Exoplanet Catalog is a project of the Planetary Habitability Laboratory @ UPR Arecibo.

    5. The Earth Similarity Index (ESI) is a measure of Earth-likeness from zero to one, with a value of one being identical to Earth. Planets with ESI values above 0.7 can be considered potentially habitable but only those with values closer to one can be considered Earth-like worlds.


Figure 1. Artistic representation of HD 40307g by PHL's SER as a water-rich world covered in clouds. The ocean surface is barely visible through the cloud and fog layers.

Figure 2. Relative size of the planets and orbits of the stellar system in HD 40307. Orbits are not to scale with planet sizes. A mass-radius relationship was used to estimate the size of the planets. The green shade is the stellar habitable zone where surface liquid water is possible for a planet with the right size and atmosphere. Note that the orbit of HD 40307g is probably highly elliptical causing extreme temperature changes similar to seasons.

Figure 3. All currently know potential habitable exoplanets listed in PHL's Habitable Exoplanets Catalog.