Astronomical Discovery: First Potential Habitable Exoplanet In A Six-Planet Star System

append delete Star System - Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307 has been discovered by a team of European-American astronomers.

"The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The system lacks massive planets and is therefore a potential candidate for having additional low-mass planetary companions," astronomers write in their paper.

The planetary system around HD 40307 has an architecture radically different from that of the solar system, which indicates that a wide variety of formation histories might allow the emergence of roughly Earth-mass objects in the habitable zones of stars...

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.

Artistic representation of HD 40307g by PHL's SER as a water-rich world covered in clouds. An ocean surface is barely visible through the cloud and fog layers. Credits: University of Puerto Rico, Arecibo

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.

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. Credits: University of Puerto Rico, Arecibo

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.

All currently know potential habitable exoplanets listed in PHL's Habitable Exoplanets Catalog.Credits: University of Puerto Rico, Arecibo

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).

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