Artist's concept of the HD 40307 planetary system featuring a close-up view of HD 40307g, which orbits within the habitable zone (Image: J. Pinfield, for the RoPACS network at the University of Hertfordshire)
Due to the masterful efforts of an international team of astronomers, a new super-Earth planet has been discovered within the habitable zone of a star just 42 light years from Earth. Part of a six planetary system, the super-Earth known as HD 40307g has several promising attributes in terms of its ability to support life and because of its relative proximity it may soon be possible to observe the planet optically.
HD 40307 is a quiet K-type dwarf star, a bit smaller and cooler than the Sun, which appears in Earth's southern skies to reside about eight degrees SSE of Canopus, the brightest star in the constellation of Carina. The apparent magnitude of HD 40307 is about 7.1, which means that it is visible in a pair of binoculars.
HD 40307 was known from European Southern Observatory measurements of its radial velocity to have three close-orbiting super-Earths, with orbital periods of 4.3, 9.6, and 20.4 days. The outermost of these planets orbits HD 40307 at a distance of 0.13 Astronomical Unit (AU), or about 12.5 million miles (20 million km), at which distance the solar radiance is considerably stronger than that at Mercury's orbit in our solar system. As yet another of the more than 100 stars found so far that host multiple exoplanets, the discovery was filed away ... but not forgotten.
Advances in analysis methods have recently lead to a re-examination of the radial velocity data of HD 40307.
“We pioneered new data analysis techniques including the use of the wavelength as a filter to reduce the influence of activity on the signal from this star," says Mikko Tuomi of the University of Hertfordshire. "This significantly increased our sensitivity and enabled us to reveal three new super-Earth planets around the star known as HD 40307, making it into a six-planet system.”
The most interesting of the new planets is the one most distant from the star, HD 40307 g. It has a mass at least seven times that of the Earth and orbits its parent star in just under 200 days at a distance of about 0.6 AU (56 million miles or 90 million km), which would put it just inside Venus' orbit. As HD 40307 is smaller and cooler than the Sun, the solar radiance is about 55 percent of that received by the Earth.
HD 40307 g is also distant enough from HD 40307 that it should rotate on its own axis without being tidally locked to its parent star. This suggests a day and night cycle suited to even heating of the planet. Given the right atmospheric conditions, this should allow for liquid water, and even a temperate climate. This is a large planet, but if it has the same average density as does the Earth, it would be about 1.9 times larger in diameter than the Earth. The surface gravity would also be about 1.9 g, a large but not insurmountable gravity.
The maximum separation between HD 40307g and its parent star as seen from Earth is about 50 milliarcseconds, a very small angle, but the upcoming James Webb Space Telescope should easily be able to detect the super-Earth optically. When that is possible, many answers about surface temperature and atmosphere will receive answers. Thus in the not too distant future we may see HD 40307g confirmed as the first solid candidate for extra-terrestrial life.
Source: University of Hertfordshire
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