In 2012, the K2.5V orange dwarf star HD 40307 was announced to have a minimum of six “super-Earth” planets in its solar system, with masses at least five times larger than Earth’s but still small enough to be rocky planets rather than gas giants. HD 40307 g, the planet which is furthest out, is in the habitable zone. There is also a high likelihood of additional smaller planets in this solar system.
Of the 6 super-Earth planets, three were originally reported in 2009 by Mayor et al. The rest, including HD 40307 g, were discovered by Mikko Tuomi and his team while they were using the radial velocity method to confirm the original discovery. This is is the largest number of super-Earth planets ever discovered in a single solar system.
To use the radial velocity method, the spectral signature of the star is examined for an extended period of time. In the case of the HD 40307 planets, the central star’s spectral signature has been recorded for five years.
During this time, a star with large orbiting planets will be pulled back and forth by those planets. The effects are not visible directly, but can be identified through spectrographic analysis of the star. In some cases, different spectral signatures can also be identified for some of its planets.
A star which wobbles because of the gravitational effects of its planets will sometimes move less quickly away from the Earth than at other times. As a result, the Doppler shift in the star’s spectrogram changes with each wobble. Intense analysis of the star’s Doppler shift and that of any planets whose light can be separated from the star’s reveals the oscillation periods of the star. In turn, this reveals the planets’ orbits.
HD 40307 g
Of the six super-Earth planets in this solar system, only HD 40307 g is in the habitable zone. Because HD 40307 is both smaller and dimmer than the Sun, the habitable zone is much closer to the star than the equivalent area around the Sun, at a distance of between 0.43 and 0.85 astronomical units.
HD 40307 g is at a distance of 0.6 astronomical units from its star. A single orbit takes 200 days to complete, which is far enough away to avoid tidal locking. Even at this distance, the planet only receives 62% of the solar radiation that Earth receives.
Nothing is known yet about the exact composition of HD 40307 g except that it is 7.1 times the mass of the Earth. The composition of its atmosphere will make a great deal of difference to its climate and its chances of supporting life. Further examination of this planet is certain.
It is currently believed that HD40307 b-d have all migrated inward toward their central star over time. If this is also the case for 40307 g, the planet may have been outside the habitable zone for most of its existence. This would reduce the chances of finding life.