Gliese 581 is a name well known and repeated in exoplanetary circles. Lying 20.3 light years from us in the constellation of Libra, it is a red dwarf star now known to have at least six planets. Twice before, the star has been in the news with one of its planets declared close to the habitable zone – the slender band of orbits around the star where the stellar radiation is high enough to melt ice, but low enough not to boil water. Planet C was close, but on the hot side, too close to the star. Planet D was close, but on the cold side. Now a planet G has appeared and it is right in the middle of the habitable zone.
Appearing in this region around a star is not in any way proof that a planet can support life, it is merely a suggestion that it is likely to fulfill at least one of the prerequisites – the balance of radiation from its host star is sufficient to sustain liquid water. But further properties derived from observations of the star are quite positive. The planet is 3-4 times the mass of the Earth, suggesting a rocky planet with a defined surface, which at a similar density would put it at 1.2-1.4 Earth masses, giving a surface gravity not that dissimilar to our own. That would imply sufficient gravity to hold an atmosphere. But the similarities with the development of our own world do seem to end there.
The planet orbits its host star in 37 days. The low luminosity of Gliese 581 means that to be in the habitable zone in terms of radiation, the planet must crowd close to the star. This means the star has sufficient gravitational effect to tidally lock its satellite – meaning as with the Moon in orbit of the Earth, the same face of the planet always points toward the star. This would imply a searingly hot one face and freezing cold dark side, with winds racing from one side to the other. At the day night boundary, buffeted by these winds, more moderate climates would be seen at the different latitudes, where life could start, possibly evolving to take advantage of less temperate spaces on the planet after leaving the cradle. Red Dwarfs are long lived stars, so the time will also be there to do it.
But gravity and photons aren’t all a star can put out. Gliese 581 isn’t a flare star, that is, it isn’t known for sudden massive bursts of material from its surface, but it will still lose mass through slow stellar winds. It is believed that planets in close orbits to their host stars tend to lose their magnetic fields, or see them closely affiliated with that of their host stars. The protective magnetosphere, the magnetic sheath that protects us against the particle radiation that results from hot matter essentially expanding off the surface of our Sun, may not be replicated in this new place.
Whether or not there is life in this increasingly diverse place, the careful measurements carried out by the team, who measured the Doppler shifts of the light from the wobbling star to an amazing precision, do show that pulling out rocky planets in the habitable zones is within reach of modern technology. However, with the actual light from the planet lost in the glare of the star, it will take time and new technology to tease out the signal of life within that light, should it be there.
The paper announcing the discovery of the planet is here and one submitted at the same time detailing modelling of planet D’s atmosphere to determine whether or not its properties are sufficient to insulate the bitter cold and foster life is here.