One of the more interesting news items I came across on Wednesday was the discovery of a fifth and sixth planet around the red dwarf star Gliese 581. The fifth planet discovered, Gliese 581f, was pushed to the side both by the discoverers and the news media, and for good reason. That’s because the sixth planet, Gliese 581g has a few characteristics that allow it to take the (hopefully short-lived) crown of most Earth-like exoplanet yet discovered, or perhaps most accurately, most potentially Earth-like exoplanet yet found.
Now, Phil Plait did an good run down of what the group did and did not find and what the implications of the findings are, so I suggest that you check it out and there’s a link to the PDF file of the paper if you want to take a look at it yourself.
Gliese 581g is a planet with a minimum mass of 3.1 times that of Earth, making it the second smallest in the system and given it’s distance from a star that is much cooler than our Sun, it likely has an average temperature only a bit below 0°C, taking into account an Earth-like greenhouse effect, making it a possible that liquid water may be possible at times on the surface.
One of the things that’s really striking about this solar system is how unlike our own it is. Here’s a diagram of the model that the researchers displayed (from Figure 6 in the paper):
The dotted ellipses represent the orbits of the three innermost planets in our solar system relative to the sun at the center, while the black orbits represent the modelled orbits of the six planets discovered so far in the Gliese 581 system. Mercury is the red orbit, Venus the green, and Earth the blue. For reference, Gliese 581g, the planet in the spotlight, is the fourth planet from its parent star, so it’s orbit is the second one in from Mercury.
That’s really striking. Gliese 581g is actually the second smallest of the bunch, with a minimum mass of 3.1 times that of Earth with the smallest being the nearest planet, having a minimum mass of 1.7 times that of Earth. This model implies that there are 5 planets more massive than the Earth well within the orbit of Mercury, the closest planet to the Sun. Jupiter, the closest planet that’s larger that the Earth is just under 5 times the average distance of the Earth from the Sun at it’s closest approach, yet this star, which is significantly smaller than the Sun, has 5 super-Earths within the orbit of Mercury.
Of course, such proximity to their parent star has an effect that has been mentioned in the news reports of this discovery, and that is that these 5 planets are tidally locked to the star, giving them a permanent day side and night side. This would seem to be a significant downside to the possibility of finding life on such a planet, as it would be harder for a tidally locked planet to support the energy gradients that allow life to thrive, as the exposure of a given area of the planet to it’s star is fixed and thus likely quite static, in the absence of some sort of global jet stream or circulating ocean currents.
One of the most important parts of the paper comes at the end and that is their discussion of η⊕, which is the percentage of stars with planets of similar size to Earth (within an order of magnitude difference in mass and within a twofold difference in radius) within a star’s “habitable zone”, which is where liquid water can form on the planet’s surface. This value has been difficult to assess because our exoplanet hunting techniques bias us toward finding planets that are very large and very close to their stars.
In finding this planet, the group claims that η⊕ has a lower bound of about 20%, meaning that habitable planets may be in abundance throughout our stellar neighborhood, which is encouraging, should we ever develop the capabilities and the will to send missions beyond the Sun to other stars. It also helps us fill in terms in the Drake equation, which will better help us understand the significance of terrestrial life on the galactic stage and bring us closer to solving the Fermi paradox.
As a final note about the paper, there is still a good deal of uncertainty in these results. The model they used explicitly favors circular orbits and while the orbits may well fit close to the model, I’d be hesitant to put too much confidence in that aspect of the model. In fact, this circular orbit model contradicts a previous paper that gave Gliese 581d, the planet just outside the orbit of Gliese 581g, a fairly eccentric orbit. As more data comes in about this star and techniques are refined, I would not be surprised to find this new model overturned for an even more complicated one.
What is not complicated, however, are some of the media narratives that have arisen out of the news. Here’s a particularly egregious example from Russia Today:
Most of the discussion is tangential and in my opinion overly optimistic at that, but more annoying to me is how it inflates this news into something it’s not. This isn’t a case of a planet that we know to have water or an atmosphere. All we know of is potentials, based upon it’s mass and its orbit. It’ll take some more careful infrared astronomy to probe those very interesting questions. I eagerly await the answer, but I’m not going to prejudge the results simply because the possibilities are exciting.