Just a few weeks ago, Steven Vogt at the University of California, Santa Cruz, and a few pals announced the discovery of a planet in the habitable zone around Gliese 581, a red dwarf star about 20 light years from here in the constellation of Libra. That’s almost next door–Gliese 581 is our 87th closest neighbour.
This planet, Gliese 581g, is about three times the mass of Earth and orbits its star every 37 days at a distance of about 15 million kilometres. That may be closer than Mercury is to our Sun, but because Gliese is a dwarf star, this puts it smack in the middle of the region in which water ought to be liquid–the so-called habitable zone.
So one question naturally arises: what is it like on Gliese 581g? Today, Vogt and his mate Kevin Heng at The Swiss Federal Institute of Technology Zurich give us the answer by simulating the atmosphere using exactly the same ocean and atmospheric modelling package that many groups use to simulate Earth 1.0’s climate
Vogt and Heng make clear that there are severe limits to the kind of model you can build, given that we know so little about Gliese 581g. “We are primarily interested in the long-term, quasi-stable, large-scale circulation patterns – the climate – as opposed to the short-term temporal variations (the weather),” they say.
So what can you say about this exo-Earth that constrains the model? First, it is almost certainly tidally locked, like our Moon. Planetary formation models suggest that Earth-sized planets that form close to stars much smaller than ours, almost certainly become tidally locked with a billion years or so of forming.
That means the planet rotates once every time it orbits its sun ensuring that the same half always faces sunward. Clearly that’s going to have a significant impact on any planet’s climate.
The model also takes into account factors such as the geometry of the planet (spherical!) and the effects of stellar irradiation. Vogt and Heng assume a surface pressure of 1 bar and assume that various other factors that have to be plugged into the model are as they are on Earth.
But Vogt and Heng have chosen not to take into account factors such as radiative transfer and atmospheric chemistry. That would be pure guesswork until more is known about this body.
The end result is that the model effectively shows what would happen were Earth scaled up to Gliese 581g’s size and tidally locked.
Even with such a simply model, Vogt and Heng produce a few interesting maps. The zonal wind patterns are shown above and the temperature below.
Of course, there’s one small fly in this exoplanet ointment. A couple of weeks ago, other astronomers announced that they’d re-examined the observations from Gliese 581 and found no sign of Vogt’s exo-Earth. Perhaps the whole thing is an artifact in the data.
Be that as it may, the maps offer an interesting insight into the kind of science that can and will be done when the discovery of an exo-Earth is finally confirmed (which should be around May next year if the current trend is anything to go by, as we discussed here.)
As Vogt and Heng put it: “Independent of whether the existence of Gliese 581g is confirmed, our study anticipates the use of meteorological solvers to quantify the atmospheric circulation on potentially habitable, Earth-sized exoplanets.”
So think of this as a kind of dry run.
Ref: arxiv.org/abs/1010.4719 Gliese 581g As A Scaled-Up Version Of Earth: Atmospheric Circulation Simulations