predictions. Now the press embargo is lifted we can take a look at what's going on around a small, nondescript, red dwarf star 20 light years away in the constellation of Libra. This star, Gliese 581 had been known to harbor 4 planets, including a couple of super-earths (less than 10 Earth masses) lurking at the very edges of the so-called orbital 'habitable zone'. Now the Lick-Carnegie Exoplanet Survey has crunched through 11 years of radial velocity spectroscopy or 'wobble' data on this star, and are announcing 2 additional planets, including a 3-4 Earth mass world smack bang in the middle of the habitable zone - GL 581 g.
This is the first world that clearly ticks off two key boxes in the laundry list for habitable planets - it's very close in mass to the Earth, and sits at a perfect distance from its parent star to stand a chance of having a temperate surface. It's a wonderful and thrilling discovery, and I'll confess to going out and staring in the direction of Libra last night - although sadly Gliese 581 is too faint to see directly with our beady human eyes. The relative ease (relative being the operative word, it took the might of the Keck observatory to pin this down) of finding this world and its sisters is a potent indicator that planets like this are quite common.
With a 37 day orbit (putting it about 0.15 AU from the 1/3rd solar mass star) there's a good chance that GL 581 g is tidally locked - with a permanent day and night side, although it's by no means clear that tidal locking is inevitable. This poses significant questions about any climate on the planetary surface - something astronomers and planetary scientists have been worrying about for a while for this kind of scenario. A thick enough atmosphere and thermal transport could help even out the drastic day/night temperature difference and keep things stable.
It's a long way from being Earth-2.0 though. The star is small, an M-dwarf, about 100 times less luminous than our Sun and strongly skewed to emitting photons in the infrared. This is also an ancient system, somewhere between 7 and 12 billion years old. GL 581 g is an old, old world bathed in red light. Although larger rocky planets than the Earth should have a more vigorous geophysical history - and the attendant chemical cycling that seems so critical for life - they too cool off with age and eventually suffer from stagnation. Whether GL 581 g has a significant water component or not is also something that we'll have to wait patiently to find out - one, or two generations of astronomical instrumentation in the future.
It's an alien place for sure. But to even be able to discuss these issues in the context of an actual, real, planet only 20 light years away is only a hairs breadth away from revolutionary - welcome to the coming age of exoplanetary science!