It's almost time to name Gliese 581d.  

Two billion years or so before our own solar system coalesced, this planet was formed around a dim red star that's now about 20 light years from Earth.  Gliese 581 d is therefore an ancient world, orbiting around a cold star.  But it may be habitable.

That's the conclusion of the latest study, by R. D. Wordsworth, F. Forget1, F. Selsis, J.-B. Madeleine, E. Millour, and V. Eymet (the paper is Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling; you can find it on archiv.org).  They ran simulations based on what we know about the planet and its star, and conclude that if d has a sufficiently thick atmosphere of CO2, it could have liquid water at its surface.  Other studies of so-called "super-earths" like d hint that many or most of them will be water planets, global oceans.  And, when you factor in a recent study of habitable zones that indicates they could be much broader than first assumed, it seems that if this world has any sort of an atmosphere at all, then it's likely habitable.  So, here's what we know about d:

• It's between 7 and 13 times the mass of the Earth, but its radius is unknown (however, likely around 1.15 times Earth's radius).  If it's as dense as the Earth, then its surface gravity is about 2 times Earth's; but Earth is the densest of the solar system's rocky planets.  If d is an ocean world, it's likely a lot less dense and its surface gravity may be the same as Earth's.  In that case, though, it is almost certainly an ocean world, with no accessible land at all.
• It's may be tidally locked to its star, meaning that the sun stays fixed in one spot in the sky, and one whole hemisphere is in permanent darkness.  This is a condition usually taken to mean that the planet in question would be lifeless because the atmosphere would all condense on the cold side; but numerous studies have now shown that tidally-locked planets can retain their atmospheres quite well.  They do, however, tend to be windy.
• It may well have a thick CO2 atmosphere (researchers suspect these are common) in which case, provided minerals are able to leach up from the depths of the planetary ocean, it may have been capable of hosting life for six billion years now.

There's a really good chance that d could support life--though you and I wouldn't want to live there, since we'd weigh twice what we do on Earth and the atmosphere would be toxic.  But it could still be a lush world, overflowing with life.

What does it look like on this world?  The sunlight of its permanent day isn't red, though we call Gliese 581 a "red dwarf."  To us, it would appear to have about the same spectrum as a 60 watt bulb, which is to say, yellowish-white; and daylight is a bit dimmer than it is on Mars, so with the naked eye, it's visually like wearing a good pair of sunglasses.  The human eye adapts to a wide range of light conditions, so you wouldn't really notice the difference.  But, if d has an atmosphere, the sky is blue.  Old as it is, d may no longer have active plate tectonics, so, like Mars, it probably doesn't have mountains or volcanoes.  But it won't be a cratered environment, either, if the atmosphere is thick enough for water to be stable.  --And speaking of water, the weathering effects of high wind and water over billions of years suggest that it's become a very flat world lately, with either a global ocean, many shallow seas and low islands, or vast dry plains.

But this is amazing--because we're talking about a real planet here, not some speculative possible world; and not some science-fictional dream.  d does exist; we'll soon know whether it really is habitable, and within a few years, may be able to detect signatures of actual life in its atmosphere.  Already, we've learned enough to know that there are billions of other planets sailing through the galaxy with ours.  If we learn that Gliese 581 d really could sustain life, we'll be able to begin estimating (roughly, at first) how many habitable planets the Milky Way contains.  Considering how close Gliese 581 is to us, that number could be huge.

So what do we name this new world?  It is ancient, far older than our own worlds; so it would be fitting to name it after one of the Titans, who are older than the Greco-Roman gods we've named our planets after.  If it's a sterile ocean, I vote for Oceanus; if it could host life, then my favoured name would be that of Oceanus's wife, the goddess of rivers and lakes: Tethys.

Welcome, Tethys, and may you divide history into two parts:  the long age in which we wondered whether we were alone in the universe--and a new epoch in which we know we are not.