It's not a fun question to ask, but anytime a discovery of this magnitude comes along, it's a good idea to hang on to a little skepticism. So let's sort through the biggest questions about Earth's newly discovered sibling.
This week, some astronomers have raised doubts over the existence of Gliese 581g, popularly known as Zarmina. What do these reports mean, exactly? Is Zarmina a cosmic goof, a phantom planet that was never really there? The short answer is that Zarmina probably does exist, but a lot of what we've heard about this planet could well be baseless speculation.
First of all, let's deal with the matter of Zarmina's existence. This is probably the only thing we can know about Zarmina with any certainty in our lifetimes, and the balance of evidence is still pretty strongly in favor of its existence. The recent objections don't say that Zarmina doesn't exist, just that other teams can't confirm its discovery.
The main challenger to the Zarmina discovery is Francesco Pepe and his team at the Geneva Observatory. He explains that his data can easily confirm the existence of the four previously discovered planets around Gliese 581 - planets b, c, d, and e - but he can't find similar evidence for the new planets f and g:
"Simulations on the real data have shown that the probability that such a signal [for Gliese 581g] is just produced 'by chance' out of the noise is not negligible, of the order of several percents. Under these conditions we cannot confirm the presence of the announced planet Gliese 581g. We haven't made a detailed analysis yet, but at first glance no statistically significant signal [for planet f] is emerging from our data set."
Now, it's important to pay close attention to what he's saying here. Pepe says that the gravitational signal of Zarmina is a faint one, and in fact it's right around the level of statistical noise. That means it's possible that Gliese 581g is just an artifact of the data noise, although Pepe himself says this is simply "not negligible." That means there's a legitimate chance the planet doesn't exist, but that chance is still smaller than the probability that the planet actually does exist.
Science doesn't like to deal in absolutes, with good reason. As such, it's important to remember that Zarmina is unconfirmed, but that doesn't mean the planet doesn't exist. Indeed, Vogt's original paper acknowledges the fact that the planet's existence isn't entirely secure, which has been largely glossed over in news reports, including here. Even so, the odds are still very good that Zarmina exists and that further investigation will confirm that it's out there, although the jury is still out on Gliese 581f.
But is there life on Zarmina?
Although I'd say there's good reason to be confident about the existence of Zarmina, whether there's life on the planet is a much trickier question. For his part, Steven Vogt has been fairly definitive on this point, saying he believes there's life there. As he pointed out in his interview with us, that's just a statement of his personal belief, and not his considered scientific opinion.
It's well worth reading his entire explanation in the original interview - particularly because, as he points out, this can't easily be condensed to a soundbite or pull-quote - but here's the gist of his reasoning:
I'm not an expert in biology but when you read about the conditions under which life took hold on this planet – it was a terrible place 4 billion years ago, with no oxygen. Yet life came on the scene quickly. Something hit Earth so hard it broke off a chunk that created the Moon. And yet life kept coming back over and over again. You learn from that that it's hard to stop life.
So when I look at a place like this planet, with strong gravity and a good temperature – all those conditions are just perfect. It would be easier for life to evolve on that planet than on Earth. So, heck I'm pretty sure there's life there. Maybe you won't be filming Corona commercials on the beach there but it will be life.
But all of that still presumes that there's liquid water on the planet, and that simply isn't a given. As Phil Plait over at Bad Astronomy pointed out when the planet was first discovered, we don't know whether the planet has liquid water or an atmosphere, and we currently don't have any means to determine either of these things.
It's worth remembering that Earth isn't the only potentially habitable planet in our Solar System. Venus and Mars both could conceivably have supported life - in fact, if you swapped their atmospheres, then a thin-skinned Venus and a thickly-covered Mars could both potentially support liquid water and, with it, life. But Venus has a supercharged greenhouse effect because of its thick atmosphere that makes life as we know it impossible, and Mars's atmosphere is too thin to support life, although it's possible it might once have had microbial life, and perhaps still does deep below the surface.
Of course, that last point is a bit of a mixed message. If Mars really is a dead world, then it's a reminder that life doesn't spring up everywhere it has a chance to, and so just because Zarmina could support life, that doesn't mean it does. On the other hand, a microbial Mars might be taken as a good indicator that life is hardier than we thought, and makes it seem more likely that there's life on Gliese 581g - and its nearby neighbor Gliese 581d, which might also be habitable if it's got a thick enough atmosphere.
All of this may seem impossibly speculative - after all, what does Mars have to do with Zarmina? It doesn't really, but the search for life in the universe relies on extrapolating as much as we can from very, very few data points. By itself, Earth doesn't tell us much other than the fact that planets just like Earth can support life. The more we know about planets that aren't like Earth - and Mars and the tidally locked Zarmina are definitely very different from Earth - then we can start getting some idea about the probabilities of life, including how frequently it actually comes into existence on habitable planets.
Some of this extrapolation is more solid than others. For instance, the fact that Zarmina is a potentially habitable planet just 20 light-years away suggests such planets are extraordinarily common in our galaxy. Phil Plait explained why in his post:
Perhaps the most interesting and exciting aspect of all this is what it implies. The Milky Way galaxy is composed of about 200 billion stars, and is 100,000 light years across. The fact that we found a planet that is even anything like the Earth at all orbiting another star only 20 light years away makes me extremely optimistic that earthlike planets are everywhere in our galaxy. 20 light years is practically in our lap compared to the vast size of our galaxy, so statistically speaking, it seems very likely it's not unique. I don't want to extrapolate from a data set of two (us and them), but if this is typical, there could be millions of such planets in the galaxy. Millions.
There's a lot we don't know about Zarmina, and a lot of the most intriguing questions - like whether it has an atmosphere, liquid water, or life - likely will remain unsolvable in our lifetimes. But we definitely should be able to confirm its existence one way or the other in the near future, and assuming this observation holds up, then we really could be looking at millions of other habitable planets in our cosmic backyard. At that point, as long as the odds of there being another planet just like Earth are better than one in a million, then we can feel very confident that we're not alone in the Milky Way, even if our galactic neighbors are just a bunch of hardy microbes.