What is it about life outside the Earth that excites us so much? This was one of the questions Ian Sample of The Guardian asked Dr Stuart Clark two weeks back on the Science Weekly podcast. Around nineteen minutes into the show comes the question, “Why are we so keen on this idea?… There is something about this which seems to be endlessly appealing to us. What is this?”
“I thought about this a lot,” replies Dr Clark, “because I wanted to imbue (the book) with some sense of the philosophy of why we do this and why this seems to capture us so much.” He is, of course, talking about his latest book, The search for Earth’s twin. I think he comes up with a rather diplomatic answer: the idea that there could be other planets with life out there, he says, is “terrifically life-affirming”. To astronomers and astrophysicists, and to physicists in general, this is a question less about faith and more about the work we do. On the one hand is how Mr Sample points out that articles written about Earth-like planets become wildly popular on the web irrespective of how many remarkably similar ones get published in even a brief span of time, which is strangely true, and how Dr Clark attributes it to human nature, which is also true, and on the other is a harder truth: the Earth will not last forever, humans want to.
What makes a planet Earth-like?
The search for other planets is a recent one, starting around 1995, and it was accelerated some years ago with the launch of NASA’s Kepler mission which led to the discovery of thousands of planets. Given that planetary systems were so commonplace, our next search was for planets that were most like Earth. The strongest argument in favour of this is because the life we know evolved in the conditions present here on Earth, we know such life best and searching for such conditions is our best bet. This does reduce the chances of finding life dramatically — or perhaps not at all — because various other unfathomable forms of life may already exist which are not hydrocarbon-based like ours is, and we may be the minority; none of this is certain, which is why we settled to search for that which we know best. Kepler, however, is no longer functioning, but has collected sufficient data that we can examine it for quite a few years until ESA’s similar mission is launched.
A new mission may be proposed to look for life on Saturn’s moon, Enceladus: https://t.co/ORCcBAQNdm
— Physics Capsule (@PhysicsCapsule) July 2, 2016
The way we search for an Earth-like planet is to look for the most ideal case: one that is orbiting a G-type star, with an orbital radius of roughly 1AU, that is rocky, and Earth-sized. In other words, a photocopy of the Earth. Once we find such an ideal case (which has eluded us till date) any deviation can be translated to the changes it would bring to potential habitation on the surface. There are specific reasons why we look for these properties. Orbiting a G-type, sun-sized star would mean all the other characteristics of the potentially Earth-like planet have to be, quite literally, the same as the Earth. A 1AU orbit would give it the same climate, an Earth-sized radius and rocky structure would give it 1g acceleration due to gravity, and in turn a similar atmosphere as the Earth and so on. And even if one of these measurements is found to be different, all other measurements will then change accordingly.
Goldilocks and Saturnian moons
The 1AU orbital radius is of particular significance for two reasons. Firstly, the climate, and possibly the seasons in cahoots with the right axial inclination, which allows for hydrocarbon-based life to thrive. Secondly, because any potential Earth-like planet we have found so far has consistently never been in this region. Instead, heavy giants like Jupiter populate the so-called Goldilocks zone. The exact reason why this happens is unknown, but it could well be the general drift of heavier planets as they slowly drift from the outer reaches of a planetary system towards their parent star. The Juno mission that will soon reach Jupiter (our Solar System’s version of the many heavy giants we find everywhere else) should give us some insight into all this.
This effectively translates to finding a planet that can sustain liquid water at room temperatures (of our Earth) so that life as we know it can evolve. Water is the key element required for hydrocarbon lifeforms like ourselves. While this is all a fine case for finding actual planets like our Earth, remember that the story itself is incomplete. Once we find the Earth’s twin comes the question of whether there is life on it. And if not, we need to start all over again and look for another potential Earth-like planet, so stretch yourself, lean back and relax. This is going to be one long millennium.
However, all is not lost. Besides planets, there could be other natural satellites that may have unique ecosystems capable of supporting life. As an article in Scientific American last week pointed out, there is growing belief that Saturn’s moon, Enceladus, may well be our best bet as of now. With an icy parental cover shielding what many believe is a liquid ocean beneath it, in Enceladus we may have a system that is able to support life closer to home after all. This does not mean there is already life on Enceladus, but if there is — and there is a remarkably slim chance of this — then it would tell us a great deal about how life on earth evolved deep in our oceans, a trump card for biologists as well. Enceladus may even be too good to be true with its geysers shooting right out of its oceans which would make it easy for a targeted mission to collect samples without even landing on the icy surface and drilling. But, of course, there is no mission planned as yet, although several astronomers are hoping for one.
At the end of it all, one cannot — unfortunately — dismiss supremely religious people. Those who take pride in not believing in evolution etc. will have a lot of fingers to point at a lot of things. Facts, however, stand their ground. If life is found, it will improve our knowledge of a lot of things; if it is not, we have to remember that we are looking for needles in universe-sized haystack.