Life on other plan­ets

We have finally taken our search for extra-ter­res­trial life seri­ously. But just how are we approach­ing this sys­tem­atic search for a needle in a haystack?

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 ago 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 that life as we know it 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.

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 1 AU, 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 1 AU 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 1 AU 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.

The assault on science

Pop­ulist poli­cies have begun to affect sci­ence adversely. The trou­ble is, they are hap­pen­ing slowly enough that nobody seems to notice.

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