Where to look for ET

The uni­verse may be teem­ing with life, but we have yet to find any beyond Earth. Th­ese are the best prospects, says STEPHEN FLEISCHFRESSER.

Cosmos - - Contents -

You may have no­ticed that there are not a lot of aliens around. Which is weird, be­cause many sci­en­tists think there should be life all over the uni­verse. There’s even a for­mula called the Drake equa­tion which tells us it prob­a­bly ex­ists.

How­ever, alien life doesn’t have to be in­tel­li­gent to be ex­cit­ing. Any ev­i­dence of life else­where in the uni­verse would be an amaz­ing dis­cov­ery! As we learn more about space, plan­ets, chem­istry and bi­ol­ogy, we get more clues as to where we should be search­ing. Here are some of the places where sci­en­tists think we might find ex­trater­res­trial life. 1 | ENCELADUS

Enceladus, named af­ter a gi­ant in Greek mythol­ogy, is ac­tu­ally a small­ish moon of Saturn. It’s only 500 km in di­am­e­ter, and when com­pared to Saturn’s largest moon Ti­tan, with a di­am­e­ter of 5,000 km, it seems pretty tiny.

Enceladus was dis­cov­ered by the English as­tronomer Wil­liam Her­schel in 1789, but all he could re­ally see was a dot through a tele­scope. For a very long time that’s all any­one knew, and most peo­ple thought this lit­tle moon was to­tally bor­ing.

But in the 1970s NASA launched the Voy­ager space­craft mis­sions and they re­vealed a rather pretty, and sur­pris­ing, icy world. Im­por­tantly the sur­face of Enceladus, com­posed of thick ice, was made up of dif­fer­ent re­gions - some were very young, and some were very old. This told sci­en­tists that there were ac­tive ge­o­log­i­cal pro­cesses go­ing on.

In 2005, NASA’S Cassini space­craft gave us a whole new out­look when it dis­cov­ered huge spouts of salty wa­ter vapour, mixed with sand and sim­ple or­ganic com­pounds of car­bon and hy­dro­gen, vent­ing from Enceladus’ sur­face. This led sci­en­tists to think that there must be a huge sub-sur­face ocean that is kept warm by some sort of en­ergy source. In 2017, sci­en­tists con­firmed the pres­ence of hy­dro­gen mol­e­cules which sug­gests the pres­ence of hy­dro­ther­mal vents in the moon’s ocean.

Taken to­gether, this means that Enceladus is prob­a­bly the most ex­cit­ing place in the so­lar sys­tem to look for life.

PS. There’s an­other moon very sim­i­lar to Enceladus or­bit­ing Jupiter, called Europa. It too could sup­port life, but its icy sur­face is much thicker than Enceladus’, mak­ing it dif­fi­cult to in­ves­ti­gate.

2 | TI­TAN

Ti­tan is Saturn’s largest moon. It’s half as big again as our moon. It’s huge! It also has a thick at­mos­phere and is the only other place in the so­lar sys­tem to have lakes and rivers. Per­fect!

But there are a few prob­lems. The at­mos­phere is mostly ni­tro­gen, the lakes and rivers are made of liq­uid methane and the tem­per­a­ture is al­most -180°C. So per­haps, not so per­fect af­ter all. Just like Enceladus, Ti­tan was vis­ited by the Voy­ager probes, but un­for­tu­nately, they couldn’t see much through the thick, hazy at­mos­phere first noted by its dis­cov­erer in 1655, the Dutch as­tronomer Chris­ti­aan Huy­gens.

Once again, the Cassini space­craft came to the res­cue. It ar­rived at Saturn in 2004 and landed a probe, ap­pro­pri­ately called Huy­gens, on Ti­tan’s sur­face in 2005. The probe, to­gether with radar map­ping from or­bit, re­vealed an­other young and ge­o­log­i­cally ac­tive world with a pos­si­ble sub-sur­face ocean.

Sub­se­quent re­search has re­vealed the pres­ence of many dif­fer­ent kinds of com­plex or­ganic mol­e­cules, a num­ber of which are nec­es­sary for life as we know it. Some sci­en­tists have sug­gested that life might be hid­ing in the oceans be­low or in the lakes and rivers, but it is so cold on Ti­tan that any­thing liv­ing there might be very dif­fer­ent from life on Earth.

3 | MARS

Mars has al­ways been fas­ci­nat­ing to Earth­lings. It is one of only three plan­ets, along with Venus and our own world, at just the right dis­tance from our sun to make it a likely place for life. Sci­en­tists call this the ‘hab­it­able zone’ be­cause it is pos­si­ble for plan­ets, with the right at­mos­phere, to have liq­uid wa­ter on their sur­faces.

We know that Mars has wa­ter, but it’s mostly frozen in po­lar ice caps, which were first seen by Her­schel. We also know that mil­lions of years ago Mars prob­a­bly had the right sort of con­di­tions for life as we know it.

In the 1970s NASA’S Vik­ing lan­ders re­ported that Mar­tian soil had been ex­posed to wa­ter and nutri­ents and that lots of car­bon diox­ide was given off, just as it would if bac­te­ria in the soil were alive. Then, in 2018, the Cu­rios­ity rover dis­cov­ered or­ganic com­pounds in the soil. In­deed, the re­cent dis­cov­ery of or­gan­isms that eat hy­dro­gen in Antarc­tica, where con­di­tions are much like Mars, has boosted hopes of find­ing life.

So that’s it for the so­lar sys­tem. But what about plan­ets or­bit­ing other stars in the galaxy, known as ‘ex­o­plan­ets’? Un­for­tu­nately, we know a lot less about ex­o­plan­ets be­cause they’re so far away, but as­tronomers have worked out that there are lots of them that might be able to sus­tain life. Let’s take a look.


Forty light-years away is a tiny star known as TRAPPIST-1. It’s a red dwarf star, which is the most com­mon type of star in the galaxy. Or­bit­ing this lit­tle luke­warm star is not one, but seven rocky plan­ets. All of th­ese plan­ets are a good size, with the small­est be­ing about half the size of Earth and the largest be­ing one fifth larger than our own world.

What’s weird is that they are all so close to their tiny star, with at least three in the star’s hab­it­able zone. While some ar­gue that only one of th­ese could re­ally sus­tain life, TRAPPIST-1 of­fers some great op­por­tu­ni­ties for fu­ture re­search.


Dis­cov­ered in 2016, the clos­est rocky ex­o­planet to Earth is Proxima Cen­tauri b. If you were trav­el­ling at light-speed it would take you four years to get there.

Proxima b, as it’s also known, is in the hab­it­able zone of the or­bit of the red dwarf star Proxima Cen­tauri and so has fairly mild sur­face tem­per­a­tures. It’s be­tween 1.5 and three times the mass of Earth and is known as a ‘su­per-earth’ planet.

Un­for­tu­nately, Proxima b’s star is ex­tremely ac­tive and the sur­face of the planet gets blasted by roughly 30 times more UV ra­di­a­tion than the Earth and is con­stantly bom­barded by a huge stream of gas known as a ‘stel­lar wind’. This wind would have ripped away any at­mos­phere that Proxima b might once have had. Nonethe­less, life might sur­vive be­neath a planet’s sur­face, and be­cause Proxima b is so close to Earth, sci­en­tists are excited to learn more about it.

6 | ROSS 128- B

This is the most re­cently dis­cov­ered ex­o­planet in a hab­it­able zone and might be the most ex­cit­ing of them all.

The star Ross 128 is 11 light-years away and doesn’t emit much ra­di­a­tion, mak­ing it one of the friendli­est places for life that sci­en­tists have found out­side our own so­lar sys­tem.

In or­bit around this small, gentle star is a rocky ex­o­planet known as Ross 128-b. The planet is 1.3 times larger than Earth and 20 times closer to its sun. But be­cause Ross 128 is so quiet, the planet would prob­a­bly be warm rather than hot. Sci­en­tists es­ti­mate that al­though Ross 128-b is only just in­side the hab­it­able zone, it might well have sur­face tem­per­a­tures rang­ing from 21°C to -60°C. While that might be on the chilly side for us, there are plenty of life­forms, par­tic­u­larly micro­organ­isms, who would think that’s balmy. Al­though there is still much to dis­cover about Ross 128-b, it’s prob­a­bly the best place to look for life out­side our own cosy cor­ner of the Milky Way.


Saturn’s moon Enceladus.

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