Where to look for extraterrestrial life


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


An artist's impression of a view of the surface of Proxima Centauri b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System.

An artist's impression of a view of the surface of Proxima Centauri b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System.

M. KORNMESSER/AFP/Getty Images

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.

4 | TRAPPIST-1 PLANETS

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.

5 | PROXIMA CENTAURI B

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.

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Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
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