Astrobiology

We know more about the universe than ever before, and we've observed all kinds of phenomena. But there's still one thing we haven't found yet: evidence of alien life.

Our own planet is teeming with life. The oceans are a soup of fish and algae; the land is covered in trees; insects hum, birds soar, humans chatter and sing.

But on other planets, we’ve never discovered so much as a microbe, let alone something more advanced.

That doesn’t mean there’s nothing out there. With every passing year, scientists increase their understanding of distant stars and planets, and we get closer and closer to finding alien life. This branch of astronomy is known as astrobiology.

The search for life on other planets can be divided into two main categories: the search for simple life forms (like bacteria) and the search for complex life forms (like ourselves).

The search for simple life forms has been underway for decades. Since 1997, NASA have landed five rovers on Mars: Sojourner (1997), Spirit and Opportunity (both 2004), Curiosity (2012) and Perseverance (2021).

These rovers can study Martian soil, searching for evidence of alien, microbial life. So far, they haven’t found anything, but it’s still exciting for astrobiologists. This is the first time in history that we’ve managed to study the surface of an alien planet up close.

The discovery of microbes on an alien planet would be groundbreaking. But it would pale in comparison to the discovery of large, complex lifeforms – maybe even an alien species with a civilization like our own.

This particular branch of astrobiology is often referred to as SETI: the Search for Extraterrestrial Intelligence.

Since the 1980s, the SETI Institute – an organization in California – has been listening out for radio signals sent to Earth from other planets. More recently, cutting-edge telescopes have been developed, with the power to study distant planets far beyond our solar system.

Again, we haven’t found anything yet. But many believe that the discovery of life is only a matter of time. When it finally happens, it would surely be the greatest discovery in the history of humankind.

The idea of planet habitability is at the heart of astrobiology. This involves looking at a planet – for example, Mars – and asking a question: could life theoretically exist there?

As far as we know, life can’t pop up anywhere. There are a couple of fundamental conditions that every life form needs. We’ll look at this in more detail later, but in simple terms: every life form needs an energy source and some kind of liquid, like water.

If a place doesn’t meet these two conditions, life couldn’t possibly exist there. Take the moon, for example. It has an energy source (sunlight) but no liquid water, so there’s no point searching for life there.

There’s an important reason why life can’t exist without access to energy or a liquid. Without them, it’s hard to perform any meaningful chemical processes – and as far as we know, chemical processes are an essential condition of life.

Some Earth-based life forms use chemical processes to combine different molecules together. For example, plants use photosynthesis to combine carbon dioxide and water into oxygen and essential sugars.

Other Earth-based life forms use chemical processes to break larger molecules apart. For example, animals use cellular respiration: breaking sugars apart into carbon dioxide and water.

The breakdown of sugars releases energy. That energy allows us to grow, and move, and reproduce. Without it, we wouldn’t properly exist. That’s why it seems that chemical processes are an essential condition of life.

The chemical processes that life forms need can’t happen without an energy source. That’s what gets a reaction started. If there’s no energy source, no reactions can happen, and by extension, life can’t exist.

On Earth, the most obvious energy source is sunlight. That’s how plants are able to photosynthesize – and this energy is passed on when animals eat those plants.

But sunlight isn’t the only way for life to acquire energy. In the depths of our oceans, where the sun doesn’t penetrate, bacteria swarm around hydrothermal vents, using chemical energy to power their reactions instead.

This process is called chemosynthesis, and it’s something that astrobiologists consider when searching for habitable planets. Maybe aliens will rely on sunlight – or maybe they’ll get their energy from another source.

As well as an energy source, most chemical reactions also need some kind of liquid medium. This liquid is used to break substances down, and help the different molecules mix together and interact.

A great example is water. It’s a brilliant solvent, which can break down almost any substance, making it the perfect medium for life-giving chemical reactions.

This is what Earth-based life forms use to facilitate their chemical reactions. From humans, to plants, to deep sea bacteria, there isn’t a single organism on the planet that doesn’t need liquid water.

In theory, water isn’t the only liquid that could be used for chemical reactions. Some astrobiologists have suggested something like liquid ammonia – which has similar properties – as an alternative.

But water is generally better than ammonia at breaking substances down. If life needs a medium for chemical reactions, water is probably the very best option around.

To sum things up: astrobiologists know that life can’t exist without chemical processes. They also know that chemical processes need an energy source and a liquid medium – probably a source of water.

By extension, they know that alien life can only exist in places that offer these two fundamental conditions. This is exactly what planet habitability is about.

Mars, for example, has an energy source (sunlight) and it might have liquid water. There’s an ice cap at the southern pole of the planet, which may have a sea beneath it.

In other words, theoretically, it might be possible for life forms to evolve on that planet. That’s why NASA send rovers there – because it seems to offer the key ingredients for life.

On Earth, we’re blessed with generous access to those two conditions that life relies on: an energy source (sunlight) and a liquid medium (water). But not every planet is so lucky – it often depends on the characteristics of a host star.

A host star is the central star with planets caught in its orbit. Our own host star is the sun – and there are trillions of others in the universe.

Without a reliable host star, it’s virtually impossible for an alien planet to provide the right conditions for alien life. First of all, it’s an excellent energy source – a star provides heat and light.

But it also helps with liquids. Without the warmth from a host star, any water on a planet will usually be frozen. In other words, the water will be solid, not liquid – and without liquid water, life forms can’t evolve.

Every host star is different, and not all of them provide the right conditions for life. Maybe a star is too weak to provide energy. Or maybe it burns with too much heat, and boils any planetary water into gas.

It often depends on the current stage of a host star’s stellar life cycle. If a host star is at the wrong stage of its life, there’s not much chance of finding life on any of its orbiting planets.

For example, a protostar is usually surrounded by clouds of space dust. These clouds of dust act like giant curtains, blocking a lot of the protostar’s energy, and making it unlikely that life could exist on a protostar’s orbiting planets.

In general, a star in the main sequence stage is thought to provide the best conditions for life, as it washes its planets with a steady stream of heat and light.

However, a main sequence star won’t always provide the right conditions for life. It also depends how much heat that star is giving off – we call this the spectral class.

If a star burns much hotter than 7000 K, it probably won’t provide the right conditions for life. Along with blistering heat, these stars also hammer any orbiting planets with deadly radiation. Hotter stars also have shorter life spans, which gives life less time to form.

As far as we know, the sweet spot for life is a K-class star – that’s when liquid water is most likely. But M, G and F-class stars are also possible options. Our sun is a G-class star.

Whatever the spectral class of a star, eventually all of them run out of fuel, when they finish converting all their hydrogen atoms into helium. After that, as we've already talked about, the stellar life cycle will go one of two ways.

A larger star (like an O-class) will explode in a violent supernova, expelling hot gases into space. This explosion will sometimes leave a black hole in its wake.

A smaller star (like our G-class sun) will expand into a red giant, before squeezing back down into a white dwarf.

A red giant or a white dwarf could potentially provide enough energy to support a planet. It isn’t as likely as a main sequence star – but it isn’t out of the question.

When searching for planets that might have life, the condition of host stars is a great place to start. But there are also other things to think about: in particular, orbital motion.

Orbital motion describes the path that a planet takes around its star. As we've mentioned, this path isn’t a perfect circle – it’s usually slightly elliptical. Below, you can see the orbital motions of four different planets: Mercury, Venus, Earth and Mars.

Orbital motion is important, because a planet with a closer orbit to a star will usually be significantly hotter than a planet further away. It’s like sitting by a fire – up close, you’ll feel more heat.

For example, Earth has an average surface temperature of 15°C. Mars, on the other hand, has an average surface temperature of -65°C. They share the exact same G-class host star, but they experience that star in totally different ways.

Every planet’s orbital path will affect its chances of life. If it’s too close to the star – and therefore too hot – life is unlikely to evolve there. If it’s too far from a star – and therefore too cold – life is unlikely too.

In our solar system, Mercury is the closest planet to the sun. Its average temperature is 167°C, and that’s a problem for life forms. Liquid water can’t exist there – at temperatures higher than 100°C, water turns to steam.

Neptune, meanwhile, is the furthest planet from the sun. Its average temperature is -200°C, and that’s a problem too. At temperatures lower than 0°C, liquid water solidifies into ice.

Earth, on the other hand, is in a bit of a sweet spot, with an average temperature of 15°C. That’s why life can exist here – because it’s just the right temperature for liquid water to occur.

That sweet spot, where a planet’s orbital motion isn’t too close to turn water to steam, and isn’t too far to turn water to ice, is often referred to as the Goldilocks Zone.

This name comes from the story of Goldilocks and the Three Bears, in which a little girl tries three bowls of porridge. The first bowl is too hot (like Mercury), the second bowl is too cold (like Neptune), but the third bowl is just right (like Earth).

A star’s Goldilocks Zone is marked by an upper and lower limit. In our solar system, the upper limit is 180 million kilometers from the sun (any further and water will freeze). The lower limit is 135 million kilometers from the sun (any closer and water will boil).

The Earth is actually the only planet comfortably within our Goldilocks Zone. Venus and Mars, on either side of us, are just beyond the outer edge.

Different stars will have different Goldilocks Zones, depending on the heat of the star. If a star burns hotter, the Goldilocks Zone will be further from the star. If a star burns cooler, the Goldilocks Zone will be closer.

A famous example is TRAPPIST-1: a smoldering host star in the Aquarius constellation. It’s an M-class star, with a surface temperature about half as high as the temperature of the sun, which means the Goldilocks Zone is closer to the star than our one.

TRAPPIST-1 has four planets with orbits that put them in the Goldilocks Zone. That’s more than we have in our own solar system: just the Earth, really, plus Mars (arguably) hovering at the outer edge.

The TRAPPIST-1 system is exciting for astrobiologists. Four planets in the Goldilocks Zone means four planets with the potential for liquid water – and by extension, the potential for life.

Even when a planet is right in the sweet spot of a Goldilocks Zone, it still might not be an appropriate place for life. One more thing that an astrobiologist needs to consider is planetary size.

In our solar system, there’s an enormous size difference between each and every planet. Just take a look at the diameters of each planet in the table down below.

There’s much less chance of finding alien life on a small planet like Mercury, than there is on a planet like Earth. When it comes to planetary habitability, size definitely matters.

In general, a smaller planet like Mercury has a lower mass than a larger planet like Earth. This isn’t always true – it depends on the composition of the planet – but it’s a good general rule.

Any planet with a lower mass will also have lower gravity. This is part of Newton’s law of universal gravitation. Heavy objects pull harder; lighter objects pull less.

Scientists can describe a planet’s surface gravity by using G as a unit. 1 G is equal to the gravitational pull on the surface of the Earth.

Mercury has a surface gravitational pull of 0.378 G. In other words, its surface gravity is weaker than Earth’s. In our solar system, the largest planet (Jupiter) has a surface gravity of 2.36 G, while the sun itself has a staggering surface gravity of 28 G.

So, what has the strength of a planet’s gravity got to do with habitability? It all comes down to that planet’s ability to maintain a stable atmosphere.

An atmosphere is basically a blanket of gases held in place by a planet’s gravity. If that gravity is too weak – for example, on Mercury – those gases will fly off into space.

While life doesn’t strictly need an atmosphere, it certainly makes things easier. Earlier, we said that chemical processes need a liquid medium and an energy source. But they also need some actual chemical components.

Photosynthesis needs carbon dioxide. Cellular respiration needs oxygen. The Earth has enough gravity to maintain these gases in a stable atmosphere – but a smaller planet would struggle to keep them around. Any water on the planet’s surface might fly into space as well.

In order to maintain an atmosphere, an alien planet would probably need a minimum mass of about 10% of Earth. Any less than that, and a meaningful atmosphere wouldn’t form.

To put that into context, the mass of the moon is about 1.2% the mass of Earth, and the mass of Mercury is about 5.5%. Neither of these have strong enough gravity to maintain a proper atmosphere.

In the other direction, there isn’t really an upper limit on viable planetary mass.

If a planet’s mass is more than 500% the mass of Earth, it might make it difficult for alien life forms to evolve. The high gravity would create a high pressure environment, where life forms might be crushed.

But life on such a planet wouldn’t be impossible. In the search for life, these Super-Earths – as giant planets are sometimes called – shouldn’t be ruled out.

With so many different factors at play, like stellar life cycles, orbits, and planetary size, you might expect it to be hard to find an alien planet that ticks all these different boxes.

But actually, planets with the potential for life are relatively abundant. In 2013, astronomers estimated that there could be as many as 40 billion Earth-sized planets in the Goldilocks Zone of viable stars – and that’s just within our own galaxy.
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There are hundreds of billions of different galaxies out there, and as far as we know, each and every one could have billions of potentially habitable planets too.

In other words, the number of potentially habitable planets is colossal. To put it into perspective: there are more habitable planets in the known universe than grains of sand on the Earth.

The mind-boggling number of habitable planets makes it hard to believe that Earth is the only place with life.

Even if the chances of life evolving are vanishingly slim – say, one in a billion – if you roll the dice on trillions of planets, surely a few of those alien worlds will evolve life forms just like we did.

We’re not just talking about simple life forms. With so many planets, it also seems likely (at least on a few) that life forms would reach a level of intelligence that rivals or exceeds our own.

In 1961, an astrobiologist called Frank Drake tried to estimate the number of advanced civilizations in our galaxy.

By looking at the number of habitable planets, and the general probability of various events, he guessed that the wider galaxy should be home to somewhere between 1000 and 100,000,000 advanced civilizations.

Drake’s calculation was only an estimate – and a lot of people have questioned it – but either way, probability suggests that humans shouldn’t be alone.

And this raises an important question. If there are so many civilizations out there, shouldn’t there also be traces of them throughout the universe?

On Earth, there are signs of human activity everywhere you look. There are ruins at the heart of the darkest jungles, and plastic at the bottom of the seas. There are camps in Antarctica, villages in the Sahara, and radio signals echoing across the skies.

If there are thousands of civilizations out there, shouldn’t the universe feel just as ‘lived in’ as the Earth? Shouldn’t we be picking up radio signals from distant planets, or finding lumps of space junk orbiting distant stars?

In the 1950s, Enrico Fermi – an American physicist – famously asked: "Where is everybody?" More than eighty years later, this question is still as relevant as it was back then.