Which stars could host alien life?

The Sun will not look different to the naked eye when, about a billion years from now, it turns our planet into a barren desert. Its big change will come much later, as our star evolves into a red giant and swells to engulf Mercury, Venus, and possibly Earth. Instead, the Sun is simply — slowly — getting brighter. Eventually, it’ll shine intensely enough to boil all the water off the surface of Earth.

Astronomers often say that we know of only one world in the entire Cosmos that hosts life: Earth. But we also know of only one star. The Sun, a middle-aged star of a type that is a minority in the galaxy, is our sole example of a star with an inhabited world.  

If planets are the stage that life plays out on, stars set the scene. Every star in the Universe creates a different environment for its worlds, and over the past few decades, scientists have begun to explore which are harsh, deadly, or cozy. One day, these discoveries could tell us how common life might be throughout the Universe.

Rough starts

The first challenge for any would-be habitable planet is surviving its star’s youth. Before a star begins fusing hydrogen and settles into “adulthood,” it churns with stormy weather. Younger stars tend to emit stronger UV radiation and fire off more frequent, intense flares. This bombardment can wear away a planet’s atmosphere and break up key chemical ingredients of life.

Even a star like the Sun poses a danger when young. If Earth did not have the right kind of chemical makeup and magnetic shielding, the Sun would have stripped away our atmosphere long before life had a chance to develop. This is part of why it is so hard to say if a given world is habitable or not: it matters not only what the system is like today, but also what it was like in the past. 

The picture becomes clearer once a star begins fusing hydrogen. During this phase, many stars will shine steadily and only gradually brighten over time. 

This phase is what Earth has to thank for the last several billion years of temperate weather. For that entire period, we have been in the Sun’s “habitable zone,” the range of orbits where a planet can absorb the right amount of energy to maintain liquid water on its surface. In other words, Earth has been threading the needle between too-hot and too-cold around the Sun, and that has given life here the stability it needs to thrive. 

Live fast, die young

But some stars don’t stay adults for billions of years. The more massive a star is, the faster it evolves.

Extremely massive stars fuse hydrogen for only millions of years before they explode as supernovae. Other heavyweight stars might not end with a bang, but they will still age faster than the amount of time life as we know it needs to develop. The shortest-lived stars that have some chance at forming life may be “F-type” stars, which are only a little more massive than the Sun. These stars fuse hydrogen for at least two billion years — roughly twice as long as life took to develop on Earth. 

Lower-mass stars, on the other hand, can steadily fuse hydrogen for hundreds of billions of years. These “M-type” stars (also known as red dwarfs) are the most common kind of star throughout the galaxy. Though they offer plenty of time for life to evolve, they also pose dangers of their own.

Featherweight but fierce

Proxima Centauri, the closest star to the Sun, may be the most famous M star of all time. Astronomers discovered an Earth-sized world around it in 2016, and they have been debating whether the planet is barren or habitable ever since. The answer may come down to the hospitality of its star.  

M stars are stormy, in similar fashion to the newborn Sun. Since M stars are dimmer, though, their habitable zones are closer in and more exposed to UV radiation. Every year, Proxima Centauri produces several “superflares” that are more energetic than the largest outbursts ever recorded from the Sun. Studies have shown that these superflares could be powerful enough to kill even radiation-resistant microbes. 

Still, no one knows for sure whether or not the planet around Proxima Centauri is habitable. Scientists are trying to learn more about the star, the planet, and how the two interact. Some astronomers argue that a world with the right geology and magnetic field might be able to remain cozy around an M star like Proxima. 

UV is not all sunburns and death rays, either. High-energy radiation might be essential to driving the reactions that form the building blocks of life, like RNA. This is why some scientists define a “UV habitable zone”: the region around a star where UV exposure is not so strong that it destroys life, but not so weak that it can’t help stir things up. 

Second chances

Once most stars stop fusing hydrogen, they move through a series of other stages that drastically change their sizes and temperatures. This chaotic environment might end any life that had already developed somewhere around the star.

But for worlds that were previously wastelands, older stars could offer a new lease on life. Such stars will tend to shine brighter, pushing their habitable zones out into the once-frigid reaches of their systems. 

That means when Earth is a charred crisp, the icy moons of Jupiter and Saturn could be melting into temperate oceans. Europa would likely not last more than a few hundred million years like this, but some of Saturn’s moons, like Titan, could end up with oceans of liquid water and ammonia for longer. Around more slowly evolving stars, this phase of habitability could last up to several billion years.

That’s not to say humanity should plan a move to Enceladus. Older stars like red giants are constantly throwing off their outer layers, and worlds would need magnetic shielding to keep them from losing their atmospheres. 

If they’re lucky, these worlds would only face a new set of challenges when their star reaches its next and final phase: a white dwarf.

About 97% of stars (including our own) will eventually become white dwarfs. It is the typical endpoint of a star’s life, shrunken to a ball about as large as Earth but thousands of times denser. Though white dwarfs are super hot, their small size means they shine dimly. Their habitable zones tend to be about dozens of times closer in than Earth’s orbit around the Sun.

It is at least theoretically possible for worlds around white dwarfs to host life, but it would be tricky. Planets would have to migrate to the new close-in habitable zone from more distant orbits. Rocky planets are unlikely to survive the trip with any surface water and atmosphere intact, though giant planets would fare better. 

If the moons of these giant planets don’t get torn apart by gravitational forces or thrown out of the system entirely, they might be hospitable in their close huddle around a white dwarf. Life could have a last gasp around the dying remains of a star — at least for a few billion years. 

A Universe of stars

These are only a handful of the many different kinds of stars. There are failed stars, called brown dwarfs, that host worlds where life could be powered by tidal heating. There are stars made up of more heavy elements, or less, which changes how they evolve and the sorts of planets they host. Some worlds orbit multiple stars, which may or may not leave them struggling to maintain stable climates. The list goes on.

Only a small fraction of these stars, like those about to go supernova, can be completely ruled out as hospitable. The rest leave only one way to truly confirm their habitability: to discover life not just on another planet, but around another star.