When science fiction becomes reality: Scientists reveal what would REALLY happen if the sun started to dim like in Project Hail Mary – with catastrophic results

What would happen if the sun started to dim? 

Scientists have revealed the terrifying answer to this question, which is the subject of the upcoming science fiction blockbuster, Project Hail Mary.

The film, based on a novel of the same name by The Martian author, Andy Weir, follows a lone scientist on a mission to uncover why the sun is dimming.

In the movie, which is set to hit cinemas in March 2026, the sun’s brightness is predicted to fall one per cent in a year and five per cent in 20 years.

These numbers might sound small.

But in reality, scientists say that these changes would be more than enough to wipe out humanity.

Professor David Stevenson, a planetary scientist from the California Institute of Technology, told Daily Mail: ‘Extinguishing life on Earth would take a long time even if you eliminated solar energy because we know of organisms that live underground.

‘But extinguishing humans could happen fast, especially since humans are not rational creatures for the most part.’

What happens when the sun starts to dim?

At a distance of around 93 million miles (150 million kilometres) from Earth, the sun delivers about 1,365 Watts per square metre of energy, which scientists call the solar constant. 

About 30 per cent of that energy is reflected back into space, while the remainder is absorbed, warming the Earth’s atmosphere and surface.

Currently, our planet is holding on to more energy than it loses – but it wouldn’t take much to tip the balance.

If the sun’s brightness were to drop or if something prevented our atmosphere from absorbing the energy, then Earth could start to rapidly cool.

Professor Lucie Green, an expert on the sun from University College London, told Daily Mail: ‘The Sun does naturally vary in brightness, but not by very much!

‘The technical term is total solar irradiance. This is slightly variable, with the variability being a result of changes during the Sun’s 11–year sunspot cycle.’

These fluctuations are barely noticeable on Earth, but there have been much more dramatic shifts in the past.

The sun’s output does naturally dip on an 11–year cycle that coincides with the number of sunspots appearing on the surface. However, these changes aren’t enough to cool Earth dramatically 

Between 1645 and 1715, the sun went through a 70–year quiet period known as the Maunder Minimum.

At the same time, the Earth entered a period known as the Little Ice Age, during which temperatures in Northern Europe fell by between 0.6°C (1.1 °F) and 2°C (3.6°F).

Although the sun was only delivering 0.22 per cent less energy, some researchers think that this change was partially responsible for the deadly chill.

If Project Hail Mary’s predictions came true and the sun’s radiation continued to fall by one per cent, the results would soon become catastrophic.

As Earth would be losing more energy into space than it gained from the sun, global temperatures would soon fall several degrees below average.

Worryingly, Earth’s history shows that even relatively small changes in the planet’s average temperature can have a massively outsized impact.

During the Little Ice Age, less than a degree Celsius of cooling led to mass famine throughout Northern Europe.

Cold winters and cool summers led to crop failures, while the sea became so cold that Norse colonies in Greenland were cut off by the ice and collapsed through starvation.

In Project Hail Mary, the teacher turned astronaut Ryland Grace, played by Ryan Gosling (pictured), learns that the sun will cool by one per cent in a year

According to a recent study, global cooling of just 1.8°C (3.25°F) would cut production of maize, wheat, soybeans and rice to fall by as much as 11 per cent.

As crop production failed due to cold weather and a lack of sunlight, mass famine would grip the world, killing 5.3 billion people in just two years.

However, if Project Hail Mary came true and the sun cooled by one to five per cent in 20 years, the effects on the climate would be even more devastating.

In Project Hail Mary, the teacher turned astronaut Ryland Grace, played by Ryan Gosling in the upcoming film, remarks: ‘That would mean an ice age. Like… right away. Instant ice age.’

That might sound dramatic, but scientists agree that it might not take much cooling for ice to reclaim the world.

According to a recent study from the University of Arizona, the average temperature during the last Ice Age, 20,000 years ago, was just 6°C (10.8°F) colder than today.

During this time, glaciers covered about half of North America, Europe and South America and many parts of Asia.

Dr Becky Smethurst, astrophysicist at the University of Oxford, told Daily Mail: ‘A drop in energy of one per cent from the Sun would trigger a new Ice Age on Earth, with the polar ice caps expanding further towards the equator.

Just like the 2004 movie ‘The Day After Tomorrow’ (pictured), these major changes to the Earth’s climate would eventually culminate in a new Ice Age that could wipe out life on Earth

According to a recent study from the University of Arizona, the average temperature during the last Ice Age, 20,000 years ago, was just 6°C (10.8°F) colder than today. This means it might not take much cooling for icy conditions to return 

‘Many ecosystems would collapse as the weather changed, farming would fail, and there would be severe food shortages. As a species, humans would likely survive this change thanks to modern technology, although we’d most likely be living underground.’

What would happen if the sun completely cooled?

Although humanity might be able to survive a global ice age, the situation would be very different if the sun completely vanished.

Within a week, the Earth’s surface would fall below –18°C (0°F) and within a year it would dip below –73°C (–100°F).

Eventually, after cooling for millions of years, the planet would stabilise at a frigid –240°C (–400°F).

However, humanity would be long gone well before the planet ever got to that point.

Some humans might be able to cling on in the deepest parts of the ocean, using hydrothermal vents for warmth.

But once the oceans freeze over, there would be very little hope for anyone to survive.

In the original novel of Project Hail Mary, written by The Martian author Andy Weir, scientists make the terrifying prediction that the sun’s brightness will fall one per cent in a year and five per cent in 20 years. If this were true, then humanity would very likely be destroyed 

Dr Alexander James, a solar scientist from University College London, told Daily Mail: ‘From a fundamental viewpoint, if the Sun completely faded, there would be no more light, meaning all our green plant life would be unable to photosynthesise.

‘That means plants would not be producing oxygen, which, of course, we need to live. Temperatures would also plummet, so I don’t see how the majority of life as we know it would be able to survive without our Sun.’

Could this ever really happen?

Thankfully, scientists say there’s no way that the sun could cool as fast as in Project Hail Mary.

Although the sun’s activity does fluctuate, even in the most extreme events and quiet periods, the effects are not dramatic.

For example, many scientists have questioned how much the Maunder Minimum really contributed to the Little Ice Age during the 17th Century.

While most experts agree that a decline in solar activity did contribute to the cooling, other factors, such as volcanic activity, likely played a bigger role.

Additionally, most of the sun’s natural variations are on a much smaller scale.

Luckily for us, the sun is so large that it cannot physically cool as quickly as Project Hail Mary suggests. Experts say the sun would only cool by one per cent in a million years if the core completely stopped producing energy

The amount of energy arriving from the sun usually only drops by 0.1 per cent during the solar cycle.

While large sunspots, cool regions on the solar surface, might cause a temporary dip as low as 0.25 per cent below average, this is nowhere near the five per cent change of Project Hail Mary.

In fact, many scientists believe that the sun cannot physically cool this fast.

Professor Michael Lockwood, a space environment physicist from the University of Reading, told Daily Mail: ‘About half of the Sun’s mass is in the radiative and convection zones outside the core – that is about a thousand, billion, billion, billion, billion kilograms.’

This enormous mass acts like a heat sink, storing colossal amounts of energy that would take billions of years to dissipate.

Professor Lockwood says: ‘Roughly speaking, if the core ceased producing any energy, the power emitted by the Sun would only have dropped by about one per cent a million years later.

‘Scientifically, anything faster than that is nonsense.’

So, even if the sun does start giving out on us, we will have plenty of time to find a better solution than sending out Ryan Gosling on a spaceship.

WHAT IS THE SOLAR CYCLE?

The Sun is a huge ball of electrically-charged hot gas that moves, generating a powerful magnetic field.

This magnetic field goes through a cycle, called the solar cycle.

Every 11 years or so, the Sun’s magnetic field completely flips, meaning the sun’s north and south poles switch places. 

The solar cycle affects activity on the surface of the Sun, such as sunspots which are caused by the Sun’s magnetic fields. 

Every 11 years the Sun’s magnetic field flips, meaning the Sun’s north and south poles switch places. The solar cycle affects activity on the surface of the Sun, increasing the number of sunspots during stronger (2001) phases than weaker (1996/2006) ones

One way to track the solar cycle is by counting the number of sunspots.

The beginning of a solar cycle is a solar minimum, or when the Sun has the least sunspots. Over time, solar activity – and the number of sunspots – increases.

The middle of the solar cycle is the solar maximum, or when the Sun has the most sunspots.

As the cycle ends, it fades back to the solar minimum and then a new cycle begins.

Giant eruptions on the Sun, such as solar flares and coronal mass ejections, also increase during the solar cycle.

These eruptions send powerful bursts of energy and material into space that can have effects on Earth.

For example, eruptions can cause lights in the sky, called aurora, or impact radio communications and electricity grids on Earth.