Under the bright lights of an aging sun

Venus can be seen as a black dot eclipsing the Sun in this image from 2012. 

Venus orbits too close to the Sun to the planet to be habitable for life as we know it. 

Venus experiences a runaway greenhouse and the average surface temperatures are thought to be around 864ºF. 

Credit: NASA/SDO & the AIA, EVE, and HMI teams; Digital Composition: Peter L. Dove

Life as we know it on Earth is linked to our star, the Sun, which provides our planet with just the right amount of heat and energy for liquid water to be stable in our lakes, rivers and oceans.

However, as the Sun ages, it is steadily growing brighter and brighter. Eventually, the sunlight that supports life will become too great, and it will bring an end to habitability on our planet.

A Star is Born and Ages
The Sun formed some 4.5 billion years ago when gravitational attraction caused a massive cloud of gas and dust to collapse.

Currently the Sun is stable and has been for billions of years. The bright ball of light in our sky goes about its days generating energy by fusing hydrogen atoms in its core.

As the Sun ages it will enter another stage of stellar evolution where it's atmosphere begins to inflate. This is when the Sun will expand into a red giant star, swallowing planets in the inner Solar System, possibly including the Earth.

As time goes on, the Sun will start shedding its atmosphere and will continue to grow into a massive planetary nebula, which is like a large cloud of gas ejected from the old star.

This is a sort of recycling stage, where elements created by the star are sent back to the interstellar medium, thereby providing new materials for more stars to form.

Next, the old core of the Sun will cool and collapse into a dense but small hunk of mass known as a white dwarf star.

Eventually, it will cool to the point where only a cold, dark husk remains.
Life as we know it is intrinsically tied to the life-cycle of the Sun because we rely on its light for energy. Right now, things are perfect for biology. In the future, this will change dramatically.

As the Sun heats up and expands, life on Earth will become increasingly difficult. Long before the Sun becomes a red giant some 4 or 5 billion years from now, our planet will be rendered uninhabitable.

Dying in a Future Solar System
The fate of the Earth as the Sun grows old is not an old topic. For decades, scientists have studied various scenarios for how an ageing Sun will affect Earth's future habitability. Writers and artists, on the other hand, have explored the idea for centuries.

However, humankind will be gone long before a red giant star fills our skies.

Rather than leading us to a rocky ball of ice, an ageing Sun will instead blast the Earth with ever-increasing heat. Before the Sun expands to a red giant, this increased heat will cause dramatic climatic change on our planet.

The Atmosphere in 3-D
Previous models have predicted that an increase of just 6 percent in the solar constant (a measure of incoming solar electromagnetic radiation) would cause a runaway greenhouse effect on Earth that would render the planet uninhabitable as the oceans boil away to space.

Based on this number, Earth's habitability could come to an end in around 650 million years from now. However, a more recent study has extended the expected lifetime of Earth as a habitable world.

Discover the lifecycle of stars with this activity and handout. 

Many people think the different stages in the life of a star are actually different types of stars, rather than just stages in the life of a single star. 

Credit: NASA/JPL, Astronomical Society of the Pacific

New research shows that the accuracy of previous studies, which were based on 'one-dimensional' models of Earth's climate, could be improved.

"One-dimensional models treat the atmosphere as a single vertical column. This single column is meant as a representative average of all points on the Earth," explains Eric Wolf of the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder.

"While one-dimensional models can treat radiative transfer well (i.e. solar energy and the greenhouse effect), they completely ignore many important aspects such as clouds, dynamics, and the pole to equator gradients of energy which ultimately describe our climate."

Wolf and his colleague Brian Toon, also of UC Boulder, used complex, three-dimensional climate models in order to bring more detail into the picture.

"Three-dimensional models, as we refer to them, are general circulation models of climate. They include a fully, spatially-resolved, rotating planet, with clouds, oceans, sea-ice, weather, etc.," Wolf told Astrobiology Magazine.

"The three-dimensional general circulation model I used has also been used for problems of modern climate. General circulation models are considered the most advanced type of climate models."

The added detail of the 3-D models showed that the Earth could remain habitable for longer than previously expected.

"According to my work, the Earth may remain 'habitable' for at least another 1.5 billion years, when the Sun is approximately 15.5 percent brighter than today," said Wolf. "This is the limit of our current study."

It's important to note that a habitable Earth in terms of astrobiology is not necessarily habitable for human beings.

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