Under the bright lights of an aging sun

July 4, 2014 by Aaron L. Gronstal, Astrobio.net, Astrobio.net
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.

[For educators and the public alike, see the lesson plan, The Lives of Stars]

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.

"I had a dream, which was not all a dream.
The bright sun was extinguish'd, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air;"

The opening lines of the poem 'Darkness,' by Lord Byron (1816)

In 1816, Lord Byron wrote the poem Darkness, which is often cited as an early example of a sub-genre of science fiction that tells the tale of a dying Earth. In his vision of Earth's future, the Sun has died and left our planet barren and ice covered, floating in a sea of black and empty space.

In 1935, H. P. Lovecraft and Robert H. Barlow came a little bit closer to what today's scientists believe might happen to the Earth with "Till A'the Seas." In this story, the Sun has expanded to a red giant and humans struggle to survive on an Earth that has been cooked into a barren desert world. However, in real life, 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 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.

"When we think about exoplanets or the future Earth, scientists refer to a planet as habitable if it has the ability to maintain at its surface," says Wolf. "However, a planet may maintain liquid water at the surface while still having a climate which is unfriendly to humans."

Today, the mean surface temperature of the Earth is around 58º F. In Wolf's scenario, 1.5 billion years from now, the mean surface temperature of the Earth is estimated to be over 100º F.

"While the oceans remain, life for land animals would be harsh," Wolf told Astrobiology Magazine. "Surviving humans would have to move towards the polar regions to escape the oppressive heat."

Comparing Habitability under a Hot Sun

Theories about the future of habitability on Earth are not simply based on models of the Sun. With astronomical observations, scientists have been able to observe stars in various stages of their life cycles. When discoveries of exoplanets entered the scene, astrobiologists began to hunt for a view of our own future by looking at rocky worlds around such stars. These distant systems can provide points of comparison between the models and real-life observations.

In a study published last year in the scientific journal Astrobiology, a team of researchers from the United Kingdom approached the question of habitability from a different angle. Rather than looking at how a planet evolves over time, they estimated the output of energy from a star as it ages.

"We developed a solar evolution model that extended the scope of a previously published model, from the original 12.6 billion year limit, to over 400 billion years, using updated observations and fits from the Dartmouth Stellar Evolution Database," said lead author Andrew Rushby of the University of East Anglia. "We used limits for the that were first presented by Jim Kasting and co-authors in a seminal paper on the subject. These stress the importance of liquid water on the surface of the planet, and assume that the planet we're investigating is a lot like the Earth."

Their simulations identified a point at which increasing radiation from the Sun would render the Earth unable to support liquid water. As with the Wolf et al. study, Rushby and his colleagues (Mark Claire of the University of St Andrews, Hugh Osborn of the University of Warwick, and Andrew J. Watson of the University of Exeter) found a longer lifespan for Earth's habitability, which they estimated to be around 1.75 billion years. They arrived at this number from the vantage point of energy output from the Sun, not by modelling how the climate of the Earth itself is affected.

"It's definitely worth the comparison, but the differences between our approaches should be noted," Rushby said. "We did not take planetary evolution into account. We looked at the star alone and neglected the ability of the planet's carbonate-silicate cycle to potentially buffer the climate against higher temperatures by increased weathering and CO2 drawdown."

In their 'climate' approach, Wolf and his colleagues also used a constant value for carbon dioxide (CO2) and methane (CH4) in their simulations, effectively taking these two elements out of the equation. There are so many factors involved in shaping Earth's climate and how it responds to changes in the Solar System environment that it is necessary to look at a few pieces of the puzzle at a time in order to build a larger picture. With further studies, the goal will be to include more factors like carbon dioxide and methane to the mix to gradually increase the accuracy of the models.

The Sun will grow into a Red Giant star in 5 billion years. This image compares the size of the Sun today (yellow dot on the left) to the size of the Sun as a Red Giant. Credit: Department of Physics, NCKU
"Along with others, I'm now working on a carbon-cycle model that would attempt to resolve the carbonate-silicate response whilst also using a more up to date climate parameterisation," said Rushby. "Preliminary results suggest that the future habitable period of the Earth may be shorter than we originally proposed, but this is not unexpected."

Comparative planetology works both ways, and studying the future of Earth can also help astronomers find exoplanets that might fit the habitability bill themselves. Rushby and colleagues studied our solar system with a model that was developed to study habitability around other stars. This is the focus of their wider research goals.

"My primary interest was other habitable planets; how long would these other worlds be temperate for?" said Rushby. "In some cases (planets around small red dwarfs), we predict over 40 billion years. We wanted to be able to help astronomers in identifying planets that could host advanced life, or at least life that could leave clues in the atmosphere, and there's no point in looking at planets that haven't been able to support life for very long because life takes billions of years to develop and evolve."

Coming from the angle of Earth's climate, the study by Wolf and colleagues also has wider implications.

"Scientists today use climate models of various types (1D and 3D) to examine the runaway glaciation and runaway greenhouse thresholds for Earth, and then we can apply these concepts to our observations of ," said Wolf.

The Earth orbits around the Sun in a region known as the 'habitable zone,' where the energy from the Sun is just right for liquid water to remain stable at the planet's surface. Life as we know it requires water to survive, so identifying the 'habitable zone' around distant stars is the first step in the hunt for Earth-like worlds.

"As of today, Earth is the only planet that we know for sure has had a habitable period," said Wolf. "Water-based life is also all that we know, so all ideas regarding habitable exoplanets (or early Mars for example) revolve around the presence of water. Thus, our studies of the habitable zones for extrasolar planets virtually all start with a water-rich, Earth-analog planet."

When a planet sits too close to a star, the energy can cause a runaway greenhouse similar to what we see today on Venus. If the planet is too far away, it becomes so cold that water is only stable as solid ice.

As a star ages and expands, the habitable zone also moves further outward in a solar system. Eventually, this zone is pushed out beyond the orbits of inner planets that were once happily orbiting inside of it. By using models developed for Earth, Wolf and Toon have shown that this process of a shifting habitable zone around a star is actually delayed.

"Earth-sized extrasolar planets can maintain habitability despite receiving relatively larger amounts of solar radiation than was previously thought," Wolf said. "This pushes the inner-edge of the habitable zone to be a little closer to the parent star. Our work provides a sort of updated guideline to the inner-edge of the habitable zone that can be used by observational astronomers."

Cooking the Climate

The two studies combined highlight the increasing crossover between sciences and the search for extrasolar planets. Tools developed to study our home planet can now be adapted to study planets in other systems, and vice versa.

"I definitely see the potential for crossover between exoplanet science and climate science here on Earth," said Rushby. "After all, researchers in both fields are looking for answers to a similar question: what is the climate of this planet like? Can we predict how the climate of this world is going to respond to a forcing, whether it be from human sources, volcanism, weathering, increased solar irradiation etc. In fact, the crossover is already happening."

The studies provide new insight into the distant future of Earth and that of millions of light years away. However, it also ties in to modern climate issues closer to home. The models used to study circulation of the Earth's climate are also some of the most prominent ones employed in the study of current climate change on our planet. While Wolf and Toon's work shows that Earth can maintain habitability long after the Sun has caused the planet to heat up, it's important to remember that this potential for life is based on liquid water and does not include humankind.

"Modern CO2 climate change is unlikely to trigger a runaway greenhouse catastrophe," said Wolf. "However, this does not imply that there is no danger due to anthropogenic [human induced] change. A human catastrophe can be caused by only a few degree temperature increase accompanied with sea level rise."

Explore further: Nearby super-Earth is best habitable candidate so far, astronomers say

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1 / 5 (3) Jul 04, 2014
Were humanity able to sustain a technical civilization for hundreds of millions of years (yeah, I know) "temporary" solutions like star shades and longer term approaches using repeated gravitational interactions with small solar system bodies over many 10s of millions of years to gradually move Earth's orbit outward could be possible.
5 / 5 (4) Jul 04, 2014
"In fact, the crossover is already happening."

The same step from 1D to 3D models is what solved the faint young Sun problem of temperate instead of snowball Archean, and nicely predict the glaciations during the global oxygenation of Earth 2.5 billion years ago. That they would be useful elsewhere wasn't a long reach.

@philw: It isn't the sustainability of a civilization that is primarily at question. But mammal species runs for ~ 1 million years on average until they have evolved (or even split) to other recognized species.

H. erectus of ~ 2 million years was the exception, no reason to think H. sapiens will become as old. Of course, you said "humanity", and depending on the sense of such sets of traits it may stick around longer.
1 / 5 (8) Jul 04, 2014
I don't believe stellar evolution si being modelled properly, because Helium is less energy dense per unit mass than hydrogen, and the equilibrium point of the core is about the same either way, which is to say if the inner core contracts enough to fuse helium, this pushes the outer core away, lowering pressure and stabilizing again. Therefore, I don't believe a Red Giant phase exists for Sun-like stars. There isn't enough energy, and in any case because of the way luminosity of stars works (related to the 4th power of mass), and the sun will lose a percent or so of it's mass just from photons from hydrogen-hydrogen fusion carrying away energy, then it stands to reason that the total luminosity of the Sun actually goes down with time, not up, particularly since Helium, and all other elements, have less energy per unit than does Hydrogen.

0.993^4 ~0.972 <<< 1^4

Suggests the Sun will cool by about 3% by the time it stops burning Hydrogen as it's primary fuel.
1 / 5 (7) Jul 04, 2014
After cooling by about 3% or so, the sun's inner core will contract by a few percent, reach a new equilibrium point, and from the outside things will be much the same, except slightly redder.

If it tries to over-expand, the pressure on the inner core decreases, cooling it back off. Thus resulting in a sinusoidal strong/weak cycle...much like what it already does.

We know the Sun already fuses some Helium, and it already fuses some CNO as well, thus it's not like some giant tipping point is building. It is already burning some of those elements even as it burns hydrogen, so even if there were an end to the hydrogen, it will have burned much of those other elements tool by then.

Let us remember that all of the elements have progressively LESS energy density, therefore it makes no sense to claim they would burn signficantly brighter.
Uncle Ira
3 / 5 (4) Jul 04, 2014
@ Returnering-Skippy you just pull all that out of your butt or is it something you learn from school. You make it sound like those scientist-astrophysicist-Skippys who spend 12 or 10 years learning the details of this stuff like they just wasting their time.

Maybe you should tell them that they don't need 12 or 10 years of school when all they got to do is ask you and you can tell them in 15 or 10 minutes how the universe works.

But that's okay I suppose, at least you not telling us about being short and fat like you were yesterday.
not rated yet Jul 05, 2014
Looks like a brown dwarf neighbourhood will be prime real estate in the future. We need to start focussing our search for an 'Earth like planet' rather than a planet around any old star that may not give us more than a couple of billion years before we need to move again.
4.4 / 5 (7) Jul 05, 2014
@Returners: The elements will sort by specific mass by gravity, which is included in all stellar models. The red giant phase is the observed evolution in Herzsprung diagrams.

Scientists can't forget about gravity and astronomy, even if you can. A classic Dunning-Kruger case; you may want to study up on those star models first. (Sigh. Science sites have them, "I'm the new Einstein"...)

@someone: I'm not sure I understand your details even if I think I get your overall message.

Unless we bioengineer us with gills so we can use ocean habitats of the tidal habitable zone (HZ) around giant planets (like Europa)*, we can use surface and later crustal based biospheres as cosmological long term habitats.

The 2nd generation of planets around white dwarfs should have the longest term HZs.

SuperEarth planets should have the longest term habitability in such HZs, maintaining both atmospheres and plate tectonics (PT) longer. PT may be the constraint on complex biospheres...

5 / 5 (6) Jul 05, 2014
... since it means large scale carbon recycling, so an oxygenated biosphere (CO2 vs plants).

*Makes me wonder, do planets circling super-massive black holes (SMBH) have a tidal HZ (by tidal heating)? Then when the universe grows old and the SMBHs has swallowed their debris disks, those may be the ultimate biospheres. But I'm afraid the tidal forces are felt well inside the event horizon of SMBHs, their gravity gradient is supposed to be low at the large radius horizon unless I'm mistaken.
1 / 5 (5) Jul 05, 2014
There's a far more pressing problem than the expansion of the sun to consider for human beings:
Our human genome is deteriorating so rapidly over the whole population that we won't last another 100k years.
So, unless the human race finds a means to firstly stop and then reverse the genetic damage that is fast accumulating in the whole human race, we don't have to concern ourselves with the millions of years the sun has before it goes a-blooming.
2.3 / 5 (3) Jul 05, 2014
Were humanity able to sustain a technical civilization for hundreds of millions of years (yeah, I know) "temporary" solutions like star shades and longer term approaches using repeated gravitational interactions with small solar system bodies over many 10s of millions of years to gradually move Earth's orbit outward could be possible.
It would make more sense to create floating cities on Saturn, Uranus or Neptune since our technologies are almost at this level now. http://www.lcpm10...PM10.pdf

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