Comets are defined as icy bodies of frozen gases, rocks and dust left over from the formation of the solar system about 4.6 billion years ago.

They orbit the sun in highly elliptical orbits that can take hundreds of thousands of years to complete. As a comet approaches the sun, it heats up very quickly causing solid ice to turn directly into gas via a process called sublimation, according to the Lunar and Planetary Institute . The gas contains water vapor, carbon monoxide, carbon dioxide and other trace substances, and is eventually swept into the distinctive comet tail.

Scientists sometimes call comets dirty snowballs or snowy dirtballs, depending on whether they contain more ice material or rocky debris according to NASA .

Related: How to view and photograph comets

According to NASA, as of January 2023, the current number of known comets is 3,743. Though billions more are thought to be orbiting the sun beyond Neptune in the Kuiper Belt and the distant Oort cloud far beyond Pluto .

Occasionally, a comet streaks through the inner solar system ; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have won’t easily forget the celestial show.

The most recent comet to be making headlines is that of recently discovered comet C/2022 E3 (ZTF) which will make a relatively close approach to Earth on Feb. 1, 2023, passing within 28 million miles (42 million km). This striking green comet was last in our neighborhood 50,000 years ago, making the last people to look up and witness this visitor from the depths of the outer solar system, likely very early Homo sapiens or Neanderthals.

Related: Amazing photos of gorgeously green Comet C/2022 E3 (ZTF)

Hoping to observe a comet for yourself? Our guides on the best telescopes and best binoculars can help. You can also check out our guide on how to view and photograph comets as well our best cameras for astrophotography and best lenses for astrophotography to get started.

What is a comet made of?

A comet primarily consists of a nucleus, coma, hydrogen envelope, dust and plasma tails. Scientists analyze these components to learn about the size and location of these icy bodies, according to ESA.

Nucleus

The nucleus of Comet 67P/Churyumov-Gerasimenko imaged by Rosetta’s OSIRIS narrow-angle camera from a distance of 177 miles (285 km). (Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

A comet nucleus is the solid core of a comet consisting of frozen molecules including water, carbon monoxide, carbon dioxide, methane and ammonia as well as other inorganic and organic molecules — dust. The nucleus of a comet is usually around 6 miles ( 10 kilometers) across or less .

Coma

As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into a gas via a process called sublimation, forming a cloud around the comet known as the coma. According to the science website howstuffworks.com the coma is often 1,000 times larger than the nucleus.

Hydrogen envelope

Surrounding the coma is a hydrogen envelope that can be up to 6.2 million miles (10 million kilometers) long and is made from hydrogen atoms according to ESA . As the comet gets closer to the sun, the hydrogen envelope gets bigger .

Tails

Comet C/1995 01 Hale-Bopp captured on March 14, 1997. In this image you can see the dust tail streaking out to the right whist the blue ion tail points away from the sun. (Image credit: ESO/E. Slawik)

There are two main types of comet tails, dust and gas. Comet tails are shaped by sunlight and the solar wind and always point away from the sun .

Comet tails get longer as a comet approaches the sun and can end up millions of miles long . The dust tail is formed when solar wind pushes small particles in the coma into an elongated curved path. Whereas the ion tail is formed from electrically charged molecules of gas.

We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of the energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope.

Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when Earth passes through the orbit of Comet Swift-Tuttle .

Comet Q&A with an expert

We asked Dr. Samatha Lawler, an astronomer at the University of Regina in the United States who studies objects at the edge of the solar system, some frequently asked questions about comets.

IMAGE CREDIT: CAMPION COLLEGE AT THE UNIVERSITY OF REGINA IMAGE CREDIT: CAMPION COLLEGE AT THE UNIVERSITY OF REGINA Samatha Lawler Dr. Samatha Lawler is an astronomer at the University of Regina in the United States who studies objects at the edge of the solar system.

What are comets made of? Comets are made of a little bit of rock, a few different types of ice (including water, carbon dioxide, ammonia, and methane), and a little bit of complex organic molecules. They are often described as “dirty snowballs,” and their shape and composition always remind me of the old, dirty, icy snow you get on the sides of city roads at the very end of winter.

When are comets visible? Comets spend most of their time far away from the sun, in the very cold outer reaches of the solar system, only coming in close to the sun every few thousand or even every few million years. Comets are most easily visible when they get close to the sun, and the many different types of ices on their surfaces start to sublimate (turn from solid into gas). This makes the comet turn from a small, dark rock into a huge, puffy cloud of gas and dust, making it much easier to see with telescopes or sometimes even with just your eyes. As the comet gets warmer, it grows a tail of gas and dust that can be millions of miles long, always pointing away from the sun.

How are comets formed? Comets are little bits of ice left over from the beginning of our solar system. They formed in the same disk of gas and dust that formed the eight planets, but because they formed in the very cold regions far away from the sun, they include a lot more ice. Astronomers are still measuring comet composition and running computer simulations to learn exactly where and how comets formed, but they likely formed in the same region of our solar system where Kuiper Belt Objects like Pluto orbit today, and got scattered out onto huge, elliptical orbits when the giant planets changed positions very early in the history of the solar system.

Where do comets come from? The comets that we occasionally see in the night sky usually come from the really distant reaches of the solar system, where they spend most of our time. This region is called the Oort Cloud, and it extends from the Kuiper Belt basically halfway to the closest star. Comets can travel out to hundreds of thousands of times the distance between the Earth and the sun, and then slowly travel back into the inner solar system on million-year-long orbits. These comets are way too small and faint to be seen until they start to melt close to the sun, and you may be lucky enough to get to see a handful of bright Oort Cloud comets over the course of your lifetime. The most famous comet, Halley’s Comet, is on a much closer orbit and is getting rapidly fainter since it melts every 76 years and has lost a lot of its volatile material.

How can amateur astronomers view comets? There are always a few faint comets visible somewhere in the night sky if you have access to decent binoculars or a small telescope. A great database with finder charts is available at in-the-sky.org. The next bright comet could be Comet Tsuchinshan-ATLAS in the fall of 2024, but we won’t know how bright it can become until it makes its close approach to the sun and starts sublimating impressively, or not.

Comet orbits

Astronomers classify comets based on the duration of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system. Recently, scientists have also discovered comets in the main asteroid belt — these main-belt comets might be a key source of water for the inner terrestrial planets .

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune’s orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. In 2017, scientists found there may be seven times more big long-period comets than previously thought.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate. Some researchers are also concerned that comets may pose a threat to Earth as well.

Comet McNaught (Comet C/2006 P1) behind Mount Paranal in the Chilean Atacama desert. This image was captured in January 2007. Comet McNaught was the brightest comet seen since 1965 and, in some places, it was even visible to the naked eye during the day! (Image credit: S. Deiries/ESO)

How do comets get their names?

Comets are generally named after their discoverer. For example, comet Shoemaker-Levy 9 got its name because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Spacecraft have proven very effective at spotting comets as well, so the names of many comets incorporate the names of missions such as SOHO or WISE.

Related: Amazing photos of Comet NEOWISE from the Earth and space

Comets through history

In antiquity, comets inspired both awe and alarm, “hairy stars” resembling fiery swords that appeared unpredictably in the sky. Often, comets seemed to be omens of doom — the most ancient known mythology, the Babylonian “Epic of Gilgamesh,” described fire, brimstone, and flood with the arrival of a comet, and the Roman emperor Nero saved himself from the “curse of the comet” by having all possible successors to his throne executed. This fear was not just limited to the distant past — in 1910, people in Chicago sealed their windows to protect themselves from what they thought was the comet’s poisonous tail.

For centuries, scientists thought comets traveled in the Earth’s atmosphere, but in 1577, observations made by Danish astronomer Tycho Brahe revealed they actually traveled far beyond the moon . Isaac Newton later discovered that comets move in elliptical, oval-shaped orbits around the sun, and correctly predicted that they could return again and again.

Chinese astronomers kept extensive records on comets for centuries, including observations of Halley’s Comet going back to at least 240 B.C., and historic annals that have proven valuable resources for later astronomers.

Missions to comets

Several missions have ventured to comets.

NASA’s Deep Impact collided an impactor into Comet Tempel 1 in 2005 and recorded the dramatic explosion that revealed the interior composition and structure of the nucleus. The mission also included a flyby of Comet Hartley 2 and the remote sensing of Comet Garradd during an extended mission.

Comet Tempel 1 67 seconds after NASA’s Deep Impact’s impactor spacecraft crashed into the comet. The image was captured by a high-resolution camera on the mission’s flyby craft. (Image credit: NASA/JPL-Caltech/UMD)

In 2009, NASA announced that samples returned from Comet Wild 2 during the Stardust mission revealed a building block of life — glycine. It was the first time an amino acid was found in a comet .

In 2014, the European Space Agency’s Rosetta spacecraft entered orbit around Comet 67P/Churyumov-Gerasimenko. The Philae lander touched down on Nov 12, 2014. Among the Rosetta mission’s many discoveries was the first detection of organic molecules on the surface of a comet; a strange song from Comet 67P/Churyumov-Gerasimenko; the possibility that the comet’s odd shape may be due to it spinning apart, or resulting from two comets fusing together; and the fact that comets may possess hard, crispy outsides and cold but soft insides, just like fried ice cream. On Sept. 30, 2016, Rosetta intentionally crash-landed on the comet, ending its mission.

June 19, 2019, ESA selected the Comet Interceptor mission as the last “fast” or F-class mission. The new mission will intercept an as-yet-undiscovered comet as it enters the inner solar system. The mission consists of three spacecraft that will capture snapshots of the comet from different angles, creating a 3D profile of the object and characterizing its surface, composition, shape and structure. Comet interceptor is due to launch in 2029 .

Famous comets

Halley’s Comet is likely the most famous comet in the world, even depicted in the Bayeux Tapestry that chronicled the Battle of Hastings of 1066. It becomes visible to the naked eye about every 75 years when it nears the sun. When Halley’s Comet zoomed near Earth in 1986, five spacecraft flew past it and gathered unprecedented details, coming close enough to study its nucleus, which is normally concealed by the comet’s coma.

The roughly potato-shaped, 9-mile-long (15 km) comet contains equal parts ice and dust, with some 80% of the ice made of water and about 15% of it consisting of frozen carbon monoxide. Researchers believe other comets are chemically similar to Halley’s Comet. The nucleus of Halley’s Comet was unexpectedly extremely dark black — its surface, and perhaps those of most others, is apparently covered with a black crust of dust over most of the ice, and it only releases gas when holes in this crust expose ice to the sun.

The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in 1994, with the giant planet’s gravitational pull ripping the comet apart for at least 21 visible impacts . The largest collision created a fireball that rose about 1,800 miles (3,000 km) above the Jovian cloud tops as well as a giant dark spot more than 7,460 miles (12,000 km) across— about the size of the Earth — and was estimated to have exploded with the force of 6,000 gigatons of TNT.

Scattered fragments of comet Shoemaker-Levy 9 were captured on May 17, 1994, by the Hubble Space Telescope. The fragments impacted Jupiter in July 1994. (Image credit: NASA/ESA/H. Weaver and E. Smith (STSci))

A highly visible comet was Hale-Bopp, which came within 122 million miles (197 million km) of Earth in 1997. Its unusually large nucleus gave off a great deal of dust and gas — estimated at roughly 18 to 25 miles (30 to 40 km) across — appeared bright to the naked eye.

Comet ISON was expected to give a spectacular show in 2013. However, the sun-grazer did not survive its close encounter with the sun and was destroyed in December of the same year.

In 2021, scientists found what could be the largest comet ever seen . Comet C/2014 UN271 or Bernardinelli-Bernstein after its discoverers, University of Pennsylvania graduate student Pedro Bernardinelli and astronomer Gary Bernstein, was officially designated a comet on June 23. Astronomers estimate this icy body has a diameter of 62 miles to 124 miles (100 km to 200 km), making it about 10 times wider than a typical comet. The comet will make its closest approach to our planet in 2031 but will remain at quite a distance even then.

Additional resources

Scientists say they’ve found a space rock for the ages in Antarctica — an extremely rare meteorite that contains some of the oldest material in the solar system.

“When we saw this one just sitting by itself in the middle of the blue ice, we all got so excited,” Chicago Field Museum researcher Maria Valdes told the Chicago Tribune.

The 17-pound meteorite, described as about “the size of a gourd,” was discovered Jan. 5 by an international team at the end of an 11-day expedition.

The extraordinary rock, which contains material from billions of years ago, is one of the largest meteorites ever found on the continent and likely originated in the Main Asteroid Belt between Mars and Jupiter, The Independent reported.

“To put the meteorite’s size in perspective, of the 45,000 meteorites retrieved from Antarctica over the last century, only 100 are this size or larger,” said Chicago’s Field Museum, which was part of the expedition.

Researchers on snowmobiles spent the better part of two weeks combing ice fields in search of meteorites when they made the stunning find just as they were about to wrap up their exploration, according to The Tribune.

3 The researchers celebrate their out-of-this-world find. Courtesy of Maria Valdes / SWNS

3 A close-up shot of the rare space rock. Courtesy of Maria Valdes / SWNS

Valdes said they were hesitant about celebrating at first “because we knew that if we found a meteorite, this was really the mother lode. On the last day, the last hour.”

The team became convinced it had indeed found a rare space rock when members discovered it was “the size of a bowling ball but twice the weight of a bowling ball,” Valdes told the paper.

The rock had what Valdes described as a “fusion crust” — a glassy outer layer that slightly melted when it entered the atmosphere. It was also worn down, a sign it had been on Earth for many ages.

The meteorite was sent to The Royal Belgian Institute of Natural Sciences in Belgium for chemical analysis.

“All meteorites have something to say about the evolution of Earth,” Valdes said. “Size doesn’t necessarily matter when it comes to meteorites, and even tiny micrometeorites can be incredibly scientifically valuable.”

3 Scientists from the US, Belgium and Switzerland spent 11 days combing the icy continent looking for space rocks. Courtesy of Maria Valdes / SWNS

Most of the 45,000 meteorites found in Antarctica over the past century have only weighed a few grams, The Independent noted.

The find came months after NASA successfully destroyed a 530-foot-wide asteroid in a test run to prepare for the possibility of a massive space rock hurling toward and threating Earth, such as the 6.2 mile-wide asteroid that scientists believe wiped out the dinosaurs millions of years ago.

In short, the universe is everything. It is everything we can touch, feel, sense, measure or detect. It includes all of space, and all the matter and energy in space. It even includes time, and it includes you too.

No one knows the exact size of the Universe because we cannot see the edge! However, the Universe has not always been the same size. It is bigger today than yesterday and it will be bigger still tomorrow- it is expanding.

Scientists believe it began in a Big Bang, which took place nearly 13.8 billion years ago. Since then, the Universe has been expanding outward at very high speed.

” ” This volcanic eruption at the Soufriere Hills volcano on the Caribbean island of Montserrat pales in comparison to what we might expect from the eruption of a supervolcano. Kevin West/Getty Images

How does global warming affect the scenario of the ice age in our future? In the long term, not much. In the near term, however, global warming could change our world drastically. The full effects of global warming will be felt in the next 200 years, say by 2200. At that time, atmospheric carbon dioxide levels will be higher than any time during the past 650,000 years [source: Thompson and Than]. The carbon dioxide will prevent solar energy from radiating back into space, warming the planet considerably. As average temperatures rise, even just a couple of degrees, glaciers will melt, sea levels will rise and coastal flooding will occur. The oceans will also be warmer and more acidic, which will cause a widespread collapse of coral reefs. Many marine species will face extinction, but they won’t be alone. On land, a quarter of all species of plants and animals will disappear forever.

This will be a critical time for our home planet, and it might seem that things couldn’t get much worse. Unfortunately, if Earth’s 4-billion-year history teaches us only one thing, it’s that global apocalypses do occur if you stretch time out far enough. In 50,000 years, we will almost certainly face an epic catastrophe that will change the planet forever. The catastrophe could take the form of an asteroid or a comet, which, upon striking the Earth, would end life as we know it. Astronomers estimate that such impacts occur every million years on average, so the odds are still in our favor, even 50,000 years into the future. A more likely cataclysm will come from the Earth itself. The same tectonic forces that cause the continents to wander across the globe also power supervolcanoes that can spew enough ash and smoke into the atmosphere to block the sun’s rays for 10 to 15 years. Geologists believe that such eruptions occur every 50,000 years, so here the odds aren’t in our favor [source: Ravilious].

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Facing the effects of an event as devastating as a supervolcano, an already hobbled Earth will certainly experience a mass extinction rivalling other extinctions marked in the fossil record. The most famous is the extinction that wiped out the dinosaurs at the end of the Cretaceous period. But the mass demise of dinosaurs paled in comparison to an extinction event that occurred at the end of the Permian period, about 251 million years ago. When the dying was over, 95 percent of all marine species and 70 percent of all land vertebrates had vanished [source: Discovery Earth]. And can you guess what caused this killing spree? Yep, it was a supervolcano — more specifically, the eruption of the Siberian Traps, which affected the global climate.

So, what are the chances Homo sapiens will be around to enjoy Earth in 50,000 years? When you consider our species has been around for just 100,000 years and the longest living human civilization has lasted only for 3,000 years, it seems unlikely we’ll be a dominant species far into the future.

And yet humans have evolved and continue to evolve today. Some scientists estimate that in the past 10,000 years, humans have evolved 100 times faster than at any other time [source: Ward]. So perhaps the chances are high that we’ll accumulate the changes necessary to adapt to Earth’s future conditions. An interesting feature on MSNBC, called “Before and After Humans,” maps out what might become of our species in the next 1 to 4 million years. One thing seems certain: If we’re around, we won’t look or act like the people who walk the Earth today.

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.

[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 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.

“When we think about exoplanets or the future Earth, scientists refer to a planet as habitable if it has the ability to maintain liquid water 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 habitable zone 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 extrasolar planets,” 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 earth 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 planets 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] climate change. A human catastrophe can be caused by only a few degree temperature increase accompanied with sea level rise.”