# How Long Will Earth's Year be When Our Sun Goes Red?

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We live in a planet that orbits 93 million miles from a G-type main-sequence star, or "yellow dwarf". That is far enough for a revolution of 365 days. Such is the case of Kepler's Third Law of Motion--the farther a body is from its parent, the longer it'd take to revolve around it.

But in the far-distant future, our sun will balloon into a red giant, a star big enough to swallow Mercury and Venus out of existence. Earth and Mars will take their place, and the habitable zone (where liquid water is possible) will be moved from between Mars and Jupiter to between Saturn and Uranus.

So, in the future, Earth will no longer be one AU from the sun, which means a far shorter year. But has anyone made any calculations as to how much shorter a Terran year will be in this future scenario?

The year length depends on the distance between the planet's centre & the Sun's centre, not the Sun's surface. So if the Sun merely expands, Earth at 1 AU will still take a standard year to perform 1 orbit.

However, when the Sun becomes a red giant, it won't just expand. As Wikipedia mentions, red giants shed a considerable amount of mass in the form of gas and dust. It's estimated that the Sun will lose around a third of its mass over the first billion years of its red giant phase, and eventually have a mass around half of its current mass; see here for details.

It's still not clear exactly what will happen to the Earth while all this is going on. It may become engulfed, like Mercury & Venus certainly will be. But even if it's not engulfed, interacting with all that gas & dust is likely to affect its orbit.

If we ignore that complication, we can easily calculate the period of a 1 AU orbit around a Sun of reduced mass. The Newtonian form of Kepler's 3rd law says that $$T^2= left(frac{4pi^2}{GM} ight)r^3$$ where $$T$$ is the period, $$G$$ is the universal gravitational constant, $$M$$ is the Sun's mass, and $$r$$ is the mean orbital radius.

If we keep $$r$$ at 1 AU but halve the Sun's mass, then the period is multiplied by $$sqrt{2}$$, giving us a year length around 516 days. That's using the current day length, by then the rotational period of Earth will be considerably longer.

I seriously doubt that the Earth will still be at 1 AU from the Sun, though, so please don't take that figure of 516 days too seriously!

## Will The Sun Become A White Dwarf? [What Is The Suns Fate?]

The Sun is at the center of our Solar System, providing the essential source of energy required for life as it is on Earth. Due to its significance, and the fact that it consists of an estimated 67 different elements all interacting with one another, its interesting to discover its overall fate.

So, will the Sun become a White Dwarf? The short answer is yes, eventually (scientists estimate this will occur in around 5 billion years). The Sun will become a White Dwarf following an interim phase of being a Red Giant. This life cycle occurs as the Suns fuel source (Hydrogen) will over time burn through, and through a process known as Nuclear Fusion (whereby Hydrogen and Helium interact), the Sun will expand to become a Red Giant. From there and after a period of time, it will then collapse into a smaller, dense White Dwarf.

White Dwarfs are small, compact yet dense stars that drop in temperature after consisting as a Red Giant. The Sun, is just like the majority of other stars, in that it will follow this similar fate. A White Dwarf is there are perfect phenomena to study to observe the trajectory and evolution of stars, and give us insights in to what will become of our Solar Systems central and pivotal one.

## How Long Will Earth's Year be When Our Sun Goes Red? - Astronomy

Now let's try to get a feel for the time scales. I will use another scale model, but instead of reducing distances, I will shrink down time. The scale model is called the cosmic calendar'' in which every second in the cosmic calendar'' corresponds to 475 real years (so 24 cosmic calendar days = 1 billion real years). If you use the classical number of 15 billion years for the age of the universe, you can squeeze the universe's entire history into one cosmic calendar year (recent measurements place the age closer to 14 billion years). The universe starts in the early morning of January 1 at midnight in the cosmic calendar and our present time is at December 31 at 11:59:59.99999 PM in the cosmic calendar. Here are some important dates in this super-compressed cosmic calendar relevant to us humans: (see also the figure below)

 Origin of the Universe--Jan. 1 Origin of our galaxy--Jan 24 Solar system origin--Sept. 9 Earth Solidifies--Sept. 14 Life on Earth--Sept. 30 Sexual reproduction advent--Nov. 25 Oxygen atmosphere--Dec. 1 Cambrian explosion (600 mil years ago when most complex organisms appear, fish, trilobites)--Dec. 17 Land plants & insects--Dec. 19, 20 First amphibians--Dec. 22 First reptiles & trees--Dec. 23 First dinosaurs--Dec. 25 KT impact, mammal age, birds--10:00 AM Dec. 30 First primates--Dec. 30 Australopithicenes (Lucy, etc.)--10:00 PM Dec. 31 Homo habilis--11:25 PM Dec. 31 Homo erectus--11:40 PM Dec. 31 Early Homo sapiens--11:50 PM Dec. 31 Neanderthal man--11:57 PM Dec. 31 Cro-Magnon man--11:58:38 PM Homo sapiens sapiens--11:58:57 PM Dec. 31 Human history--11:59:39 PM Ancient Greeks to now--last five seconds Average human life span--0.15 seconds

It is rather surprising that we have been able to discover so much about the long term evolution of the universe and the things in it, especially when you consider that we have only been seriously observing the universe for about 100 years, which is only a very slight fraction of the universe's lifetime. About 100 years ago is when photography was first used in astronomy, making truly systematic observation programs possible. How can astronomers say that the Sun will go through a red giant phase in about 5 billion years from now with confidence? Is it hubris to confidently talk about the Earth's formation process 4.6 billion years ago?

To give you an idea of the difficulties in studying long timespans consider this analogy: An alien comes to Earth to search for life and to understand how it evolved. ET has a camera and has just 15 seconds to take as many photographs as possible. Fifteen seconds is the same proportion of a human lifetime as the 100 years is to the universe's age (15 seconds/human lifetime = 100 years/universe age). ET returns home and her colleagues try to understand Earth from this 15 second period of snapshots. They won't see any important evolutionary changes. How will they determine the dominant life form? They could use a variety of criteria: 1) Size: leads them to choose whales or elephants 2) Numbers: choose insects 3) amount of land space controlled by one species: choose automobiles.

Suppose they somehow decide humans are dominant. They now have further problems. There is considerable diversity among the humans (though to ET with 10 tentacles, 200 eyes, and a silicon outer shell, the humans all look alike!). ET and colleagues try to systematically classify the humans. The humans come in a variety of sizes. In a coarse classification scheme, they break the sizes down into small, medium, and large. They also come in variety of optical colors for their outer shell: red, black, brown, yellow, and white. There appears to be 2 separate sexes (ET is both male and female). After some false starts with theories that used hair length and eye color, they are ready to ask themselves, Do small, brown, female humans evolve into large, red, male humans?'' Do the small stay small and the large stay large?'' Why is there a tendency for small humans to be with one or two large humans?'' With the three characteristics [size (3 divisions), color (5 divisions), and sex (2 divisions)], ET has 3࡫ࡨ = 30 different combinations and 30吚 = 900 possible evolutionary schemes to consider! Well, the universe has a lot more characteristics and, therefore, many more combinations to consider!

## Tech wipeout

“Back then, there was not very much technology so the damage was not very significant, but if it happened in the modern world, the damage could be trillions of dollars,” says Loeb. “A flare like that today could shut down all the power grids, all the computers, all the cooling systems on nuclear reactors. A lot of things could go bad.”

#### Experience Argentina’s 2020 total solar eclipse: Witness a rare celestial event on a New Scientist Discovery Tour

Loeb says an event as powerful as the 1859 one could cause about $10 trillion of damage to power grids, satellites and communications. A flare just a bit stronger could even damage the ozone layer. Previous work has shown that such an event seems likely to occur in the next century, with a 12 per cent chance of it happening in the next decade, but nobody seems to be all that worried, Loeb says. Asteroid impacts get all the attention when it comes to life-threatening space events, but Loeb and Lingam found that superflares would be just as deadly and are just as likely. “I’m not lying awake in bed at night worrying about solar superflares, but that doesn’t mean that someone shouldn’t be worrying about it,” says Greg Laughlin at Yale University. Last month, Loeb and Lingam came up with one potential way to protect Earth from superflares both large and small: an enormous loop of conductive wire between us and the sun that could act as a magnetic shield and deflect flares’ particles away. Unfortunately, launching such a shield into space would cost upwards of$100 billion. “I think that seriously diverting resources to build a wire loop in space would not be the best way to spend money,” says Laughlin. “But thinking more about how solar superflares work and getting a sense of how our sun fits in with its peers would be a very valuable effort.”

Journal reference: The Astrophysical Journal, DOI: 10.3847/1538-4357/aa8e96

## Time Will End in Five Billion Years, Physicists Predict

The universe will cease to exist around the same time our sun is slated to die, according to new predictions based on the multiverse theory.

Our universe has existed for nearly 14 billion years, and as far as most people are concerned, the universe should continue to exist for billions of years more.

But according to a new paper, there's one theory for the origins of the universe that predicts time itself will end in just five billion years—coincidentally, right around the time our sun is slated to die.

The prediction comes from the theory of eternal inflation, which says our universe is part of the multiverse. This vast structure is made up of an infinite number of universes, each of which can spawn an infinite number of daughter universes. (Related: "New Proof Unknown 'Structures' Tug at Our Universe.")

The problem with a multiverse is that anything that can happen will happen an infinite number of times, and that makes calculating probabilities—such as the odds that Earth-size planets are common—seemingly impossible.

"Normal notions of probability—where you say, Event A happens twice and Event B happens four times, so Event B is twice as likely—don't work, because instead of two and four, you have infinity," said Ken Olum of Tufts University in Massachusetts, who was not involved in the study.

And calculating probabilities in a multiverse wouldn't just be a problem for cosmologists.

"If inﬁnitely many observers throughout the universe win the lottery, on what grounds can one still claim that winning the lottery is unlikely?" theoretical physicist Raphael Bousso of the University of California, Berkeley, and colleagues write in the new study.

Physicists have been circumventing this problem using a mathematical approach called geometric cutoffs, which involves taking a finite swath of the multiverse and calculating probabilities based on that limited sample.

But in the new paper, published online last month at the Cornell University website arXiv.org, Bousso's team notes that this technique has an unintended and, until now, overlooked consequence.

"You cannot use [cutoffs] as mere mathematical tools that leave no imprint," Bousso said. "The same cutoff that gave you these nice and possibly correct predictions also predicts the end of time.

"In other words, if you use a cutoff to compute probabilities in eternal inflation, the cutoff itself"—and therefore the end of time—"becomes an event that can happen."

Universe Is One Bubble in a Boiling Pot

Despite this odd wrinkle, Bousso and colleagues think eternal inflation is a solid concept. Most of the theory's underlying scientific assumptions—such as Albert Einstein's theories of relativity—"all seem kind of innocuous, and it's hard to see what could replace them," Bousso said.

In fact, many physicists think eternal inflation is a natural extension of the theory of inflation, which solved some of the problems with the original big bang theory.

According to early models of the big bang, groups of matter that are now on opposite ends of distant reaches of the universe are too far apart to have ever been in contact with each other. That means the early universe should have been clumpy.

What's more, at the rate our universe is now expanding, its overall shape should have curved over time. Also, the initial moment of creation should have filled the universe with heavy, stable particles called magnetic monopoles.

But observations in the past few years of radiation left over from the big bang say otherwise: The early universe was uniform, the shape of the current universe is flat, and magnetic monopoles have never been conclusively observed.

Standard inflation theory accounts for all this by saying the universe experienced a period of extremely rapid expansion in its first few moments, eventually leveling off to create the flat, uniform universe we see today.

Eternal inflation is a next step in inflation theory, and it allows scientists to avoid some other tricky cosmology questions, such as what existed before our universe (answer: other universes) and why our universe appears to have properties fine-tuned for life (answer: everything is possible).

"Although we don't have a theory [to explain the earliest moments of the universe], we have some pretty good ideas about what such a theory would look like . and these ideas seem to necessarily include other universes," said Charles Lineweaver, an astrophysicist at Australian National University, who was not a member of the study team.

"A good analogy would be that our theories predict a boiling pot of water, and the origin of our universe is the formation of one of the bubbles at the bottom of the pot. The theory strongly suggests the existence of other bubbles, because when you boil water, you never get just one bubble."

Time Coming to an Abrupt End?

But eternal inflation still isn't perfect, as the problem with probabilities in the multiverse illustrates.

If probabilities are to work in a multiverse, there must be actual cutoffs that bring various universes to their ends, study leader Bousso says. According to the formulas used to calculate cutoffs, a universe that is 13.7 billion years old will reach its cutoff in about 5 billion years, his team concludes.

For most people, the idea that a mathematical tool could be elevated to a real-world event might seem strange, but there are precedents for it in physics.

For example, Tufts University's Olum said, there was a time when many physicists resisted the idea that protons—subatomic particles with positive charges—are themselves made up of smaller particles called quarks. (Related: "Proton Smaller Than Thought—May Rewrite Laws of Physics.")

Mathematically, quarks help explain the so-called strong force in the nucleus of an atom—and in the real world they now help account for the "zoo" of strange particles that's been discovered in accelerators.

"People said this idea that there are particles inside of a proton that can never get out and that we can't ever see in isolation is crazy," Olum said. "There was a long time when people thought quarks were just a useful calculation tool, but they didn't really believe in them. Nowadays, though, everybody believes quarks are real fundamental particles."

Along the same vein, if theorists believe in eternal inflation, they either need to believe that cutoffs are not valid techniques for computing probabilities—or that cutoffs are real events that predict the end of time, Bousso and colleagues say.

What a real-world cutoff would look like and what form the end of time would take are unclear, the team says. If it happens, it would probably be sudden and unexpected.

And even if humans could see a cutoff coming, we almost certainly wouldn't be viewing it from Earth.

Scientists think our sun—now a middle-age star at about 4.57 billion years old—will be reaching the end of its life in about five billion years. At that point in time, the sun will run out of fuel in its core and will start to shed its outer layers of gas, inflating to become a red giant and ultimately a planetary nebula.

Earth's exact fate during this event is unclear, but few scientists would argue that life on the planet could survive the sun's death.

End of Time Not Inevitable

Although Australian National University's Lineweaver agrees that calculating probabilities in an eternal multiverse is problematic, he doesn't think predicting a real-world cutoff is the solution.

"I never rule out anything completely, but I don't take this very seriously," Lineweaver said. "I'm going to take questioning the assumptions [behind eternal inflation] more seriously."

Tufts University's Olum also doesn't think physicists should accept the end of time as inevitable.

"Nobody knows why [eternal inflation] should be wrong, but nobody knows exactly why time should come to an end either. To me, these things are on equal footing," he said.

Inflation aside, there are many theories in physics for how the cosmos might end. In a "big crunch," for example, the universe would reverse its current expansion and shrink into a black hole.

Then there's the "heat death" theory, in which the universe expands forever until it reaches a state of thermal equilibrium, in which nothing can happen.

Yet another idea is called the big rip, in which the accelerated expansion of the universe eventually rips all matter apart, atom by atom. (Also see "Einstein and Beyond" in National Geographic magazine.)

If the theory of eternal inflation is correct, then even when our universe ceases to be, the larger multiverse will continue.

No matter which scenario sounds most plausible, "there's no need to go off and sell your stocks because the universe is going to end in five billion years," Olum said.

## Earth’s fiery demise

It is widely understood that the Earth as a planet will not survive the sun’s expansion into a full-blown red giant star. The surface of the sun will probably reach the current orbit of Mars – and, while the Earth’s orbit may also have expanded outwards slightly, it won’t be enough to save it from being dragged into the surface of the sun, whereupon our planet will rapidly disintegrate.

Life on the planet will run into trouble well before the planet itself disintegrates. Even before the sun finishes burning hydrogen, it will have changed from its present state. The sun has been increasing its brightness by about 10% every billion years it spends burning hydrogen. Increased brightness means an increase in the amount of heat our planet receives. As the planet heats up, the water on the surface of our planet will begin to evaporate.

An increase of the sun’s luminosity by 10% over the current level doesn’t sound like a whole lot, but this small change in our star’s brightness will be pretty catastrophic for our planet. This change is a sufficient increase in energy to change the location of the habitable zone around our star. The habitable zone is defined as the range of distances away from any given star where liquid water can be stable on the surface of a planet.

Magnificent coronal mass eruption. NASA, CC BY

With a 10% increase of brightness from our star, the Earth will no longer be within the habitable zone. This will mark the beginning of the evaporation of our oceans. By the time the sun stops burning hydrogen in its core, Mars will be in the habitable zone, and the Earth will be much too hot to maintain water on its surface.

## The Truth Behind the Rogue Planet Nibiru

Doomsday prophecies can often find receptive ears. Sure they're grim, but for various reasons, some people actually take comfort in apocalyptic predictions. That doesn't, however, make these prophecies true. A lot of widespread ideas about end times rely on faulty science and nonexistent "evidence."

Take the Nibiru cataclysm. It's perhaps one of the worst doomsday offenders. Most believers say that Nibiru is a mysterious planet that orbits the sun, completing a new trip around the star every 3,600 Earth years. And supposedly, Nibiru is on a collision course with us. The story goes that Nibiru will someday crash into our home world or, failing that, get close enough to trigger a mass outbreak of natural disasters that'll end human civilization as we know it.

Don't worry Nibiru is pure fiction. If it was real, there'd be traces of its gravitational influence all over the solar system. No such clues exist. Besides, any planet with Nibiru's alleged orbit likely would've kissed our sun goodbye ages ago, leaving mankind in peace.

### In the Beginning.

Nibiru entered the public consciousness in 1976 with the publication of "The 12th Planet" by Zecharia Sitchin. We should note that Sitchin himself didn't believe Nibiru posed any immediate threat to mankind. On the contrary, he thought it was linked to the creation of our species. Yeah, there's a lot to unpack here.

The late Sitchin was a journalist and a student of Sumerian cuneiform — ancient writings of Mesopotamia and Persia mainly on clay tablets. Somewhere down the line, he became convinced that Homo sapiens are not the product of natural selection — at least, not entirely. According to his (questionable) interpretations of ancient Mesopotamian texts and inscriptions, the first humans were bio-engineered by some aliens called the Annunaki, who once colonized southeastern Africa.

Sitchin claimed these beings hailed from Nibiru, a hitherto-undiscovered planet. His writings state that Nibiru approaches Earth once every 3,600 years and then retreats to the depths of space.

"The 12th Planet" and Sitchin's follow-up books were never taken seriously by scientists or historians, but they sold millions of copies nonetheless.

As for Nibiru, it was destined to become an object of fear. Starting in the mid-1990s, the mythic planet was incorporated into a slew of doomsday theories. One psychic predicted that Nibiru would fly past us in the year 2003, causing mass destruction en route. Obviously, this didn't happen. But Nibiru kept making headlines.

Many proponents of the faux 2012 apocalypse thought Nibiru was going to strike Earth that December, vindicating their beliefs about the Mayan Long Count calendar. More recently, in 2017, some Christian fundamentalists declared that Nibiru or a similar object was fast approaching and would soon herald the apocalypse.

### Sayonara, Solar System!

Let's take this opportunity to try and put some minds at ease. To recap, Nibiru supposedly has an orbital period of 3,600 Earth years. On its face, that claim seems plausible. After all, it takes the minor planet Sedna (which actually exists) an incredible 11,400 Earth years to finish one trip around our sun. But Sedna gives the sun a wide berth. Scientists use astronomical units, or AUs, to measure some of the vast distances in the cosmos. One AU is equal to about 93 million miles (150 million kilometers), which is the average distance between Earth and the sun.

Even at its closest point to the sun, Sedna is 76 AUs away from the life-giving star — putting it far beyond Neptune, Uranus and the much-maligned Pluto. Yet Nibiru is supposed to make regular forays into the inner solar system, which is the domain of Mercury, Venus, Earth and Mars.

Using these criteria, Bruce McClure at Earthsky.org calculated that the far end of Nibiru's orbital path would be about 469 AUs away from the sun. So in the span of 3,600 years, poor old Nibiru would have to travel all the way from planet Earth to this very distant location — and back. To stay on schedule, the planet would need a ridiculously narrow, almost stick-shaped orbit.

And it'd be moving really, really fast. As it passed by the Earth, we'd expect Nibiru to have a dizzying travel speed of 26.1 miles per second (42.1 kilometers per second). That spells trouble. A planet cruising at such a high velocity — and along such an unstable orbit — would be at risk of getting ejected out of the solar system entirely. Bye, Felicia!

### The Gravity of the Situation

OK, so what would happen if Nibiru actually stayed the course and maintained its weird orbit around the sun? Well, if that were the case, we'd have found telltale evidence.

Long before Neptune was discovered in 1846, astronomers suspected there might be a large planet in its general vicinity. Why? Because observers noticed that Uranus — which was first sighted in 1781 — kept deviating from its expected orbit. Mathematicians hypothesized that this was because a nearby planet was influencing Uranus. Lo and behold, these predictions were spot-on. The mystery planet turned out to be the gas giant we now call Neptune.

Likewise, if Nibiru was real, its influence on the other planets in our solar system would be plain to see. And if — as many apologists claim — Nibiru was Jupiter-sized or bigger, that influence would be all the more obvious because massive planets exert strong gravitational pulls.

Today, all the planets from Venus to Neptune orbit the sun on the same general plane (give or take a few degrees). But according to astronomer David Morrison, if a Nibiru-esque body was careening past Earth every 3,600 years, its gravity would've driven at least some of those planets way off the plane — leaving them with severely tilted orbital pathways.

(Also, spare a thought for Earth's natural satellite. Nibiru would have presumably stolen our moon away by now.)

### Seeing Is Believing

Finally, there's the issue of direct observation — or, more accurately, the lack thereof. Astronomers would be able to detect Nibiru several years before it reached Earth. And several months prior to the wayward planet's arrival, it'd shine brighter than some of the stars that are currently visible to the naked eye. But nobody's ever seen the prophesized planet, and there's no scientific reason to think that anyone ever will. The jury is in: Nibiru's just a hoax.

On new age websites, the term "Planet X" is sometimes used interchangeably with "Nibiru." But they aren't synonyms. "Planet X" is a label that scientists occasionally give to theoretical planets (and similar bodies) whose existence has yet to be proven. Pluto once went by this title.

## How Long Will Earth's Year be When Our Sun Goes Red? - Astronomy

ITHACA, N.Y. - Scientists at Cornell University and the American Museum of Natural History have identified 2,034 nearby star-systems - within the small cosmic distance of 326 light-years - that could find Earth merely by watching our pale blue dot cross our sun.

That's 1,715 star-systems that could have spotted Earth since human civilization blossomed about 5,000 years ago, and 319 more star-systems that will be added over the next 5,000 years.

Exoplanets around these nearby stars have a cosmic front-row seat to see if Earth holds life, the scientists said in research published June 23 in Nature.

"From the exoplanets' point-of-view, we are the aliens," said Lisa Kaltenegger, professor of astronomy and director of Cornell's Carl Sagan Institute, in the College of Arts and Sciences.

"We wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun's light," she said. "And because stars move in our dynamic cosmos, this vantage point is gained and lost."

Kaltenegger and astrophysicist Jackie Faherty, a senior scientist at the American Museum of Natural History and co-author of "Past, Present and Future Stars That Can See Earth As A Transiting Exoplanet," used positions and motions from the European Space Agency's Gaia eDR3 catalog to determine which stars enter and exit the Earth Transit Zone - and for how long.

"Gaia has provided us with a precise map of the Milky Way galaxy," Faherty said, "allowing us to look backward and forward in time, and to see where stars had been located and where they are going."

Of the 2,034 star-systems passing through the Earth Transit Zone over the 10,000-year period examined, 117 objects lie within about 100 light-years of the sun and 75 of these objects have been in the Earth Transit Zone since commercial radio stations on Earth began broadcasting into space about a century ago.

"Our solar neighborhood is a dynamic place where stars enter and exit that perfect vantage point to see Earth transit the Sun at a rapid pace," Faherty said.

Included in the catalog of 2,034 star-systems are seven known to host exoplanets. Each one of these worlds has had or will have an opportunity to detect Earth, just as Earth's scientists have found thousands of worlds orbiting other stars through the transit technique.

By watching distant exoplanets transit - or cross - their own sun, Earth's astronomers can interpret the atmospheres backlit by that sun. If exoplanets hold intelligent life, they can observe Earth backlit by the sun and see our atmosphere's chemical signatures of life.

The Ross 128 system, with a red dwarf host star located in the Virgo constellation, is about 11 light-years away and is the second-closest system with an Earth-size exoplanet (about 1.8 times the size of our planet). Any inhabitants of this exoworld could have seen Earth transit our own sun for 2,158 years, starting about 3,057 years ago they lost their vantage point about 900 years ago.

The Trappist-1 system, at 45 light-years from Earth, hosts seven transiting Earth-size planets - four of them in the temperate, habitable zone of that star. While we have discovered the exoplanets around Trappist-1, they won't be able to spot us until their motion takes them into the Earth Transit Zone in 1,642 years. Potential Trappist-1 system observers will remain in the cosmic Earth transit stadium seats for 2,371 years.

"Our analysis shows that even the closest stars generally spend more than 1,000 years at a vantage point where they can see Earth transit," Kaltenegger said. "If we assume the reverse to be true, that provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet."

The James Webb Space telescope - expected to launch later this year -- is set to take a detailed look at several transiting worlds to characterize their atmospheres and ultimately search for signs of life.

The Breakthrough Starshot initiative is an ambitious project underway that is looking to launch a nano-sized spacecraft toward the closest exoplanet detected around Proxima Centauri - 4.2 light-years from us - and fully characterize that world.

"One might imagine that worlds beyond Earth that have already detected us, are making the same plans for our planet and solar system," said Faherty. "This catalog is an intriguing thought experiment for which one of our neighbors might be able to find us."

The Carl Sagan Institute, the Heising Simons Foundation and the Breakthrough Initiatives program supported this research.

## When will the Sun run out of fuel?

All life on Earth owes its existence to the Sun, whose rays have showered the planet with energy for billions of years. But, like all things, the Sun has its days numbered. Every star has a life cycle consisting of formation, main sequence, and ultimately death when it runs out of fuel — the Sun is no exception.

The good news is that before this will happen, our species should have evolved into something entirely different or long become extinct. According to scientists, the sun has enough fuel to keep it running for another 5 billion years. When that happens, the solar system will be transformed forever.

### The life cycle of the Sun

The star is classed as a G-type main-sequence star, also known as a yellow dwarf. Like other G-type main-sequence stars, the Sun converts hydrogen to helium in its core through nuclear fusion. Each second, it fuses about 600 million tons of hydrogen to helium. The term yellow dwarf is a misnomer since G stars actually range in color from white to slightly yellow. The Sun is, in fact, white but appears yellow because of Rayleigh scattering caused by the Earth’s atmosphere.

The Sun and its planets have been around for about 4.57 billion years. They were all formed out of the same giant cloud of molecular gas and dust which, at some critical point, collapsed under gravity at the center of the nebula.

Due to a nonuniform distribution of mass, some pockets were denser, consequently attracting more and more matter. At the same time, these clumps of matter that were increasing in mass began to rotate due to the conservation of momentum. The increasing pressure also caused the dense regions of gas and dust to heat up.

Scientists’ models suggest that the initial cloud of dust and gas eventually settled into a huge ball of matter at the center, surrounded by a flat disk of matter. The ‘ball’ would eventually turn into the Sun once the temperature and pressure were high enough to trigger nuclear fusion, while the disk would go on to form the planets.

Scientists estimate that it took the Sun only 100,000 years to gather enough mass in order to begin fusing hydrogen into helium. For roughly a few million years, the Sun shone very brightly as a T Tauri star, before it eventually settled into its current G-type main-sequence configuration.

Like most other stars in the universe, the Sun is currently living through its ‘main sequence’ phase. Every second, 600 million tons of matter are converted into neutrinos and roughly 4 x 10 27 Watts of energy.

What happens to Earth after the sun dies

There is only a finite amount of hydrogen in the Sun which means it must eventually run out. Since its formation, scientists estimate the Sun consumed as much hydrogen as about 100 times the mass of the Earth.

As the Sun loses hydrogen, its fuel-holding core shrinks, allowing the outer layers to contract towards the center. This puts more pressure on the core, which responds by increasing the rate at which it fuses hydrogen into helium. Naturally, this means the Sun will get brighter with time.

Scientists estimate that the Sun’s luminosity increases by 1% every 100 million years. Compared to when it turned into a G-type main-sequence star 4.5 billion years ago, the Sun is now 30% more luminous.

All of this means that the Sun will slowly turn the heat up on Earth. About 1.1 billion years from now, the Sun will be 10% brighter, triggering a greenhouse effect on Earth similar to the warming that made Venus into a hellish planet.

As the Sun approaches the end of its life cycle, Earth’s ocean will boil and the planet will become uninhabitable as early as 1 billion years from now. Credit: Wikimedia Commons.

The heat transfer with Earth’s atmosphere would be huge by this point in time, causing the oceans to boil and the ice caps to melt. As the atmosphere becomes saturated with water, high energy radiation from the Sun will split apart the molecules, allowing water to escape into space as hydrogen and oxygen until the whole planet becomes a barren wasteland.

Life would stand no chance, permanently sealing Earth’s fate as the next Venus or Mars. Speaking of which, at this point into the future, Mars’ orbit would move into the habitable zone, which might become a second Earth for a short while before it too would become unsalvageable.

Some 3.5 billion years from now, the Sun will be 40% brighter than today.

And, in about 5.4 billion years, the Sun will run out of hydrogen fuel, marking the end of its main sequence phase. What will inevitably happen next is that the built-up helium in the core will become unstable and collapse under its own weight. Since the Sun first started fusing hydrogen, all of the helium it has produced has accumulated in the core with no way to get rid of it.

At this point, the Sun will be ready to enter its “Red Giant” phase, characterized by an enormous swelling in size due to gravitational forces that compress the core and allow the rest of the sun to expand. The Sun will grow so large that it will encompass the orbits of Venus and Mercury, and quite possibly even Earth. Some astronomers estimate it might grow to 100 times its current size.

What this means is that even if life on Earth somehow miraculously survives the tail-end of the Sun’s main sequence, it will most certainly be destroyed by a Red Sun so large it will touch our planet.

### Don’t be blue, even stars have to die

The Sun will remain in a Red Giant phase for about 120 million years. At this point, the core of the Sun, when it reaches the right temperature and pressure, will start fusing helium into carbon, then carbon and helium into oxygen, neon and helium into magnesium, and so on all the way up to iron. This reaction is triggered when the last remaining shell of hydrogen that envelops the core is burned.

The Sun will then eventually expel its outer layers and then contract into a white dwarf. Meanwhile, all the Sun’s outer material will dissipate, leaving behind a planetary nebula.

“When a star dies it ejects a mass of gas and dust – known as its envelope – into space. The envelope can be as much as half the star’s mass. This reveals the star’s core, which by this point in the star’s life is running out of fuel, eventually turning off and before finally dying,” explained astrophysicist Albert Zijlstra from the University of Manchester in the UK.

“It is only then the hot core makes the ejected envelope shine brightly for around 10,000 years – a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen from extremely large distances measuring tens of millions of light years, where the star itself would have been much too faint to see.”

If it were much more massive, the Sun’s final fate would have been much more spectacular exploding into a supernova and perhaps forming a black hole. Due to its relatively small size, however, the Sun will likely live as a white dwarf for trillions of years before finally fading away entirely leaving the solar system in pitch-black darkness. The Sun has now become a black dwarf.

In summary: the sun has about 5-7 billion years left of its main sequence phase — the most stable part of its life. However, life on Earth might become extinct as early as 1 billion years from now due to the Sun becoming hot enough to boil the oceans.

## What Will Happen to Earth When the Sun Dies?

Stars are born, they live, and they die. The sun is no different, and when it goes, the Earth goes with it. But our planet won't go quietly into the night.

Rather, when the sun expands into a red giant during the throes of death, it will vaporize the Earth.

Perhaps not the story you were hoping for, but there's no need to start buying star-death insurance yet. The time scale is long — 7 billion or 8 billion years from now, at least. Humans have been around only about 40-thousandth that amount of time if the age of the Earth were compressed into a 24-hour day, humans would occupy only the last second, at most. If contemplating stellar lifetimes does nothing else, it should underscore the existential insignificance of our lives. [What If Earth Were Twice as Big?]

So what happens when the sun goes out? The answer has to do with how the sun shines. Stars begin their lives as big agglomerations of gas, mostly hydrogen with a dash of helium and other elements. Gas has mass, so if you put a lot of it in one place, it collapses in on itself under its own weight. That creates pressure on the interior of the proto-star, which heats up the gas until it gets so hot that the electrons get stripped off the atoms and the gas becomes charged, or ionized (a state called a plasma). The hydrogen atoms, each containing a single proton, fuse with other hydrogen atoms to become helium, which has two protons and two neutrons. The fusion releases energy in the form of light and heat, which creates outward pressure, and stops the gas from collapsing any further. A star is born (with apologies to Barbra Streisand).

There's enough hydrogen to keep this process going for billions of years. But eventually, almost all of the hydrogen in the sun's core will have fused into helium. At that point, the sun won't be able to generate as much energy, and will start to collapse under its own weight. That weight can't generate enough pressure to fuse the helium as it did with the hydrogen at the beginning of the star's life. But what hydrogen is left on the core's surface wil fuse, generating a little additional energy and allowing the sun to keep shining.

That helium core, though, will start to collapse in on itself. When it does, it releases energy, though not through fusion. Instead it just heats up because of increased pressure (compressing any gas increases its temperature). That release of energy results in more light and heat, making the sun even brighter. On a darker note, however, the energy also causes the sun to bloat into a red giant. Red giants are red because their surface temperatures are lower than stars like the sun. Even so, they are much bigger than their hotter counterparts.

A 2008 study by astronomers Klaus-Peter Schröder and Robert Connon Smith estimated that the sun will get so large that its outermost surface layers will reach about 108 million miles (about 170 million kilometers) out, absorbing the planets Mercury, Venus and Earth. The whole process of turning into a red giant will take about 5 million years, a relative blip in the sun's lifetime. [50 Interesting Facts About Earth]

On the bright side, the sun's luminosity is increasing by a factor of about 10 percent every billion years. The habitable zone, where liquid water can exist on a planet's surface, right now is between about 0.95 and 1.37 times the radius of the Earth's orbit (otherwise known as astronomical units, or AU). That zone will continue to move outward. By the time the sun gets ready to become a red giant, Mars will have been inside the zone for quite some time. Meanwhile, Earth will be baking and turning into a steam bath of a planet, with its oceans evaporating and breaking down into hydrogen and oxygen.

As the water gets broken down, the hydrogen will escape to space and the oxygen will react with surface rocks. Nitrogen and carbon dioxide will probably become the major components of the atmosphere — rather like Venus is today, though it's far from clear whether the Earth's atmosphere will ever get so thick. Some of that answer depends on how much volcanism is still going on and how fast plate tectonics winds down. Our descendants will, one hopes, have opted to go to Mars by then — or even farther out in the solar system. [What If Every Volcano on Earth Erupted at Once?]

But even Mars won't last as a habitable planet. Once the sun becomes a giant, the habitable zone will move out to between 49 and 70 astronomical units. Neptune in its current orbit would probably become too hot for life the place to live would be Pluto and the other dwarf planets, comets and ice-rich asteroids in the Kuiper Belt.

One effect Schröder and Smith note is that stars like the sun lose mass over time, primarily via the solar wind. Planets' orbits around the sun will slowly expand. It won't happen fast enough to save the Earth, but if Neptune edges far enough out it could become a home for humans, with some terraforming.

Eventually, though, the hydrogen in the sun's outer core will get depleted, and the sun will start to collapse once again, triggering another cycle of fusion. For about 2 billion years the sun will fuse helium into carbon and some oxygen, but there's less energy in those reactions. Once the last bits of helium turn into heavier elements, there's no more radiant energy to keep the sun puffed up against it's own weight. The core will shrink into a white dwarf. The distended sun's outer layers are only weakly bound to the core because they are so far away from it, so when the core collapses it will leave the outer layers of its atmosphere behind. The result is a planetary nebula.

Since white dwarfs are heated by compression rather than fusion, initially they are quite hot — surface temperatures can reach 50,000 degrees Fahrenheit (nearly 28,000 degrees Celsius) — and they illuminate the slowly expanding gas in the nebula. So any alien astronomers billions of years in the future might see something like the Ring Nebula in Lyra where the sun once shone.