The Sun is getting hotter. Not significantly on the scale of human lifetimes, and not even enough to account for global warming. But on a geological scale it’s happening, and it has dire consequences for life on Earth. 

The Sun’s changing luminosity, radius and temperature.

The Sun gets hotter over time because of the gradual transformation of elements over time in the Sun’s core. As hydrogen is fused into helium, the core becomes more dense, and thus gravity squeezes it a bit more and the core gets hotter. As the core temperature continues to rise, hydrogen fusion (p-p chain) becomes more efficient, and a secondary fusion reaction known as the CNO cycle also starts to kick in. This heats the core even further. As a result the outer layer of the Sun swells, making the Sun slightly larger. But it also gets brighter, so the end result is that the Sun produces more energy as it ages.

For most of its lifetime this increase is pretty gradual. Over a hundred million years the luminosity of the Sun will increase by about 1%. But over billions of years this is significant. When life first appeared on Earth about 3.5 billion years ago, the Sun was about 75% as bright as it is now. In a couple billion years it will be about 20% brighter than it is now.

The changing habitable zone of the Sun.

Because of Sun’s increase in energy over time, the habitable zone of the solar system is gradually thinning and getting further from the Sun. A few billion years ago both Earth and Mars were in a reasonable zone of habitability, but now only Earth remains. In another billion years or so even the Earth will leave the habitable zone and will likely be too hot to sustain life. This has interesting consequences for the possibility of life on other worlds. While many young stars may have planets conducive to life, as the stars age the range changes. The universe may be littered with planets that once had life, but are now barren. Planets where life survives long enough to develop civilization and technology may be rare, even if life is common throughout the universe.

But of course the days are numbered even for civilizations like ours. Either we become extinct like so many species before us, or we adapt and change, eventually leaving our Sun for greener pastures. To do otherwise is to face our end of days.

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In the Star Trek fictional universe, habitable Earth-like planets are designated as Class M planets, where M stands for the Vulcan word Minshara. In the real universe such planets would also be known as class M, but for different reasons.For exoplanets, we can currently only estimate their potential habitability by broad physical properties such as mass, size, and possible temperature. It’s difficult to determine the temperature of an exoplanet, since so many factors can come into play. If Mars were at the orbit of Earth, it would still be cold and hostile. If Venus were at Earth distance it would still be hot and inhospitable. But we can broadly categorize temperature by the amount of radiant energy it receives from its star. From this we can group planets by temperature into class-p psychroplanets (cold), class-t thermoplanets (hot) and class-m mesoplanets (medium). Earth is therefore a class m planet, since it is in the “goldilocks zone” for our Sun, but then so are Venus and Mars.

We can also categorize exoplanets by their size or mass by how they compare to known planets. Thus we can have mercurian, subterran (Venus and Mars), terran, superterran (super Earths), neptunian (small gas giants), and jovian (Jupiter-like). It’s generally thought that only the terran classes would be of sufficient mass to have an Earth-like atmosphere, but it’s also possible that larger planets might have a terran-massed moon.

Visit another planet? One can dream. Credit: NASA/JPL-Caltech

The holy grail of exoplanet hunters is to find an Earth-massed planet in the goldilocks zone of a Sun-like star. We haven’t found one yet, but we’re getting closer. Early last year it was announced that we’d discovered the most Earth-like planet so far, known as Kepler-186f. It was found to be only about 10% larger than Earth and in its star’s habitable zone. The big downside is that it orbits a red dwarf. Red dwarf stars tend to have strong flares and stellar wind, which would act to strip closer planets of their atmosphere. They also tend to be much hotter in their youth, only later cooling down to their reddish orange hue. So it’s very possible that even Earth-sized planets in a red dwarf’s goldilocks zone would be far from habitable.

This week at the AAS meeting the Kepler team added 554 exoplanets to the confirmed list, including 3 new “Earth-like” worlds. Earth-like in this case simply means they are less than twice Earth size and in the theoretical habitable zone of their star. Of these Kepler-438b is now considered the most Earth-like exoplanet. It is only about 12% larger than Earth and gets about 40% more heat from its star than Earth does from the Sun.

All three of these new planets orbit red dwarf stars, and not Sun-like stars. That’s not surprising given that it is much easier to find larger planets around smaller stars than the other way around. But this new announcement shows that we’re getting closer to the holy grail of a truly Earth-like exoplanet. Given the known distribution of exoplanets, there are likely 8 to 20 billion of them in the Milky Way.

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Recently there’s been news of the discovery of the closest potentially habitable exoplanet yet.  Named Gliese 832 c, it is a “super-Earth” orbiting in the habitable zone of a red dwarf star.  In April of this year, there was news of the “most Earth-like planet yet”. Named Kepler 186 f, it is a super-Earth orbiting a red dwarf star. In 2012, there was an announcement of the “most habitable exoplanet yet” named Gliese 581 g.  It is a …wait for it… super-Earth orbiting a red dwarf. Whenever a new potentially habitable world is announced, it will likely be a super-Earth orbiting a red dwarf. But why?

It first has to do with the nature of stars.  The majority of stars are red dwarfs.  They make up about 3/4 of all main sequence stars, compared to only 8% that are Sun-like (G-class).  From what we’ve seen, pretty much all stars have planets, so all things being even most planets in the universe orbit red dwarf stars. It is to be expected then that most potentially habitable planets are around red dwarfs.

It also has to do with the nature of planet hunting.  There are two major ways to find a planet around another star.  The first is known as the transit method, where a planet passes in front of its star, causing it to dim slightly.  This dip in brightness is pretty small, which can make it difficult to observe.  But the smaller the star and the larger the planet, the bigger the dip in brightness. So large planets around small stars are easier to find than smaller planets around larger stars. The second method is known as the radial velocity method. This technique measures the Doppler shift of a star as it wobbles toward and away from us. The wobble is due to the gravitational pull of a planet as it orbits the star.  The bigger the planet the greater the pull, and the smaller the star the more it reacts to that pull, so larger planets around smaller stars produce more wobble than smaller planets around larger stars.  So again it is easier to find the former than the latter.

Finally, it has to do with our definition of “potentially habitable”. In this case potentially habitable means the planet has to be within the right distance range for liquid water to be possible. In our own solar system that is about the range from Venus to Mars.  It also requires that the planet must be rocky like Earth. This means a planet can’t be larger than about 2 Earth diameters, or a few Earth masses.  Basically within the Mars to super-Earth range of size.

Of the planets that meet those conditions, super-Earths around red dwarfs are the easiest to find. So that’s why they are so often in the news.  Of course none of this says anything about whether the planets actually have life. One big downside of red dwarfs is that they tend to produce large solar flares and x-rays that would be pretty hostile to life on a closely orbiting planet.  Red dwarfs likely have the most potentially habitable planets, but they might not have the most actually inhabited planets.

We are still in the early days of planet discovery. What you’ll see over time is that new “potentially habitable” planets will be more Earth-like in size, and orbiting more Sun-like stars, until eventually we will find one very similar to our own.

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NASA and JPL have announced the discovery of an Earth-sized planet orbiting in the habitable zone of its star.  This has led to some popular press announcements that Earth’s twin has been discovered, but these planets are twins more in line with Danny DeVito and Arnold Schwarzenegger than identical twins. It should also be emphasized that being in the “Goldilocks Zone” of a star does not mean the planet harbors life, or even liquid water.  So what do we know about this planet so far?

A comparison of the Kepler-186 system and the inner solar system. Credit: NASA/JPL

The planet is named Kepler-186f, and is the 5th planet from its star, Kepler 186. The star itself is a red dwarf about half the mass of the Sun. The Kepler study shows now evidence of large solar (stellar) flares over the 4-year observation period, but we do know the star is active, and even that it has starspots. This is an important factor when considering habitability, because red dwarf stars tend to have strong flares and stellar wind, which would act to strip closer planets of their atmosphere.  Red dwarf stars also tend to be much hotter in their youth, only later cooling down to their reddish orange hue.  So planets of such a star may be baked dry as well.

But it’s not all bad news. Since the star is smaller and thus cooler than the Sun, it’s habitable zone is closer to the star.  In this case, Kepler-186f has an orbit roughly the size of Mercury’s orbit in our own solar system. At that distance the planet might not be tidally locked, so it could have a daily cycle similar to Earth’s. It is only about 10% larger than Earth, and while we can’t determine its mass directly, if it is about the same composition as Earth it would have a mass about 1.4 times that of Earth.  That would give it a surface gravity only 15% stronger than Earth’s, so it isn’t likely to have a thick hydrogen-helium atmosphere.

If Kepler-186f has a strong magnetic field, then it is possible that it has a more Earth-like atmosphere, and would be capable of having liquid water on its surface.  Given its size, it could also be geologically active.  It is possible that the planet is the most Earth-like exoplanet discovered so far.  However the most likely scenario is that it is dry and cold.  More Mars-like than Earth-like.

Of course none of this should minimize the importance of this discovery.  It shows that Earth-sized planets do exist within the habitable zones of their stars.  We figured they must exist, but now we know. It is a first step toward discovering a truly Earth-like planet around another star.

Paper:  Elisa V. Quintana et al. An Earth-Sized Planet in the Habitable Zone of a Cool Star. Science, Vol. 344 no. 6181 pp. 277-280 (2014)  DOI: 10.1126/science.1249403 

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Earth is the one planet we know of that is well suited for life.  Of course this is a sample size of only one, and it’s a biased sample, since we’re it.  This means we should take any speculation on the existence of life on other planets with a grain of salt, but there are some things we can at least tentatively speculate on.

One of these is the possibility of liquid water on a planet.  Life on Earth can survive in a wide range of environments, but generally needs to have access to water, so it seems like a decent starting requirement for life similar to Earth life.  This means at the very least we need to consider the temperature of a planet.

The temperature of a planet depends on several things.  Most importantly is the temperature of the star it orbits.  Red dwarf stars are smaller and cooler, so planets would need to be closer than Earth to be equally warm.  For stars larger and hotter than the Sun, the planets would need to be farther away.  But it also depends on things light albedo (how much light a planet reflects back) and warming effects of its atmosphere (Earth would be an ice planet without the warming effects of water and carbon dioxide in its atmosphere).  This can vary significantly.  A planet could have the right distance from its star, but have an atmosphere that makes it too hot or too cold.

Still, a basic starting point would be to consider our own solar system.  Earth is habitable, Venus is far too warm, and Mars is too cold.  If we could tweak the atmospheres of Venus and Mars we might be able to make them habitable, so let’s take the average distances of Venus and Mars as the closest and farthest limit for a habitable planet around a star like our Sun.  Within that range would then be the “Goldilocks Zone” for planets orbiting the Sun.

For stars hotter or cooler than the sun, this distance would need to be adjusted outward or inward.  But this is easily done, and there is a simple for this based on a star’s temperature.  From this equation you can define the Circumstellar Habitable Zone or CHZ.  Given this equation you can then plot the distances of known extra-solar planets (exoplanets) by distance relative to the CHZ of their star to get an idea of just how many of them might be habitable.

Distribution of known exoplanets zone. Credit: Borucki et. al.

The result can be seen in the figure above.  As you can see, most exoplanets are far too close to their star to be habitable.  Of more than 1200 exoplanets, only 39 are in the CHZ of their star.  Part of this is due to the bias inherent in finding planets.  It is easier to detect planets with shorter orbital periods, and such planets are more likely to be too close to their star to be habitable.  Still, this graph implies that about 3% of planets might be habitable.

Of course just because a planet is the right distance to be habitable doesn’t mean it will be.  It might not have large quantities of water, or there might be other conditions that make it hostile to life, such as an extremely dense atmosphere.  Even if the conditions are perfect, life simply might not arise on the planet.  On the other hand, this also ignores moons around gas giants.  Jupiter lies far outside the habitable zone of our solar system, but its moon Europa does have liquid water.  It’s possible that life could arise on the moons of gas planets outside the habitable zone.

Of course this is all based on the assumption that life needs liquid water.  Maybe it doesn’t.  Maybe it doesn’t need to be based on carbon.  We can speculate, but for now we still only have a sample size of one planet.  Based on what we know exo-life certainly seems possible, but for now all we can do is keep looking.

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