Recent news tour of our Solar System from Mercury to FarFarOut

Some recent news tour of our Solar System:

• Mercury: Study Reveals MESSENGER Watched a Meteoroid Strike Mercury - Combing through data from NASA's MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission to Mercury reveals it likely watched a meteor slamming into the planet.
• Venus: Life on Venus? The Picture Gets Cloudier, doubtful - After a sensational announcement about the possibility of life on Venus, new doubts arise.
• Earth: Astronomers Think They've Found Another Trojan Asteroid Lurking in Earth's Orbit - A recently discovered object sharing Earth's orbital path around the Sun could actually be a trojan (shares a planet's orbit) asteroid, astronomers have found.
• Mars: Mars Mission From the U.A.E. Begins Orbit of Red Planet - U.A.E.'s probe arrives to Mars soon.
• Asteroid Belt: The Asteroid Belt: Wreckage of a Destroyed Planet or Something Else? - Samples of asteroids that have fallen to Earth reveals that the Asteroid Belt likely wasn't formed from planet that was smashed a part.
• Ceres and other Dwarf Planets:Top 10 Giant Facts About the Dwarf Planets - "Despite being the smallest of the five dwarf planets, Ceres was the first one discovered."
• Jupiter: Jupiter Is Bigger Than Some Stars, So Why Didn't We Get a Second Sun? - Jupiter and Sun have a lot in common, so how did Jupiter end up not being a planet?
• Saturn: Saturn's moon Titan: Largest sea is 1,000-feet deep - "Far below the gaseous atmospheric shroud on Saturn's largest moon, Titan, lies Kraken Mare, a sea of liquid methane. Cornell University astronomers have estimated that sea to be at least 1,000-feet deep near its center -- enough room for a potential robotic submarine to explore."
• Uranus: 35 Years Ago: Voyager 2 Explores Uranus - 35 years ago, Uranus was examined by Voyager's 11 instruments.
• Neptune: Next-generation planetary missions could hunt for gravitational waves, say astronomers - "Spacecraft heading to Uranus and Neptune in the next decade could be used to investigate gravitational waves as they venture into the outer Solar System."
• Pluto: Pluto's atmosphere gets its blue haze from icy organic compounds, study suggests - "The haze shrouding Pluto might be made up of ice crystals possessing cyanide hearts, a new study finds."
• Quaoar: How 50000 Quaoar changed the way we look at our solar system - "The ball-shaped planetoid, approximately half the size of Pluto, is the largest object found in the solar system since Clyde Tombaugh discovered Pluto in 1930."
• "FarFarOut": Astronomers Just Confirmed The Most Distant Known Object in The Solar System - "The most distant known object in the Solar System is now confirmed. FarFarOut, a large chunk of rock found in 2018 at a whopping distance of around 132 astronomical units from the Sun, has been studied and characterised, and we now know a lot more about it, and its orbit."

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Video: What Would Trees Look Like On Other Planets?

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Super blood wolf moon eclipse 2019, as best as I can get by handholding my phone

Tonight's Lunar Eclipse.  I didn't even know about it until I saw the Earth's shadow upon the moon while casually looking out a living room window.  As soon as I saw, I started watching in the backyard; and experimenting with my phone camera to capture the moments.

Via Instagram http://bit.ly/2DoOPMe

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Almost to the total Lunar Eclipse. This is about as good of a shot I can get handholding my phone. Peak of the total eclipse is in about 15min; via Instagram http://bit.ly/2MssKQ1

This photo was taken near the peak.  It took a fair amount of Instagram filter use to bring this photo to the point of being marginally recognizable; via Instagram http://bit.ly/2sEQRSl

Meyer–Womble Observatory on Mount Evans.

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Planetizing Pluto

It seems that main reason why we care so much about what is and is not a planet is due to the Astrological origins of Astronomy. The word "planet" is derived from a Greek word for "wanderer", as in a star that wanders around the sky. Earth wasn't considered a planet by this ancient definition.

Today, the term planet isn't special, and things we call planets shouldn't be special either. The current definition of the word "planet" by IAU doesn't make any sense since it pretty much invalidates all the planets of "planet" status with the 2006 addition "A planet is a celestial body that has cleared the neighborhood around its orbit."  For example, Pluto is tied to Neptune's orbit. So, technically, Neptune's orbit hasn't been cleared of other objects (the biggest being Pluto's system), so Neptune isn't a planet. IAU definition presents us with a mess. We need to stop treating the word "planet" like it's some special term. "Planet" is just a word and it should be used to describe a well defined class of objects that aren't determined by ancient superstitions.

Always emotions

One of the common statements by those who argue against the application of the term "planet" to Pluto is that those who want to categorize Pluto as a planet want this for "mostly for emotional reasons".  However, the very reason Pluto was "demoted" was for mostly emotional reasons. Here's how:

The definition of "planet" adopted in 2006 by the IAU actually invalidates all other planets from the class as well, particularly the big ones.  Also, the IAU still named Pluto a type of planet called "Dwarf Planet" while in the same breath saying that it is no longer a planet.  These oversights were due to the 2006 definition not being vetted by scientists in the field of study (which is supposedly a violation of IAU policy).  Why was this definition pushed so hard that it by-passed normal procedures?  Someone was trying to game the system.

In my opinion, the whole thing is a mess because Michael Brown (self-described "Pluto Killer") and others of similar opinions wanted to have some fun trolling the IAU.  He has reportedly stated many times how much fun he had with the reclassification of Pluto.  If anything is emotional, it was the whole effort to "demote" Pluto for a bit of fun.

A Different Perspective

What would our thoughts be if we evolved on a rocky orb that revolved around some Gas Giant?  If we looked out at this alternative solar system, we'd see all these other rocky orbs; some orbiting one of several Gas Giants and some orbiting the primary star.  Would all the Gas Giants be known by the same name that we'd called our own rocky orb?  No. We'd call the Gas Giants something else, and all the rocky orbs would be called by the same classification as our own world, regardless to them orbiting a Gas Giant or orbiting the star.  There wouldn't even be a concept of "moon".

The main reason we have trouble with this whole "moon" and "planet" classification is because of our own Earth-centric view of our solar system. The reason some people try to protect the word "planet" is rooted in ancient superstitions that people don't realize they are still perpetuating.  We need to break free of this ancient beliefs and just use science to categorize things in a neutral and fact based manner.  We need a real definition for the word "planet" which isn't implemented for unvetted and emotional reasons, but rather being based on hard facts.  We need a definition developed by a body of Planetary Scientists who base their conclusions on geophysical traits.

For more information about Pluto and New Horizons, please read Chasing New Horizons.

Maybe we are the first

I've said several times that it is possible that the human species is the first species in the Milky Way galaxy to evolve to our level of intelligence and technology.  This opinion is based on information about just how much needs to happen to allow for the spark of life in conjunction with the apparent rarity of our own solar system.  The recent study Relative likelihood for life as a function of cosmic time seems to confirm this idea.

A basic premise is that life requires stars for two different purposes.  The study states,

Life requires stars for two reasons. Stars are needed to produce the heavy elements (carbon, oxygen and so on, up to iron) out of which rocky planets and the molecules of life are made. Stars also provide a heat source for powering the chemistry of life on the surface of their planets.^[001]

This means that rogue planets aren't likely to spark or support life.  This also means that Population III and most Population II stars systems will not have life either, because they are unlikely to have the elements necessary to form terrestrial planets.  That pretty much leaves us with Population I stars.

Rogue planet, artist concept

Population III or II stars, artist concept

What's all this about "Population"? It's a name for stars at various stages of galaxy development.  

• Population III stars are the stars that likely formed right after the Big Bang.  They have not been directly observed, so their existed is estimated.  They were made up of mostly Hydrogen and Helium.  As such, they are unlikely to have any planets.
• Population II stars are stars that are still made up of mostly Hydrogen and Helium, but have higher concentrations of elements such as Oxygen, Silicon, Neon, etc.  Typically, such star systems are still unlikely to contain terrestrial planets.  Many Population II stars still exist in our galaxy, though in regions without access to many heavier elements.
• Population I stars are stars that are yet again still made up of mostly Hydrogen and Helium, but have much higher concentrations of the more stable element Iron and other heavy elements.  Population I star systems are much more likely to contain terrestrial planets.  The Sun (Sol) is a Population I star.

Why is this discussion about "Populations" important to the discussion about the arrival of human-like intelligence?  At the risk of oversimplifying this a bit, I'll state that Population III stars lead to the formation of Population II stars, and Population II stars lead to the formation of Population I stars.  As each generation of stars lived out their lifespans, they made way for the next generation to arise.  Population I stars could not have formed 13.5B years ago; there weren't enough heavy elements around.  Just as today, it is extremely unlikely that Population III stars could arise now; there's too much heavy elements around.

Life is very unlikely to have occurred until Population I stars formed and supported terrestrial planets.  Terrestrial planets in the Goldilocks Zone around their star then had to have the necessary events and composition to allow for the spark of life to occur, and subsequently support life until species of higher intelligence evolve.

Is Earth ahead of the curve for the development of life?

The previously mentioned study suggests that Earth may have developed life to the human-level a bit earlier than average.  The study concludes that, "life around low mass stars in the distant future is much more likely than terrestrial life around the Sun today."^[001]  Life throughout the galaxy may be far more common billions of years from now than it is today.  That also means that there may not be any/many other alien species with which we can contact and interact right now.  The study puts our odds at 0.1%.^[001]

This could explain why we've not seen evidence of extraterrestrial intelligent life in our galaxy.  Maybe we are among the very first. Others like us are so rare, we will not be able to contact each other.

Maybe a billion years from now, a future intelligent species will evolve on some future (yet to exist) world, and when they point radio telescopes into their  night sky, they receive a song of hundreds of thousands radio signals from just as many other civilizations.  Maybe, if our species is able to continue evolving, our long-from-now-posterity becomes the evil invaders of other worlds, rather than our world being the one constantly invaded, as Hollywood would have us imagine.  Maybe we are the monsters in waiting.

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Article Series:

• Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
• Small stars may have stable Habitable Zones, but habitable planets might not be common there
• Habitable Planets around White Dwarfs
• Habitable Worlds Around Binary Star Systems might not match Sci-fi
• How many Earth-like planets are orbiting Sun-like stars?
• First round of life in the Universe might have been possible extremely early
• Factors a planet needs for suitability of life; perhaps
• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first

"Goldilocks zone of metallicity" on a galactic scale

What does the night sky look like to a planet within the Galactic Bulge?  Cool Cosmos describes it as,

Stars in the Galactic Center are so concentrated that they typically are only a few light weeks away from each other. In contrast, our local neighborhood of stars are separated from one another by light years. If we found ourselves on a planet near the Galactic Center, our nighttime sky would light up in a blazing display every night, filled with stars as bright as the planet Venus looks to us.^[001]

However, would there be a habitable planet from which to see this sight?  Is it possible to have life-supporting planets near the Galaxy's center?

The concept of Habitable Zones around stars has been studied for a couple of decades.  Life similar to ours can only exist on planets that are a certain distance from their sun.  This is due to the amount of energy from the sun that is received by the planet.  Too much energy, the planet is too hot.  Too little energy, the planet is too cold, hence the Goldilocks reference.

There's another type of Habitable Zone at the galactic scale which uses a somewhat different set of criteria.  Solar systems which have planets that can support life must themselves be made from material that has a lot of elements that are heavier than Helium.  In astronomy, elements heavier than Helium are often referred to as metals.  Metal content of a star is called its metallicity.  The danger is that is if a solar system is made from material that is too rich in metallicity, Earth-sized planets may not be able to exist due to the likelihood of much larger (heavier) worlds displacing those Earth-size planets.  Hence, "Goldilock zone of metallicity" is the idea that certain regions of a galaxy may be too metal-rich and other regions may be metal-poor in order to allow for the presence of Earth-like worlds.^[002]

It's not just the metals

Metallicity is not the only factor, however.

Early intense star formation toward the inner Galaxy provided the heavy elements necessary for life, but the supernova frequency remained dangerously high there for several billion years.^[002]

If a solar system is too close to the galactic core, the intense supernova frequency in a young galaxy might've keep many worlds from supporting life.  This is because they would have experienced numerous blast waves, cosmic rays, gamma rays and x-rays that are fatal to lifeforms.^[002]  As the collective of solar systems age and die, they would have contributed to increasing metallicity.  This means, the right conditions for life on Earth-like planets may have never happened near the galactic core.  Stars that are too close to the galactic core never had and never will have the right conditions to support Earth-like worlds with Earth-like life.

Where can solar systems with habitable planets reside within the Milky Way?  According to the study The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, the inner bulge component, diffuse halo component, and a thick disk component of our Milky Way Galaxy would not likely allow for Earth-size planets to exist within the right timeframe.^[002]  So, the Habitable Zone of our Milky Way Galaxy isn't even really based on distance from the galactic core.  It's a somewhat washer-shape region in between all the places that Earth-sized planets cannot exist within solar systems.

Current Habitable Zone of Milky Way 

Given all of these factors, the authors of the study The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way state,

We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago.^[002]

Knowing our Milky Way's Habitable Zone helps us in the search for life on other worlds.  We can focus more efforts on this space.  This isn't to say that this is the only space where life can and does reside.  The Galactic Habitable Zone is just our safest bet for finding evidence of life.

Primary reference:
C. H. Lineweaver,Y. Fenner, B. K. Gibson, Science 303:59–62, DOI: 10.1126/science.1092322, The galactic habitable zone and the age distribution of complex life in the Milky Way

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Article Series:

• Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
• Small stars may have stable Habitable Zones, but habitable planets might not be common there
• Habitable Planets around White Dwarfs
• Habitable Worlds Around Binary Star Systems might not match Sci-fi
• How many Earth-like planets are orbiting Sun-like stars?
• First round of life in the Universe might have been possible extremely early
• Factors a planet needs for suitability of life; perhaps
• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first

Factors a planet needs for suitability of life; perhaps

There are many notions about what life might be like on other worlds.^[001]  However, from the limited examples of what we know about life, it would seem to us that there is a preference for life that is based on carbon and water.  As the study On the probability of habitable planets says,

...exploring the wide field of modern chemistry and challenging the most open-minded chemists reveals that with our present knowledge it is difficult to imagine any alternative chemistry approaching the combination of diversity, versatility and rapidity afforded by liquid water-based biochemistry. This results from the unique ability of carbon to form complex species, and the unique characteristics of water as a liquid solvent...^[002]

Another factor is that that carbon and the molecules that are formed from carbon seem to be very common in our galaxy, being found in interstellar space, other planets, comets, asteroids and space dust.  Organic material seems to be everywhere.^[002]  There is a nebula that is practically made of alcohol.^[003]

Perhaps are search for exoplanet life needs to extend beyond simply looking for water.  Maybe our search should include crosschecking with a search for carbon.

Concept illustration of Kepler-22b, which may be a good
place to search for life

According to On the probability of habitable planets, there are four types of habitability on planets that may harbor life in some form.

• Class I - Habitats where conditions allow for water on the surface, and where energy is primarly provided by the planet's sun.  This is the most Earth-like class.
• Class II - Habitats where the planet may have had water on the surface early on, but conditions did not allow the planet to retain that water.  This is most Mars-like class.
• Class III - Habitats where significant water exists below the surface, and where such underground oceans can interact with a silicate-rich core.  This planets may be too far from their sun to have surface liquid water, but via some process, such as geothermal heating, liquid water is present within the planet.  This is the most Europa-like class.
• Class IV - Habitats where a lot of liquid water exists above an icy layers. Oceans may actually be sandwiched between ice layers.  Ganymede and Callisto may represent this class.

Is complex and even intelligent life possible on any of these classes?  It seems that the most likely class that would have complex life is Class I.  But, of course that is based on assumptions and biases born from our own example.  Classes II, III and IV may extend the limits of what is considered to be the Habitable Zone around a star.

Another factor is the CO₂ cycle.  Perhaps the CO₂ content of a planet will allow that planet to retain more heat from its sun.  

It turns out that a thick CO₂ atmosphere may be one of the most efficient solutions for keeping a planet warm. This is not only due to the properties of the CO₂ gas itself.

However, taking into account the radiative effects of the CO₂ ice clouds, which tend to form in such thick CO₂ atmospheres allows further increases in the warming of the surface thanks to a cloud “scattering greenhouse effect”.  Taking into account this process, the outer edge of the habitable zone has been extended as far as 2.5 AU.^[002]

In other words, CO₂ in the right mixture within a planet's atmosphere may extend the outer limit of how far a way a planet can be from its sun and still be warm enough to support life.  But, other factors must be explored.

[A planet] staying in the habitable zone is obviously not sufficient for a planet to continuously maintain liquid water on its surface: it must have an atmosphere which keeps the surface pressure and the surface temperature (through its greenhouse effect) in the right range, for billions of years.^[002]

In addition to forming the correct atmosphere necessary to support life, a planet must also be able to keep that atmosphere for a very long time. Also, that atmosphere may need to change over time in order to adjust to changes in stellar output.  For example, a planet has to be large enough (or have enough gravity) to keep its atmosphere from escaping, not just as a result of simply drifting away, but also to counter the effect of stellar wind and other star related phenomenon.^[002]

Plate tectonics is another factor that may be important to a planet's ability to support life.  Plate tectonics manage planetary cycles, such as CO₂.^[004]  The process of how a planet develops plate tectonics on a global scale is not well understood.  However, when examining the two examples of planets of similar size within our own solar system, Earth and Venus, the key difference appears to be water.  Perhaps the higher water content of Earth enables plate tectonics.  How special is Earth, after-all?^[002]  

Would an equivalent to plate tectonics be necessary on class III and IV planets?  For those same classes, atmospheres may not be a factor at all, since oceans would be underground.  What other cycles would be necessary in such classes?  How many class I planets with a long term atmosphere and plate tectonics are in Habitable Zones?  There's a lot of open questions.  Another question I have, would we be able and willing to seed Terran lifeforms on these other classes planets (and moons), even within our own solar system, even if we do not intend to colonize them for ourselves?

Pirmary reference:
F. Forget. International Journal of Astrobiology, 13, Issue 3, July 2013, pp. 177-185, arXiv:1212.0113 [astro-ph.EP], On the probability of habitable planets

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Article Series:

• Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
• Small stars may have stable Habitable Zones, but habitable planets might not be common there
• Habitable Planets around White Dwarfs
• Habitable Worlds Around Binary Star Systems might not match Sci-fi
• How many Earth-like planets are orbiting Sun-like stars?
• First round of life in the Universe might have been possible extremely early
• Factors a planet needs for suitability of life; perhaps
• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first

First round of life in the Universe might have been possible extremely early

I've posted other articles about the possibility of life in our Galaxy based on what is known right now.  For this article, after going into some concepts from a somewhat recent study, I'm going to speculate a bit based on the suggestion by that study that life was possible for a very specific period of 10 million years to 17 million years after the formation of the Universe.  The study is The Habitable Epoch of the Early Universe.

What is significant about this very specific period after our Universe's formation?  According to the study, the cosmic microwave background provided a uniform heating source that was between 0 to 100°C (the melting point and boiling point of water at 1atm) during 10-17 million years after the formation of our Universe.  This means that there was no Habitable Zone around stars since the entire Universe was one gigantic habitable zone (except maybe being too close to a star).^[001]

Hypothetical earliest stars in our Universe

Challenges for Earliest Stars and Planets

There's a catch.  Stars that formed immediately after the Big Bang were very different than the stars we now see.  The only two elements available in the Universe were Hydrogen and Helium.  These early stars are referred to as being metal-poor, lacking access to elements heavier than than Helium.  There is speculation that the very first stars where actually extremely metal-poor.  Material from which terrestrial planets could have formed simply wasn't available yet.  When these first stars died, they produced the elements necessary for the formation of planets and metal-rich stars.  The death of these stars had to happen very quickly in order to meet the criteria necessary to consider life being possible so early in our Universe's existence.

In order for rocky planets to exist at these early times, massive stars with tens to hundreds of solar masses, whose lifetime is much shorter than the age of the Universe, had to form and enrich the primordial gas with heavy elements through winds and supernova explosions.^[001]

Cosmic simulations suggest the formation of massive early stars that explode relatively quickly.^[001]  Gravitational lensing also suggests the formation of such stars in the earliest galaxies.^[002]  Given the possibility for such stars and such explosions of such stars, planet formation early in the Universe was also possible.^[001] Given the cosmic microwave background heat of the Universe, the likelihood of planets with water on their surface was again also possible.

On the plus side for these planets, once the cosmic microwave background cooled down after the 17 million year mark, the planets themselves may have been able to keep warm enough on their own for quite awhile, even without a nearby star.

[Thermal gradients needed for life] can be supplied by geological variations on the surface of rocky planets. Examples for sources of free energy are geothermal energy powered by the planet’s gravitational binding energy at formation and radioactive energy from unstable elements produced by the earliest supernova. These internal heat sources (in addition to possible heating by a nearby star), may have kept planets warm even without the cosmic microwave background, extending the habitable epoch...^[001]

Speculation

Although the study The Habitable Epoch of the Early Universe suggests that life may have been possible in the early Universe, much of that life may not have survived past 17 millions years after the Big Bang unless it was lucky enough to be in the Habitable Zone within a solar system that included a very stable star.  However, even if the life didn't survive, the organic matter from which the life formed may have survived.  The survival of this life or its material could have seeded the later Universe, drastically increasing the chances of life reemerging.   Some speculate life on Earth originates from extra-solar system sources.  Perhaps the material necessary for the emergence of life was already in the mix from which our Sun formed.  The mechanism for such transference of life and materials is called Panspermia, or specifically, Pseudo-panspermia and Lithopanspermia.

It seems there would have been a substantial gap between the first wave of early life and the next wave of life; this next wave presumably being the epoch within which we find ourselves now.  How might species from the early epoch be viewed by species of the current epoch?

From a Science Fiction perspective, such early life may have evolved to sentience very early in our Universe's existence.  Being so close to our Universe's beginning and having so long to evolve may have allowed these early species to development god-like powers by now.  Such species may be Q of Star Trek: TNG, Time Lords of Doctor Who, Nibblonians of Futurama, and perhaps less god-like Precursors of Star Control II and Progenitors, also of Star Trek: TNG.

Would signs of god-like species be discernible to us young species?  We wouldn't likely see evidence in the form of direct radio signals, as such species would have long since evolved beyond such primitive methods of communication.  Perhaps we could catch a glimpse of these early species in the earliest days of their development via EM signals they emitted billions of years ago, from distance galaxies.

We'd have to know where to point our detectors.  Signals from ancient civilizations within our own galaxy would have passed us by billions of years ago.  However, signals from ancient civilizations in galaxies billions of light years away would be reaching us at the same time as the rest of the light from those galaxies.  Such signals would be faint and scattered, but they may be just distinct enough to discern from the background noise.  For example, at certain times of the year, Earth glows at certain EM frequencies much brighter than any other object in our galaxy.  A similar civilization billions of years ago in a galaxy billions of lights away might be obvious to us once we start looking for such phenomenon.

The idea that life may have developed so early in our Universe's existence opens up a Universe of possibilities.  Our understanding of our origins may be even effected by this concept.  On the other hand, maybe life in our Universe wasn't possible at all until very recently.  Maybe we are one of the first species to develop sentience in all of the Universe.  I'll cover more about this in a later article.

Primary reference:
A. Loeb, International Journal of Astrobiology, 13, no. 4, (Sept., 2014), arXiv:1312.0613 [astro-ph.CO], The Habitable Epoch of the Early Universe

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Article Series:

• Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
• Small stars may have stable Habitable Zones, but habitable planets might not be common there
• Habitable Planets around White Dwarfs
• Habitable Worlds Around Binary Star Systems might not match Sci-fi
• How many Earth-like planets are orbiting Sun-like stars?
• First round of life in the Universe might have been possible extremely early
• Factors a planet needs for suitability of life; perhaps
• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first

Habitable Worlds Around Binary Star Systems might not match Sci-fi

It's hard to talk about planets in binary (dual) star systems without mentioning Star Wars: A New Hope.  The famous scene of Luke Skywalker standing and looking off into the distance while two suns appear near the horizon is iconic.  However, there's a problem with that iconic scene.  The problem is a not full-fledged error, just an unlikelihood: both suns appear as the same size in the sky in very close proximity to each other.  The reason this is unlikely is due to the two types of planetary orbits in binary star systems: s-type and p-type.  A planet in either type of binary system would rarely see both suns as the same size in the sky and that close to each other, even if the suns are the same size.

S-type is the name for the orbit of a planet that revolves around just one of the two stars within a binary system. P-type is the name for the orbit of a planet that revolves around both stars within a binary system, having a common barycenter (or center of mass) with the suns as the suns orbit around each other.^[001]

S-type orbits are interesting, but for this article, I'll cover P-type because this is more interesting to me when talking about Habitable Zones, particularly where the suns have similar masses.  Of course, even with p-type orbits, there exist many possible varieties for how the suns can orbit each other.

The ability of a planet to maintain liquid water depends on the interaction between stellar radiation and the top of its atmosphere.  Also, that interaction is complicated in a binary system.  There can be substantial difference in energy received by the dual suns.  Sometimes both suns appear side by side in the sky, providing maximum energy.   However, when one sun is eclipsed by the other, the amount of energy is lessened due to the closer sun blocking the stellar radiation from its partner.^[001]

In either case, this variation in stellar radiation can limit how small a planet's orbit can be around the dual suns and still be capable of harboring life.   As stated in Calculating the Habitable Zone of Binary Start Systems II: P-Type Binaries:

This interaction strongly depends on the stellar spectral energy distributions implying that stars with different energy distributions will contribute differently to the absolute incident flux at the top of the planet’s atmosphere.^[001]

Even with all of these factors, planet formation within the Habitable Zone of a binary system would be similar to that of a singular star system.^[001]

Two suns of similar mass with elliptical orbits around a common barycenter

   Two suns of similar mass in the same orbit around a common barycenter

Where two suns are of the same size, their location to the planet can vary greatly depending on the size of their orbit around their common barycenter.  If there is a wide orbit, it is safe to assume that one sun will provide more energy than the other sun which is farther away.  In this case, the further sun would appear smaller in size within the sky, even though both suns are of the same mass.  The graphics above suggest why the iconic Star Wars scene isn't likely accurate.  The scene is possible maybe one or two times per year if the suns are in similar orbits which are tight and circular; or in rare instances where the suns have elliptical orbits and the planet just happens to be in the right place at the right time.

Sidebar

Here's a close up of Alpha Centauri A and B. Their distance from each other can be as much as 11 AU's, which would make a p-type planetary orbit so large, that habitable planets would be unlikely.  Planets have been discovered in s-type orbits around Alpha Centauri B and their sibling Proxima Centauri.

HZ reference:
N. Haghighipour, L. Kaltenegger, The Astrophysical Journal, 777 (Nov., 2013) 166, arXiv:1306.2890 [astro-ph.EP], Calculating the Habitable Zone of Binary Star Systems II: P-Type Binaries 

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• Habitable Worlds Around Binary Star Systems might not match Sci-fi
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• First round of life in the Universe might have been possible extremely early
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• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first