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[Music]
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planets are almost as common as stars in
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the galaxy
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of the many hundreds of billions of
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stars in the milky way
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a significant portion of them have
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gravitationally bound
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systems of planets orbiting them and
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while many are radically different to
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ours
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they all share some key characteristics
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which can give us clues about how they
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came
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to be formed knowing the life cycles of
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stars and their planetary systems
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provides us with crucial insight into
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the mechanisms that gave rise to them
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and it is with this insight that we are
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finally beginning to understand
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the events that unfolded at the birth of
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the solar system
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before we dive into all that we first
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need to know about the solar system
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because the arrangement of a planetary
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system can give us clues about its
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origins
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today the solar system is about four and
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a half billion years old
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and is home to eight known planets four
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smaller rocky terrestrial planets on the
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inside
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and four large gas and ice giant planets
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further out
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six out of these eight planets are home
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to at least one moon
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but the largest have dozens some so
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large and complex that they would be
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worlds in their own right were it not
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for their captivity around their giant
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host
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in addition to the planets and moons of
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the solar system there are dozens of
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small dwarf planets
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millions of rocky asteroids and
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trillions of icy chunks arranged into
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bands and belts around the sun
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the outer part of the solar system
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consists of three regions
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the kuiper belt the scattered disc and
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significantly farther away
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the hypothetical ought cloud three
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collections of volatile ice chunks which
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formed closer to the sun before being
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scattered by gravitational disturbances
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the kuiper belt begins just beyond the
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orbit of neptune
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and it extends to around 55 astronomical
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units
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an astronomical unit is the distance at
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which earth orbits the sun
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just under 150 million kilometers
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in comparison the scattered disk extends
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much further
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to over a hundred astronomical units
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away
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and the hypothetical ought cloud is
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thought to start at around
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600 astronomical units extending for
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over a light year into interstellar
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space
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in reality the full scale of the solar
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system
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is so enormous that even light the
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fastest thing
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in the universe takes more than 18
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months to escape the outer edge
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however the vast majority of the
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system's interesting features
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including the most peculiar feature life
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are concentrated inside an area within
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about 50
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astronomical units of the sun an area
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protected from harmful
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interstellar and intergalactic radiation
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by the sun's heliosphere
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the solar system today lies in a
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peaceful balance
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with each of its eight planets bound in
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predictable near circular orbits
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the small rocky planets sit neatly on
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the inside
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with the larger gas giants further out
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and so for a long time we thought that
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this was the natural arrangement of most
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planetary
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systems however since 1995
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we have been able to peer into the
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planetary neighborhoods of other stars
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and what we've realized is that systems
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of planets are very rarely
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this well structured and settled planets
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of all sizes and types can be scattered
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throughout their respected systems and
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they don't necessarily stay in circular
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orbits
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in fact it is a fundamental property of
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planetary systems to be dynamic
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which can profoundly alter the system's
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destiny
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and it appears that we are no exception
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to this
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despite the neat arrangements of the
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planets today
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it's likely that the solar system once
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looked like a very
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different place
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sadly we as humans woke up far too late
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to witness the dynamic young solar
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system
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first hand but we are able to see the
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effects of certain key
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events that occurred early in its life
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which allows us to start to put together
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a timeline
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and thanks to telescopes like hubble and
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kepler
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we've surveyed catalogued and deduced
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the characteristics of thousands of
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other star systems
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at every stage of their life cycle from
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their gaseous births to their violent
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deaths
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this observed life cycle has helped us
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to connect the dots on a number of key
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questions regarding the formation of the
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solar system
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the culmination of these decades of
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research is presented
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in the nebula hypothesis our model for
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describing and explaining the birth of
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the solar system
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and all other star systems and it is
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generally agreed upon by most scientists
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it's not without its unanswered
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questions and areas of ambiguity
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but for the stuff that we do know we
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have large amounts of observational data
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to back it up
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the nebula hypothesis tells us that star
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formation begins
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inside massive cold clouds of gas in
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space
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containing many different elements and
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compounds
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mostly hydrogen and helium but often
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with a small portion of more complex
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stuff
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organic molecules hydrocarbons
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silicas and various forms of volatile
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substances
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like water and methane
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because these clouds are so enormous
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stars rarely form
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in isolation around 90 of stars
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including the sun were most likely born
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within a stellar birth cluster
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which formed together and then later
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dispersed
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the thing that makes these clouds so
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good at producing stars is the fact that
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they are gravitationally unstable
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highly asymmetric with an unevenly
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distributed mass
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meaning the cloud is always moving and
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interacting within itself
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as such these clouds are prone to
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internal fragmentation
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internal areas of the nebula with a
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higher particle density detach from the
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overall structure of the cloud and begin
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contracting due to gravity
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acting as a smaller denser nebula
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within these denser fragments pockets of
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gas ranging in sizes from 2000
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astronomical units to 20 000
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astronomical units start to form it
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is within these pockets also known as
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protostellar nebulae
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that star systems like the solar system
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form
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the birth of such a system always begins
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with angular momentum
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while all the necessary ingredients for
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stars are present within proto-stellar
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nebulae
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something needs to wake the cloud so
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that it can collapse and form a star
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essentially giving the nebulous mixture
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a gravitational stir
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to set the processes into motion
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once something has acted upon a pocket
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of gas like this
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it begins to rotate pulling its contents
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into a vortex
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the speed at which the nebula rotates
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flings the gas within the protostellar
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nebula
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out on its orbital plane over thousands
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of years
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resulting in a fast swelling accretion
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disk forming from the matter
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which feeds gas into the center
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gas orbiting in this disc accelerates
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the closer it gets to the core
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compressing matter around the centre and
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concentrating mass
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this process continues for about a
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hundred thousand years
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until a central stable sphere of hot gas
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which is no longer contracting
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forms at the heart of the disk the seed
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from which the star will arise
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as gas continues to be fed onto the seed
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a dense
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envelope of in-falling matter collects
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around the seed
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acting as a gaseous oven for the star by
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raising the mass
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compression density and ultimately the
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temperature
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eventually the mass and pressure is
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sufficient enough that a proto-star
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ignites within the first stage of a
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star's great furnace
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[Music]
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at this point the star is not hot or
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massive enough to begin nuclear fusion
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the process powering main sequence stars
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like the sun
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rather at this point the only energy and
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heat source comes from the gravitational
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compression
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of the envelope which compresses hot
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swelling gas
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around the star increasing its mass and
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density
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often accompanied by characteristic twin
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polar jets of outflow gas
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before long a substantial amount of the
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proto-stellar nebula's mata is
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concentrated onto the protostar
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what remains of the accretion disk
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feeding the envelope will not exceed
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more than about one-fifth of the star's
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total mass
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and as material is used up within the
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system the envelope around the star
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begins to deplete and the infant star
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becomes observable for the first time
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first in the far infrared spectrum and
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then later in the visible spectrum
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it has become a young stellar object
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around this time providing that the star
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is at least 13 times heavier than
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jupiter
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it reaches the stage at which it can
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begin nuclear fusion
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regardless of its total mass it cannot
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fuse hydrogen yet
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but instead fuses deuterium which
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generates much less outward pressure
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if the mass accumulated by the protostar
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during incubation
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does not exceed around 80 times the mass
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of jupiter then the star will not evolve
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beyond this deuterium burning stage
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and will remain floating through space
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as a small dim brown dwarf star
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a failed star whose light fades away
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over billions of years
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however for the stars that are above
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this mass limit
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they will be able to fuse hydrogen
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eventually and will start evolving
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into their main sequence burning phase
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slowly over tens of millions of years
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in the meantime the young star is at
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last generating
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some kind of outward pressure and begins
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erupting jets from its photosphere
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which blows away the last of the gaseous
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envelope
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this envelope takes about a million
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years to become transparent and clear
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completely
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the young star is now fully visible and
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is classed as a t
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towery star t towery stars are named
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after the first of their kind that was
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discovered
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they are young stars which generate a
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broad range of emissions
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owing to what remains of the accretion
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disk that has not yet had time to form
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into planets
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this material hits the now fully exposed
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star's surface
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on the poles creating a variety of
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spectral emissions
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magnetic activity and often twin polar
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jets which we can detect from the earth
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the classical t tauri stage lasts around
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10 million years
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by the end of which time the accretion
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disk has cleared completely
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therefore during this stage a brief
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window opens for planetary formation
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the accretion disk becomes a
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protoplanetary disk
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this is the make or break stage for the
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star's system
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while planets can take billions of years
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to change and evolve
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every planet within a star system has to
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form within this brief 10 million year
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window
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or else it will never form at all
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at the very start of the titari stage
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the disc is far too hot for anything to
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form
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the temperature within four astronomical
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units of the star will be around 400
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kelvin
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and the temperature within the distance
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from earth to the sun is around a
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thousand kelvin
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at this temperature volatiles like water
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and methane are easily vaporized and
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ejected towards the outer regions of the
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disk
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but as the t tyre stage progresses
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material clears and the disc
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cools and less volatile substances like
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silicate rock and iron begin condensing
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forming tiny crystalline dust grains
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[Music]
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in a dense rotating disc environment
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dust grains are drawn to one another
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by various natural processes the main
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one being gravity
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which causes them to clot and form
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clumps of dust grains up to a centimeter
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or two wide these clumps then float
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through space
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attracting and sticking to other clumps
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growing at a rate of a few centimeters
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per year
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after a million years or so of this
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process large
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chunks up to a kilometer wide have
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formed known as planetesimals
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once the planetesimals have formed they
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too begin attracting an accreting
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material from the disk
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this process favors larger bodies and
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within a hundred thousand years
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runaway accretion processes have given
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rise to rocky bodies more than a
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thousand kilometers in diameter
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these bodies then dominate their region
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of the disk sweeping up much of the
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leftover material at the expense of
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smaller bodies
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by the time the planetary formation
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window closes after 10 million years
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the majority of the system's mass is
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concentrated within a few dozen large
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rocky bodies
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known as planetary embryos
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the disc has dissipated and accretion is
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no longer practical
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collisions take over planetary embryos
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smash into one another in chaotic
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encounters
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generating enormous amounts of heat and
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friction before eventually
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sticking together once these rocky
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bodies reach a certain size
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they become so massive that their
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heavier elements and metals begin
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sinking towards the heated core and the
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planetary embryos slowly becomes a
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differentiated planet with a hot
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internal layered structure
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this is how rocky terrestrial planets
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like earth venus
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mars and mercury form and as mentioned
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they tend to be found closer to their
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parent star
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where the disc used to be much hotter an
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area where only silica rock and metals
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can condense to form planetesimals
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however silicates only account for a
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negligible portion
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of the proto-stellar nebula's mass and
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so there simply isn't that much silica
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rock to allow terrestrial planets to
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grow
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exceptionally large
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the majority of the proto-stellar
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nebula's contents is hydrogen and helium
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gas
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and this remains within the disk as
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planets begin to form
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and further away from the young star's
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heat and radiation
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space becomes cold enough for the
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volatiles that were ejected from the hot
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part of the disk to condense into a
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solid form
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known as ices and volatile ices provide
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an abundance of extra material for
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planetary formation
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further away from the star more remote
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planetary embryos which can attract
00:15:56
these ices can quickly grow to dozens of
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times the mass of rocky terrestrial
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planets
00:16:01
heavy enough to begin attracting
00:16:03
hydrogen and helium from the surrounding
00:16:05
disk
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before it is driven away by the young
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star
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these attracted atoms form a thick
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atmosphere around the icy planetary core
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eventually becoming so large and heavy
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that the whole thing becomes this
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indistinguishable dense ball
00:16:20
of extremely compressed liquefied gases
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also known as a gas giant
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on the other hand ice giants such as
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uranus and neptune
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are thought to be failed gas giants
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which formed later on in the system's
00:16:35
planetary formation window
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and subsequently we're able to
00:16:38
accumulate much less hydrogen and helium
00:16:42
however they still retain thick
00:16:43
coverings of volatiles
00:16:45
and can be dozens of times the mass of
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the earth
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be it gas or ice giant these massive
00:16:51
planets often barrel around their
00:16:53
respected star systems after they form
00:16:56
crossing orbital lines and providing
00:16:58
gravitational kicks
00:16:59
which sends other planets orbits into
00:17:02
chaos
00:17:04
planetesimal material that has not been
00:17:06
used in planetary formation
00:17:07
thus far is scattered by these
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influences
00:17:11
often collecting into large
00:17:13
circumstellar belts of rock and ice
00:17:15
further away from the parent star
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like the kuiper belt for example
00:17:21
eventually the protoplanetary disk is
00:17:23
either recruited by planets
00:17:25
vaporized by ultraviolet radiation or
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scattered out of the system by the
00:17:29
star's emissions
00:17:32
one way or another no more than 10
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million years after the ttari stage
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begins
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the planetary formation window closes
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for good
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what remains is either a fully formed
00:17:43
system of planets
00:17:45
or a lone star with a failed dusty disk
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for us of course a vast and beautiful
00:17:51
planetary system was left behind
00:17:54
a system that gave rise to life so
00:17:56
complex that it is capable of
00:17:58
comprehending these incredible events
00:18:00
that occurred billions of years ago
00:18:04
so that's the science of a star system
00:18:07
but what about the story
00:18:10
what is it that makes this place unique
00:18:12
from the rest
00:18:14
what happened when our solar system was
00:18:16
born
00:18:26
as the nebula hypothesis tells us the
00:18:29
solar system began with a giant
00:18:31
interstellar molecular cloud around 65
00:18:34
light years in diameter
00:18:35
within which smaller pockets of gas were
00:18:38
fragmenting and acting as
00:18:39
isolated proto-stellar nebulae
00:18:43
one such pocket known as the solar
00:18:45
nebula
00:18:46
or pre-solar nebula at this point was
00:18:49
where the solar system would begin
00:18:50
its journey as we know
00:18:54
star formation begins with angular
00:18:56
momentum
00:18:58
something must have provided the kicking
00:19:00
to life to initiate the formation of the
00:19:02
sun
00:19:04
we believe that this thing was the
00:19:06
supernova of a nearby dying star
00:19:08
which generated a huge shock wave that
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cascaded through space
00:19:13
this shock wave struck the molecular
00:19:15
cloud and pre-solar nebula
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causing them both to rotate and
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showering it with heavier elements
00:19:21
released from the dead star's core
00:19:24
this shockwave reverberated throughout
00:19:27
the entire cloud
00:19:28
awakening hundreds of individual
00:19:30
proto-stellar nebulae
00:19:32
a whole host of new stars including the
00:19:35
sun
00:19:35
were forming inside what is known as the
00:19:38
solar birth cluster
00:19:41
in spite of the fact that we are far
00:19:42
away from our stellar neighbors today
00:19:45
current simulations of the solar system
00:19:47
suggest it began within a group
00:19:49
containing 200 to 400 stars that formed
00:19:52
from material in the same molecular
00:19:54
cloud
00:19:56
these stars formed in close proximity
00:19:58
but were not as close
00:19:59
dense or populous as say a globular
00:20:02
cluster
00:20:03
and so it wouldn't have taken much for
00:20:05
these stars to escape one another's
00:20:07
gravity
00:20:09
stars within these kinds of clusters are
00:20:11
born with momentum
00:20:12
which carries them away from their
00:20:14
original position while the system
00:20:16
within
00:20:16
takes shape eventually constituent stars
00:20:20
in the cluster are carried far enough
00:20:22
away that they begin to independently
00:20:24
latch onto
00:20:25
and orbit the galactic core of the milky
00:20:27
way which continues to carry them around
00:20:29
the galaxy along their own path
00:20:32
until the cluster is completely
00:20:33
dispersed
00:20:37
it took hundreds of millions of years
00:20:39
for the solar system to migrate
00:20:40
out of its birth cluster long after the
00:20:43
planets had taken shape
00:20:46
around a hundred thousand years after
00:20:48
the solar nebula began to collapse
00:20:50
the sun had begun forming inside its
00:20:52
gaseous envelope
00:20:54
and eventually the surrounding gas in
00:20:56
the nebula was flattened into a
00:20:58
protoplanetary disk around 200
00:21:00
astronomical units in diameter
00:21:03
within a million years its contents had
00:21:06
crystallized to form dust grains
00:21:08
which quickly clumped together forming
00:21:10
both rocky and icy planetesimals soon
00:21:13
after
00:21:14
thus ushering in the planetesimal era of
00:21:16
the solar system
00:21:17
and opening the brief window for
00:21:19
planetary formation
00:21:22
and only a few million years after this
00:21:24
window
00:21:25
opened the solar system knew its first
00:21:28
world
00:21:37
as we briefly touched on earlier in
00:21:40
every star system
00:21:41
there is a boundary a certain distance
00:21:43
from the parent star
00:21:45
the area beyond which it is cold enough
00:21:47
to allow volatile ices to condense
00:21:49
into a solid form the solar system is no
00:21:53
exception
00:21:54
and its snow line lies around five
00:21:56
astronomical units
00:21:58
from the sun it is no surprise that the
00:22:01
oldest and largest planet of them all
00:22:04
lies just around this boundary when the
00:22:07
young sun was forming
00:22:08
vaporized volatiles ejected from the
00:22:11
inner solar system
00:22:12
began condensing around the snow line
00:22:15
this led to the rapid accumulation of
00:22:17
water vapor within the vicinity
00:22:19
which created a circumstellar region of
00:22:21
low pressure
00:22:23
this meant that particles in this area
00:22:25
could orbit faster
00:22:27
preventing them from falling further in
00:22:29
towards the sun
00:22:30
effectively creating an orbital barrier
00:22:33
which enabled a faster buildup of dust
00:22:36
this band of dust then coalesced into a
00:22:38
planetary core about 10 times the mass
00:22:41
of the earth today
00:22:42
massive enough to start attracting the
00:22:44
early hydrogen and helium atoms from the
00:22:46
surrounding disk
00:22:49
within about a hundred thousand years of
00:22:51
the planetary core forming
00:22:53
it now weighed more than 150 times the
00:22:56
mass of the earth
00:22:57
and for the first time the sun rose on
00:23:00
the horizon
00:23:00
of its first born planet jupiter
00:23:12
jupiter formed around three million
00:23:14
years after the solar system
00:23:16
and in addition to being the first
00:23:17
planet to see the young thitari sun rise
00:23:20
it also had access to the untapped
00:23:23
supply of dust in the protoplanetary
00:23:25
disk
00:23:27
as jupiter began to accrete abundances
00:23:30
of material
00:23:31
after formation this in turn dragged the
00:23:34
planet slightly inwards
00:23:35
causing it to veer off its orbital path
00:23:38
and spiral towards the sun
00:23:40
it barrelled through the young solar
00:23:42
system clearing vast trails in the
00:23:45
protoplanetary dust
00:23:46
allowing it to become far larger and
00:23:48
heavier than all of the other planets of
00:23:50
the solar system
00:23:52
and today it is around 320 times the
00:23:55
mass of the earth
00:23:56
two and a half times heavier than all of
00:23:58
the other planets combined
00:24:03
shortly after jupiter formed another gas
00:24:05
giant was coalescing from the dust
00:24:07
around the snow line
00:24:09
saturn but not with the brilliant ring
00:24:13
system that we know it to have today
00:24:16
while saturn is a similar size to
00:24:18
jupiter it is considerably lighter
00:24:22
most likely because jupiter had already
00:24:24
consumed most of the material that could
00:24:26
have been used to grow
00:24:27
saturn the same can be said for the
00:24:29
solar system's ice giant planets
00:24:32
uranus and neptune they both formed
00:24:35
shortly after their gas giant siblings
00:24:38
and by this point the young sun's
00:24:39
emissions had scattered most of the
00:24:41
hydrogen and helium gas
00:24:42
that was left in the disk and so it is
00:24:45
not surprising that the ice giants are
00:24:47
much smaller than the gas giants
00:24:49
given when they are thought to have
00:24:51
formed the real mystery
00:24:53
is where in their current remote
00:24:56
positions there wouldn't have been
00:24:58
enough time during the planetary
00:24:59
formation window to allow the ice giants
00:25:02
to form and accrete to such
00:25:03
sizes there simply wasn't enough dust
00:25:06
that far away
00:25:08
what this suggests is that the two ice
00:25:11
giants formed further inside the solar
00:25:13
system
00:25:14
around or perhaps even between the
00:25:16
orbits of saturn and jupiter
00:25:19
and it wasn't until much later that they
00:25:21
migrated outwards
00:25:24
meanwhile in the inner solar system once
00:25:27
the planetary formation epoch had ended
00:25:29
we estimate that there would have been
00:25:31
between 50 to 100
00:25:33
moon to mars sized planetary embryos
00:25:36
which then began colliding to merge and
00:25:38
grow
00:25:38
in a chaotic process of planetary
00:25:40
natural selection
00:25:43
it's likely that a number of would-be
00:25:46
planets from this age have since been
00:25:48
lost
00:25:48
due to various violent events mercury is
00:25:52
one such example
00:25:54
it is very small compared to the other
00:25:56
terrestrials
00:25:57
and its internal structure constitutes
00:25:59
mostly a planetary core
00:26:02
suggesting it was once a batting
00:26:04
protoplanet in the making
00:26:05
before another planetary embryo struck
00:26:07
it
00:26:09
this collision likely tore off its outer
00:26:11
mantle and sent the exposed planetary
00:26:13
core
00:26:14
hurtling in towards the sun where it has
00:26:16
settled
00:26:17
healed and remains to this day
00:26:21
mercury is an example of a failed world
00:26:25
but slightly further away from the sun
00:26:27
the fortunes of three other terrestrial
00:26:29
planets were much different
00:26:32
venus the earth and mars all formed from
00:26:34
collisions of planetary embryos
00:26:36
which heated their cores and allowed
00:26:38
them to grow to the point at which they
00:26:40
gained internal layered structures
00:26:42
allowing for magnetic fields and
00:26:44
geological activity
00:26:46
which protected their early atmospheres
00:26:49
after the planetary formation window
00:26:51
closed it took around 50 million years
00:26:54
for the sun to fully evolve
00:26:55
into a hydrogen-fusing main sequence
00:26:58
yellow dwarf
00:26:59
and another 50 million years for the
00:27:01
terrestrial planets to form completely
00:27:03
as we know them today
00:27:05
thus completing the solar system's
00:27:07
present-day structure
00:27:10
after this planetary migration took over
00:27:13
the fate of the solar system for the
00:27:15
next billion years or so
00:27:17
and the gravitational influences of the
00:27:19
giant planets
00:27:20
sculpted the solar system into the
00:27:22
arrangement we see today
00:27:25
between 500 or 600 million years after
00:27:28
they formed
00:27:29
jupiter and saturn fell into a two to
00:27:31
one orbital resonance
00:27:33
with jupiter completing two orbits for
00:27:35
every one orbit of saturn
00:27:39
the combined co-moving tidal influences
00:27:41
of the two giants
00:27:43
acted as a gravitational slingshot and
00:27:46
it is thought that this influence is
00:27:47
what kicked
00:27:48
neptune and uranus out from the snowline
00:27:50
region towards the positions that we see
00:27:53
today
00:27:54
when this happened the young neptune
00:27:56
plowed into what was the kuiper belt
00:27:58
back then
00:27:59
which was much closer to the sun
00:28:01
starting at 15 astronomical units and
00:28:04
extending to only 20 astronomical units
00:28:08
but neptune's influence coursing into
00:28:11
this region disrupted the belt and
00:28:13
scattered its objects in all directions
00:28:16
some of the debris was sent hurtling
00:28:18
into the outer solar system
00:28:20
where it collected into what is now the
00:28:22
present-day more remote kuiper belt
00:28:25
and the larger scattered disc but other
00:28:28
planetesimals were less fortunate
00:28:31
they were sent inwards towards the sun
00:28:33
by neptune where they then
00:28:35
encountered the gravitational influence
00:28:37
of saturn and jupiter's resonance
00:28:40
this ejected the planetesimals back out
00:28:42
of the solar system with
00:28:44
much more force than neptune could
00:28:45
muster sending them
00:28:47
far into interstellar space into what is
00:28:50
now believed to be the oughts cloud
00:28:54
further in and the cascading influence
00:28:56
of neptune's departure to the outer
00:28:58
solar system was wreaking havoc on the
00:29:00
asteroid belt
00:29:02
the sparse circumstellar band of rocky
00:29:04
asteroids that lies between two to four
00:29:06
astronomical units from the sun
00:29:10
today its mass is barely a fraction of a
00:29:12
percent of earth's
00:29:14
but back in the early solar system it
00:29:17
was much more densely populated
00:29:19
with enough material to form another two
00:29:21
to three earth-sized planets
00:29:24
this material was in the process of
00:29:26
collecting interplanetary embryos
00:29:28
before neptune and jupiter's influences
00:29:31
sent the belt into disarray
00:29:34
each of the terrestrials was pounded by
00:29:36
asteroids for around 300 million years
00:29:39
in an event known as the late heavy
00:29:41
bombardment
00:29:42
causing chaos to rain down on each of
00:29:45
the planets
00:29:47
but among these many asteroids something
00:29:49
else was arriving
00:29:51
water as the unrelenting chaos of the
00:29:55
early solar system
00:29:56
ensued icy planetesimals scattered by
00:29:59
the gas giants from further out
00:30:01
came hurtling into the inner solar
00:30:03
system as comets
00:30:04
many of which also smashed into the
00:30:07
terrestrial planets
00:30:09
the water contained within these comets
00:30:12
instantly vaporized on impact
00:30:14
but this vapor was retained within each
00:30:16
planet's early atmosphere
00:30:19
gradually each of the terrestrial
00:30:21
planets cooled down from their
00:30:22
primordial states
00:30:24
and as more and more water arrived the
00:30:26
balance of each world's atmosphere
00:30:28
changed
00:30:29
allowing the vapor to condense in the
00:30:31
sky and fall as rain
00:30:35
for thousands of years it rained on each
00:30:37
of the terrestrial planets
00:30:39
filling streams basins and eventually
00:30:42
entire oceans with liquid water
00:30:48
nowadays we know of earth as the blue
00:30:50
planet
00:30:52
but in the early solar system it's more
00:30:54
than likely that there are actually
00:30:56
three blue planets not necessarily
00:31:00
occurring at the same time
00:31:01
but we believe that both mars and venus
00:31:04
also once harboured oceans of liquid
00:31:06
water
00:31:06
like on earth today water certainly once
00:31:11
flowed on mars's surface
00:31:13
we can see evidence of smoothing
00:31:14
weathering and erosion
00:31:16
all over the face of the planet and many
00:31:19
of our efforts to venus
00:31:20
since the 1980s suggest that it may have
00:31:23
had
00:31:23
shallow surface coverings of liquid
00:31:25
water for around 2 billion years
00:31:29
all three planets were once home to
00:31:31
similar temperate ocean environments
00:31:33
likely made habitable by the same
00:31:35
complex geological processes on each's
00:31:38
sea floor
00:31:41
of course both of our neighbors have
00:31:43
since lost such wondrous properties
00:31:46
as the smallest of the three planets
00:31:48
mars cooled down from its primordial
00:31:50
state much quicker than its siblings
00:31:53
and thus may have been the first world
00:31:54
to become blue
00:31:57
however because mars is so small its
00:32:00
core
00:32:00
also cooled down very quickly its
00:32:03
insides went cold
00:32:05
thus causing the convection currents
00:32:07
within its mantle to subside
00:32:10
this caused the planet's protective
00:32:12
magnetic field to switch off
00:32:14
allowing harmful solar emissions to
00:32:16
strip back the martian atmosphere
00:32:18
and today all of its oceans have
00:32:20
evaporated
00:32:24
mars would have only had habitable
00:32:26
conditions for a few hundred million
00:32:28
years at most
00:32:29
probably not enough time for anything
00:32:31
more than simple cellular life
00:32:33
such as bacteria to emerge but even then
00:32:37
there's no guarantees
00:32:41
venus on the other hand managed to
00:32:43
retain its habitable conditions for
00:32:45
longer
00:32:46
it is much larger than mars and thus its
00:32:49
interior remains active
00:32:51
even to this day early in the sun's life
00:32:55
when it was less bright and hot venus
00:32:57
sat in a position
00:32:59
analogous to where earth lies today
00:33:02
just the right distance for temperate
00:33:04
conditions which supported liquid water
00:33:06
for more than 2 billion years
00:33:09
multicellular life on earth is thought
00:33:11
to have taken
00:33:12
around 3 billion years to evolve but
00:33:15
it's still
00:33:15
interesting to think what could have
00:33:17
occurred on venus during its habitable
00:33:19
epoch
00:33:22
unfortunately for venus the sun
00:33:25
continued to evolve
00:33:26
gradually burning ever hotter and
00:33:28
brighter which gradually raised the
00:33:30
planet's temperatures causing its oceans
00:33:32
to evaporate
00:33:34
this led to a buildup of water vapour in
00:33:37
its atmosphere
00:33:38
followed by a runaway greenhouse effect
00:33:40
of atmospheric thickening
00:33:42
which caused the planet's water to boil
00:33:44
away completely
00:33:46
now the planet is hostile and
00:33:48
experiences the hottest temperatures of
00:33:50
any world in the solar
00:33:52
system thanks to its thick toxic
00:33:54
atmosphere and its close proximity to
00:33:56
the sun
00:33:59
our two planetary neighbours were once
00:34:01
earth-like
00:34:02
but neither boasts anything close to the
00:34:04
complex array of idealistic properties
00:34:07
that make earth so
00:34:08
special today earth's natural balance
00:34:12
is maintained by the life processes it
00:34:14
has given rise to
00:34:16
and it has remained an optimal distance
00:34:18
from the sun to keep its
00:34:20
climate temperate for billions of years
00:34:22
much longer than either of our neighbors
00:34:26
earth is a standout world and a success
00:34:29
story
00:34:30
a thriving oasis among scores of dried
00:34:32
up deserts
00:34:35
but no matter how well suited we are our
00:34:38
barren companions are a constant
00:34:40
reminder that balance
00:34:41
simply cannot last forever
00:34:45
though the sun remains peaceful today a
00:34:47
new age is coming for the solar system
00:34:50
an age of change destruction and
00:34:53
evolution
00:34:54
an age for new worlds and new stories
00:34:57
but one in which the earth
00:34:59
has no place
00:35:06
[Music]
00:35:11
[Music]
00:35:29
you
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