SIGNS IN THE HEAVENS: The Celestial Convergence - Planet Jupiter Out All Night; Planets Venus, Saturn and Mercury Before Sunrise!

November 25, 2012 - SOLAR SYSTEM - The coming weeks are an awesome time to look for planets! The chart at the top of this post shows the planets Venus and Saturn closest together in the east before dawn on Monday, November 26. The planet Mercury is crawling into view now, too, in the eastern predawn sky, gearing up for a very interesting alignment between Mercury, Venus and Saturn next week. Plus, you can see the planet Jupiter anytime on these late November 2012 nights. Earth will pass between Jupiter and the sun next week, placing Jupiter in its best place to observe this year. All four planets – including Mercury – should remain in fine view for the next several weeks.
If you’re an acute observer, and have binoculars, you might even catch Mars low in the southwest sky after sunset. So it’s possible for you to see all five visible planets on this November night. By visible planet, we mean any planet that’s readily visible without an optical aid and which has been observed by our ancestors since time immemorial. In their outward order from the sun, the visible planets are Mercury, Venus, Earth, Mars, Jupiter and Saturn. Brilliant Jupiter will be super easy to spot. It’ll be low in the east at nightfall and rising upward during the evening hours. It’ll be highest up for the night around midnight and low in the west at morning dawn. If it’s clear, you simply can’t miss Jupiter because it’s the most brilliant star-like object to light up the evening sky.

Jupiter is bright! You can see it easily. It will be up more or less all night, shining more brightly than any
of the surrounding stars. This photo of Jupiter is from November 18, 2012. Image: Carlos Colon Sr.

The only planet to outshine Jupiter is Venus, the morning “star.” At mid-northern latitudes, dazzling Venus and the fainter planet Saturn rise together in the east about two and one-half hours before sunrise. If you can’t see Saturn next to Venus with the eye alone, use binoculars or a low-powered telescope. Venus and Saturn will remain within the same binocular field of view in the predawn and dawn sky from about November 24 to December 1. Day by day, look for Venus to fall downward as Saturn climbs upward.

You’ll need an unobstructed horizon and clear sky – and possibly binoculars – to spot Mercury, the innermost planet of the solar system. This world rises about one and one-half hours before sunrise tomorrow (Monday, November 26) at mid-northern latitudes. Try looking for Mercury close to the sunrise point on the horizon some 90 to 60 minutes before sunrise. If you miss Mercury in late November, keep in mind that the closest planet to the sun will be coming up even sooner before sunrise for the next few weeks. Moreover, Mercury will brighten all the while. Bottom line: Starting tonight (November 25, 2012) and for the next several weeks, look for Jupiter to shine all night long, Mars low in the southwest at dusk and nightfall, and for Venus, Saturn and Mercury in the eastern predawn and dawn sky. - EarthSky. 

 http://thecelestialconvergence.blogspot.gr/

Goldilocks, and other Habitable Zones for Life

Heard of the Goldilocks zone?
It’s the idea that an area of space around a star will be at the right temperature for life to exist. Not too hot, not too cold, hence Goldilocks.
It’s a bit like standing around a campfire on a very cold night. Stand too far away and you freeze, stand too close and you catch on fire and burn to death.
It’s the same with planets orbiting stars too, if they’re too far away then water freezes and life can’t emerge, and if they orbit too close the planet is roasting hot and nothing can live.
It gets a bit more complex than this though, but complex in a fun way. Oh and its also got some pretty big implications for the search for extraterrestrial life…

This Goldilocks zone is more usually called a habitable zone (HZ for short). It’s the distance around a star at which a planet can maintain surface liquid water.
Scientists care about liquid water, as all life on Earth needs liquid water to survive (life on Earth is basically bags of water with a few other ingredients thrown in). Scientists’ care about the idea of a HZ as it guides our thinking as to where in our Solar System life could potentially be found, and where it could be found in other solar systems too. And we all care about finding alien life, right?
Earth is in the HZ of our star, obviously, whereas Venus is too close to the Sun, as it’s surface is almost hot enough to glow, and Mars is probably right at the outer edge of the habitable zone, as its surface is too cold for water to remain liquid for long.

A simplified representation of our Sun’s habitable zone

A habitable zone is therefore defined as the region around a star between the distance at which water would evaporate and the distance at which surface water begins to freeze. (Sometimes the outer edge is set at the distance at which carbon dioxide would freeze out of an atmosphere, as CO₂ is a greenhouse gas that can heat a planet, meaning that planets rich in CO₂ could be warm enough to enjoy liquid water at a distance a bit further out than we would normally expect to find it).
The HZ doesn’t just depend on the distance from a star though; it also depends upon the features of the planet. If Mars had been slightly bigger it would have been able to maintain an atmosphere (it lost most of it’s initial atmosphere to space as its gravity isn’t strong enough to capture it permanently, more here) and if this atmosphere contained enough greenhouse gassed Mars could have a warm and wet surface today. Thus a HZ is typically defined as the region around a star in which an Earth-like planet could maintain surface liquid water.
A HZ also depends upon the star too. Larger stars emit much more heat, thus the zone in which an Earth-like planet could maintain surface liquid water would be much further out than for our Sun, and much closer in for stars smaller than ours.

Like this (click to enlarge)

Habitable zones are also affected by time. Over their lifetimes the heat output of stars changes. Our Sun has increased in luminosity since it first formed and is roughly 30% hotter today than it was 4.6 billion years ago. This means that the habitable zone must have moved outwards throughout the life of our star. Astronomers and astrobiologists believe that Earth has always been inside our Sun’s habitable zone, but it inspired a scientist called Michael Hart to come up with the idea of the Continuously Habitable Zone (CHZ). This is the region around a star in which an Earth-like planet can sustain surface liquid water for most of the lifetime of its star.
The idea of a CHZ is important, as the fossil record indicates that it took a long time for complex life to evolve on Earth. Palaeontologists have discovered that single-celled life emerged early in Earth’s history, possibly as far back as 4 billion years ago, but that it took more than 3.5 billion years for this bacterial life to evolve into the first animals. If the Earth had formed 5% closer to the Sun, or 15% further away, its likely that it would have been outside of this CHZ and thus animal life would not have been able to evolve on Earth (yes, that includes us).
This leads to a really cool habitable zone idea, that there may be different HZs for different types of life, an Animal Habitable Zone (AHZ) and a Microbial Habitable Zone (MHZ).
It’s likely that the AHZ would be very narrow, and would be confined to a star’s CHZ, as the planet would need to have surface liquid water for billions of years to allow animals time to evolve.

The animal habitable zone, narrow

The MHZ will likely be much wider, as microbial life may well take a mere few hundred millions years or so to emerge, thus can live on planets that may only spend a short time in a star’s HZ. Venus and Mars may well have had their own microbial life early in their histories, and thus may have been inside our Sun’s MHZ for a time.
Two other discoveries have also expanded the possible boundaries of a MHZ. The first of these was the discovery of extremophiles in the 1970s. Extremophiles are single-celled life forms that thrive in extreme conditions such as boiling water, sulphuric acid or inside rocks deep within the Earth’s crust. Extremophiles expanded the range of conditions in which life can be found and thus expand the range of the MHZ.
The second discovery is that liquid water can exist below the surface of planetary bodies that orbit way outside of a star’s HZ. Evidence suggests that some of the moons orbiting gas giant planets in our Solar System, such as Europa and Enceladus, may have vast subsurface oceans that could support life. I won’t go into the details here (if you want to know more than please see these posts; Europa, Enceladus) but it’s possible that these moons may have their own biospheres in underground oceans, but its more likely that these biospheres are microbial rather than animal. The existence of these moons suggests that the MHZ may be huge, and could potentially span between the orbits of Venus and Saturn in our Solar System.

Europa, within our Sun’s microbial habitable zone?

So what does this mean for the search for alien life?
Firstly, it means that if we hunt for advanced alien life, such as alien civilisations with radio technology, then we need to confine our searches to exoplanets that orbit in a very narrow CHZ around their stars.
Secondly, it suggests that microbial life may be relatively common in our Galaxy, as the MHZ is potentially so wide, but that complex life may be extremely rare, as it likely requires a planet of the right size and composition to orbit stars with a stable temperature at a precise distance. This means that planets that can support animal life in our Galaxy may be rare.
So maybe we aren’t alone in our Galaxy, but maybe most of our alien cousins are simple bacteria.
 http://astrobioloblog.wordpress.com/

How the Earth was born

In a previous post I wrote about how we discovered the age of the Earth, and I mentioned that our planet formed at the same time as the other rocky bodies in our Solar System.
I didn’t say HOW this happened though. So now I will.
Happy now?

It all begins with a cloud of dust and gas out in space that astronomers call a molecular cloud. Space is generally pretty empty. There’s the occasional star, planet or comet, but the vast voids between these objects have little in them, perhaps some molecules of hydrogen and helium gas, and maybe some “space dust”, which is mostly bits of silica, carbon, iron and some other elements (basically very small bits of rock), but mostly space is filled with empty, erm, space. In some regions inside galaxies dust and gas accumulates though, these are the molecular clouds I just mentioned.

The Horsehead Nebula is actually part of a molecular cloud. And you thought they sounded a bit boring too. Shame on you

In our Universe, everything that has mass attracts everything else that has mass, this is gravity. So all these molecules of gas and dust within the molecular clouds are gravitationally attracting the other molecules of gas and dust in the cloud. The force of this gravitational attraction is pretty weak though, as molecules have comparatively low mass and gravity weakens quickly over distance.
However, if the cloud can become large and dense enough a limit is breached and gravitational attraction begins to kick-in, pulling the molecules towards each other. As the molecules move closer to each other the force of gravity increases, and a runaway effect begins, causing the molecular cloud to start collapsing, and as it collapses it begins to spin.
What causes the cloud to reach this required density is not known. It’s thought that the dust and gas in the cloud could be compressed by passing gravity waves (a pretty cool phenomena, but one I’ll save for a later post), but no ones ever actually seen a gravity wave, so we can’t be sure.
The collapsing molecular cloud begins to spin faster and faster as it gets smaller. This happens as the cloud has momentum, and if a body with a fixed momentum becomes smaller, it spins faster, a nifty trick called the “Conservation of Angular Momentum”. You can perform a quick experiment yourself to test this. Stand up and put your arms out and begin to spin. If you continue spinning and then pull your arms in towards your body you should find that you’ll spin a little bit faster. Then vomit.

A bit like this. No vomiting though. I promise

As the molecular cloud continues to spin faster it will first form a spherical shape but then will start to flatten into a disc, as if you rotate a sphere at a uniform speed its equator will spin faster than its poles.
You can also try this at home too. Take a ball, like an orange or a tennis ball or something, and draw one dot at its equator and one near the pole. If you spin the orange so that it takes five seconds to turn full circle (with the poles facing up and down) then both dots will take five seconds to rotate, but the dot near the pole will complete a much smaller lap in terms of distance covered than the one at the orange’s equator. Thus the equator dot will have covered more distance in the same time and therefore will have moved faster. This is why a spherical piece of pizza dough becomes flat when spun by a pizza chef, the centre of the sphere spins faster and moves outwards, compressing into a flat pizza base. (I actually used to be a pizza chef for a bit, I used a spinning machine though, so I was a cheap fraud.)
At the core of the molecular cloud the contraction will be the most intense, as the core will be the densest region and will thus experience the greatest gravitational attraction. As the core contracts it becomes denser still, experiences stronger gravitational attraction, contracts some more, becomes denser, and so on and so on, I think we can all see where this is heading.
Eventually the core of the cloud will become so dense that another critical point is breached, the point at which hydrogen atoms in the cloud begin to fuse together to create larger atoms in a process called nuclear fusion. Nuclear fusion releases energy in the form of light and is the process that powers our Sun. Thus in the heart of the collapsing molecular cloud a star is born. This is how our Sun came into existence around 4.56 billion years ago.

An artist’s rendition of a protostar surrounded by its circumstellar disk of gas and dust. Yeah I know, its pretty damn cool looking

Around 99% of the mass of the molecular cloud will have ended up in the Sun, whilst the remaining 1% of the gas and dust will have continued to spin as a circumstellar disc around our young star. Over time the dust molecules in this cloud will have begun to collide with each other as they bounced around chaotically within the circumstellar disk. In most collisions the molecules will have bounced-off each other, but sometimes they will have stuck together, and over the course of hundreds of thousands of years they will have begun to form larger and larger clumps of rock. I think a similar process causes the dust under your bed to coalesce into larger and larger clumps (dust bunnies!)
Over time some of these clumps of rock will become pretty big, and after they became larger than roughly one kilometre across another trigger point will have been breached as their gravitational attraction became strong enough to begin attracting other larger clumps of rock. In this fashion larger and larger pieces of rock begin to smash into each other and are remoulded to form planetesimals, the first step in the creation of planets.

Planetesimals drifting around a young star. Purdy.

Over the next tens of thousands of years these planetesimals will have continued to grow larger and larger by the process of gravitational focussing, with the largest planetesimals greedily cannibalising the smaller ones and growing the fastest.
Eventually the accretion of these planetesimals will have led to the formation of a small number of planetary embryos of around 1000 km in diameter, and further collisions over the next 100 to 300 thousand years will have caused these embryos to accrete into an even small number of planets around the size of the Moon and Mars. Collisions will still have occurred between these bodies, but as there were so few of them at this point the collisions would have been far less frequent, and it may have taken another hundred million years or so for the inner rocky planets of our Solar System to form, eventually leaving us with Mercury, Venus, the Earth and Mars.
A fifth rocky planet likely tried to form between the orbits of Mars and Jupiter, but Jupiter’s strong gravitational attraction will have ensured that the rocky bodies in these regions will have collided with too much force to properly coalesce, leaving a belt of asteroids rather than a fifth planet.
Nearer the centre of the disc the temperature will have been hotter than the outer regions. Thus substances with a high boiling temperature will have solidified nearer to the Sun, and substances with a low boiling temperature will have solidified further out. The inner planets and asteroid belt are therefore mostly formed of iron, nickel and silicate rocks, but further out ices of different types could also solidify, such as water, ammonia and methane ice. Planets like Jupiter, Saturn, Uranus and Neptune therefore formed initially from mixtures of metal, rock and ice.
At some point in your life someone probably confidently told you that you could drive a bus straight through one of the giant planets as they’re entirely made of gas. This is pure madness. As at the heart of each of these planets is one of these central kernels of metal, rock and ice.
The outer regions of the circumstellar disk also contained much more hydrogen and helium gas, that these icy and rocky kernels could hoover-up in vast quantities due to the power of gravity, creating the thick covering of hydrogen and helium we see on the gas giants today, hence the name.

That’s the inside of Jupiter you’re looking at. You can see the inner kernel of metal, rock and ice, then a layer of liquid hydrogen & helium (you do not want to drive a bus through that), and then an outer layer of gaseous hydrogen & helium, you could probably get a bus through that, if you didn’t mind dying

Back to the Earth.
As the Earth first formed it will originally have been a ball of molten metal and magma. The denser elements of this liquid inferno, such as iron and nickel, will have begun to solidify first and will have sunk towards the core of the Earth. As the Earth solidified further it will have differentiated by this process into a solid metallic inner-core (although it was extremely hot in this core the pressure was sufficient to cause the iron and nickel to solidify), a liquid metal outer-core where the pressure wasn’t quite so intense, surrounded by a rocky outer mantle.
The early Earth will have been a hellish place to visit, and its first half a billion years or so are referred to as the Hadean (Hell like) Eon by geologists. The intense heat inside the Earth will have caused large regions of the rocky mantle to melt and burst onto the Earth’s surface as massive volcanic eruptions. As the mantle melted, the magma it produced will have had a slightly different composition to the mantle rock, as not all of the minerals in the mantle will have melted, and as the magma erupted on the surface and cooled a differentiated crust will have began to form around the Earth. Thus the structure of the Earth we know today was formed, with a rocky crust and mantle, and a metalic inner and outer core.
The Solar System will still have been full of large chunks of rock in the Hadean, many of which will have rained down on Earth’s surface as meteorite impacts. But early in the Earth’s life, around 4.533 billion years ago, in an act of supreme planetary vandalism she was struck by a much larger object, a proto-planet the size of Mars called Theia. A bit like your house colliding with a slightly smaller house, at the speed of a bullet.

Or, a bit like this. Forget the whole house thing.

Such a monumentally enormous collision will have released a vast amount of energy, fracturing and melting large chunks of the Earth and probably completely destroying Theia. As the Earth recovered, the debris from this collision will have been gravitationally bound to the Earth, initially forming a ring around our planet similar to that of Saturn’s. But over time the chunks in the ring will have began to coalesce, accreting into a separate planetary body that orbits our planet today, thus the Moon was born. So the Theia collision was actually a blessing, if an initially very destructive one.
So there you go, the Earth, our very own planet is basically made out of a bunch of dust that stuck together, melted, cooled again, and then got hit by another planet to provide us with the Moon we see in the sky above us today. As Professor Brian Cox would say, brilliant.
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