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Does Distance Affect the SolarRadiation the Planet Receives?

The amount of solar radiation the Earth receives is very closely related to its distancefrom the sun. And although the sun's output has varied over it's long lifetime, Earth'sdistance from the sun and orbital characteristics have the greatest effect on theamount of radiation our planet receives. But not all sunlight is absorbed by the Earth.Some is reflected back into space instead of being converted to heat.

Inverse Square LawThe inverse square law is a fundamental concept in physics that applies to manyphenomena including gravity, electrostatics and the propagation of light. The lawstates that a given quantity or intensity is inversely proportional to the square of thedistance from the source. For example, the intensity of solar radiation on the surfaceof Mercury is nearly nine times that of Earth's, but Mercury is only around three timescloser to the Sun. Tripling the distance to the sun decreases the amount of radiationreaching Earth's surface to one-ninth the light level on Mercury.

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Orbital VariationsAccording to Kepler's first law of planetary motion, the law of orbits, Earth moves inan elliptical path around the sun. The distance between Earth and sun varies slightlythroughout the year. At aphelion, the farthest distance from the sun, Earth is 152million km away. But at perihelion, the closest distance from the sun, the Earth is 147million km away. As a result, throughout the year, the amount of light reaching Earth'ssurface changes by a few percent.

Solar RadiationThrough the years, scientists have directly monitored solar radiation by usinginstruments and satellites like the Total Irradiance Monitor, part of the SORCE satellitemission. Studies show that solar output varies minute to minute and changesdrastically over thousands of years. These variations can influence changes in Earth'sclimate. Sunspots are also related to solar output, although it's not understood how.Historical records of sunspot activity indicate that solar output is higher wheneverthere are more sunspots.

Planetary AlbedoScientists can calculate the amount of solar output Earth receives at a given distancefrom the sun. Earth reflects some of this light into space, reducing the total radiationabsorbed. This effect is described by the term albedo, which is a measure of theaverage amount of light reflected by an object.

Albedo is measured on a scale from zero to one. An object with an albedo of onewould reflect all light reaching it, while at zero albedo, all light would be absorbed.Earth's albedo is about 0.39, but changes over time such as cloud cover, ice caps orother surface features change this value.

1. There are moons orbiting Jupiter and Saturn that we think could possibly harbor life. These moons are obviously outside of the habitable zone, so why would scientists think that life may exist there?

2. The orbit of Earth is an ellipse, but it is very close to being a circle with the change in distance only a few percent throughout the year. How might life be different on Earth if our orbit was more elliptical and we got much closer and much farther from the sun as we orbited?

What makes one Earth-like planet more habitable than others?

When it comes to finding the right kind of planet to target in the search for life elsewhere in the universe, the size

of the planet matters.

All planets are believed to form by a process of competitive cannibalism, in a disk of material around a nascent star.

Small pieces of dust collide and grow, devouring their neighbors. As they get larger, their ability to consume all around

them increases and their growth accelerates.

If a planet swells to more than around ten times the mass of the Earth before the nuclear furnace within its host ignites,

it will be able to accrete vast amounts of gas. It will grow to become a gas giant, like Jupiter and Saturn in our own solar

system. Such planets would definitely not be good places to look for life.

At the other end of the spectrum, an object that is too small and of too low a mass (such as Mercury, or Earth’s moon)

won’t have enough gravity to hold an atmosphere.

Typically, the more massive the planet, the more massive the atmosphere it can acquire and maintain.

This is important because the mass of a planet’s atmosphere will directly influence its climate. The location of the “habitable

zone” around a star will therefore be a function of the mass of the planet in question.

A more massive planet, with a more massive atmosphere, will likely have a stronger greenhouse effect. Such a planet

would most likely be habitable at distances that would result in smaller planets icing over.

Similarly, a smaller planet, with a thinner atmosphere, will likely be habitable at distances at which the oceans of a

more massive world would boil.

Magnetic field

While a planet needs to be sufficiently massive to acquire and host a thick enough atmosphere to support life, there is

more to the story.

Particularly in their youth, stars are violent hosts. They fling material to space and continually bombard their planets

with radiation.

To some degree, that is vital for life. Without the energy the Earth receives from the sun, there would be no light and

our planet would be a frozen ice-ball.

But with the blessing of light comes a curse – the wind blowing from a star can tear away the atmosphere of a planet,

stripping it to nothingness.

Artist’s impression of the effect of a vast stellar flare on the atmosphere of the exoplanet HD 189733b. Stellar activity can strip away the atmosphere of a planet, leaving it barren and lifeless.

Fortunately for life on Earth, our planet is protected from the worst vagaries of the solar wind by its strong magnetic

field.

As the solar wind strikes Earth’s magnetic field, it is deflected around, shielding the planet within. Only the strongest

solar storms, and the most energetic particles, can penetrate that shielding.

Cascades of charged particles that penetrate the Earth’s magnetic shield pour down to the poles, causing the

magnificent Aurora Boraelis and Australis.

The Aurora Borealis, imaged here from northern Canada, is the result of charged particles flung outward by the Sun smashing into Earth’s atmosphere.

But what if the Earth had no magnetic field? The situation would be grave. Our nearest planetary neighbour, Mars, lacks a strong magnetic field and most likely has been unshielded since the early youth of the solar system. That lack of shielding has caused the planet to pay a terrible price – Mars’s once thick atmosphere has been whittled away to almost nothing.

The presence of a magnetic field is particularly important during a planet’s youth – when its host star is still young and

excitable, and energetically shedding material to space. As stars age they usually mellow but remain capable of slowly

stripping planets of their gaseous shrouds.

It is clear, therefore, that for a planet to be considered a promising target in the search for life, it must possess a strong

magnetic shield to protect its atmosphere. But how does a planet like the Earth maintain a magnetic field sufficiently

strong, and for sufficiently long, to offer that kind of protection?

Plate T ectonics The Earth’s magnetic field is driven by a colossal dynamo, deep in the Earth’s outer core. It is thought that this dynamo is driven by convection currents in that region of our planet’s interior. But therein lies a problem – how can you maintain convection inside a planet over periods of billions of years?

The Earth’s magnetosphere protects our atmosphere from the erosive power of the Solar Wind. But without convection in Earth’s core, that field would not exist.

The interior of the Earth is very hot, primarily as a result of the decay of radioactive elements trapped in the interior.

Our planet slowly sheds that heat to space through its crust, with the result that the surface layers of our planet’s

interior are cooler than those near the core.

In order to setup and maintain convection, you need a large temperature difference between two locations. So the

convection currents in the Earth’s mantle are the result of the fact that the part of the mantle just beneath the crust is far

cooler than that next to the core.

But here’s the strange thing. If the Earth didn’t have plate tectonics, the upper layers of the mantle would not be able to

cool anywhere near as efficiently. With a warmer upper mantle, the temperature difference between that region and the

Earth’s deep interior would be smaller, and convection would eventually cease.

And without that convection, the Earth’s dynamo would die, and its magnetic field would weaken and disappear.

It is thought that this is precisely what happened to Mars. At first, the red planet may well have been sufficiently hot

inside for a certain degree of tectonic activity to occur. Certainly, the crust of Mars bears evidence of an ancient

magnetic field, frozen into the rocks.

The topography of Mars, based on observations taken by the Mars Orbiter Laster Altimeter. Despite its famous volcanoes, Mars does not exhibit plate techtonics, which may be tied to the loss of its atmosphere over the past 4 billion years.

But Mars is smaller than Earth and so lost its interior heat to space much more quickly. Eventually, it cooled

sufficiently that it could not maintain convection, and its magnetic field died down.

At that point, the solar wind began to strip the atmosphere away, while at the same time, chemical processes on the

surface were pulling the atmosphere down into the rocks. Without tectonics to recycle the rocks, and to drive the

magnetic field, Mars slowly became the world we see today.

In addition to maintaining our planet’s magnetic field, plate tectonics are important for many other reasons, particularly

when it comes to the creation of the atmosphere we breath today.

Without water

Tectonic activity is clearly important for life on Earth and so would certainly be a key criterion in the search for life

elsewhere. But some researchers suggest that the story may be even more complicated than we once thought.

Over the years, a number of studies have suggested that without water, the Earth wouldn’t have plate tectonics – a

theory that remains heavily debated.

The general idea is that water acts as a lubricant – either between colliding plates (helping them overcome the

friction between them), or within Earth’s mantle (increasing its fluidity, helping it to move, and easing the

convection currents therein).

Without water, a planet might have to be somewhat more massive than the Earth to support plate tectonics. Had Earth

formed much drier the motion of its plates might have seized long ago, causing convection in the mantle to die, and our

atmosphere to be been purged, just like that of Mars.

So, with this theory, we’ve come full circle. Life itself needs water, but for a planet to keep its atmosphere, it needs a magnetic field. To maintain a magnetic field, a planet needs a dynamo. To have a dynamo, it needs plate tectonics. And to keep plate tectonics, it needs water.

1. This article mentions water helping support life on Earth in a completely different way than most people think of. Are there other factors to making a planet habitable that may have more than one role in supporting life?

2. Mars doesn't have a magnetic field, Earth does. What would happen to Earth if it suddenly lost its magnetic field? What a happen to Earth if suddenly its magnetic field became much stronger?