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Our Solar System
The structure of our solar system
The four planets closest to the Sun – Mercury, Venus, Earth, and Mars – all have a similar composition. Their cores are made primarily of iron with a silicate mantle. They formed from dense dust clouds that were closest to the Sun.
Farther away from the Sun, the leftover material was made mostly of hydrogen gas. Temperatures were much lower and the gas clouds were not large enough to form their own stars. Instead, they formed huge gas planets, or gas giants, called Jupiter and Saturn. These planets have no solid surface.
At the outermost part of the ring of gas surrounding the Sun, temperatures are very cold. The material here condensed into huge balls of ice and hydrogen gas. The planets formed were Uranus and Neptune. Like the gas giants, they do not have a solid surface.
Why is this the way that it is?
Each star radiates energy in all directions. As you get father from the star, the amount of energy in each area gets smaller (the same energy exists, but its more spread out). This is due to something called the inverse square law.
The Habitable Zone
Since the energy spreads out, the temperature would decrease as you get farther from the star. This puts a limit on where life (as we know it) should be.
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The habitable zone exists around all stars. For hot stars, the habitable zone starts further from the star and is much larger than around cooler stars.
This habitable zone is the region around which liquid water could exist.
Boundaries
The inner boundary of a habitable zone is where water would be lost as a result of a runaway , in which greenhouse gases in a planet’s atmosphere would trap incoming , leading to the planet’s becoming hotter and hotter until the water boiled away.
The outer boundary is where such greenhouse warming would not be able to maintain surface temperatures above freezing anywhere on the planet.
Astronomers have calculated the extent of the habitable zone for many different types of stars. For example, at present, the habitable zone of the
is estimated to extend from about 0.9 to 1.5 s (the distance between Earth and the Sun).A star’s luminosity increases with time, both the inner and outer boundaries of its habitable zone move outward. Thus, a planet that is in the habitable zone when a star is young may subsequently become too hot. may have been such a planet; however, because it is geologically active, its current surface is too young to show any evidence that a more neutral climate may have existed billions of years ago. Other planets could be too cold for liquid water to exist when their star is young but might warm up enough to have liquid water on their surface later as their star’s luminosity increases. This may happen to
a few billion years hence. Thus, the most promising region to find Earth-like life would be in a “continuously habitable zone,” where liquid water could have been present from early in the star’s life up to the current . The continuously habitable zone of the Sun (from four billion years ago to the present) is from about 0.9 to 1.2 astronomical units.Changes in the Sun’s habitable zone
Earth has had liquid water on its surface for much of the past four billion years.
However, four billion years ago the Sun’s luminosity was only about 75 percent as intense as it is at present, and climate models suggest that Earth should have been frozen over at such a low solar luminosity.
This apparent disagreement between theory and observation is known as the “.” Another planet to which the faint young Sun problem might apply is
. On that planet the oldest regions of the surface show signs of running water while younger regions do not, which suggests that Mars had a warmer and thicker atmosphere in the past, when the Sun was less luminous, than it has now that the Sun is brighter. The warmth of Earth and Mars during their early periods (and thus the to the faint young Sun problem) can be attributed to the presence of abundant greenhouse gases in their atmospheres, with , water, and possibly and playing major roles.Habitable zones for high- and low-mass stars
The location of a habitable zone also depends upon its mass. Smaller stars like the
survive far longer than do high-mass stars. High-mass stars have lifetimes of only millions of years, whereas advanced life took billions of years to develop on . Thus, even if Earth-like planets formed around high-mass stars at distances where liquid water was stable, it is unlikely that conditions would exist long enough on these planets for life to form and evolve into advanced organisms.At the other end of the mass spectrum, the smallest, faintest stars can last for trillions of years. However, these cool s emit almost all of their luminosity at infrared wavelengths, which may be difficult for life to harness, and they typically display larger luminosity variations than do Sun-type stars. In addition, in order for a planet to remain within the habitable zone of a faint star, it would have to orbit so close that raised on the planet would cause the same hemisphere always to face the star (just as the
’s near side always faces Earth). As a result, there would be no day-night cycle, and the planet’s atmosphere, unless it was sufficiently thick, would freeze onto the surface of the cold, perpetually dark hemisphere. (However, if the planet had a sufficiently massive atmosphere, winds would redistribute heat and the atmosphere would not freeze.) Moreover, the high temperatures within the habitable zones of faint stars suggest that such planets are likely to lack the atmospheric gases required by life.Galactic habitable zone
The of a stellar habitable zone has been extended to a planet’s location in the . Near the centre of the Milky Way, stars are typically much closer to one another than they are farther out on the spiral arms, where the Sun is located. At the galactic centre, therefore, phenomena such as might present a greater hazard to life than they would in the region where Earth is located. On the other hand, in the outer regions of the Milky Way beyond the location of Earth, there are fewer stars. Since the bulk of a terrestrial planet is composed of s that were produced within stars, the material out of which new stars are being formed may not have enough of those elements necessary for Earth-like planets to grow. Considerations of this type have led to the concept of a galactic habitable zone, to a stellar habitable zone. The concept of a ’s habitable zone may well be viable, but the extent and boundaries of such a region are far more difficult to quantify than those of a star’s habitable zone.
How can we use this info to look beyond our solar system?
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