Enigmatic Universe

September 7, 2010

If its nighttime where you’re reading this, and if you’re blessed to live in an area that isn’t swamped by light pollution, you might be able to see the stars in our night sky. Just west of Taurus and just east of Pisces is the constellation Aries. Hiding between those stars is the galaxy dubbed NGC 772, an unbarred spiral galaxy 130 million light years away. Because it’s 130 million light years away, we can only get a picture of what this galaxy looked like 130 million years ago. What if with an especially-powerful telescope we saw a planet forming in some star in this galaxy? By the time that light got into our telescopes, that solar system today would be remarkably different from the image we see. Our view of these deep space objects are so out-of-date that we have virtually no way of knowing what is going on in these places in real-time.

So what if we had a telescope powerful enough to detect a certain dim star in that galaxy? Assuming that the star we were looking at was lined up in just the right way, we might be able to detect a planet orbiting in that system. And we point our telescope at this star and we find that periodically, that star gets even dimmer at a regular interval. We conclude that this dimming is caused by a planet that makes a transit between our telescope and that star, blocking some of our light and it passes in front. Astronomers call this the transit method of detection, and with this method, planet hunters have found hundreds of planets, some of them are even rocky like our planet Earth. For years, we check the regular dimming of this star and we take note of where and when the planet passes in front of its star. This occurs on regular intervals and we use it to determine the volume of the planet based on how much light is being blocked. This method called transit photometry; our current technology only allows us to find planets within a certain distance of the star it orbits, though, but it’s effective at seeking out smaller-mass planets like the rocky terrestrial worlds in our own inner solar system.

From there, we could figure out the planet’s mass based on its gravitational pull and radial velocity measurements to quickly we deduce from all of this data that our planet is made of rock, slightly more massive than earth, and orbiting close to its star. Because we have a powerful instrument that could detect the composition of this planet’s atmosphere, we learn to our surprise that this planet’s atmosphere is nitrogen-based with carbon dioxide, water vapor and other gases. Imagine our surprise when we discover this planet is similar to Earth in so many ways that it could be habitable to life as we know it.

Planet habitability is the measure of a planet’s potential to sustain life, which is determined by a wide range of factors. For instance, where a star is located in the galaxy can determine how likely it is life will arise on a planet orbiting that star. Stars located closer to the galactic center are more likely to be irradiated by the cosmic rays emitted by galaxy’s massive core. Also, stars form much denser clusters in the center of our galaxy, so it would be likely that these planets would get caught in the supernova explosions by their neighboring stars. This would mean that planets would get doused with high-energy radiation that would rip apart organic molecules and make it difficult for life to form. The Earth is located on the outer edge of the Milky Way galaxy, so we’re in less danger of this radiation bombardment. However, as we look at farther from the center of the galaxy, elements heavier than hydrogen and helium get rarer because a lower density of stars mean supernovas happen more infrequently and fewer metals get formed by nucleosynthesis, so building a planet made out of anything other than hydrogen gas is harder at the edge of our galaxy. Assuming a star is located in the area in between either extreme, it could be host to a habitable planet, one that is not only conducive to life as we know, but one that stays habitable long enough to spawn life.

Check out our next post when we take a closer look at a Solar system in the Arietis galactic habitable zone, home to a planet that will serve as the subject of our upcoming thought experiment.

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