It is theorized that our moon was formed when our planet was struck a glancing blow by an object roughly the size of Mars. The notion is known as the giant impact hypothesis. If such a large impact happened to our planet, what about the other planets. How common are giant impacts within our solar system? From the loss of the outer layers of Mercury, to the two-faced appearance of Mars, to the tipped over condition of Uranus, it seems that about half our planets were struck hard enough, by large enough objects, to have a major effect on how the planets appear today.
According to the rare Earth hypothesis, see episode 59, a large moon is needed for the development of complex life. Our moon isn’t the biggest moon in our solar system; but the moons that are bigger are orbiting much larger planets. Our moon is the largest as a percentage of the planet it orbits. Measured in that way, no other moon comes close. So how rare is such a large moon? Where did our big beautiful moon even come from?
Here are some articles about our moon and where it may have come from.
Today, we consider all the large planets orbiting stars other than our sun, and their tendency to adopt eccentric orbits. The possible reasons include close encounters with other stars, interactions between the planets themselves, and different ways the large planets may have formed.
Here’s an article on the large size of Jupiter, and the core accretion model, versus disk instability as possible explanations.
Here are several articles on computer simulations of planet to planet interaction, which could cause orbits to become eccentric, or planets to adopt a short-term nearly circular orbit, or even flip over.
According to the “rare earth” hypothesis, see episode 59, one of the requirements for the development of complex life is a stable, and nearly circular orbit. If the orbit is too eccentric, the planet would be cooked during one part of its year, and frozen most of the rest of the time. That would mean the temperature extremes would be too much for complex life to arise. Since our solar system has planets with roughly circular orbits, it was assumed that such orbits were common. Once we started detecting planets around other stars, we found many planets whose orbits were anything but circular. As it happened, the methods first used to detect exoplanets, see episode 56, were much better at finding very large planets. More recent methods, see episode 57, have allowed us to look at planets that are closer in size to our own Earth. A recent study suggests that Earth sized planets are far more likely to adopt roughly circular orbits, a hopeful sign for the occurrence of complex life elsewhere in our Galaxy.
Here are a couple of articles about the study. Both of them say about the same thing, but each one includes slightly different details.
not quite a star, not quite a planet, not quite life
After a somewhat disjointed primer on organic chemistry, we talk about how the radiation of a protostar, bathing the protoplanetary disk, See the previous episode, can create the early chemical building blocks of life. This has happened in laboratory experiments, and the chemicals have been observed around young stars, in the material of comets, and in meteorites. This suggests that the very beginning of what would become life, happened during the very beginning of what would become our solar system, while we were not quite a star, not quite a planet and not quite life.
Here are a couple of articles about the laboratory experiments.
Burning through the atmosphere can get warm enough, but here’s an article that describes a meteorite that was heated much more, before it wiazed through the air and hit the surface of Earth. And yet, it still brought amino acids along for the ride.
As a nebula collapses, there are forces which resist the collapse. Things like rotation, ionization and heat can overwhelm gravity and keep a given chunk of dust and gas from ever managing to start nuclear fusion and become a star. Those same forces, if the cloud manages to become a star, can help to form planets.
Here’s an article on how our solar system got the infusion of heavy elements needed to form rocky planets like our Earth.
Here’s an article on an early stage collapsing cloud of dust and gas in the Eagle Nebula that has roughly the same amount of material as our solar system.
In episode 59, we talked about the “rare Earth hypothesis.” According to that school of thought, when and where a star is born, and when and where it lives, matters. Our Sun apparently showed up after a very active epic of star formation. This may have protected our baby solar system from being bathed by too much radiation. In addition, we orbit our galaxy in an area that has enough heavy elements for making rocky planets, but not too close to the overly hot and violent center of the milky way. Only, that’s not where we started. In fact, nobody is quite certain where we started, or how we got where we are now.
Here’s a NASA press release about the evolution of spiral galaxies, and evidence that suggests that stars were being born, around 10,000,000,000 years ago, at roughly 30 times the rate they are being born now.
What we need is our rare and wonderful Earth, and approximately 4.54 billion years. Of course, that begs the question. How did we end up with our Earth, and how important is it that a planet is like our Earth to create intelligent tool users? According to the “Rare Earth Hypothesis,” to get minds, many highly improbable things, things that happened to our planet, must take place. Otherwise you don’t even get to create anything as complicated as a flatworm, let alone intelligent tool users. Over the next several episodes, as we examine how our planet gave rise to our species, we’ll revisit this hypothesis and consider which of many factors are necessary, rare, or common in our cosmos.
Here are a couple of articles with further details on the “Rare Earth Hypothesis.”
In episode 56 and episode 57, we looked at a couple of methods of detecting planets that are orbiting around stars other than our own sun. These methods involve a good deal of analysis and inference. Today, we learn about how astronomers can look directly at a planet around another star, once the overwhelming glare of the star is blocked out.
Since this is the last episode on finding exoplanets, here are a couple of links to pages about finding them, should you desire to dig a bit deeper.
Unlike the method described in the previous episode, the transit method allows one to look at many stars at a time. When a planet crosses between us and the star it’s orbiting, the star’s light dims very slightly. If we can detect that dimming, we can detect said planet.
Here’s a link to the Kepler mission, that used the transit method to detect many extra solar planets, including some that are roughly Earth sized, apparently somewhat Earth like, and even orbiting in their star’s habitable zone—not too far or too close and thus possibly with liquid water, a prerequisite for life.