Will it have a civilization? How can we know which stars are the best places to look? Before we dive into the details in order to answer this last question, let us look at a useful fairy tale which can be our guide by analogy.
Have you ever heard the story of Goldilocks and the Three Bears? In the most popular telling of the story, a lost young girl with golden-colored locks discovers the home of three bears—a momma bear, a papa bear and baby bear. Inside she finds three chairs, three bowls of porridge sitting on a table and three beds. Being hungry from her hours of being lost, she tries the largest bowl and finds the porridge too hot. Next, she tries the smallest bowl and finds it to be too cold. Finally, she eats from the middle bowl and finds it to be just right. With great relief, she gobbles down the medium bowl’s contents. There’s more to the original story, but we have the part that concerns us.
Stars are like giant fireplaces. The closer we get to them, the warmer we become.
Get too close, and we become too hot. Move too far away, and we become too cold. In between these zones of too much heat and too much cold, we find a zone that is just right. Scientists call this the “Goldilocks zone.” This is where we would find, for any one star, a planet that can sustain liquid water, like the oceans on our own planet Earth.
These are some of the basics of astronomy and extrasolar planetary science.
Requirements of Age and Chemistry
There are two other key requirements for our desired star system:
To go back to our Goldilocks example, the maturity of the star system is a bit like the age of the bowl of porridge. Say the oatmeal was poured into the boiling water only moments before, each grain of oats has not had any time to soak up the water. The meal is still tough and hard to eat. In a young star system, the planets are not yet fully formed.
Any planet younger than 2 or 3 billion years is still in the process of formation. Such planets are more likely to be bombarded by large meteors, asteroids or comets. This makes planets in young star systems dangerous places for life.
What do we mean by “rich in chemistry?” Using our Goldilocks analogy, again, a poor bowl of porridge would be more like a weak form of gruel—extremely watered down. Imagine a bowl of water with only three grains of oatmeal. That’s not very delicious. In a chemically poor star system, the planets are likely made of hydrogen and helium and very little of anything else. If a planet were to form in the Goldilocks zone, it might have to be a giant in order to keep from being blown away by the stellar wind. Or it might form an Earth-sized planet made almost entirely out of water and other light
elements and compounds. Such a planet would not have any land and likely would never develop any kind of life because of the lack of proper chemistry.
Today, scientists can tell whether the star is rich or poor in its chemistry. Quite often, they will rate a star by the amount of iron they find in its atmosphere. A star with lots of iron is more likely to have planets, and those planets are more likely to have iron cores like our own Earth. They call the metal richness of the star its “metallicity.”
Scientist are also becoming more adept at determining the ages of individual stars. In other words, they can tell for some stars whether they are old enough to have mature, fully-formed planets.
Protecting the Goldilocks Zone
The best kind of star system for an Earth-like planet has only one sun. An Earth- like world needs to have a stable orbit within the Goldilocks zone. But a compatible star system can also have two suns very close to one another—a very close binary star, frequently called a “spectral binary,” because of the way they are detected. These are systems where the two stars are so close together that they cannot be seen as separate stars through an optical telescope. An Earth-like planet could orbit both stars without much danger that the two stars would disrupt the planet’s orbit.
A wide binary star system could also support an Earth-like world, but that planet would orbit only one of the stars.
We run into a problem with star systems that are between “wide” and “close” binaries. Any Earth-like planet orbiting one of the stars in a binary system with medium separation would be pulled out of orbit by the other star. Or, if the planet were to orbit both stars, the distance to a stable circular orbit would have to be so great that the planet would be far too cold—too far from the Goldilocks zone.
To protect the Goldilocks zone, we need a star system that will allow a stable circular orbit within our “just right” distance.
With the right camera and telescope you can study wide and medium close binary stars. There is nothing quite as exciting as seeing real live binary star systems through your own telescope and documenting their current positions as the stars orbit one another. Imagine what the planets in each of those star systems must be like. When the conditions are “just right,” the system may very well have an Earth-like world.