Planets for Humanity
The Ultimate in Extrasolar Planets: Worlds Like Earthby Carl Martin
Islands in the Dark
In the early 1970's, I read a book by Stephen Dole and Isaac Asimov entitled, "Planets for Man." Okay, so the title is not exactly politically correct these days. But try to understand the idea from a child's point-of-view. Understand the exotic adventure of it, and how exciting the idea is, finding these islands in the dark of space.
The book laid the foundation for what planet hunting is all about, at least for me. You can keep all your gas giants. They may be interesting to look at or to think about, but as a tourist stop, how long does that last? About thirty seconds for some"And it took how many months to get here?" Yikes! Forty-eight months travel for a thirty second thrill does not seem worth the effort. That's for some people. For me though, the thrill of planet approach and departure would last for several days, if not a lifetime. And I would never tire of it. But I'm unusual. Space has been in my blood for nearly sixty years.
Dole and Asimov's book fed a hunger which had been building throughout my childhood and teenage yearssince before Sputnik. I had found the book in the downtown library in Phoenix, Arizona. I don't remember how many times I had checked it out, but I had savored each page and had taken copious notes.
I began to think about terraforming Mars and Venus, so that humanity would have nearby targets for space adventure or even commerce. I studied the mathematics of planetary atmospheres and calculated the time it would take to transport sufficient quantities of gases and water to each world. I devised schemes to cool off Venus and to warm up Mars so that the terraforming might have a chance of success.
I also began to wonder about other stars, near our own, which might harbor habitable worlds like Earth.
Stars Which are "Just Right"
Earth-like worlds won't be found in just any star system. Certain ingredients are needed to fulfill our quest.
Civilization, and even life, would not last long if the sun varied much in its light output. Imagine freezing temperatures one day in the tropics, and the oceans boiling in the Arctic on the next. Some stars vary enough to destroy any chance of habitability. On the cooler end of the stellar spectrum, some red dwarfs are also called "flare" stars because they flare up in brightness many times their usual norm. On the other end of the scale, when more massive stars first enter old-age, they go through turbulent periods of fluctuation as they move toward burning helium instead of hydrogen.
Most stars come in multiple star systems, and most multiples are binary stars. It wouldn't do to have two sun-like stars orbiting each other at the distance found between Earth and Sol (our own sun). Where would an Earth-like world fit? Simple answer: it wouldn't. Only if the stars are very close or very far apart can one or more planets exist in stable orbits, otherwise things get crazy. Planets could be tugged into wild orbits of wildly swinging temperatures, or ejected from the system into the lonely coldness of interstellar space, or worse, set on a collision course with one of its sibling worlds or one of its suns. Having a stable orbit is so important to a habitable world.
A parent star, typically called the system "primary," must be at least a certain age. Why? The age of the star closely translates to the age of the system, and young systems still have a great deal of formation going on. That means collisions! Lots of them. It won't do to settle on a planet which experiences heavy meteor showers on a semi-regular basis. This means that the system needs to be relatively clean of planet crossing asteroids and comets. And it takes two to three billion years to accomplish this.
Some stars are automatically disqualified. The brighter, more massive main sequence stars will never reach the proper age. They will balloon out to become red giants long before then. We find the right candidates from about spectral type F2 V on down to the cooler varieties of stars. Stars in this range can still prove to be young, but only these less massive stars will ever reach the proper age before becoming unstable in their old age.
Some stars don't have all of the right stuff. They are said to be "metal poor," which means they lack oxygen, carbon, nitrogen and other essential ingredients for life. A star may have some of these, but might be poor enough in these elements that their planets are also relatively metal poor. Planets may have mostly hydrogen and helium and thus provide no place for habitation. A less extreme system might have worlds which are largely water and no landa sterile sea without the nutrients needed by even the simplest forms of life. A good astronomer's gauge for this is something called "Fe/H"the ratio of iron to hydrogen compared to the same ratio in another star (typically our own sun).
The Goldilocks Zone
If you happen to be out camping, you know how nice a campfire can be on a particularly cold night. Get too close, though, and you could burn yourself. Stay too far away, and you could freeze. Planets are like that too. Just because the system primary consists of twin suns does not guarantee its planets a toasty future. Only those at the right distance can stay comfortably warm.
This "just right" range is sometimes called the "shell of habitability," "eco-zone" or "Goldilocks zone." Like most things in nature, there is no hard boundary. A planet with thicker atmosphere and larger oceans can stay warm at a greater distance than one without those qualities. This range in a star system also depends on the total brightness of the stars which act as primary suns. Planets orbiting red dwarfs have to huddle far closer to its weak fire than would planets in a brighter "mid-dwarf" sun.
Gliese 581g (Unconfirmed)
The closest planet to "Earth-like" that we have found yet might just be orbiting the red dwarf, Gliese 581. The as yet unconfirmed planet, labeled "g" by scientists, orbits within the star's "Goldilocks Zone." Also, the planet possesses one of the lightest masses yet known for an extrasolar planetthree to four times the mass of Earth.
Though the planet may possess life, if all the right factors work in its favor, Gliese 581g proves to be too massive for most humans. The surface gravity would be uncomfortable to endure and visitors would be at greater risk from life-threatening accidents. Yet, if the planet has a density similar to that of Earth, the surface gravity would be only slightly greater than Dole's recommended upper limit of 1.5g (where this "g" is the gravitational acceleration at the surface of Earth).
Gliese 581, the star, tidally locks planet "g" so that one side perpetually faces its sun, while the other side never sees the light of daybaked on one side, frozen on the other. Only near the terminator (the line between night and day) can the temperatures approach those where life as we know it thrives. This, of course, assumes that Gliese 581g has an atmosphere similar to that on Earth. If it possesses one more like Mars, then the extremely thin air could not hold or transport heat, and would not allow any surface water to exist. If Gliese 581g holds a thick blanket similar to that surrounding Venus, then the entire planet may be wrapped in a super-heated, greenhouse climate where liquid water would be impossible on the surface.
Though Gliese 581g is an exciting find (and still unconfirmed), it remains unsatisfactory to this adventurer. I'm still holding out for the grail of all extrasolar planetsone that is truly Earth-likecomfortable gravity, rotation, temperature and plenty of water.
Where to Look
In the Solar vicinity, we find quite a few stars which fulfill our requirements. They are both ancient and chemically rich. These "ancient suns" are just right for habitable worlds like Earth.
The hunger for extrasolar, Earth-like planets drove me to push the envelope of my own personal creativity. In 2001, I designed and programmed the 3D astronomy space software, "Stars in the NeighborHood." I wanted to see for myself the "lay of the land" in our neck of the Milky Way galaxy. As I poured over the journal articles on astronomical science, I learned how our scientists are learning more and more about the aging process of stars. I learned that we now have methods for determining the approximate ages of individual starssomething we did not have when Stephen Dole wrote his seminal paper for the Rand Corporation, "Habitable Planets for Man." Now we have enough information to peg which stars are "just right" for finding planets like Earthsomething I call in the software, "Garden Spots of the Galaxy."
3D astronomy space software, Stars in the NeighborHood, available at www.SpaceSoftware.Net.